Hawkins Electrical Guide v. 08 (of 10)
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Title: Hawkins Electrical Guide v. 08 (of 10)
Author: N. Hawkins
Release date: September 28, 2015 [eBook #50068]
Most recently updated: October 22, 2024
Language: English
Other information and formats: www.gutenberg.org/ebooks/50068
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*** START OF THE PROJECT GUTENBERG EBOOK HAWKINS ELECTRICAL GUIDE V. 08 (OF 10) ***
Transcriber's Notes:
Underscores "_" before and after a word or phrase indicate _italics_
in the original text.
Equal signs "=" before and after a word or phrase indicate =bold=
in the original text.
Small capitals have been converted to SOLID capitals.
Illustrations have been moved so they do not break up paragraphs.
Misprints in the table SAVING DUE TO HEATING THE FEED WATER, Pg. 1936
have been corrected, they are:
Init. Temp. Pressure Old Value New Value
130 40 .0954 .0934
200 40 .0900 .0999
210 40 .1000 .1010
230 100 .0117 .1012
In the original text, there are two Fig. 2769's and two Fig. 2770's.
The second of each has had an "a" suffix added, i.e. 2769a and 2770a.
On line 7397 the word "impedence" was corrected to "impedance".
Superscripts are shown as ^{d} and subscripts are shown as _{d}, where
"d" is an integer.
Inconsistent spelling and hyphenation has been left as in the original.
THE THOUGHT IS IN THE QUESTION
THE INFORMATION IS IN THE ANSWER
HAWKINS ELECTRICAL GUIDE NUMBER EIGHT
QUESTIONS ANSWERS & ILLUSTRATIONS
A PROGRESSIVE COURSE OF STUDY FOR ENGINEERS,
ELECTRICIANS, STUDENTS AND THOSE DESIRING TO
ACQUIRE A WORKING KNOWLEDGE OF
ELECTRICITY AND ITS APPLICATIONS
A PRACTICAL TREATISE by HAWKINS AND STAFF
THEO AUDEL & CO. 72 FIFTH AVE. NEW YORK.
COPYRIGHTED, 1915, BY THEO. AUDEL & CO.,
NEW YORK.
Printed in the United States.
TABLE OF CONTENTS
GUIDE No. 8
=WAVE FORM MEASUREMENT= 1,839 to 1,868
Importance of wave form measurement--=methods=:
step by step; constantly recording--=classes of
apparatus=: wave indication; _oscillographs_--=step
by step methods=--Joubert's; four part commutator;
modified four part commutator; ballistic galvanometer;
zero; Hospitalier ondograph--=constantly recording
methods=: cathode ray; glow light; moving iron; moving
coil; hot wire--=oscillographs=--moving coil type;
construction and operation; production of the time scale;
oscillograms--falling plate camera; its use.
=SWITCHBOARDS= 1,869 to 1,884
=General principles=: diagram--small plant a.c.
switchboard--=switchboard panels=; =generator
panel=; diagram of connections--simple method of
determining bus bar capacity--feeder panel--diagrams of
connection for two phase and three phase installations.
=ALTERNATING CURRENT WIRING= 1,885 to 1,914
Effects to be considered in making
calculations--=induction=; self- and mutual; mutual
induction, how caused--=transpositions=--inductance
per mile of three phase circuit, table--=capacity=;
table--=frequency--skin effect=; calculation;
table--=corona effect=; its manifestation;
critical voltage; spacing of wires--=resistance of
wires--impedance--power factor=; apparent current;
usual power factors encountered; example--=wire
calculations--sizes of wire--table of the property of
copper wire--drop=; example--current--example covering
horse power, watts, apparent load, current, size of wire,
drop, voltage at the alternator, and electrical horse power.
=POWER STATIONS= 1,915 to 1,988
Classification--=central stations=; types: a.c.,
d.c., and a.c. and d.c.; reciprocating engine vs.
turbine--=location of central stations=; price of
land; trouble after erection; water supply; service
requiring direct current--=size of plant=; nature
of load; peak load; load factor; machinery required;
example; factors of evaporation; grate surface per
horse power--=general arrangement of station=;
belt drive with counter shaft; desirable features of
belt drive; conditions, suitable for counter shaft
drive; location of engine and boilers; the steam pipe;
piping between engine and condenser; number and type
of engine; superheated steam; switchboard location;
individual belt drive; direct drive--=station
construction=--=foundations=--=walls=--
=roofs=--=floors=--=chimneys=;
cost of chimneys and mechanical draft; high chimneys
ill advised--=steam turbine=; types: impulse
and reaction; why high vacuum is necessary; the
working pressure--=hydro-electric plants=--water
turbines; types: impulse, reaction--=isolated
plants=--=sub-stations=; arrangement; three phase
installations; reactance coils in sub-stations; portable
sub-stations.
=MANAGEMENT= 1,989 to 2,114
The term "management"--=selection=; general
considerations--=selection of generators=;
efficiency of generators; size and number;
regulation--=installation=; precautions;
handling of armatures; assembling a machine; speed
of generators; calculation of pulley sizes; gear
wheels--=belts=; various belt drives; horse
power transmitted by belts; velocity of belt; endless
belts--=switchboards=; essential points of difference
between single phase and three phase switchboard wiring;
assembling a switchboard; usual equipment.
=Operation of Alternators--alternators in parallel=;
synchronizing; lamp methods; action of amortisseur winding;
synchronizing three phase alternators; disadvantage of
lamp method--=cutting out alternator=; precautions;
hunting--=alternators in series=.
=Transformers=; selection; efficiency; kind of oil
used; detection of moisture; drying oil; regulation;
transformers in parallel; polarity test--=motor
generators=; various types and conditions requiring
same--=dynamotors=; precautions--=rotary
converters=; objections to single phase type; operation
when driven by direct current, by alternating current; most
troublesome part; efficiency; overload; starting; starting
current.
=Electrical measuring instruments=; location;
readings; station voltmeters; points relating to
ammeters; attention necessary; usual remedies to correct
voltmeter--=how to test generators=; commercial
efficiency; various tests.
=Station Testing:= resistance measurement by "drop"
method--methods of connecting ammeter voltmeter and
wattmeter for measurement of power--=motor testing:=
single phase motor--three phase motor, voltmeter and
ammeter method; two wattmeter method; polyphase wattmeter
method; one wattmeter method; one wattmeter and Y box
method--three phase motor with neutral brought out; single
wattmeter method--temperature test, three phase induction
motor--=three phase alternator testing:= excitation
or magnetization curve test--synchronous impedance
test--load test--three phase alternator or synchronous
motor temperature test--=direct current motor= or
=generator testing:= magnetization curve--(shunt)
external characteristic--direct current motor testing;
load and speed tests--temperature test, "loading back"
method--=compound dynamo testing=: external
characteristic, adjustable load--=transformer testing=:
external characteristic, adjustable load--=transformer
testing=: core loss and leakage or exciting current
test--copper loss--copper loss by wattmeter measurement
and impedance--temperature--insulation--internal
insulation--insulation resistance--polarity--winding
or ratio tests.
CHAPTER LXIII
WAVE FORM MEASUREMENT
The great importance of the wave form in alternating current work
is never denied, though it has sometimes been overlooked. The
application of large gas engines to the driving of alternators
operated in parallel requires an accurate knowledge of the wave form,
and a close conformation to a sine wave if parallel operation is
to be satisfactory. It is also important that the fluctuations in
magnetism of the field poles should be known, especially if solid
steel pole faces be used.
If an alternator armature winding be connected in delta, the
presence of a third harmonic becomes objectionable, as it gives
rise to circulating currents in the winding itself, which increase
the heating and lowers the efficiency of the machine.
That the importance of having a good wave form is being
realized, is proved by the increasing prevalence in alternator
specifications of a clause specifying the maximum divergence
allowable from a true sine wave. It is however perhaps not always
realized that an alternator which gives a good pressure wave on
no load may give a very bad one under certain loads, and the
ability of the machine to maintain a good wave form under severe
conditions of load is a better criterion of its good design than
is the shape of its wave at no load.
The question of wave form is of special interest to the power
station engineer. Upon it depends the answer to the questions:
whether he may ground his neutral wires without getting large
circulating currents; whether he may safely run any combination
of his alternators in parallel; whether the constants of his
distributing circuit are of an order liable to cause dangerous
voltage surges due to resonance with the harmonics of his
pressure wave; what stresses he is getting in his insulation due
to voltage surges when switching on or off, etc. It has been
shown by Rossler and Welding that the luminous efficiency of the
alternating current arc may be 44 per cent. higher with a flat
topped than with a peaked pressure wave, while on the other hand
it is well known that transformers are more efficient on a peaked
wave. Also the accuracy of many alternating current instruments
depends upon the wave shape.
In making insulation breakdown tests on cables, insulators, or
machinery, large errors may be introduced unless the wave form at
the time of the test be known. It is not sufficient even to know
that the testing alternator gives a close approximation to a sine
wave at no load; since if the capacity current of the apparatus
under test be moderately large compared with the full load current
of the testing alternator, the charging current taken may be
sufficient to distort the wave form considerably, thus giving
wrong results to the disadvantage of either the manufacturer or
purchaser.
[Illustration: FIG. 2,583.--General Electric simultaneous record of
three waves with common zero.]
The desirability of a complete knowledge of the manner in which the
pressure and current varies during the cycle, has resulted in various
methods and apparatus being devised for obtaining this knowledge.
The apparatus in use for such purpose may be divided into two general
classes,
1. Wave indicators;
2. Oscillographs.
and the methods employed with these two species of apparatus may be
described respectively as,
1. Step by step;
2. Constantly recording.
that is to say, in the first instance, a number of instantaneous
values are obtained at various points of the cycle, which are plotted
and a curve traced through the several points thus obtained. A
constantly recording method is one in which an infinite number of
values are determined and recorded by the machine, thus giving a
complete record of the cycle, leaving no portion of the wave to be
filled in.
[Illustration: FIG. 2,584.--General Electric simultaneous record of
three waves with separate zeros.]
[Illustration: Figs. 2,585 and 2,586.--Oscillograms (from paper by
Morris and Catterson-Smith, Proc. I. E. E., Vol. XXXIII, page 1,023),
showing _how the current varies_ =in one of the armature coils of
a direct current motor=. Fig. 2,585 was obtained with the brushes
in the neutral position, and fig. 2,586 with the brushes shifted
forward.]
The various methods of determining the wave form may be further
classified as:
{ Joubert's method;
{ Four part commutator method;
{ Modified four part commutator method;
1. Step by step { Ballistic galvanometer method;
{ Zero method;
{ By Hospitalier ondograph.
[Illustration: FIG. 2,587.--=Oscillogram= by Bailey and Cleghorne
(Proc. I.E.E., Vol. XXXVIII), =showing= _the sparking pressure or
pressure between the brush and the commutator segment at the moment
of separation_. The waves fall into groups of three owing to the fact
that there were three armature coils in each slot.]
{ cathode ray;
{ by use of various types { glow light;
2. constantly recording { of =oscillograph=, { moving iron;
{ such as { moving coil;
{ hot wire.
[Illustration: FIG. 2,588.--Various wave forms. The sine wave
represents a current or pressure which varies according to the sine
law. A distorted wave is due to the properties of the circuit, for
instance, the effect of hysteresis in an iron core introduced into a
coil is to distort the current wave by adding harmonics so that the
ascending and descending portions may not be symmetrical. A peaked
wave has a large maximum as compared with its virtual value. A peaked
wave is produced by a machine with concentrated winding.]
=Joubert's Method.=--The apparatus required for determining the
wave form by this step by step method, consists of a galvanometer,
condenser, two, two way switches, resistance and adjustable contact
maker, as shown in fig. =2,589=.
The contact maker is attached to the alternator shaft so that
it will rotate synchronously with the latter. By means of the
adjustable contact, the instant of "making" that is, of "closing"
the testing circuit may be varied, and the angular position of the
armature, at which the testing circuit is closed, determined from
the scale, which is divided into degrees.
A resistance is placed in series with one of the alternator
leads, such that the drop across it, gives sufficient pressure for
testing.
=Ques. Describe the method of making the test.=
[Illustration: FIG. 2,589.--Diagram illustrating Joubert's =step by
step method= of wave form measurement.]
Ans. For current wave measurement switch No. 1 is placed on contact
F, and for pressure wave measurement, on contact G, switch No. 2 is
now turned to M and the drop across the resistance (assuming switch
No. 1 to be turned to contact F) measured by charging the condenser,
and then discharging it through the galvanometer by turning the
switch to S. This is repeated for a number of positions of the
contact maker, noting each time the galvanometer reading and position
of the contact maker. By plotting the positions of contact maker as
abscissæ, and the galvanometer readings as ordinates, the curve drawn
through them will represent the wave form.
[Illustration: FIG. 2,590.--=Four part commutator method= of wave
form measurement. The contact device consists of two slip rings and
a four part commutator. One slip ring is connected to one terminal
of the source, the other to the voltmeter, and the commutator to the
condenser. By adjusting R when a known direct current pressure is
impressed across the terminals, the voltmeter can be rendered direct
reading.]
[Illustration: FIG. 2,591.--=Modified four part commutator method=
of wave form measurement (Duncan's modification). By this method
one contact maker can be used for any number of waves having the
same frequency. Electro-dynamometers are used and the connections
are made as here shown. The moving coils are connected in series to
the contact maker, and the fixed coils are connected to the various
sources to be investigated, then the deflection will be steady and
by calibration with direct current can be made to read directly in
volts.]
The apparatus is calibrated by passing a known constant current
through the resistance.
[Illustration: FIG. 2,592.--Diagram illustrating the =ballistic
galvanometer method= of wave form measurement. =The test may be made=
as described in the accompanying text, or in case the contact breaker
is belted instead of attached rigidly to the shaft, it could be
arranged to run slightly out of synchronism, then by taking readings
at regular intervals, points will be obtained along the curve without
moving the contact breaker. If this method be used, a non-adjustable
contact breaker suffices. =In arranging the belt drive= so as to run
slightly out of synchronism, if the pulleys be of the same size, the
desired result is obtained by pasting a thin strip of paper around
the face of one of the pulleys thus altering the velocity ratio of
the drive slightly from unity.]
=Ballistic Galvanometer Method.=--This method, which is due to
Kubber, employs a _contact breaker_ instead of a _contact maker_.
The distinction between these two devices should be noted: A contact
maker keeps the circuit _closed_ during each revolution for a short
interval only, whereas, a contact breaker keeps the circuit _open_
for a short interval only.
Fig. 2,592, shows the necessary apparatus and connections for
applying the ballistic galvanometer method. The contact breaker
consists of a commutator having an ebonite or insulating segment
and two brushes.
_In operation_ the contact breaker keeps the circuit closed
during all of each revolution, except the brief interval in which
the brushes pass over the ebonite segment.
The contact breaker is adjustable and has a scale enabling its
various positions of adjustment to be noted.
=Ques. Describe the test.=
[Illustration: FIGS.. 2,593 and 2,594.--=Two curves= _representing
pressure and current respectively of a rotary converter._ Fig. 2,593,
pressure wave V, fig. 2,594 current wave C. These waves were obtained
from a converter which was being driven by an alternator by means of
an independent motor. The rotary converter was supplying idle current
to some unloaded transformers and the ripples clearly visible in the
pressure wave V, correspond to the number of teeth in the armature of
the rotary converter.]
Ans. The contact breaker is placed in successive positions and
galvanometer readings taken, the switch being turned to F, fig.
2,592, in measuring the current wave, and to G in measuring the
pressure wave. The results thus obtained are plotted giving
respectively current and pressure waves.
=Ques. How is the apparatus calibrated?=
Ans. By sending a constant current of known value through the
resistance R.
=Zero Method.=--In electrical measurements, a zero method is one _in
which the arrangement of the testing devices is such that the value
of the quantity being measured is shown when the galvanometer needle
points to_ =zero=.
In the zero method either a contact maker or contact breaker may be
used in connection with a galvanometer and slide wire bridge, as
shown in figs. 2,595 and 2,596.
[Illustration: FIG. 2,595.--Diagram illustrating zero method of wave
measurement with _contact_ =maker=. The voltage of the battery must
be at least as great as the maximum pressure to be measured and must
be kept constant.]
=Ques. What capacity of battery should be used?=
Ans. Its voltage should be as great as the maximum pressure to be
measured.
=Ques. What necessary condition must be maintained in the battery?=
Ans. Its pressure must be kept constant.
=Ques. How are instantaneous values measured?=
Ans. The bridge contact A is adjusted till the galvanometer shows no
deflection, then the length AS is a measure of the pressure.
The drop between these points can be directly measured with a
voltmeter if desired.
=Ques. How did Mershon modify the test?=
Ans. He used a telephone instead of the galvanometer to determine the
correct placement of the bridge contact A.
[Illustration: FIG. 2,596.--Diagram illustrating zero method of wave
measurement with _contact_ =breaker=. The voltage of the battery must
be at least as great as the maximum pressure to be measured and must
be kept constant.]
=Ques. How can the instantaneous values be recorded?=
Ans. By attaching to the contact A, a pencil controlled by an
electro-magnet arranged to strike a revolving paper card at the
instant of no deflection, the paper being carried on a drum.
=Hospitalier Ondograph.=--The device known by this name is a
development of the Joubert step by step method of wave form
measurement, that is to say, the principle on which its =action is
based=, consists in _automatically charging a condenser from each
100th wave, and discharging it through a recording galvanometer, each
successive charge of the condenser being automatically taken from a
point a little farther along the wave._
[Illustration: FIG. 2,597.--Diagram of Hospitalier ondograph showing
mechanism and connections. It represents a development of Joubert's
step by step method of wave form measurement.]
As shown in the diagram, fig. 2,597, the ondograph consists of
a synchronous motor A, operated from the source of the wave form
to be measured, connected by gears B to a commutator D, in such a
manner that while the motor makes a certain number of revolutions,
the commutator makes a like number diminished by unity; that is to
say, if the speed of the motor be 900 revolutions per minute, the
commutator will have a speed of 899.
The commutator has three contacts, arranged to automatically
charge the condenser _cc'_ from the line, and discharge it through
the galvanometer E, the deflection of which will be proportional
to the pressure at any particular instant when contact is made.
In fig. 2,597, GG' are the motor terminals, HH' are connected to
the condenser _cc'_ through a resistance (to prevent sparking at
the commutator) and I, I' are the connections to the service to be
measured.
A permanent magnet type of recording galvanometer is employed.
Its moving coil E receives the discharges of the condenser in
rapid succession and turns slowly from one side to the other.
[Illustration: FIG. 2,598.--View of Hospitalier ondograph. =In
operation=, a long pivoted pointer carrying a pen and actuated by
electro-magnets, records on a revolving drum a wave form representing
the alternating current, pressure or current wave.]
The movable part operates a long needle (separately mounted)
carrying a pen F, which traces the curve on the rotating cylinder
C. This cylinder is geared to the synchronous motor to run at such
a speed as to register three complete waves upon its circumference.
By substituting an electromagnetic galvanometer for the
permanent magnet galvanometer, and by using the magnet coils as
current coils and the moving coil as the volt coil, the instrument
can be made to draw watt curves. Fig. 2,598 shows the general
appearance of the ondograph.
=Cathode Ray Oscillograph.=--This type of apparatus for measuring
wave form was devised by Braun, and consists of a cathode ray tube
having a fluorescent screen at one end, a small diaphragm with a
hole in it at its middle, and two coils of a few turns each, placed
outside it at right angles to one another. These coils carry currents
_proportional to the_ =pressure= _and_ =current= _respectively_ of
the circuit under observation.
[Illustration: FIG. 2,599.--General Electric =moving coil
oscillograph= complete =with tracing table=. The tracing table is
employed for observing the waves, and by using a piece of transparent
paper, the waves under observation appear as a continuous band of
light which can be traced, thus making a permanent record. This is
not, however, to be regarded as a recording attachment, and can not
be used where instantaneous phenomena are being investigated. =The
synchronous motor= for operating the synchronous mirror in connection
with tracing and viewing attachment is wound for 100 to 115 volts, 25
to 125 cycles, and should, of course, be run from the same machine
which furnishes power to the circuit under observation. A rheostat
for steadying and adjusting the current should be connected in series
with the motor. =The beam from the vibrator mirrors= _striking this
synchronous mirror moves back and forth over the curved glass, and
gives the length of the wave; the movement of the vibrator mirror
gives the amplitude, and the combination gives the wave complete_. An
arc lamp or projection lantern produces the image reflected by the
mirrors upon the film, tracing table or screen. For the rotation of
the photographic film, a small direct current shunt wound motor is
ordinarily used.]
The ray then moves so as to produce an energy diagram on the
fluorescent screen.
[Illustration: FIG. 2,600.--General Electric moving coil
oscillograph. =The moving elements= _consist of single loops of flat
wire carrying a small mirror and held in tension by small spiral
springs_. The current passing down one side and up the other, forces
one side forward and the other backward, thus causing the mirror to
vibrate on a vertical axis. The vibrator elements fit into chambers
between the poles of electro-magnets, and are adjustable, so as to
move the beam from the mirror, both vertically and horizontally. A
sensitized photographic film is wrapped around a drum and held by
spring clamps. The drum, with film, is placed in a case and a cap
then placed over the end, making the case light, when the index is
either up or down. The loading is done in a dark room. A driving dog
is screwed into the drum shaft, and which, when the drum and case are
in place, revolves the film past a slot. =When an exposure is to be
made=, the index is moved from the closed position, thus opening the
slot in the case and exposing the film to the beam of light from the
vibrating mirrors when the electrically operated shutter is open. The
slot is then closed by moving the index to "=Exposed=." A slide with
ground glass can be inserted in place of the film case or roll holder
to arrange the optical system when making adjustments. The shutter
operating mechanism is arranged so as to hold the shutter open during
exactly one revolution of the film drum. There are two devices
connected to the shutter operating mechanism; one opens the shutter
at the instant the end of the film passes the slot; the other opens
immediately, at any part of the film, and both give exposure during
one revolution. The first is useful when making investigations in
which the events are either recurring, or their beginnings known or
under control, and the second when the time of the event is not under
control, such as the blowing of fuses or opening of circuit breakers.]
The instrument is much used in wireless telegraphy, as it is capable
of showing the characteristics of currents of very high frequency.
[Illustration: FIG. 2,601.--General Electric =moving coil
oscillograph= _with case removed_, =showing= _interior construction
and arrangement of parts_. The oscillograph is furnished complete
with a three element electro-magnet galvanometer, optical system,
shutter and shutter operating mechanism, film driving motor and cone
pulleys, photographic and tracing attachments, 6 film holders, and
the following repair parts, for vibrators: 6 extra suspension strips;
6 vibrator mirrors; 1 box gold leaf fuses; 1 bottle mirror cement; 1
bottle damping liquid.]
[Illustration: FIG. 2,602.--Oscillogram showing the direct current
pressure of a 25 cycle rotary converter (below), and (above) the
pressure wave taken between one collector ring and one commutator
brush. The 12 ripples per cycles in the direct current voltage are
due to a 13th harmonic in the alternating current supply.]
=Glow Light Oscillograph.=--This device consists of two aluminum rods
in a partially evacuated tube, their ends being about two millimeters
apart. When an alternating current of any frequency passes between
them a sheath of violet light forms on one of the electrodes, passing
over to the other when the current reverses during each cycle. The
phenomenon may be observed or photographed by means of a revolving
mirror.
[Illustration: FIG. 2,603.--Curves by Morris, _illustrating the_
=dangerous rush of current which may occur when switching on a
transformer=. The circuit was broken at F and made again at G. The
current was so great as to carry the spot of light right off the
photographic plate due to the fact that a residual field was left in
the core after switching off, and on closing the switch again the
direction of the current was such as to tend to build up the full
flux in the same direction as this residual flux. =The dotted lines=
have been drawn in _to show how the actual waves were distorted from
the normal_.]
=Moving Iron Oscillograph.=--This type is due to Blondel, to whom
belongs the credit of working out and describing in considerable
detail the principles underlying the construction of oscillographs.
The moving iron type of oscillograph consists of a very thin
vane of iron suspended in a powerful magnetic field, thus forming
a polarized magnet. Near this strip are placed two small coils
which carry the current whose wave form is to be measured.
The moving iron vane has a very short period of vibration and
can therefore follow every variation in the current.
[Illustration: FIG. 2,604.--Siemens-Blondel =moving coil type=
oscillograph. The coil is in the shape of a loop of thin wire, which
is suspended in the field of an electro-magnet excited by continuous
current. The current to be investigated is sent through this loop,
which in consequence of the interaction of current and magnetic
field, begins to vibrate. The oscillations are rendered visible by
directing a beam of light from a continuous current arc lamp onto a
small mirror fixed to the loop. The light reflected by the mirror is
in the form of a light strip, but by suitable means this is drawn
out in respect of time, so that a curve truly representing the
current is obtained. The loop of fine wire is stretched between two
supports and is kept in tension by a spring. As the spring tension
is considerable, the directive force of the vibrating system is
large, and its natural periodicity very high. The mirror is fixed in
the center of the loop, and has an area of 1 square mm. In order to
protect the loops from mechanical injury they are built into special
frames. The mirrors are of various sizes, the loop for demonstration
purposes (projection device) being provided with the largest
mirror and the most sensitive loop with a mirror of the smallest
dimensions.]
Attached to the vane is a small mirror which reflects a beam of
light upon some type of receiving device.
The Siemens-Blondel oscillograph shown in fig. 2,604, is of
the _moving coil_ type, being a development of the moving iron
principle.
=Moving Coil Oscillograph.=--The operation of this form of
oscillograph is based _on the behaviour of a movable coil in a
magnetic field_.
[Illustration: FIGS. 2,605 and 2,606.--=Oscillograms= reproduced from
a paper by M. B. Field on "A Study of the =Phenomena of Resonance=
by the Aid of Oscillograms" (_Journal_ of _E. E._, Vol. XXXII). =The
effect of resonance= on the wave forms of alternators has been the
subject of much investigation and discussion; it is a matter of vital
importance to the engineer in charge of a large alternating current
power distribution system. Fig. 2,605 shows the pressure curve of an
alternator running on a length of unloaded cable, the 11th harmonic
being very prominent. Fig. 2,606 shows the striking alteration
produced by reducing the length of cable in the circuit and thus
causing resonance with the 13th harmonic.]
It consists essentially of a modified moving coil galvanometer
combined with a rotating or vibrating mirror, a moving
photographic film, or a falling photographic plate. The
galvanometer portion of the outfit is usually referred to as the
oscillograph as illustrated in figs. 2,608 to 2,612, representing
diagrammatically the moving system.
In the narrow gap between the poles S, S of a powerful magnet
are stretched two parallel conductors formed by bending a thin
strip of phosphor bronze back on itself over an ivory pulley P.
A spiral spring attached to this pulley serves to keep a uniform
tension on the strips, and a guide piece L limits the length of
the vibrating portion to the part actually in the magnetic field.
A small mirror M bridges across the two strips as shown. The
effect of passing a current through such a "vibrator" is to cause
one of the strips to advance while the other recedes, and the
mirror is thus turned about a vertical axis.
[Illustration: FIG. 2,607.--General view of electro-magnet
form of Duddell moving coil oscillograph, showing oil bath and
electro-magnet. This instrument is specially designed to have a very
high natural period of vibration (about 1/10,000 of a second) so as
to be suitable for accurate research work. It is quite accurate for
frequencies up to 300 per second. In the figure, A is the brass oil
bath in which two vibrators are fixed; B, core of electro-magnet
which is excited by two coils, one of which, C, is seen. The ends of
these two coils are brought out to four terminals at D, so that the
coils may be connected in series for 200 volt, or in parallel for 100
volt circuits. The bolts, E,E, hold the oil bath in position between
the poles of the magnet. F,F,F (one not seen), are levelling screws;
G,G, terminals of one vibrator; H, fuse; K, thermometer with bulb in
center of oil bath.]
[Illustration: FIGS. 2,608 to 2,612.--=Vibrator= of Duddell moving
coil oscillograph and =section through oil bath= of electro-magnet
oscillograph. =The vibrator consists of= a brass frame W, which
supports two soft iron pole pieces P,P. Between these, a long narrow
groove is divided into two parts by a thin soft iron partition,
which runs up the center. The current being led in by the brass wire
U, passes from an insulated brass plate to the strip, which is led
over an ivory guide block, down one of the narrow grooves and over
another guide block, the loops round the ivory pulley O, which puts
tension on the strip by the spring N, back to the guide block again,
up the other narrow groove, and out by way of the insulated brass
plate and lead U. Halfway up the grooves the center iron partition R
is partially cut away to permit of a small mirror M, bridging across
from one strip to the other, being stuck to the strips by a dot of
shellac at each corner. The figure illustrates one type of vibrator
in which P is removable from W for ease in repairing. In type 1,
these pole pieces P,P are not removable. =The vibrators= are placed
side by side in the gap between the poles S,S of the electro-magnet,
see fig. 2,610. Each vibrator is pivoted about vertical centers, the
bottom center fitting in the base of the oil bath, and the one at
the top being formed by a screw in the cock piece Y. It can thus be
easily turned in azimuth, its position being fixed by the adjusting
screw L, a spiral spring serving to keep the vibrator always in
contact with this screw. Since each cock piece can be independently
moved forward or backward, each vibrator can be tipped slightly in
either of these directions so that complete control over the mirrors
is obtained and reflected spots of light may be made to coincide with
that reflected from the fixed zero mirror, which latter is fixed to a
brass tongue in between the two vibrators. =A plano-convex lens= of
50 cm. focal length is fixed on the oil bath in front of the vibrator
mirrors to converge the reflected beams of light. It will be noticed
that this lens is slightly inclined so that no trouble will be given
by reflections from its own surface. The normal distance from the
vibrator mirrors to the scale of photographic plate is 50 cm., and
at this distance, a convenient working deflection on each side of
the zero line is 3 to 4 cm. This is obtained with a R.M.S. current
through the strips of from .05 to .1 of an ampere according to wave
form, etc. =The maximum deflection= on each side of the zero line
should not exceed 5 cm. while the maximum R.M.S. current through the
strips should in no case exceed .1 ampere.]
Each strip of the loop passes through a separate gap (not shown
in the figure). The whole of the "vibrator," as this part of the
instrument is called, is immersed in an oil bath, the object
of the oil being to damp the movement of the strips, and make
the instrument dead beat. It also has the additional advantage
of increasing by refraction the movement of the spot of light
reflected from the vibrating mirrors.
The beam of light reflected from the mirror M is received on
a screen or photographic plate, the instantaneous value of the
current being proportional to the linear displacement of the spot
of light so formed.
With alternating currents, the spot of light oscillates to and
fro as the current varies and would thus trace a straight line.
To obtain an image of the wave form, it is necessary to traverse
the photographic plate or film in a direction at right angles to
the direction of the movement of the spot of light.
[Illustration: FIG. 2,613.--Duddell =moving coil oscillograph= _with
projection and tracing desk outfit_. The outfit is designed for
teaching and lecture purposes. =In operation=, _after the beam of
light from the arc lamp has been reflected from the oscillograph
mirrors, it falls on a vibrating mirror which gives it a deflection
proportional to time in a direction at right angles to the deflection
it already has and which is proportional to the current passing
through the oscillograph_. It is therefore only necessary to place a
screen in the path of the reflected beam of light to obtain a trace
of the wave form. Since the vibrating mirror is vibrated by means
of a cam on the shaft of a synchronous motor, which motor is driven
from, or synchronously with, the source of supply whose wave form is
being investigated, the wave form is repeated time after time in the
same place on the screen, and owing to the "persistence" of vision,
the whole wave appears stationary on the screen. The synchronous
motor with its vibrating mirror, mentioned above, is located
underneath the "tracing desk." When used in this position a wave a
few centimeters in amplitude is seen through a sheet of tracing paper
which is bent round a curved sheet of glass. A permanent record of
the wave form can thus easily be traced on the paper. A _dark box_
which is designed to hold a sheet of sensitized paper in place of
the tracing paper, can be fitted in place of the tracing desk. Thus
an actual photographic record of the wave form is obtained. If the
synchronous motor be transferred from its position underneath the
tracing desk to the space reserved for it close to the oscillograph,
the beam of light is then received on a large mirror which is placed
at an angle of about 45 degrees to the horizontal and so projects the
wave form onto a large vertical screen which should be fixed about
two and a half meters distant. Under these conditions a wave form of
amplitude 50 cm. each side the zero line may be obtained which is
therefore visible to a large audience.]
=Ques. How are the oscillograms obtained in the Duddell moving coil
oscillograph?=
Ans. In all cases the oscillograms are obtained by a spot of light
tracing out the curve connecting current or voltage with time. The
source of light is an arc lamp, the light from which passes first
through a lens, and then, excepting when projecting on a screen,
through a rectangular slit about 10 mm. long by 1 mm. wide. The
position of the lamp from the lens is adjusted till an image of the
arc is obtained covering the three (two moving, one fixed) small
oscillograph mirrors. The light is reflected back from these mirrors
and, being condensed by a lens which is immediately in front of them,
it converges till an image of the slit is formed on the surface where
the record is desired. All that is necessary now to obtain a bright
spot of light instead of this line image is to introduce in the path
of the beam of light a cylindrical lens of short focal length.
[Illustration: FIGS. 2,614 and 2,615.--Sectional view of =permanent
magnet form= of Duddell =moving coil oscillograph.= This instrument
has a lower natural period of vibration (1/3000 second) than the
type shown in fig. 2,612, and therefore is not capable of accurately
following wave forms of such high frequency, but it is sufficiently
quick acting to follow wave forms of all ordinary frequencies with
perfect accuracy. It is easier to repair, and more portable owing to
the fact that the magnetic field is produced by a permanent magnet
instead of an electro-magnet. This also renders the instrument
suitable for use on high tension circuits without earth connection,
as, owing to the fact that no direct current excitation is required,
the instrument is more easily insulated than other types.]
=Ques. What is the function of the mirrors on the vibrating vane?=
[Illustration: FIG. 2,616.--Diagram of connections of Duddell
oscillograph =to high pressure circuit.= The modification necessary
for high pressure circuit only applies to the vibrator which gives
the pressure wave and consists in adding two more resistances, R_{4}
and R_{5}. Referring to fig. 2,617, it will be seen that in case fuse
f_{2} blows, or the vibrator be accidentally broken, the full supply
voltage is immediately thrown on the instrument itself. This is not
permissible in high voltage work and therefore the resistance R_{5}
is introduced as a permanent shunt to the oscillograph vibrator.
The resistance R_{4} is an exact duplicate of R_{2} being a 21 ohm
plug resistance box for adjusting the sensitivity of the vibrator
to an even figure. =In practice= R_{5} is usually a part of R_{1},
and in most of the high voltage resistances, two taps are brought
out near one end to serve as R_{5}. One of these taps is usually 50
ohms distant from the end terminal and the other only 5 ohms from
the end. =The use of these taps is as follows:= The large resistance
consisting of R_{1} + R_{5} is so chosen with respect to the voltage
of the circuit under investigation that the current through R_{1} is
about .1 ampere. _It should never be more than this continuously._
Then R_{4} is connected to the 50 ohm tap, and since the resistance
of the oscillograph vibrator circuit is variable from about 5 to
26 ohms by means of R_{4}, the current can be controlled through
the oscillograph from about .066 to .091 of an ampere, enabling an
open wave form to a convenient scale to be obtained. =If it now
be desired to record large rises of pressure,= such as may occur
in cases of resonance, _the height of the wave must be reduced in
order to keep these rises on the plate_. This is accomplished by
disconnecting R_{4} from the 50 ohm tap and connecting it to the 5
ohm tap, when the current through the vibrator will be from .05 to
.016 of an ampere according to whether the resistance R_{4} is in
or out of circuit. When, instead of using the _falling plate_, the
_cinematograph_ camera is being used, it becomes necessary always
to work on the 5 ohm tap since the width of the film is much less
than that of the plate, and the current must therefore be less.
=In experiments where sudden rises of voltage are expected= _it is
often advisable to keep_ R_{1} _as great as possible._ That end of
the resistance R_{1} referred to as R_{5} in the diagram should be
securely connected to the supply main and no switch or fuse used. A
switch may, if desired, be used in series with R_{1}, provided it be
inserted at the point where R_{1} joins the supply main remote from
R_{5}. It will be seen that fuses f_{1} and f_{2} are shown. Provided
that the connections are always made in accordance with the diagram,
and the vibrators are always shunted by R_{5} or R_{3} respectively,
there is not much objection to the use of these fuses, but on general
principles it is wise to avoid fuses in high tension work and
accordingly with each permanent magnet oscillograph, dummy fuses are
supplied, which can be inserted in place of the ordinary fuses when
desired. _The remark previously made about keeping both vibrators
and the frame of the instrument at approximately the same pressure
applies with additional emphasis in high pressure work._]
Ans. They simply control the direction of a beam of light in a
horizontal plane in such a manner that its deflection from a zero
position depends on the current passing through the instrument, and
it is therefore evident that the oscillograph is not complete without
means of producing a time scale.
[Illustration: FIG. 2,617.--Diagram of connections of Duddell
oscillograph =to low pressure circuit=, R_{1} is a high non-inductive
resistance connected across the mains in series with one of
the vibrators. S_{2} is a switch, and f_{2}, the fuse (on the
oscillograph in this circuit). The resistance of R_{1} in ohms
should be rather more than ten times the voltage of the circuit,
so that a current of a little less than .1 of an ampere will pass
through it. The vibrator will then give the curve of the circuit
on an open scale. (For the projection oscillograph, the resistance
R_{1} should be only twice the supply voltage, since .5 of an ampere
is required to give full scale deflection on a large screen.) =To
obtain the current wave form=, _the shunt_ R_{3} _is connected in
series with the circuit under investigation and the second vibrator
is connected across this shunt_. Here also f_{1} is a fuse, S_{1} a
switch, and R_{2} an adjustable resistance box. The switch S_{1} is
however unnecessary if the plug resistance box supplied for R_{2}
be used, since an infinity plug is included in this box. The shunt
R_{3} should have a drop of about 1 volt across it in order to give
a suitable working current through the vibrator. The resistance
R_{2} is not absolutely essential, but it is a great convenience in
adjusting the current through the vibrator. It is a plug resistance
box, the smallest coil being .04 of an ohm and the total 21 ohms.
Being designed to carry .5 ampere continuously it can be used
with any other type of Duddell oscillograph, and by its use the
sensitiveness of the vibrator can be adjusted so that a round number
of amperes in the shunt gives 1 mm. deflection. This adjustment is
best made with direct current. =It should be noted= in connecting
the oscillograph in circuit, that _the two vibrators should be so
connected to the circuit that it is impossible that a higher pressure
difference than_ 50 _volts should exist between one vibrator and the
other, or between either vibrator and the frame_. To ensure attention
to this important point, a brass strap is provided which connects
the two vibrators together and to the frame of the instrument. This
does not mean that this point must necessarily be earthed since the
frame of the instrument is insulated from the earth. It is advisable,
however, to earth it when possible.]
[Illustration: FIGS. 2,618 and 2,619.--=Two curves= _obtained with
the_ =falling plate camera= and illustrating _the discharge of a
condenser through an inductive circuit_. =When taking curve A= the
resistance in the circuit was very small compared to the inductance,
while =before taking curve B= an additional non-inductive resistance
was inserted in the circuit so that the oscillations were damped out
much more rapidly although the periodic time remained approximately
constant.]
=Ques. How is the time scale produced?=
Ans. Either the surface on which the beam of light falls may be
caused to move in a vertical plane with a certain velocity, so that
the intersection of the beam and the plane surface traces out a curve
connecting current with time (a curve which becomes a permanent
record if a sensitized surface be used); or, the surface may remain
stationary and in the path of the horizontally vibrating beam may be
introduced a mirror which rotates or vibrates about a horizontal
axis, thus superposing a vertical motion proportional to time on the
horizontal vibration which is proportional to current, and causing
the beam of light to trace out a curve connecting current and time on
the stationary surface.
=Ques. What kind of recording apparatus is used with the Duddell
oscillograph?=
Ans. A falling plate camera, or a cinematograph film camera.
[Illustration: FIG. 2,620.--Synchronous motor with vibrating mirror
as used with Duddell moving coil oscillograph. =Since the motor must
run synchronously= with the wave form it is required to investigate,
_it should be supplied with current from the same source_. The motor
can be used over a wide range of frequencies (from 20 to 120). When
working at frequencies below 40, it is advisable to increase the
moment of inertia of the armature, and for this purpose a suitable
brass disc is used. =The armature carries a sector=, _which cuts off
the light from the arc lamp during a fraction of each revolution, and
a cam which rocks the vibrating mirror_. =It makes one revolution
during two complete periods=, and the cam and sector are so arranged
that during 1½ periods, the mirror is turning with uniform angular
velocity, while during the remaining half period, the mirror is
brought back quickly to its angular position, the light being cut off
by the sector during this half period.]
=Ques. Explain the operation of the falling plate camera.=
Ans. In this arrangement a photographic plate is allowed to fall
freely by the force of gravity down a dark slide. At a certain point
in its fall it passes a horizontal slit through which the beams of
light from the oscillograph pass, tracing out the curves on the plate
as it falls.
[Illustration: FIGS. 2,621 to 2,623.--Interior of cinematograph
camera as used on Duddell moving coil oscillograph =for obtaining
long records=. The loose side of case is shown removed and one of
the reels which carry the film lying in front. =The spool of film=
which is placed on the loose reel A, passes over the guide pulley
B, then vertically downward between the brass gate D (shown open in
the figure), and the brass plate C. =The exposure aperture= is in
the plate C and can be opened or closed by a shutter controlled by
the lever M. The groove in the plate C, and the springs which press
the gate D flat on the plate C, prevent the film having any but a
vertical motion as it passes the exposure slit. E is the sprocket
driving pulley which engages with the perforations on the film and
unwinds it from the reel A to reel H. Outside the case on the far
side of it is secured to the axle G a three speed cone pulley. This
is driven by a motor of about 1/7 horse power, which also drives,
through the gears shown, the sprocket pulley E. Close to the grooved
cone pulley is a lever carrying a jockey pulley L, and a brake, which
latter is normally held onto the cone pulley by a spring and so
causes the loose belt to slip. By pressing a lever which is attached
to the falling plate camera case, the brake can be suddenly released
and at the same time the jockey pulley caused to tighten the belt
onto the grooved cone pulley, so that the starting and stopping of
the film is controlled independently of the driving motor, and being
quickly accomplished avoids waste of film. =Both reels= are alike and
each is made in two pieces. =The upper reel= is loose on its axle and
its motion is retarded slightly by a friction brake. =The lower reel=
is also loose on its axle, but it is driven by means of a friction
clutch, the clutch always rotating faster than the reel so that the
used film delivered by the sprocket pulley E is wound up as fast as
delivered. K is the front face of one reel, the boss on it pushes
into the tube on the other half H, which serves not only to unite the
two halves, but also to secure the end of the film which is doubled
through J.]
The mean speed of the plate at the moment of exposure is about
13 feet per second. This speed is very suitable for use with
frequencies of from 40 to 60 periods per second. A cloth bag is
used to introduce the plate to the slide.
A catch holds the plate until it is desired to let it fall.
Inside the case, is a small motor, 100 or 200 volts direct
current, driving four mirrors which are fixed about a common axis
with their planes parallel to it.
[Illustration: FIG. 2,624.--Portion of oscillograph record taken with
cinematograph film camera, =showing the rush of current= and =sudden
rise of voltage= _at the moment of switching on a high pressure
feeder_.]
By looking through a small slot in the end of the camera into
these rotating mirrors, the observer sees the wave form which the
oscillograph is tracing out and is thus able to make sure that
he is obtaining the particular wave form or other curve desired
before exposing the plate.
[Illustration: FIG. 2,625.--Portion of oscillograph record taken with
a cinematograph film camera =showing the effect of switching off
a high pressure feeder= and illustrating the violent fluctuations
produced by sparking at the switch contacts.]
The plate falls into a second red cloth bag which is placed on
the bottom of the slide. The plates used are "stereoscopic size",
6¾" × 3¼" (17.1 × 8.3 cm.).
=Ques. For what use is the cinematograph camera adapted?=
Ans. For long records.
For instance, in investigations, such as observation on the
paralleling of alternators, the running up to speed of motors,
and the surges which may occur in switching on and off cable,
etc. The cinematograph camera fits on to the falling plate case
and by means of which a roll of cinematograph film can be driven
at a uniform speed past the exposure aperture, enabling records
up to 50 metres in length to be obtained. An interior view of the
cinematograph camera is shown in fig. 2,621.
[Illustration: FIG. 2,626.--Curves reproduced from an article by J.
T. Morris in the _Electrician_. "On recording transitory phenomena by
the oscillograph."]
[Illustration: FIG. 2,627.--First rush of current from an alternator
when short circuited, showing unsymmetrical initial wave of current,
becoming symmetrical after a few cycles. 25 cycles.]
[Illustration: FIG. 2,628.--Pressure wave obtained from narrow
exploring coil on alternator armature, indicating distribution of
field flux. The terminal voltage of the alternator is very nearly a
sine wave, 60 cycles; about 17 volts.]
SOME OSCILLOGRAPH RECORDS
[Illustration: FIG. 2,629.--The waves of voltage and current of an
alternating arc. A, voltage wave; B, current wave showing low power
factor of the arc without apparent phase displacement. 60 cycles.]
[Illustration: FIG. 2,630.--Rupturing 650 volt circuit. A, current
wave; B, 25 cycle wave to mark time scale.]
[Illustration: FIG. 2,631.--First rush of current from alternator
when short circuited, showing unsymmetrical current wave, also wave
of field current caused by short circuit current in armature. Upper
curve, armature current; lower curve, field current.]
[Illustration: FIG. 2,632.--Mazda (tungsten) lamp, showing rapid
decrease to normal current as filament heats up. 25 cycles.]
[Illustration: FIG. 2,633.--Current wave in telephone line
corresponding to sustained vowel sound "_i_," as in machine; voice
pitched at A 110.]
[Illustration: FIG. 2,634.--Carbon lamp, showing rapid increase to
normal current as filament heats up. 25 cycles.]
[Illustration: FIG. 2,635.--Short circuit current on direct current
end of rotary converter, 21,500 amperes maximum. Upper curve, direct
current voltage; lower curve, direct current amperage. Duration of
short circuit about .1 second.]
CHAPTER LXIV
SWITCHBOARDS
=General Principles of Switchboard Connections.=--The interconnection
of generators, transformers, lines, bus bars, and switches with
their relays, in modern switchboard practice is shown by the
diagrams, figs. 2,636 to 2,645. The figures being lettered A to J
for simplicity, the generators are indicated by black discs, and the
switches by open circles, while each heavy line represents a set of
bus bars consisting of two or more bus bars according to the system
of distribution. It will be understood, also, in this connection,
that the number of pole of the switches and the type of switch will
depend upon the particular system of distribution employed.
Diagram A, shows the simplest system, or one in which a single
generator feeds directly into the line. There are no transformers
or bus bars and only one switch is sufficient.
In B, a single generator supplies two or more feeders through a
single set of bus bars, requiring a switch for each feeder, and a
single generator switch.
In C, two generators are employed and required and the addition
of a bus section switch.
D, represents a number of generators supplying two independent
circuits. The additional set of bus bars employed for this purpose
necessitates an additional bus section switch, and also additional
selector switches for both feeders and generators.
E, shows a standard system of connection for a city street
railway system having a large number of feeders.
[Illustration: FIGS. 2,645 and 2,646.--Diagrams illustrating general
principles of switchboard connections.]
This arrangement allows any group of feeders to be supplied from
any group of generators.
[Illustration: FIG. 2,646.--Fort Wayne switchboard panel for one
alternator and one transfer circuit. Diagram giving dimensions,
arrangement of instruments of board, and method of wiring. The
different forms of standard alternating current switchboard panels
for single phase circuits made by the Fort Wayne Electric Works are
designed to fulfill all the usual requirements of switchboards for
this class of work. The line includes panels equipped for a single
generator; for one generator and two circuits; one generator and one
transfer circuit; one generator, an incandescent and an arc lighting
circuit; and also feeder panels of different kinds.]
It also permits the addition of a generator switch for each
generator.
F, represents the simplest system with transformers.
It requires a single generator transformer bank, switch and
line. The arrangement as show at F is used where a number of
plants supply the same system.
G, represents a system having more than one line.
In this case a bus bar and transformer switch is used on the
high tension side.
H, shows a number of generators connected to a set of low
tension bus bars through generator switches, and employing a low
tension transformer switch.
I, shows the connections of a system having a large number of
feeders supplied by several small generators. In this case, the
plant is divided into two parts, each of which may be operated
independently.
[Illustration: FIG. 2,647.--General Electric =small plant alternating
current switchboard=, _designed for use in small central stations and
isolated plants_. They are for use with one set of bus bars, to which
all generators and feeders are connected by means of single throw
lever switches or circuit breakers, suitable provision being made for
the parallel operation of the generators.]
J, represents the arrangement usually employed in modern plants
where the generator capacity is large enough to permit of a
generator transformer unit combination with two outgoing lines. By
operating in parallel on the high tension side only, any generator
can be run with any transformer. The whole plant can be run in
parallel, or the two parts can be run separately.
[Illustration: FIG. 2,648.--Crouse-Hinds =voltmeter and ground
detector radial switch=, arranged for mounting on the switchboard.
The switch proper is placed on the rear of the board with hand wheel,
dial, and indicator only on the front side. The current carrying
parts are of hard brass, with contact surfaces machined after
assembling. The contact parts are of the plunger spring type, and the
cross bar has fuse connections. Ground detector circuits are marked
G+ and G- for two wire system, and G+, G-, GN+ and GN- for three wire
system. When the voltmeter switch is to be used as a ground detector,
two circuits are required for a two wire system, and four circuits
for a three wire system, that is, a six circuit voltmeter and ground
detector switch for use on a two wire system has two circuits for
ground detector and four circuits for voltmeter readings. A six
circuit voltmeter and ground detector switch, for use on a three wire
system, has four circuits for ground detector and two circuits for
voltmeter readings.]
=Switchboard Panels.=--The term "panel" means the slab of marble or
slate upon which is mounted the switches, and the indicating and
controlling devices. There are usually several panels comprising
switchboards of moderate or large size, these panels being classified
according to the division of the system that they control, as for
instance:
1. Generator panel;
2. Feeder panel;
3. Regulator panel, etc.
In construction, the marble or slate should be free from
metallic veins, and for pressures above, say, 600 volts, live
connections, terminals, etc., should preferably be insulated from
the panels by ebonite, mica, or removed from them altogether, as
is generally the case with the alternating gear where the switches
are of the oil type.
[Illustration: FIGS. 2,649 and 2,650,--Wiring diagrams of
Crouse-Hinds voltmeter and ground detector switches. Fig. 2,649
voltmeter switch; fig. 2,650 voltmeter and ground detector switch. A
view of the switch is shown in fig. 2,648; it is designed for use on
two or three wire systems up to 300 volts.]
The bus bars and connections should be supported by the
framework at the back of the board, or in separate cells, and the
instruments should be operated at low pressure through instrument
transformers.
The panels are generally held in position by bolting them to an
angle iron, or a strip iron framework behind them.
=Generator Panel.=--This section of a switchboard carries the
instruments and apparatus for measuring and electrically controlling
the generators. On a well designed switchboard each generator has, as
a rule, its own panel.
[Illustration: FIGS. 2,651 to 2,653.--Diagrams of connections for
generator panels. =Key to symbols=: =A=, ammeter; =A.S.=, ammeter
switch; =C.T.=, current transformer; =F.=, fuse; =F.A.=, direct
current field ammeter; =F.S.=, field switch; =G.C.S.=, governor
control switch; =L.S.=, limit switch (included with governor motor);
=O.S.=, oil switch; =P.I.W.=, polyphase indicating wattmeter;
=P.W.M.=, polyphase watthour meter; =P.R.=, pressure receptacle;
=P.P.=, pressure plug; =Rheo.=, rheostat; =S.=, shunt; =S.R.=,
synchronizing receptacle; =S.P.=, synchronizing plugs; =T.B.=,
terminal board for instrument leads; =V=, alternating current
voltmeter.]
[Illustration: FIGS. 2,654 and 2,655.--Diagrams illustrating =a
simple method of determining bus capacity= as suggested by the
General Electric Co. Fig. 2,654 relates to any panel; the method is
as follows: =1.= Make a rough plan of the _entire board_, regardless
of the number of panels to be ordered. _The order of panels_
shown is recommended, it being most economical of copper and best
adapted to future extensions. =2.= To avoid confusion keep on one
side of board everything pertaining to exciter buses, and on other
side everything pertaining to A. C. buses. =3.= With single lines
represent the exciter and A. C. buses across such panels as they
actually extend and by means of arrows indicate that portion of each
bus which is connected to feeders and that portion which is connected
to generators. _Remember that "Generator" and "Feeder" arrows must
always point toward each other_, otherwise the rules given below do
not hold. Note also that the field circuits of alternator panels are
treated as D. C. feeders for the exciter bus. =4.= On each panel
mark its ampere rating, that is, the maximum current it supplies
to or takes from the bus. For A. C. alternator panels the D. C.
rating is the excitation of the machines. =5.= Apply the following
rules _consecutively_, and note their application in fig. 2,654.
(For the sake of clearness ampere ratings are shown in light face
type and bus capacities in large type.) =A.= _Always begin with the
tail of the arrow and treat "generator" and "feeder" sections of
the bus separately._ =B.= _Bus capacity for first panel = ampere
rating of panel._ =C.= _Bus capacity for each succeeding panel =
ampere rating of panel plus bus capacity for preceding panel._ (See
sums marked above the buses in fig. 2,654.) =D.= _For a panel not
connected to a bus extending across it, use the smaller value of
the bus capacities already obtained for the two adjoining panels._
(See exciter bus for panel C.) =E.= _The bus capacity for any feeder
panel need not exceed the maximum for the generator panels_ (see A.
C. bus for panel G) _and vice versa_ (see exciter bus for panel B).
Hence the corrections made in values obtained by applying rules =B=
and =C=. The arrangement of panels shown in fig. 2,654 is the one
which is mostly used. The above method may, however, be applied to
other arrangements, one of which is shown in fig. 2,655. Here the
generators must feed both ways to the feeders at either end of the
board so that in determining A. C. bus capacities it is necessary to
first consider the generators with the feeders at one end, and then
with the feeders at the other end as shown by the dotted A. C. buses.
The required bus capacities are then obtained by taking the maximum
values for the two cases.]
[Illustration: FIG. 2,656.--End view showing =general arrangement of
switchboards= for 240, 480, and 600 volt alternating current. The cut
shows a single throw oil switch mounted on the panel.]
In the case of a dynamo, a good representative panel would have
mounted upon it a reverse current circuit breaker, an ammeter, a
double pole main switch (or perhaps a single pole switch, since
the circuit breaker could also be used as a switch) a double pole
socket into which a plug could be inserted to make connection with
a voltmeter mounted on a swinging bracket at the end of the board;
a rheostat handle, the spindle of which operates the shunt rheostat
of the machine, the rheostat being placed either directly behind
the spindle, if of small size, or lower down with chain drive from
the hand wheel spindle, if of larger size, a field discharge switch
and resistance, a lamp near the top of the panel for illuminating
purposes, a fuse for the voltmeter socket, and, if desired, a
watthour meter. If the dynamo be compound wound, the equalizing
switch will generally be mounted on the frame of the machine, and in
some cases the field rheostat will be operated from a pillar mounted
in front of the switchboard gallery. If the generator be for traction
purposes, the circuit breaker is more often of the maximum current
type, and a lightning arrester is often added, without a choke coil,
the latter as well as further lightning arresters being mounted on
the feeder panels.
[Illustration: FIGS. 2,657 and 2,658.--Two views of a =feeder panel=,
showing general arrangement of the devices assembled thereon. A,
circuit breaker; B, ammeter; C, voltmeter; D, switches.]
In the case of a high pressure alternating current plant of
considerable size, the bus bars oil switches, and the current and
pressure transformers are generally mounted either in stoneware
cells, or built on a framework in a space guarded by expanded metal
walls, and no high pressure apparatus of any sort is brought on to
the panels themselves.
[Illustration: FIGS. 2,659 to 2,666.--Diagram of connections for
three phase feeder panels. =Key to symbols=: A, ammeter; A.S.,
three way ammeter switch; B.A.S., bell alarm switch; C.T., current
transformer; F, fuse; O.S., oil switch; P.I.W., polyphase indicating
wattmeter; P.W.M., polyphase watthour meter; T.B., terminal board;
T.C., trip coils for oil switch.]
=Feeder Panel.=--The indicating and control apparatus for a feeder
circuit is assembled on a panel called the feeder panel.
The most common equipment in the case of a direct current feeder
panel comprises an ammeter, a double pole switch, and double pole
fuses or instead of the fuses, a circuit breaker on one or both
poles; in the case of a traction feeder a choke coil and a lightning
arrester are often added.
[Illustration: FIGS. 2,667 and 2,668.--Diagrams of connections for
two phase and three phase installations: A and A1, ammeter; C.C.,
constant current transformer; C.T., current transformer; D.R.,
discharge resistance; F, fuse; F.S., field switch; L.A., lightning
arrester; O.S., oil switch; P.P., pressure plug; P.R., pressure
receptacle; P.T., pressure transformer; S and S1, plug switches;
T.C., oil switch trip coil; V, voltmeter.]
The equipment of a typical high pressure three phase feeder panel is
an ammeter (sometimes three ammeters, one in each phase) operated
by a current transformer, and oil break switch with two overload
release coils, or three if the neutral of the circuit be earthed, the
releases being operated by current transformers.
[Illustration: FIG. 2,669.--Crouse-Hinds radial ammeter switch,
arranged for mounting directly on the switchboard. It is designed
for use with external shunt ammeters of any make or capacity, and in
connection with the required number of shunts, makes possible the
taking of current readings of a corresponding number of circuits by
means of one ammeter. The wiring diagram is shown in fig. 2,670.]
The switch when on a large system is often in a cell some distance
behind the panel, and is then controlled by a system of levers, or by
a small motor which is started and stopped by a throw over switch on
the panel, in which case there is generally a lamp or lamps on the
panel to show whether the switch is open or closed.
Air brake switches or links are placed between the bus bars
and the oil switch to allow of the latter being isolated for
inspection purposes, and as a general rule no apparatus carrying
high pressure current is allowed on the front of the panel. With
both direct and alternating current feeders, a watthour meter is
often added to show the total consumption of the circuit.
[Illustration: FIG. 2,670.--Wiring diagram for Crouse-Hinds radial
ammeter switch as illustrated in fig. 2,669. The switch proper is on
the rear of the switchboard, and the hand wheel dial and indicator on
the front.]
A typical three phase generator panel is provided with three
ammeters, one in each phase, operated from three current
transformers, one to each ammeter, a volt meter, a power factor
indicator, and an indicating watthour meter, all operated from
one or more pressure transformers, and the necessary current
transformers, the operating handle of the oil switch, which is
connected to the switch itself by means of rods, two maximum
releases operated by current transformers, or a reverse relay
for automatically tripping the switch, lamps for indicating when
the switch is tripped, a socket for taking the plug which makes
connection between the secondary of a pressure transformer and
the synchronizer on the synchronizing panel, and a lamp for
illuminating purposes, while on the base of the panel or on a
pillar at the front of the gallery is mounted the gear for the
field circuit. This consists of a double pole field switch and a
discharge resistance, an ammeter, a handle for the rheostat in
the generator field, and (if each alternator have its own direct
coupled exciter) possibly also a small rheostat for the exciter
field.
NOTE.--In some cases where the capacity of
the plant is not very great, the oil switch
is mounted on the back of the panel, and the
bus bars, current transformers, &c., on the
framework, also just at the back of the panel,
but under no circumstances, in good modern
practice, is high pressure apparatus permitted
on the front of the board. Where the capacity of
the plant is very large, the oil switches are
operated electrically by means of small motors,
and in this case the small switch gear for
starting and stopping this motor is mounted on
the generator panel, also the lamp or lamps to
indicate when the switch is open, and when closed.
CHAPTER LXV
=ALTERNATING CURRENT WIRING=
In the case of alternating current, because of its peculiar
behaviour, there are several effects which must be considered in
making wiring calculations, which do not enter into the problem with
direct current.
Accordingly, in determining the size of wires, allowance must be made
for
1. Self-induction;
2. Mutual-induction;
3. Power factor;
4. Skin effect;
5. Corona effect;
6. Frequency;
7. Resistance.
Most of these items have already been explained at such length,
that only a brief summary of facts need be added, to point out
their connection and importance with alternating current wiring.
=Induction.=--The effect of induction, whether self-induction
or mutual induction, is to set up a back pressure of _spurious
resistance_, which must be considered, as it sometimes materially
affects the calculation of circuits even in interior wiring.
_Self-induction is the effect produced by the action of the
electric current upon itself during variations in strength._
=Ques. What conditions besides variations of current strength governs
the amount of self-induction in a circuit?=
Ans. The shape of the circuit, and the character of the surrounding
medium.
If the circuit be straight, there will be little self-induction,
but if coiled, the effect will become pronounced. If the
surrounding medium be air, the self-induction is small, but if it
be iron, the self-induction is considerable.
[Illustration: FIGS. 2,671 to 2,676.--=The effect of self-induction.=
In a non-inductive circuit, as in fig. 2,672, the whole of the
virtual pressure is available to cause current to flow through the
lamp filament, hence it will glow with maximum brilliancy. If an
inductive coil be inserted in the circuit as in fig. 2,674, the
reverse pressure due to self-induction will oppose the virtual
pressure, hence the effective pressure (which is the difference
between the virtual and reverse pressures), will be reduced and
the current flow through the lamp diminished, thus reducing the
brilliancy of the illumination. The effect may be intensified to such
degree by interposing an iron core in the coil as in fig. 2,676, as
to extinguish the lamp.]
=Ques. With respect to self-induction, what method should be followed
in wiring?=
Ans. When iron conduits are used, the wires of each circuit should
not be installed in separate conduits, because such arrangement will
cause excessive self-induction.
The importance of this may be seen from the experience of one
contractor, who installed feeders and mains in separate iron
pipes. When the current was turned on, it was found that the
self-induction was so great as to reduce the pressure to such an
extent that the lamps, instead of giving full candle power, were
barely red. This necessitated the removal of the feeders and main
and re-installing them, so that those of the same circuit were in
the same pipe.
=Ques. What is mutual induction?=
Ans. Mutual induction is the effect of one alternating current
circuit upon another.
[Illustration: FIG. 2,677.--Measurement of self induction when
the frequency is known. The apparatus required consists of a high
resistance or electrostatic a.c. voltmeter, d.c. ammeter, and a
non-inductive resistance. Connect the inductive resistance to be
measured as shown, and close switch M, short circuiting the ammeter.
Connect alternator in circuit and measure drop across R and across
X_{_i_}. Disconnect alternator and connect battery in circuit, then
open switch M and vary the continuous current until the drop across R
is the same as with the alternating current, both measurements being
made with the same voltmeter; read ammeter, and measure drop across
X_{_i_}. Call the drop across X_{_i_} with alternating current E, and
with direct current E_{_i_}, and the reading of the ammeter J. Then
____________________
L = √E^{2} + E_{_i_}^{2} ÷ 2π _f_ I. If the resistance X_{_i_}
be known, and the ammeter be suitable for use with alternating
current, the switch and R may be dispensed with.
______________________________
Then L = √E^{2} - X_{_i_}^{2} I_{_i_}^{2} ÷ 2π _f_ I,
where I_{_i_} is the value of the alternating current. The resistance
of the voltmeter should be high enough to render its current
negligible as compared with that through X_{_i_}.]
=Ques. How is it caused?=
Ans. It is due to the magnetic field surrounding a conductor cutting
adjacent conductors and inducing back pressures therein.
This effect as a rule in ordinary installations is negligible.
=Transpositions.=--The effect of mutual induction between two
circuits is proportional to the inter-linkage of the magnetic fluxes
of the two lines. This in turn depends upon the proximity of the
lines and upon the general relative arrangement of the conductors.
[Illustration: FIG. 2,678.--Transposition diagram for two parallel
lines consisting of two wires each.]
[Illustration: FIG. 2,679.--Transposition diagram for three phase,
three wire line, transposing at the vertices of an equilateral
triangle. The line is originally balanced and becomes unbalanced on
transposing, a procedure which should be resorted to only to prevent
_mutual induction_.]
[Illustration: FIG. 2,680.--Transposition diagram of three phase,
three wire line, with center arranged in a straight line.]
The inductive effect of one line upon another is equal to the
algebraic sum of the fluxes due to the different conductors of the
first line, considered separately, which link the secondary line.
The effect of mutual induction is to induce surges in the line where
a difference of frequency exists between the two currents, and to
induce high electrostatic charges in lines carrying little or no
current, such as telephone lines.
INDUCTANCE PER MILE OF THREE PHASE CIRCUIT
---------+-------+----------+------------
| Diam. | Distance | Self
Size | in | _d_ in | Inductance
B. & S. | inch. | inches. | L henrys.
---------+-------+----------+------------
0000 | .46 | 12 | .00234
| | 18 | .00256
| | 24 | .00270
| | 48 | .00312
| | |
000 | .41 | 12 | .00241
| | 18 | .00262
| | 24 | .00277
| | 48 | .00318
| | |
00 | .365 | 12 | .00248
| | 18 | .00269
| | 24 | .00285
| | 48 | .00330
| | |
0 | .325 | 12 | .00254
| | 18 | .00276
| | 24 | .00293
| | 48 | .00331
| | |
1 | .289 | 12 | .00260
| | 18 | .00281
| | 24 | .00308
| | 48 | .00338
| | |
2 | .258 | 12 | .00267
| | 18 | .00288
| | 24 | .00304
| | 48 | .00314
| | |
3 | .229 | 12 | .00274
| | 18 | .00294
| | 24 | .00310
| | 48 | .00351
| | |
4 | .204 | 12 | .00280
| | 18 | .00300
| | 24 | .00315
| | 48 | .00358
| | |
5 | .182 | 12 | .00286
| | 18 | .00307
| | 24 | .00323
| | 48 | .00356
| | |
6 | .162 | 12 | .00291
| | 18 | .00313
| | 24 | .00329
| | 48 | .00369
| | |
7 | .144 | 12 | .00298
| | 18 | .00310
| | 24 | .00336
| | 48 | .00377
| | |
8 | .128 | 12 | .00303
| | 18 | .00325
| | 24 | .00341
| | 48 | .00384
| | |
9 | .114 | 12 | .00310
| | 18 | .00332
| | 24 | .00348
| | 48 | .00389
| | |
10 | .102 | 12 | .00318
| | 18 | .00340
| | 24 | .00355
| | 48 | .00396
---------+-------+----------+------------
This effect may be nullified by separating the lines and by
transposing the wires of one of the lines so that the effect produced
in one section is opposed by that in another. Of two parallel lines
consisting of two wires each, one may be transposed to neutralize the
mutual inductance.
Fig. 2,678 shows this method. The length L' should be an even
factor of L so that to every section of the line transposed there
corresponds an opposing section.
[Illustration: FIG. 2,681.--Capacity effect in single phase
transmission line. The effect is the same as would be produced by
shunting across the line at each point an infinitesimal condenser
having a capacity equal to that of an infinitesimal length of
circuit. For the purpose of calculating the charging current, a very
simple and sufficiently accurate method is to determine the current
taken by a condenser having a capacity equal to that of the entire
line when charged to the pressure on the line at the generating end.
The effect of capacity of the line is to reduce the pressure drop,
that is, improve the regulation, and to decrease or increase the
power loss depending on the load and power factor of the receiver.]
[Illustration: FIG. 2,682.--Capacity effect in a three phase
transmission line. It is the same as would be produced by shunting
the line at each point by three infinitesimal condensers connected in
star with the neutral point grounded, the capacity of each condenser
being twice that of a condenser of infinitesimal length formed by
any two of the wires. The effect of capacity on the regulation and
efficiency of the line can be determined with sufficient accuracy
in most cases by considering the line shunted at each end by three
condensers connected in star, the capacity of each condenser being
equal to that formed by any two wires of the line. An approximate
value for the charging current per wire is the current required to
charge a condenser, equal in capacity to that of any two of the
wires, to the pressure at the generating end of the line between any
one wire and the neutral point.]
The self inductance of lines is readily calculated from the
following formula:
L = .000558 {2.303 log (2A ÷ _d_) + .25} per mile of circuit
where
L = inductance of a loop of a three phase circuit in henrys.
_Note._--The inductance of a complete single phase
circuit = L × 2 ÷ √3.
A = distance between wires;
_d_ = diameter of wire.
CAPACITY IN MICRO-FARADS PER MILE OF CIRCUIT
FOR THREE PHASE SYSTEM
---------+-------+----------+--------------
| Diam. | Distance | Capacity
Size | in | A in | C in
B. & S. | inch. | inches. | micro-farads
---------+-------+----------+--------------
0000 | .46 | 12 | .0226
| | 18 | .0204
| | 24 | .01922
| | 48 | .01474
| | |
000 | .41 | 12 | .0218
| | 18 | .01992
| | 24 | .01876
| | 48 | .01638
| | |
00 | .365 | 12 | .0124
| | 18 | .01946
| | 24 | .01832
| | 48 | .01604
| | |
0 | .325 | 12 | .02078
| | 18 | .01898
| | 24 | .01642
| | 48 | .01570
| | |
1 | .289 | 12 | .02022
| | 18 | .01952
| | 24 | .01748
| | 48 | .0154
| | |
2 | .258 | 12 | .01972
| | 18 | .01818
| | 24 | .01710
| | 48 | .01510
| | |
3 | .229 | 12 | .01938
| | 18 | .01766
| | 24 | .01672
| | 48 | .01480
| | |
4 | .204 | 12 | .01874
| | 18 | .01726
| | 24 | .01636
| | 48 | .01452
| | |
5 | .182 | 12 | .01830
| | 18 | .01690
| | 24 | .01602
| | 48 | .01426
| | |
6 | .162 | 12 | .01788
| | 18 | .01654
| | 24 | .01560
| | 48 | .0140
| | |
7 | .144 | 12 | .01746
| | 18 | .01618
| | 24 | .01538
| | 48 | .01374
| | |
8 | .128 | 12 | .01708
| | 18 | .01586
| | 24 | .01508
| | 48 | .01350
| | |
9 | .114 | 12 | .01660
| | 18 | .01552
| | 24 | .01478
| | 48 | .01326
| | |
10 | .102 | 12 | .01636
| | 18 | .01522
| | 24 | .01452
| | 48 | .01304
---------+-------+----------+--------------
=Capacity.=--In any given system of electrical conductors, a
pressure difference between two of them corresponds to the presence
of a quantity of electricity on each. With the same charges, the
difference of pressure may be varied by varying the geometrical
arrangement and magnitudes and also by introducing various
dielectrics. The constant connecting the charge and the resulting
pressure is called the capacity of the system.
All circuits have a certain capacity, because each conductor
acts like the plate of a condenser, and the insulating medium,
acts as the dielectric. The capacity depends upon the insulation.
For a given grade of insulation, the capacity is proportional
to the surface of the conductors, and universally to the distance
between them.
A three phase three wire transmission line spaced at the corners
of an equilateral triangle as regards capacity acts precisely
as though the neutral line were situated at the center of the
triangle.
The capacity of circuits is readily calculated by applying the
following formulae:
C = 38.83 sc 10^{-3} / log (D ÷ d) per mile,
insulated cable with lead sheath;
C = 38.83 × 10^{-3} / log (4h ÷ d) per mile,
single conductor with earth return;
C = 19.42 × 10^{-3} / log (2A ÷ d) per mile of
parallel conductors forming metallic circuit;
in which
C = Capacity in micro-farads; for a metallic circuit, C =
capacity between wires;
sc = Specific inductive capacity of insulating material; = 1 for
air, and 2.25 to 3.7 for rubber;
D = Inside diameter of lead sheath;
d = Diameter of conductor;
h = Distance of conductors above ground;
A = Distance between wires.
=Frequency.=--The number of cycles per second, or the frequency,
has a direct effect upon the inductance reactance in an alternating
current circuit, as is plainly seen from the formula.
X_{i} = 2π_f_L
In the case of a transmission line alone; the lower frequencies
are the more desirable, in that they tend to reduce the inductance
drop and charging current. The inductance drop is proportional to
the frequency.
The natural period of a line, with distributed inductance and
capacity, is approximately given by
__
P = 7,900 / √LC
where L is the total inductance in millihenrys, and C, the total
capacity in micro-farads. Accordingly some lower odd harmonic of
the impressed frequency may be present which corresponds with the
natural period of the line. When this obtains, oscillations of
dangerous magnitude may occur. Such coincidences are less likely
with the lower harmonics than with the higher.
=Skin Effect.=--The tendency of alternating current to confine itself
to the _outer_ portions of a conductor, instead of passing uniformly
through the cross section, is called _skin effect_. The effect is
proportional to the size of the conductor and the frequency.
=Ques. What effect has "skin effect" on the current?=
Ans. It is equivalent to an increase of ohmic resistance and
therefore opposes the current.
[Illustration: FIGS. 2,683 to 2,687.--Skin effect and shield effect.
Fig. 2,683, section of conductor illustrating skin effect or tendency
of the alternating current to distribute itself unequally through the
cross section of a conductor as shown by the varied shading, which
represents the current flowing most strongly in the outer portions
of the conductor. For this reason it has been proposed to use hollow
or flat instead of solid round conductors; however, with frequency
not exceeding 100, the skin effect is negligibly small in copper
conductors of the sizes usually employed. In figs. 2,684 and 2,685,
or 2,686 and 2,687, if two adjacent conductors be carrying current
in the same direction, concentration will occur on those parts of
the two conductors remote from one another, and the nearer parts
will have less current, that is to say, they will be =shielded=.
In this case, the induction due to one conductor will exert its
opposing effect to the greatest extent on those parts of the other
conductor nearest to it; this effect decreasing the deeper the latter
is penetrated. After crossing the current axis, the induction will
still decrease in magnitude, but will now aid the current in the
conductor. Hence, the effect of these two conductors on one another
will make the current density more uniform than is the case where
the two conductors adjacent to one another are carrying current in
opposite directions, as in figs. 2,685 and 2,686, therefore, the
resistance and the heating for a given current will be smaller. If
the two return conductors be situated on the line passing through
the center of the conductors just considered, the effect will be to
still further concentrate the current; the distribution symmetry will
be further disturbed, and the resistance of the conductor system
increased. It is therefore difficult to say which of the two cases
considered holds the advantage so far as increasing the resistance is
concerned. The case, however, in which the phases are mixed has much
the smaller reactive drop.]
If the conductor be large, or the frequency high, the central
portion of the conductor carries little if any current, hence the
resistance is therefore greater for alternating current than for
direct current.
=Ques. For what condition may "skin effect" be neglected?=
Ans. For frequencies of 60 or less, with conductors having a diameter
not greater than 0000 B. & S. gauge.
=Ques. How is the "skin effect" calculated for a given wire?=
Ans. Its area in circular mils multiplied by the frequency, gives the
ratio of the wire's ohmic resistance to its combined resistance.
That is to say, the factor thus obtained multiplied by the
resistance of the wire to direct current will give its combined
resistance or resistance to alternating current.
The following table gives these ratio factors for large
conductors.
RATIO FACTOR FOR COMBINED RESISTANCE
+---------------+---------+---------------+---------+
| Cir. mils. | Ratio | Cir. mils. | Ratio |
| × frequency | factor | × frequency | factor |
+---------------+---------+---------------+---------+
| 10,000,000 | 1.00 | 70,000,000 | 1.13 |
| 20,000,000 | 1.01 | 80,000,000 | 1.17 |
| 30,000,000 | 1.03 | 90,000,000 | 1.20 |
| 40,000,000 | 1.05 | 100,000,000 | 1.25 |
| 50,000,000 | 1.08 | 125,000,000 | 1.34 |
| 60,000,000 | 1.10 | 150,000,000 | 1.43 |
+---------------+---------+---------------+---------+
=Corona Effect.=--When two wires, having a great difference of
pressure are placed near each other, a certain phenomenon occurs,
which is called _corona effect_. When the spacing or distance
between the wires is small and the difference of pressure in the
wires very great, a continuous passage of energy takes place through
the dielectric or atmosphere, the amount of this energy may be an
appreciable percentage of the power transmitted. Therefore in laying
out high pressure transmission lines, this effect must be considered
in the spacing of the wires.
=Ques. How does the corona effect manifest itself?=
Ans. It is visible at night as a bluish luminous envelope and audible
as a hissing sound.
=Ques. What is the critical voltage?=
Ans. The voltage at which the corona effect loss takes place.
=Ques. Upon what does the critical voltage depend?=
Ans. Upon the radius of the wires, the spacing, and the atmospheric
conditions.
[Illustration: FIG. 2,688.--Electromagnetic and electrostatic
fields surrounding the conductors of a transmission line. The
electromagnetic field is represented by lines of magnetic force
that surround the conductors in circles, (the solid lines), and
the electrostatic field by (dotted) circles passing from conductor
to conductor across at right angles to the magnetic circles. For
any given size of wire and distance apart of wires there is a
certain voltage at which the critical density or critical gradient
is reached, where the air breaks down and luminosity begins--the
critical voltage where corona manifests itself. At still higher
voltages corona spreads to further distances from the conductor
and a greater volume of air becomes luminous. Incidentally, it
produces noise. Now to produce light requires power and to produce
noise requires power. Air is broken down and is heated in breaking
down, and to heat also requires power; therefore, as soon as corona
forms, power is consumed or dissipated in its formation. When this
phenomenon occurs on the conductors of an alternating current circuit
a change takes place in relation to current and voltage. On the
wires of an alternating current transmission line, at a voltage
below that where corona forms--at a voltage where wires are not
luminous--considerable current, more or less depending on voltage
and length of wire, flows into the circuit as capacity current or
charging current.]
The critical voltage increases with both the diameter of the
wires, and the spacing.
The losses due to corona effect increase very rapidly with
increasing pressure beyond the critical voltage.
The magnitude of the losses as well as the critical voltage is
affected, by atmospheric conditions, hence they probably vary with
the particular locality, and the season of the year. Therefore,
for a given locality, a voltage which is normally below the
critical point, may at times be above it, depending upon changes
in the weather.
Such elements as smoke, fog, moisture, or other particles (dust,
snow, etc.) floating in the air, increase the losses; rain,
however, apparently has no appreciable effect upon the losses.
It follows then that in the design of a transmission line, the
atmospheric conditions of the particular locality through which
the line passes should be considered.
=Ques. How should live wires be spaced?=
Ans. They should be so spaced as to lessen the tendency to leakage
and to prevent the wires swinging together or against towers. The
spacing should be only sufficient for safety, since increased spacing
increases the self-induction of the line, and while it lessens the
capacity, it does so only in a slight degree.
The following spacing is in accordance with average practice.
SPACING FOR VARIOUS VOLTAGES
+---------+----------+
| Volts | Spacing |
+---------+----------+
| 5,000 | 28 ins. |
| 15,000 | 40 ins. |
| 30,000 | 48 ins. |
| 45,000 | 60 ins. |
| 60,000 | 60 ins. |
| 75,000 | 84 ins. |
| 90,000 | 96 ins. |
| 105,000 | 108 ins. |
| 120,000 | 120 ins. |
+---------+----------+
=Resistance of Wires.=--For quick calculation the following method
of obtaining the resistance (approximately) of wires will be found
convenient:
1,000 feet No. 10 B. & S. wire, which is about .1 inch in diameter
(.1019), has a resistance of one ohm, at a temperature of 68° F. and
weighs 31.4 pounds. A wire three sizes larger, that is No. 7, has
twice the cross section and therefore one-half the resistance. A wire
three sizes smaller than No. 10, that is No. 13, has one-half the
cross section and therefore twice the resistance.
Thus, starting with No. 10, any number three sizes larger will double
the cross sectional area and any wire three sizes smaller will halve
the cross sectional area of the preceding wire. This is true to the
extreme limits of the table, so that the area, weight and resistance
of any wire may be at once calculated to a close approximation from
this rule, intermediate sizes being obtained by interpolation.
For alternating current, the combined resistance, that is, the total
resistance, including skin effect, is obtained by multiplying the
resistance, as found above by the "ratio factor" (see table page
1,894).
[Illustration: FIGS. 2,689 to 2,692.--Triangles for obtaining
graphically, impedance, impressed pressure, etc., in alternating
current circuits. For a full explanation of this method the reader is
referred to Guide 5, Chapter XLVII on Alternating Current Diagrams. A
thorough study of this chapter is recommended.]
=Impedance.=--_The total opposition to the flow of electricity in an
alternating current circuit_, or the impedance may be resolved into
two components representing the ohmic resistance and the spurious
resistance; these components have a phase difference of 90°, and
they may be represented graphically by the two legs of a right angle
triangle, of which the hypothenuse represents the impedance.
Similarly, the volts lost or "drop" in an alternating circuit may be
resolved into two components representing respectively
1. The loss due to resistance.
2. The loss due to reactance.
These components have a phase difference of 90° and are represented
graphically similar to the impedance components. This has been
explained at considerable length in Chapter XLVII (Guide V).
[Illustration: FIG. 2,693.--Mechanical analogy of power factor, as
exemplified by a locomotive "poling" a car off a siding. The car and
locomotive are shown moving in parallel directions, and the pole
AB, inclined at an angle ϕ. Now, if the length of AB be taken to
represent the pressure exerted on the pole by the locomotive, then
the imaginary lines AC and BC, drawn respectively parallel and at
right angles to the direction of motion will represent respectively
the useful and no energy (wattless) components; that is to say, if
the pressure AB be applied to the car at an angle ϕ, only part of it,
AC, is useful in propelling the car, the other component, BC, being
wasted in tending to push the car off the track at right angles to
the rails, being resisted by the flanges of the outer wheels.]
=Power Factor.=--When the current falls out of step with the
pressure, as on inductive loads, the power factor becomes less than
unity, and the effect is to increase the current required for a
given load. Accordingly, this must be considered in calculating the
size of the wires. As has been explained, the current flowing in
an alternating current circuit, as measured by an ammeter, can be
resolved into two components, representing respectively the _active
component_ and the _wattless component_ or idle current. These are
graphically represented by the two legs of a right triangle, of which
the hypothenuse represents the current measured by the ammeter.
This _apparent_ current, as is evident from the triangle, exceeds
the _active_ current and lags behind the pressure by an amount
represented by the angle ϕ between the hypothenuse and leg
representing the energy current as shown in fig. 2,694.
[Illustration: FIG. 2,694.--Diagram showing that the apparent current
is more than the active current, the excess depending upon the angle
of phase difference.]
[Illustration: FIG. 2,695.--Diagram showing components of impedance
volts. Compare this diagram with figs. 2,689 and 2,671, and note that
the term "reactance" is the difference between the inductance drop
and the capacity drop if the circuit contain capacity, for instance,
if inductance drop be 10 volts and capacity drop be 7 volts then
reactance 10-7 = 3 volts.]
=Ques. What determines the heating of the wires on alternating
current circuits with inductive loads?=
Ans. The apparent current, as represented by the hypothenuse of the
triangle in fig. 2,694.
=Ques. How is the apparent current obtained?=
Ans. Divide the true watts by the product of the power factor
multiplied by the voltage.
Example.--A certain circuit supplies 20 kw. to motors at 220
volts and .8 power factor. What is the apparent current?
true watts 20,000
Apparent Current = -------------------- = -------- = 113.6 amperes
power factor × volts .8 × 220
=Ques. What else, besides power factor, should be considered in
making wire calculations for motor circuits?=
Ans. The efficiency of the motor, and the heavy starting current.
The product of the efficiency of the motor multiplied by the
power factor gives the _apparent efficiency_, which governs the
size of the wires, apparatus, etc., necessary to feed the motors.
Allowance should be made for the heavy starting current required
for some motors to avoid undue drop.
TABLE OF APPROXIMATE AMPERES PER TERMINAL
FOR INDUCTION MOTORS
Column headings: A-110 volts B-220 volts C-440 volts D-550 volts
+-----+----------------+------------------+------------------------+
| | | Two phase | Three phase |
|Horse| Single phase | four wire | three wire |
|power+----+-----+-----+-----+-----+------+-----+-----+------+-----+
| | A | B | C | A | B | C | A | B | C | D |
+-----+----+-----+-----+-----+-----+------+-----+-----+------+-----+
| .5| 6.6| 3.4 | 1.8| 3.3 | 1.7 | .9 | 3.7 | 1.8 | 1 | |
| 1 | 14 | 7 | 3.5| 6.4 | 3.2 | 1.6 | 7.4 | 3.7 | 1.9 | |
| 2 | 24 | 12 | 6 | 11 | 5.7 | 2.9 | 13 | 6.6 | 3.3 | 2.5 |
| 3 | 34 | 17 | 8.5| 16 | 8.1 | 4.1 | 19 | 9.3 | 4.7 | 3.5 |
| 4 | 52 | 26 | 13 | 26 | 13 | 6.5 | 30 | 15 | 7.5 | 6 |
| 5 | 74 | 37 | 18.5| 38 | 19 | 9.5 | 44 | 22 | 11 | 9 |
| 10 | 94 | 47 | 23.5| 44 | 22 | 11 | 50 | 25 | 12.5 | 11 |
| 15 | | | | 66 | 33 | 16.5 | 76 | 38 | 19 | 16 |
| 20 | | | | 88 | 44 | 22 | 102 | 51 | 25.5 | 22 |
| 25 | | | | 111 | 55 | 28 | 129 | 64 | 32 | 25 |
| 30 | | | | 134 | 67 | 33.5 | 154 | 77 | 38.5 | 32 |
| 40 | | | | 178 | 89 | 44.5 | 204 | 107 | 53.5 | 44 |
| 50 | | | | 204 | 102 | 51 | 236 | 118 | 59 | 52 |
| 75 | | | | 308 | 154 | 77 | 356 | 178 | 89 | 77 |
| 100 | | | | 408 | 204 | 102 | 472 | 236 | 118 | 100 |
+-----+----+-----+-----+-----+-----+------+-----+-----+------+-----+
=Ques. What are the usual power factors encountered on commercial
circuits?=
Ans. A mixed load of incandescent lamps and induction motors will
have a power factor of from .8 to .85; induction motors above .8 to
.85; incandescent and Nernst lamps .98; arc lamps, .85.
=Wire Calculations.=--In the calculation of alternating current
circuits, the two chief factors which make the computation different
from that for direct current circuits, is _induction_ and _power
factor_. The first depends on the frequency, and physical condition
of the circuit, and the second upon the character of the load.
=Ques. Under what conditions may inductance be neglected?=
[Illustration: FIGS. 2,696 to 2,698.--Example of wiring showing where
inductance is negligible, and where it must be considered in wire
calculations.]
Ans. In cases where the wires of a circuit are not spaced over an
inch apart, or in conduit work, where both wires are in the same
conduit.
Under these conditions the calculation is the same as for direct
current after making proper allowance for power factor.
=Ques. Under what conditions must induction be considered?=
Ans. On exposed circuits with wires separated several inches,
particularly in the case of large wires.
=Sizes of Wire.=--The size of wire for any alternating circuit may be
determined by slightly modifying the formula used in direct current
work, and which, as derived in Guide No. 4, page 748, is
amperes × feet × 21.6
circular mils = --------------------- (1)
drop
The quantity 21.6, is twice the resistance (10.8) of a foot of
copper wire one mil in diameter (_mil foot_). This resistance
(10.8) is multiplied by 2, giving the quantity 21.6, because the
length of a circuit, or feet in the formula, is given as the "run"
or distance one way, that is, one-half the total length of wire in
the circuit, must be multiplied by 2 to get the total drop, viz.:
amperes × feet × 10.8 × 2 amperes × feet × 21.6
circular mils = ------------------------- = ---------------------
drop drop
It is sometimes however convenient to make the calculation in terms
of watts. Formula (1) may be modified for such calculation.
In modifying the formula, the "drop" should be expressed in
percentage instead of actual volts lost, that is, instead of the
difference in pressure between the beginning and the end of the
circuit.
In any circuit the loss in percentage, or
drop
% loss = ------------------ × 100
impressed pressure
from which
% loss × impressed pressure
drop = --------------------------- (2)
100
Substituting equation (2) in equation (1)
amperes × feet × 21.6
circular mils = -----------------------------
% loss × imp. pressure
------------------------
100
amperes × feet × 2,160
= -------------------------- (3)
% loss × imp. pressure
Equation (3) is modified for calculation in terms of watts as
follows: The power in watts is equal to the _applied voltage_
multiplied by the current, that is to say, the power is equal to
the _volts at the consumer's end of the circuit_ multiplied by the
current, or simply
watts = volts × amperes
from which
watts
amperes = ----- (4)
volts
[Illustration: FIGS. 2,699 to 2,703.--Stranded copper cables. For
conductors of large areas and in the smaller sizes where extra
flexibility is required it becomes necessary to employ stranded
cables made by grouping a number of wires together in either
concentric or rope form. The concentric cable as here illustrated is
formed by grouping six wires around a central wire thereby forming
a seven wire cable. The next step is the application in a reverse
direction of another layer of 12 wires and a nineteen wire cable is
produced. This is again increased by a third layer of eighteen wires
for a 37 wire cable and a fourth layer of 24 wires for a 61 wire
cable. Successive layers, each containing 6 more wires than that
preceding, may be applied until the desired capacity is obtained. The
cuts show sectional views of concentric cables each formed from No.
10 B. & S. gauge wires.]
Substituting this value for the current in equation (3) and
remembering that the pressure taken is the volts at the consumer's
end of the line
watts
----- × feet × 2,160
volts
circular mils = ---------------------
% loss × volts
watts × feet × 2,160
= -------------------- (5)
% loss × volts^{2}
This formula (5) applies to a direct current two wire circuit, and to
adapt it to any alternating current circuit it is only necessary to
use the letter M instead of the number 2,160, thus
watts × feet × M
circular mils = ------------------ (6)
% loss × volts^{2}
in which M is a coefficient which has various values according to the
kind of circuit and value of the power factor. These values are given
in the following table:
=VALUES OF M=
--------+-----------------------------------------------------------
| POWER FACTOR
SYSTEM +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
| 1.00| .98| .95| .90| .85| .80| .75| .70| .65| .60
--------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
Single |2,160|2,249|2,400|2,660|3,000|3,380|3,840|4,400|5,112|6,000
phase | | | | | | | | | |
--------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
Two |1,080|1,125|1,200|1,330|1,500|1,690|1,920|2,200|2,556|3,000
phase | | | | | | | | | |
(4 wire)| | | | | | | | | |
--------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
Three |1,080|1,125|1,200|1,330|1,500|1,690|1,920|2,200|2,556|3,000
phase | | | | | | | | | |
(3 wire)| | | | | | | | | |
--------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
NOTE.--The above table is calculated as
follows: For =single phase= M = 2,160 ÷ power
factor^{2} × 100; for =two phase= four wire, or
three phase three wire, M = ½ (2,160 ÷ power
factor^{2})× 100. Thus the value of M for a
single phase line with power factor .95 = 2,160 ÷
.95^{2} × 100 = 2,400.
It must be evident that when 2,160 is taken as the value of M,
formula (6) applies to a two wire direct current circuit and also to
a single phase alternating current circuit when the power factor is
unity.
In the table the value of M for any particular power factor is found
by dividing 2,160 by the square of that power factor for single phase
and twice the square of the power factor for two phase and three
phase.
=Ques. For a given load and voltage how do the wires of a single and
two phase system compare in size and weight, the power factor being
the same in each case?=
Ans. Since the two phase system is virtually two single phase
systems, the four wires of the two phase systems are half the size of
the two wires of the single phase system, and accordingly, the weight
is the same for either system.
=VALUES OF T=
+-------------------+--------------------------------------------+
| | POWER FACTOR |
| SYSTEM +--------+--------+--------+--------+--------+
| | 1.00 | .98 | .90 | .80 | .70 |
+-------------------+--------+--------+--------+--------+--------+
| Single phase | 1.00 | .98 | .90 | .80 | .70 |
+-------------------+--------+--------+--------+--------+--------+
| Two phase, 4 wire | 2.00 | 1.96 | 1.80 | 1.60 | 1.40 |
+-------------------+--------+--------+--------+--------+--------+
|Three phase, 3 wire| 1.73 | 1.70 | 1.55 | 1.38 | 1.21 |
+-------------------+--------+--------+--------+--------+--------+
NOTE.--This table is for finding the value
of the current in line, using the formula I =
W ÷ (E × T), in which I = current in line; E =
voltage between main conductors at receiving or
consumers' end; W = watts. For instance, what
is the current in a two phase line transmitting
1,000 watts at 550 volts, power factor .80? I =
1,000 ÷ (550 × 1.60) = 1.13.
=Ques. Since there is no saving in copper in using two phases, what
advantage has the two phase system over the one phase system?=
Ans. It is more desirable on power circuits, because two phase motors
are self-starting.
That is to say, the rotating magnetic field that can be
produced by a two phase current, permits an induction motor to
start without being equipped with any special phase splitting
devices which are necessary on single phase motors, because the
oscillating field produced by a single phase current does not
produce any torque on a squirrel cage armature at rest.
=Ques. For equal working conditions, what is the comparison between
the single, two and three phase system as to size and weight of
wires?=
Ans. Each wire of the three phase system is half the size of one of
the wires of the single phase system, hence the weight of copper
required for the three phase system is 75% of that required for the
single phase system. Since in the two phase system half of the load
is carried by each phase, each wire of the three phase system is the
same size as one of the wires of the two phase system, hence, the
copper required by the three phase system is 75% of that required by
the two phase system.
=MISCELLANEOUS FORMULÆ FOR COPPER WIRES=
Diameter squared = circular mils
Circular mils × .7854 = square mils
.000003027 × circular mils = pounds per foot
.003027 × circular mils = pounds per 1,000 feet
.0159847 × circular mils = pounds per mile
.003879 × square mils = pounds per 1,000 feet
.33033 ÷ circular mils = feet per pound
.0000002924 × circular mils = pounds per ohm
.342 ÷ circular mils = ohms per pound
.096585 × circular mils = feet per ohm
10.353568 ÷ circular mils = ohms per foot
Breaking weight of wire ÷ area = breaking weight per square inch.
Breaking weight per square inch × area = breaking weight of wire.
The weight of copper wire is 1-1/7 times the weight of iron wire of
same diameter.
EXAMPLE.--What size wires must be used on a single phase circuit
2,000 feet in length to supply 30 kw. at 220 volts with loss of
4%, the power factor being .9?
The formula for circular mils is
watts × feet × M
circular mils = ------------------ (1)
% loss × volts^{2}
Substituting the given values and the proper value of M from the
table, in (1)
30,000 × 2,000 × 2,660
circular mils = ---------------------- = 82,438
4 × 220^{2}
Referring to the accompanying table of the properties of copper
wire, the nearest _larger_ size wire is No. 1 B. & S. gauge having
an area of 83,690 circular mils.
=TABLE OF THE PROPERTIES OF COPPER WIRE=
Giving weights, length and resistances of wires of Matthiessen's
Standard Conductivity for both B. & S. G. (Brown & Sharpe Gauge)
and B. W. G. (Birmingham Wire Gauge) from Transactions October
1903, of the American Institute of Electrical Engineers.
__________________________________________________________________
| | |
Gauges. To the nearest fourth | | Length.|Resistance.
significant digit. | | |
_____________________________________| |________|_________
| | | | Weight. | |
| | Diameter. | Area. | Lbs. | Feet | Ohms per
| | | | per | per lb.|1,000 ft.
______|_______|___________|__________| 1,000 |________|__________
| | | | feet. | |
| | | Circular | | |
B.& S.| B.W.G.| Inches. | mils. | | | @ 68° F.
______|______ |___________|__________|_________|________|_________
| | | | | |
0000 | | 0.460 | 211,600 | 640.5 | 1.561 | .04893
| 0000 | 0.454 | 206,100 | 623.9 | 1.603 | .05023
| 000 | 0.425 | 180,600 | 546.8 | 1.829 | .05732
| | | | | |
000 | | 0.4096 | 167,800 | 508.0 | 1.969 | .06170
| 00 | 0.380 | 144,400 | 437.1 | 2.288 | .07170
00 | | 0.3648 | 133,100 | 402.8 | 2.482 | .07780
| | | | | |
| 0 | 0.340 | 115,600 | 349.9 | 2.858 | .08957
0 | | 0.3249 | 105,500 | 319.5 | 3.130 | .09811
| 1 | 0.3000 | 90,000 | 272.4 | 3.671 | .1150
| | | | | |
1 | | 0.2893 | 83,690 | 253.3 | 3.947 | .1237
| 2 | 0.2840 | 80,660 | 244.1 | 4.096 | .1284
| 3 | 0.2590 | 67,080 | 203.1 | 4.925 | .1543
| | | | | |
2 | | 0.2576 | 66,370 | 200.9 | 4.977 | .1560
| 4 | 0.2380 | 56,640 | 171.5 | 5.832 | .1828
3 | | 0.2294 | 52,630 | 159.3 | 6.276 | .1967
| | | | | |
| 5 | 0.2200 | 48,400 | 146.5 | 6.826 | .2139
4 | | 0.2043 | 41,740 | 126.4 | 7.914 | .2480
| 6 | 0.2030 | 41,210 | 124.7 | 8.017 | .2513
| | | | | |
5 | | 0.1819 | 33,100 |100.2 | 9.98 | .3128
| 7 | 0.1800 | 32,400 | 98.08 | 10.20 | .3196
| 8 | 0.1650 | 27,230 | 82.41 | 12.13 | .3803
| | | | | |
6 | | 0.1620 | 26,250 | 79.46 | 12.58 | .3944
| 9 | 0.1480 | 21,900 | 66.30 | 15.08 | .4727
7 | | 0.1443 | 20,820 | 63.02 | 15.87 | .4973
| | | | | |
| 10 | 0.1340 | 17,960 | 54.35 | 18.40 | .5766
8 | | 0.1285 | 16,510 | 49.98 | 20.01 | .6271
| 11 | 0.1200 | 14,400 | 43.59 | 22.94 | .7190
| | | | | |
9 | | 0.1144 | 13,090 | 39.63 | 25.23 | .7908
| 12 | 0.1090 | 11,880 | 35.96 | 27.81 | .8715
10 | | 0.1019 | 10,380 | 31.43 | 31.82 | .9972
| | | | | |
| 13 | 0.0950 | 9,025 | 27.32 | 36.60 | 1.147
11 | | 0.09074 | 8,234 | 24.93 | 40.12 | 1.257
| 14 | 0.08300 | 6,889 | 20.85 | 47.95 | 1.503
| | | | | |
12 | | 0.08081 | 6,530 | 19.77 | 50.59 | 1.586
| 15 | 0.07200 | 5,184 | 15.69 | 63.73 | 1.997
13 | | 0.07196 | 5,178 | 15.68 | 63.79 | 1.999
| | | | | |
| 16 | 0.06500 | 4,225 | 12.79 | 78.19 | 2.451
14 | | 0.06408 | 4,107 | 12.43 | 80.44 | 2.521
| 17 | 0.0580 | 3,364 | 10.18 | 98.23 | 3.078
| | | | | |
15 | | 0.05707 | 3,257 | 9.858 | 101.4 | 3.179
16 | | 0.05082 | 2,583 | 7.818 | 127.9 | 4.009
| 18 | 0.04900 | 2,401 | 7.268 | 137.6 | 4.312
| | | | | |
17 | | 0.045260 | 2,048 | 6.200 | 161.3 | 5.055
| 19 | 0.042000 | 1,764 | 5.340 | 187.3 | 5.870
18 | | 0.040300 | 1,624 | 4.917 | 203.4 | 6.374
| | | | | |
19 | | 0.035890 | 1,288 | 3.899 | 256.5 | 8.038
| 20 | 0.035000 | 1,225 | 3.708 | 269.7 | 8.452
| 21 | 0.032000 | 1,024 | 3.100 | 322.6 | 10.11
| | | | | |
20 | | 0.031960 | 1,022 | 3.092 | 323.4 | 10.14
21 | | 0.028460 | 810.1 | 2.452 | 407.8 | 12.78
| 22 | 0.028000 | 784.0 | 2.373 | 421.4 | 13.21
22 | | 0.025350 | 642.4 | 1.945 | 514.2 | 16.12
| 23 | 0.025000 | 625.0 | 1.892 | 528.6 | 16.57
23 | | 0.022570 | 509.5 | 1.542 | 648.4 | 20.32
| | | | | |
| 24 | 0.022000 | 484.0 | 1.465 | 682.6 | 21.39
24 | | 0.020100 | 404.0 | 1.223 | 817.6 | 25.63
| 25 | 0.020000 | 400.0 | 1.211 | 825.9 | 25.88
| | | | | |
| 26 | 0.018000 | 324.0 | .9808 | 1,020 | 31.96
25 | | 0.017900 | 320.4 | .9699 | 1,031 | 32.31
| 27 | 0.016000 | 256.0 | .7749 | 1,290 | 40.45
| | | | | |
26 | | 0.015940 | 254.1 | .7692 | 1,300 | 40.75
27 | | 0.014200 | 201.5 | .6100 | 1,639 | 51.38
| 28 | 0.014000 | 196.0 | .5933 | 1,685 | 52.83
| | | | | |
| 29 | 0.013000 | 169.0 | .5116 | 1,955 | 61.27
28 | | 0.012640 | 159.8 | .4837 | 2,067 | 64.79
| 30 | 0.012000 | 144.0 | .4359 | 2,294 | 71.90
| | | | | |
29 | | 0.011260 | 126.7 | .3836 | 2,607 | 81.70
30 | | 0.010030 | 100.5 | .3042 | 3,287 | 103.0
| 31 | 0.010000 | 100.0 | .3027 | 3,304 | 103.5
| | | | | |
| 32 | 0.009000 | 81.0 | .2452 | 4,078 | 127.8
31 | | 0.008928 | 79.70 | .2413 | 4,145 | 129.9
| 33 | 0.008000 | 64.0 | .1937 | 5,162 | 161.8
| | | | | |
32 | | 0.007950 | 63.21 | .1913 | 5,227 | 163.8
33 | | 0.007080 | 50.13 | .1517 | 6,591 | 206.6
| 34 | 0.007000 | 49.0 | .1483 | 6,742 | 211.3
| | | | | |
34 | | 0.006305 | 39.75 | .1203 | 8,311 | 260.5
35 | | 0.005615 | 31.52 | .09543 |10,480 | 328.4
36 | 35 | 0.005000 | 25.0 | .07568 |13,210 | 414.2
| | | | | |
37 | | 0.004453 | 19.83 | .06001 |16,660 | 522.2
| 36 | 0.004000 | 16. | .04843 |20,650 | 647.1
38 | | 0.003965 | 15.72 | .04759 |21,010 | 658.5
| | | | | |
39 | | 0.003531 | 12.47 | .03774 |26,500 | 830.4
40 | | 0.003145 | 9.888 | .02993 |33,410 |1047.
--------------------------------------------------------------------
=Drop.=--In order to determine the drop or volts lost in the line,
the following formula may be used
drop = ((% loss × volts) / 100) × S (1)
in which the % loss is a percentage of the applied power, that is,
the power delivered to the consumer and not a percentage of the power
at the alternator. "Volts" is the pressure at the consumer's end of
the circuit.
=VALUE OF "S" FOR 60 CYCLES=
-----------------+------------------------+------------------------+
| .98 power factor | .90 power factor |
--------+--------+------------------------+------------------------+
Size of | Area | Spacing of | Spacing of |
wire | in | conductors | conductors |
B.&S. |circular| | |
gauge | mils. | | |
| +----+----+----+----+----+----+----+----+----+----+
| | 1" | 3" | 6" | 12"| 24"| 1" | 3" | 6" | 12"| 24"|
--------+--------+----+----+----+----+----+----+----+----+----+----+
500,000 |500,000 |1.21|1.45|1.61|1.77|1.92|1.32|1.80|2.11|2.44|2.75|
300,000 |300,000 |1.15|1.29|1.38|1.48|1.57|1.19|1.47|1.66|1.84|2.02|
0,000 |211,600 |1.12|1.22|1.28|1.34|1.41|1.13|1.33|1.45|1.58|1.63|
000 |167,800 |1.09|1.18|1.22|1.28|1.29|1.08|1.23|1.33|1.44|1.53|
00 |133,100 |1.07|1.14|1.18|1.21|1.25|1.03|1.16|1.24|1.32|1.40|
0 |105,500 |1.05|1.10|1.14|1.17|1.20|1.00|1.09|1.16|1.22|1.28|
1 | 83,690 |1.04|1.08|1.10|1.13|1.15|1.00|1.05|1.09|1.14|1.19|
2 | 66,370 |1.02|1.05|1.08|1.10|1.12|1.00|1.00|1.04|1.08|1.12|
3 | 52,630 |1.02|1.04|1.06|1.07|1.09|1.00|1.00|1.00|1.03|1.06|
| | | | | | | | | | | |
4 | 41,740}|1.00|1.02|1.03|1.04|1.07|1.00|1.00|1.00|1.00|1.00|
5 | 33,100}| | | | | | | | | | |
| | | | | | | | | | | |
6 | 26,250}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|
7 | 20,820}| | | | | | | | | | |
| | | | | | | | | | | |
8 | 16,510}| | | | | | | | | | |
9 | 13,090}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|
10 | 10,380}| | | | | | | | | | |
--------+--------+----+----+----+----+----+----+----+----+----+----+
--------+--------+----+----+----+----+----+----+----+----+----+----+
Size | | .80 power factor | .70 power factor |
of | Area +------------------------+------------------------+
wire | in | Spacing of | Spacing of |
B.&S. |circular| conductors | conductors |
gauge | mils. +----+----+----+----+----+----+----+----+----+----+
| | 1" | 3" | 6" | 12"| 24"| 1" | 3" | 6" | 12"| 24"|
--------+--------+----+----+----+----+----+----+----+----+----+----+
500,000 | 500,000|1.27|1.89|2.25|2.64|3.03|1.14|1.72|2.12|2.53|2.92|
300,000 | 300,000|1.11|1.46|1.68|1.90|2.12|1.00|1.33|1.56|1.78|2.01|
0,000 | 211,600|1.03|1.27|1.43|1.58|1.75|1.00|1.14|1.29|1.45|1.69|
000 | 167,800|1.00|1.16|1.28|1.41|1.53|1.00|1.02|1.15|1.28|1.50|
00 | 133,100|1.00|1.07|1.15|1.22|1.00|1.00|1.00|1.03|1.13|1.21|
0 | 105,500|1.00|1.00|1.07|1.15|1.00|1.00|1.00|1.00|1.01|1.09|
1 | 83,690}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|
2 | 66,370}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|
3 | 52,630}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|
| | | | | | | | | | | |
4 | 41,740}| | | | | | | | | | |
5 | 33,100}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|
| | | | | | | | | | | |
6 | 26,250}| | | | | | | | | | |
7 | 20,820}| | | | | | | | | | |
| | | | | | | | | | | |
8 | 16,510}| | | | | | | | | | |
9 | 13,090}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|
10 | 10,380}| | | | | | | | | | |
--------+--------+----+----+----+----+----+----+----+----+----+----+
The coefficient S has various values as given in the accompanying
tables. As will be seen from the table, the value of S to be used
depends upon the size of wire, spacing, power factor and frequency.
These values are accurate enough for all practical purposes, and may
be used for distances of 20 miles or less and for voltages up to
25,000.
The capacity effect on very long high voltage lines, makes this
method of determining the drop somewhat inaccurate beyond the limits
above mentioned.
=VALUE OF "S" FOR 25 CYCLES=
---------+--------+------------------------+------------------------+
Size | | .98 power factor | .90 power factor |
of | Area +------------------------+------------------------+
wire | in | Spacing of | Spacing of |
B.&S. |circular| conductors | conductors |
gauge | mils. +----+----+----+----+----+----+----+----+----+----+
| | 1" | 2" | 6" | 12"| 24"| 1" | 3" | 6" | 12"| 24"|
---------+--------+----+----+----+----+----+----+----+----+----+----+
500,000 |500,000 |1.01|1.17|1.23|1.29|1.36|1.02|1.22|1.35|1.43|1.61|
300,000 |300,000 |1.04|1.10|1.13|1.18|1.21|1.00|1.08|1.16|1.25|1.31|
0,000 |211,600 |1.03|1.07|1.09|1.11|1.14|1.00|1.02|1.07|1.13|1.15|
000 |167,800 |1.00|1.05|1.06|1.09|1.10|1.00|1.00|1.02|1.07|1.11|
00 |133,100 |1.00|1.03|1.05|1.06|1.08|1.00|1.00|1.00|1.02|1.05|
0 |105,500 |1.00|1.01|1.02|1.03|1.04|1.00|1.00|1.00|1.00|1.00|
| | | | | | | | | | | |
1 | 83,690}| | | | | | | | | | |
2 | 66,370}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|
3 | 52,630}| | | | | | | | | | |
| | | | | | | | | | | |
4 | 41,740}| | | | | | | | | | |
5 | 33,100}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|
6 | 26,250}| | | | | | | | | | |
| | | | | | | | | | | |
7 | 20,820}| | | | | | | | | | |
8 | 16,510}| | | | | | | | | | |
9 | 13,090}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|
10 | 10,380}| | | | | | | | | | |
---------+--------+----+----+----+----+----+----+----+----+----+----+
Size | | .80 power factor | .70 power factor |
of | Area +------------------------+------------------------+
wire | in | Spacing of | Spacing of |
B.&S. |circular| conductors | conductors |
gauge | mils. +----+----+----+----+----+----+----+----+----+----+
| | 1" | 3" | 6" | 12"| 24"| 1" | 3" | 6" | 12"| 24"|
---------+--------+----+----+----+----+----+----+----+----+----+----+
500,000 | 500,000|1.00|1.15|1.30|1.47|1.62|1.00|1.00|1.16|1.33|1.49|
300,000 | 300,000|1.00|1.00|1.09|1.16|1.25|1.00|1.00|1.00|1.02|1.12|
0,000 | 211,600|1.00|1.00|1.00|1.03|1.10|1.00|1.00|1.00|1.00|1.00|
000 | 167,800|1.00|1.00|1.00|1.00|1.01|1.00|1.00|1.00|1.00|1.00|
00 | 133,100|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|
0 | 105,500|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|
| | | | | | | | | | | |
1 | 83,690}| | | | | | | | | | |
2 | 66,370}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|
3 | 52,630}| | | | | | | | | | |
| | | | | | | | | | | |
4 | 41,740}| | | | | | | | | | |
5 | 33,100}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|
6 | 26,250}| | | | | | | | | | |
| | | | | | | | | | | |
7 | 20,820}| | | | | | | | | | |
8 | 16,510}| | | | | | | | | | |
9 | 13,090}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|
10 | 10,380}| | | | | | | | | | |
---------+--------+----+----+----+----+----+----+----+----+----+----+
EXAMPLE.--A circuit supplying current at 440 volts, 60
frequency, with 5% loss and .8 power factor is composed of No. 2
B. & S. gauge wires spaced one foot apart. What is the drop in the
line?
According to the formula
% loss × volts
drop = -------------- × S
100
Substituting the given values, and value of S as obtained from
the table for frequency 60
5 × 440
drop = ------- × 1 = 22 volts
100
=Current=.--As has been stated, the effect of power factor less than
unity, is to increase the current; hence, in inductive circuit
calculations, the first step is to determine the current flowing in a
circuit. This is done as follows:
apparent load
current = ------------- (1)
volts
and
watts
apparent load = ------------ (2)
power factor
Substituting (2) in (1)
watts
------------
power factor watts
current = ------------ = --------------------- (3)
volts power factor × volts
[Illustration: FIG. 2,704.--Rope type of stranded copper cable
which is used when a high degree of flexibility is required. The
construction of this cable is the stranding together of seven groups,
each containing seven wires and producing a total of 49 wires.
In cases when a greater carrying capacity is desired than can be
obtained through the use of the 7 × 7 or 49 wire cable, the number of
groups is increased to nineteen thereby making a total of 133 wires
(19 × 7).]
EXAMPLE.--A 50 horse power 440 volt motor has a full load
efficiency of .9 and power factor of .8. How much current is
required?
Since the brake horse power of the motor is given, it is
necessary to obtain the electrical horse power, thus
brake horse power 50
E.H.P. = ----------------- = -- = 55.5
efficiency .9
which in watts is
55.5 × 746 = 41,403
which is the actual load, and from which
actual load 41,403
apparent load = ------------ = ------ = 51,754
power factor .8
The current therefore at 440 volts is
apparent load 51,754
------------- = ------ = 117.6 amperes
volts 440
EXAMPLE.--A 50 horse power single phase 440 volt motor, having
a full load efficiency of .92 and power factor of .8, is to be
operated at a distance of 1,000 feet from the alternator. The
wires are to be spaced 6 inches apart and the frequency is 60, and
% loss 5. Determine: =A=, _electrical horse power_; =B=, _watts_;
=C=, _apparent load_; =D=, _current_; =E=, _size of wires_; =F=,
_drop_; =G=, _voltage at the alternator_.
=A=. _Electrical horse power_
brake horse power 50
E. H. P. = ----------------- × --- = 54.3
efficiency .92
or,
54.3 × 746 = 40,508 watts
=TABLE OF WIRE EQUIVALENTS=
(Brown and Sharpe gauge)
------+--------+--------+--------+---------+---------+---------
0000 | 2 # 0 | 4 # 3 | 8 # 6 | 16 # 9 | 32 # 12 | 64 # 15
000 | 2 " 1 | 4 " 4 | 8 " 7 | 16 " 10 | 32 " 13 | 64 " 16
00 | 2 " 2 | 4 " 5 | 8 " 8 | 16 " 11 | 32 " 14 | 64 " 17
0 | 2 " 3 | 4 " 6 | 8 " 9 | 16 " 12 | 32 " 15 | 64 " 18
1 | 2 " 4 | 4 " 7 | 8 " 10 | 16 " 13 | 32 " 16 | 64 " 19
2 | 2 " 5 | 4 " 8 | 8 " 11 | 16 " 14 | 32 " 17 | 64 " 20
3 | 2 " 6 | 4 " 9 | 8 " 12 | 16 " 15 | 32 " 18 | 64 " 21
4 | 2 " 7 | 4 " 10 | 8 " 13 | 16 " 16 | 32 " 19 | 64 " 22
5 | 2 " 8 | 4 " 11 | 8 " 14 | 16 " 17 | 32 " 20 | 64 " 23
6 | 2 " 9 | 4 " 12 | 8 " 15 | 16 " 18 | 32 " 21 | 64 " 24
7 | 2 " 10 | 4 " 13 | 8 " 16 | 16 " 19 | 32 " 22 | 64 " 25
8 | 2 " 11 | 4 " 14 | 8 " 17 | 16 " 20 | 32 " 23 | 64 " 26
9 | 2 " 12 | 4 " 15 | 8 " 18 | 16 " 21 | 32 " 24 | 64 " 27
10 | 2 " 13 | 4 " 16 | 8 " 19 | 16 " 22 | 32 " 25 | 64 " 28
11 | 2 " 14 | 4 " 17 | 8 " 20 | 16 " 23 | 32 " 26 | 64 " 29
12 | 2 " 15 | 4 " 18 | 8 " 21 | 16 " 24 | 32 " 27 | 64 " 30
13 | 2 " 16 | 4 " 19 | 8 " 22 | 16 " 25 | 32 " 28 |
14 | 2 " 17 | 4 " 20 | 8 " 23 | 16 " 26 | 32 " 29 |
15 | 2 " 18 | 4 " 21 | 8 " 24 | 16 " 27 | 32 " 30 |
16 | 2 " 19 | 4 " 22 | 8 " 25 | 16 " 28 | |
17 | 2 " 20 | 4 " 23 | 8 " 26 | 16 " 29 | |
18 | 2 " 21 | 4 " 24 | 8 " 27 | 16 " 30 | |
19 | 2 " 22 | 4 " 25 | 8 " 28 | | |
20 | 2 " 23 | 4 " 26 | 8 " 29 | | |
21 | 2 " 24 | 4 " 27 | 8 " 30 | | |
------+--------+--------+--------+---------+---------+---------
=B.= _Watts_
watts = E.H.P. × 746 = 54.3 × 746 = 40,508
=C.= _Apparent load_
apparent load or kva = (actual load or watts ÷ power factor)
= 40,508 ÷ .8 = 50,635
=D.= _Current_
current = (apparent load or kva ÷ volts)
= 50,635 ÷ 440
= 115 amperes
=E.= _Size of wires_
cir. mils = (watts × feet × M) ÷ (% loss × volts^{2})
= (40,508 × 1,000 × 3,380) ÷ (5 × 440^{2})
= 141,443
From table page 1,907, nearest size _larger_ wire is No. 00 B.&S.
gauge.
=F.= _Drop_
drop = ((% loss × volts) ÷ 100) × S
= ((5 × 440) ÷ 100) × 1.17
= 25.74 volts
NOTE.--Values of S are given on page 1910.
=G.= _Voltage at alternator_
alternator pressure = (volts at motor + drop)
= 440 + 25.74
= 465.7 volts.
CHAPTER LXVI
POWER STATIONS
The term _power station_ is usually applied to any building
containing an installation of machinery for the conversion of energy
from one form into another form. There are three general classes of
station:
1. Central stations;
2. Sub-stations;
3. Isolated plants.
These may also be classified with respect to their function as
1. Generating stations;
2. Distributing stations;
3. Converting stations.
and with respect to the form of power used in generating the electric
current, generating stations may be classed as
1. Steam electric;
2. Hydro-electric;
3. Gas electric, etc.
=Central Stations.=--It must be evident that the general type of
central station to be adapted to a given case, that is to say, the
general character of the machinery to be installed depends upon the
kind of natural energy available for conversion into electrical
energy, and the character of the electrical energy required by the
consumers.
This gives rise to a further classification, as
1. Alternating current stations;
2. Direct current stations;
3. Alternating and direct current stations.
The alternators or dynamos may be driven by steam or water turbines,
reciprocating engines, or gas engines, according to the character of
the natural energy available.
[Illustration: FIG. 2,705.--Elevation of small station with direct
drive, showing arrangement of the boiler and engine, piping, etc.]
=Ques. Why is the reciprocating engine being largely replaced by the
steam turbine, especially for large units?=
Ans. Because of its higher rotative speed, and absence of a
multiplicity of bearings which in the case of a high speed,
reciprocating engine must be maintained in close adjustment for the
proper operation of the engine.
The higher speed of rotation results in a more compact unit,
desirable for driving high frequency alternators.
=Ques. Is the steam turbine more economical than a high duty
reciprocating engine?=
Ans. No.
=Location of Central Stations.=--As a rule, central stations should
be so located that the average loss of voltage in overcoming the
resistance of the lines is a minimum, and this point is located
at the center of gravity of the system. In fig. 2,706 is shown a
graphical method of locating this important spot.
[Illustration: Fig. 2,706.--Diagram illustrating graphical method
of determining the _center of gravity_ of a system in locating the
central station.]
Suppose a rough canvass of prospective consumers in a district
to be supplied with electric light or power shows the principal
loads to be located at A, B, C, D, E, etc., and for simplicity
assume that these loads will be approximately equal, so that each
may be denoted by 1 for example:
The relative locations of A, B, C, D, E, etc., should be drawn to
scale (say 1 inch to the 1,000 feet) after which the problem resolves
itself into finding the location of the station with respect to this
scale.
[Illustration: FIG. 2,707.--Exterior of central station at Lewis,
Ia.; example of very small station located in the principal business
section of a town. It also illustrates the use of a direct connected
gasoline electric set. The central station is located on Main Street,
which is the principal thoroughfare, and is installed in a low one
story building for which a mere nominal rental charge is paid, the
company having the option to buy the property later at the value of
the land plus the cost of the improvements and simple interest on
the same. To the front of an old frame building about 16 feet by
28 feet has been built a neat, well lighted concrete block room,
about 16 feet by 16 feet, carrying the building to the lot line and
affording ample space for the generating set and switchboards, and
such desk room as is needed for the ordinary office business of
the company. In this room, which is finished in natural pine with
plastered walls, has been installed a standard General Electric 25
kw. gasoline electric generating set consisting of a four cylinder,
four cycle, vertical water cooled, 43-54 H.P. gasoline engine, direct
connected to a three phase, 2,300 volt, 600 R.P.M. alternator with
a 125 volt exciter mounted on the same shaft and in the same frame.
With the generating set is a slate switchboard panel equipped with
three ammeters, one voltmeter, an instrument plug switch for voltage
indication, one single pole carbon break switch, one automatic oil
circuit breaker line switch and rheostats. Instrument transformers
are mounted above and back of the board. For street lighting service
a 4 kw. constant current transformer has been installed, and with
it a gray marble switchboard panel mounted on iron frames and
carrying an ammeter and a four point plug switch. On a board near
the generator set are mounted in convenient reach suitable wrenches,
spanners, and repair parts and tools. To cool the engine cylinders
five 6 × 8 steel tanks have been installed in the old building, a
pump on engine giving forced circulation.]
The solution consists in first finding the center of gravity
of any two of the loads, such as those at A and B. Since each of
these is 1, they will together have the same effect on the system
as the resultant load of 1 and 1, or 2, located at their center of
gravity, this point being so chosen that the product of the loads
by their respective distances from this point will in both cases
be equal.
The loads being equal in this case the distances must be equal
in order that the products be the same, so that the center of
gravity of A + B is at G, which point is midway between A and B.
Considering, next, the resultant load of 2 at G and the load of
1 at C, the resultant load at the center or gravity of these will
be 3, and this must be situated at a distance of two units from C
and one unit from G so that the distance 2 times the load 1 at C
equals the distance 1 times the load 2 at G. Having thus located
the load 3 at H, the same method is followed in finding the load 4
at I. Then in like manner the resultant load 4 and the load 1 at E
gives a load 5 at S.
The point S being the last to be determined represents,
therefore, the position of the center of gravity of the entire
system, and consequently the proper position of the plant in order
to give the minimum loss of voltage on the lines.
=Ques. Is the center of gravity of the system, as obtained in fig.
2,706, the proper location for the central station?=
Ans. It is very rarely the best location.
=Ques. Why?=
Ans. Other conditions, such as the price of land, difficulty of
obtaining water, facilities for delivery of coal and removal of
ashes, etc., may more than offset the minimum line losses and copper
cost due to locating the station at the center of gravity of the
system.
[Illustration: FIG. 2,708.--Map of Cia Docas de Santos hydro-electric
system; an example of station location remote from the center of
distribution. In the figure A is the intake; B, flume; C, forebay;
D, penstocks; E, power house; F, narrow gauge railway; G, general
store; H, point of debarkation; I, transmission line; J, dead ends;
K, sub-station. Santos, in the republic of Brazil, is one of the
great coffee shipping ports of the world, and for the development of
its water front has required an elaborate system of quays. These have
been developed by the Santos Dock Company, which holds a concession
for the whole water front. The company, needing electric power for
its own use, has developed a system deriving its power from a point
about thirty miles from the city, where a small stream plunges
down the sea coast from the mountain range that runs along it. The
engineers have estimated that 100,000 horse power can be obtained
from this source.]
=Ques. How then should the station be located?=
Ans. The more practical experience the designer has had, and the
more common sense he possesses, the better is he equipped to handle
the problem, as the solution is generally such that it cannot be
worked out by any rule of thumb method.
[Illustration: FIG. 2,709.--Station location. The figure shows
two distribution centers as a town A and suburb B supplied with
electricity from one station. For minimum cost of copper the location
of the station would be at G, the center of gravity. However, it is
very rarely that this is the best location. For instance at C, land
is cheaper than at G, and there is room for future extension to the
station, as shown by the dotted lines, whereas at G, only enough
land is available for present requirements. Moreover C is near the
railroad where coal may be obtained without the expense of cartage,
and being located at the river, the plant may be run condensing thus
effecting considerable economy. The conditions may sometimes be such
that any one of the advantages to be secured by locating the station
at C may more than offset the additional cost of copper.]
=Ques. What are the general considerations with respect to the price
of land?=
Ans. The cost for the station site may be so high as to necessitate
building or renting room at a considerable distance from the district
to be supplied.
If the price of land selected for the station be high, the
running expenses will be similarly affected, inasmuch as more
interest must then be paid on the capital invested.
The price or rent of real estate might also in certain
instances alter the proposed interior arrangement of the station,
particularly so in the case of a company with small capital
operating in a city where high prices prevail. In general,
however, it may be stated that whatever effect the price of real
estate would have upon the arrangement, operation and location
of a central station it can quite readily and accurately be
determined in advance.
=Ques. With respect to the cost of the land what should be especially
considered?=
Ans. Room for the future extension of the plant.
Although such additional space need not be purchased at the time
of the original installation it is well, if possible, to make
provision whereby it can be obtained at a reasonable figure when
desired. The preliminary canvass of consumers will aid in deciding
the amount of space advisable to allow for future extensions; as a
rule, however, it is wise to count on the plant enlarging to not
less than twice its original size, as often the dimensions have to
be increased four and even six times those found sufficient at the
beginning.
[Illustration: FIG. 2,710.--Section of the central station or
"electricity works" at Derby, showing boiler and engine room and
arrangement of bunkers, conveyor, ash pit, grates, boilers (drum,
heating surface and superheater), economizer, flue, turbines,
condenser pumps, etc.; also location of switchboard gallery and
system of piping.]
=Ques. What trouble is likely to be encountered with an illy located
plant after it is in operation?=
Ans. It may be considered a nuisance by those residing in the
vicinity, occasioning many complaints.
[Illustration: FIG. 2,711.--View of old and new Waterside stations.
The new station at the right has an all turbine equipment of ten
units, some Curtis and some Parsons machines, two have a capacity of
14,000 kw., and the remaining eight are of 12,000 kw. each. The old
Riverside station, seen at the left is described on page 1940.]
Thus, if the plant be placed in a residential section of the
community the smoke, noise and vibration of the machines may
become a nuisance to the surrounding inhabitants, and eventually
end in suits for damage against the company responsible for the
same. For these and the other reasons just given a company is
sometimes forced to disregard entirely the location of a central
station near the center of gravity of the system, and build at a
considerable distance; such a proceeding would, if the distance be
great, necessitate the installation of a high pressure system.
There might, however, be certain local laws in force restricting
the use of high pressure currents on account of the danger
resulting to life, that would prevent this solution of the
problem. In such cases there could undoubtedly be found some site
where the objections previously noted would be tolerated; thus,
there would naturally be little objection to locating next to a
stable, a brewery, or a factory of any description.
=Ques. Why is the matter of water supply important for a central
station?=
Ans. Because, in a steam driven plant, water is used in the boilers
for the production of steam by boiling, and if the engines be of the
condensing type it is also used in them for creating a vacuum into
which the exhaust steam passes so as to increase the efficiency of
the engine above what it would be if the exhaust steam were obliged
to discharge into the comparatively high pressure of the atmosphere.
The force of this will be apparent by considering that the
water consumption of the engine ordinarily is from 15 to 25 lbs.
of "feed water" per horse power per hour, and the amount of
"circulating water" required to maintain the vacuum is about 25 to
30 times the feed water, and in the case of turbines with their 28
or 29 inch vacuum, much more. For instance, a 1,000 horse power
plant running on 15 lbs. of feed water and 30 to 1 circulating
water would require (1,000 × 15) × (30 + 1) = 465,000 lbs. or
55,822 gals. per hour at full capacity.
=Ques. Besides price what other considerations are important with
respect to water?=
Ans. Its quality and the possibility of a scarcity of supply.
It is quite necessary that the water used in the boilers should be
as free as possible from impurities, so as to prevent the deposition
within them of any scale or sediments. The quality of the water used
for condensing purposes, however, is not quite so important, although
the purer it is the better.
If the plant is to be located in a city, the matter of water supply
need not generally be considered, because, as a rule, it can be
obtained from the waterworks; there will then, of course, be a water
tax to consider and this, if large, may warrant an effort being
made to obtain the water in some other way. In any event, however,
the possibility of a scarcity in the supply should be reduced to a
minimum.
If the plant be located in the country, some natural source of water
would be utilized unless the place be supplied with waterworks, which
is not generally the case. It is usual, however, to find a stream,
lake or pond in the vicinity, but if none such be conveniently near,
an artesian or other form of well must be sunk.
If abundance of water exist in the vicinity of the proposed
installation, not only would the location of the plant be governed
thereby, but the kind of power to be used for its operation would
depend thereon. Thus, if the quantity of the water were sufficient
throughout the entire year to supply the necessary power, water
wheels might be installed and used in place of boilers and steam
engines for driving the generators. The station would then, of
course, be situated close to the waterfall, regardless of the center
of gravity of the system.
[Illustration: FIG. 2,712.--View illustrating the location of a
station as governed by the presence of a water falls. In such cases
the natural water power may be at a considerable distance from the
center of gravity of the distribution system because of the saving
in generation. In the case of long distance transmission very
high pressure may be used and a transformer step down sub-station
be located at or near the center of gravity of the system, thus
considerably reducing the cost of copper for the transmission line.]
=Ques. What should be noted with respect to the coal supply?=
Ans. The facility for transporting the coal from the supply point to
the boiler room.
In this connection, an admirable location, other conditions
permitting, is adjacent to a railway line or water front so that
coal delivered by car or boat may be unloaded directly into the
bins supplying the boilers.
If the coal be brought by train, a side or branch track will
usually be found convenient, and this will usually render any
carting of the fuel entirely unnecessary.
In whatever way the coal is to be supplied, the liability of a
shortage due to traffic or navigation being closed at any time of
the year should be well looked into, as should also the facility
for the removal of ashes, before deciding upon the final location
for the plant.
[Illustration: FIG. 2,713.--View of a station admirably located with
respect to transportation of the coal supply. As shown, the coal may
be obtained either by boat or rail, and with modern machinery for
conveying the coal to the interior of the station, the transportation
cost is reduced to a minimum.]
[Illustration: FIG. 2,714.--Floor plan of part of the turbine central
station erected by the Boston Edison Co., showing two 5,000 kw.
Curtis steam turbines in place. The complete installation contains
twelve 5,000 kw. Curtis steam turbines, a sectional elevation being
shown in fig. 2,758, page 1,971.]
=Choice of System.=--The chief considerations in the design of a
central station are economy and capacity. When the current has to be
transmitted long distances for either lighting or power purposes,
economy is attainable only by reducing the weight of the copper
conductors. This can be accomplished only by the use of the high
voltage currents obtainable from alternators.
Again, where the consumers are located within a radius of two miles
from the central station, thereby requiring a transmission voltage of
550 volts or less, dynamos may be employed with greater economy.
Alternating current possesses serious disadvantages for certain
important applications.
For instance, in operating electric railways and for lighting it is
often necessary to transmit direct current at 500 volts a distance
of five or ten miles. In such cases, the excessive drop cannot be
economically reduced by increasing the sizes of the line wire, while
a sufficient increase of the voltage would cause serious variations
under changes of load. Hence, it is common practice to employ some
form of auxiliary generator or booster, which when connected in
series with the feeder, automatically maintains the required pressure
in the most remote districts so long as the main generators continue
to furnish the normal or working voltage.
The advantage of a direct current installation in such cases over a
similar plant supplying alternating current line is the fact that a
storage battery may be used in connection with the former for taking
up the fluctuations of the current, thereby permitting the dynamo to
run with a less variable load, and consequently at higher efficiency.
=Ques. Name some services requiring direct current.=
Ans. Direct current is required for certain kinds of electrolytic
work, such as electro-plating, the electrical separation of
metals, etc., also the charging of storage batteries for electric
automobiles.
[Illustration: FIG. 2,715.--Example of central station located
remote from the distributing center and furnishing alternating
current at high pressure to a sub-station where the current is passed
through step down transformers and supplied at moderate pressure
to the distribution system. In some cases the sub-station contains
also converters supplying direct current for battery charging,
electro-plating, etc.]
=Ques. How is direct current supplied?=
Ans. Sometimes the central station is equipped with suitable
apparatus for supplying both direct and alternating current. This may
be accomplished in several different ways: By installing both direct
and alternating current generators in the central station; by the use
of double current generators or dynamotors, from which direct current
may be taken from one side and alternating current from the other
side; or by installing, in the sub-station of an alternating current
central station, in addition to the transformers usually placed
therein, a rotary converter for changing or converting alternating
current into direct current.
Thus, it is evident that the character of a central station
will be governed to a great extent by the class of services to be
supplied.
An exception to this is where the entire output has to be
transmitted a long distance to the point of utilization.
In such cases a copper economy demands the use of high tension
alternating current, and its distribution to consumers may be made
directly by means of step down transformers mounted near by or
within the consumers' premises, or it may be transformed into low
voltage alternating current by a conveniently located sub-station.
Where the current is to be used chiefly for lighting and there
are only a few or no motors to be supplied, the choice between
direct current and alternating current will depend greatly upon
the size of the installation, direct current being preferable
for small installations and alternating current for large
installations.
If the current is to be used primarily for operating machinery,
such as elevators, travelling cranes, machine tools and other
devices of a similar character, which have to be operated
intermittently and at varying speeds and loads, direct current is
the more suitable; but if the motors performing such work can be
operated continuously for many hours at a time under practically
constant loads, as, for instance in the general work of a pumping
station, alternating current may be employed with advantage.
[Illustration: FIG. 2,716.--Diagram illustrating diversity factor.
By definition _diversity factor = combined actual maximum demand of
a group of customers divided by the sum of their individual maximum
demands_. Example, a customer has fifty (50) watt lamps and, of
course, the sum of the individual maximum demands of the lamps is 2.5
kw. watts ("connected load"). The customer's maximum demand, however,
is 1.5 kw. Hence, the diversity factor[A] of the customer's group of
lamps is 1.5 ÷ 2.5 = .6. In the diagram the ordinates of the curves
show the ratio _maximum demand_ to _connected load_ for various kinds
of electric lighting service in Chicago.]
[A] NOTE.--The diversity factor of a customer's group of lamps,
namely, the ratio of maximum demand to connected load is usually
called the _demand factor_ of the customer.
=Size of Plant.=--Before any definite calculation can be made, or
plans drawn, the engineer must determine the probable load. This is
usually ascertained in terms of the number and distances of lamps
that will be required, by making a thorough canvass of the city or
town, or that portion for which electrical energy is to be supplied.
The probable load that the station is to carry when it begins
operation, the nature of this load, and the probable rate of increase
are matters upon which the design and construction chiefly depend.
[Illustration: FIG. 2,717.--Load curve for one day.]
=Ques. What is the nature of the load carried by a central station?=
Ans. It fluctuates with the time of day and also with the time of
year.
=Ques. How is a fluctuating load best represented?=
Ans. Graphically, that is to say by means of a curve plotted on
coordinate paper of which ordinates represent load values and the
corresponding abscissæ time values, as in the accompanying curves.
=What is the nature of a power load?=
Ans. Where electricity is supplied for power purposes to a number of
factories, the load is fairly steady, dropping, of course, during
meal hours. In the case of traction, the average value of the load is
fairly steady but there are momentarily violent fluctuations due to
starting cars or trains.
[Illustration: FIG. 2,718.--Load curve for one year.]
=Ques. What is the peak load?=
Ans. The maximum load which has to be carried by the station at any
time of day or night as shown by the highest point of the load curve.
=Ques. Define the load factor.=
Ans. The machinery of the station evidently must be large enough to
carry the peak load, and therefore considerably in excess of that
required for the average demand. The ratio of the average to the
maximum load is called the load factor.
There are two kinds of load factor: the annual, and the daily.
The annual load factor is obtained as a percentage by
multiplying the number of units sold (per year) by 100, and
dividing by the product of the maximum load and the number of
hours in the year. The daily load factor is obtained by taking the
figures for 24 hours instead of a year.
[Illustration: FIG. 2,719.--Load curve of plant supplying power for
the operation of motors in a manufacturing district. The horizontal
dotted lines show suitable power ratings. A properly designed steam
plant has a large overload capacity, a hydraulic plant has a small
overload capacity, and a gasoline engine plant has no overload
capacity. Accordingly, the peak of the load (maximum load) may be 25
or 30 per cent. in excess of the rated capacity of a steam plant,
not more than 5 or 10 per cent. in excess of the rated capacity of a
hydraulic plant, not at all in excess of the rated capacity of a gas
engine plant.]
=Ques. What must be provided in addition to the machinery required to
supply the peak load?=
Ans. Additional units must be installed for use in case of repairs or
break down of some of the other units.
EXAMPLE.--What would be the boiler horse power required to
generate 5,000 kw. under the following conditions: Efficiency of
generators 85%; efficiency of engines 90%; feed water of engines
and auxiliaries 15 lbs. per I. H. P.; boiler pressure 175 lbs.;
temperature of feed water 150° Fahr? With a rate of combustion of
15 lbs. of coal per sq. foot of grate per hour and an evaporation
(from and at 212°) of 8 lbs. of water per lb. of coal, what area
of grate would be required and how much heating surface?
5,000 kw. = 5,000 ÷ .746 = 6,702= electrical horse power
To obtain this electrical horse power with alternators whose
efficiency is 85% requires
6,702 ÷ .85 = 7,885 brake horse power at the engine
This, with mechanical efficiency of 90% is equivalent to
7,885 ÷ .9 = 8,761 indicated horse power
Since 15 lbs. of feed water are required for the engines and
auxiliaries per indicated horse power per hour, the total feed
water or evaporation required to generate 5,000 kw. is
15 × 8,761 = 131,415 lbs. per hour.
that is to say, the boilers must be of sufficient capacity
to generate 131,415 lbs. of steam per hour from water at a
temperature of 150° Fahr. This must be multiplied by the _factor
of evaporation_ for steam at 175 lbs. pressure from feed water at
a temperature of 150°, in order to get the equivalent evaporation
"_from and at 212_°."
The formula for the factor of evaporation is
H - _h_
factor of evaporation = ------- (1)
965.7
in which
H = total heat of steam at the observed pressure;
_h_ = total heat of feed water of the observed temperature;
965.7 = latent heat, of steam at atmospheric pressure.
Substituting in (1) values for the observed pressure and
temperature as obtained from the steam table
1,197 - 118
factor of evaporation = ------------ = 1.117
965.7
for which the equivalent evaporation "_from and at 212_°" is
131,415 × 1.117 = 146,791 lbs.= per hour
=FACTORS OF EVAPORATION=
-------------+-----------------------------------------------------+
Temp of | STEAM PRESSURE BY GAUGE |
feed water. +-----+-----+-----+-----+-----+-----+-----+-----+-----+
Deg. Fahr. | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 | 130 |
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
32 |1.214|1.216|1.220|1.222|1.225|1.227|1.229|1.231|1.232|
40 |1.206|1.209|1.212|1.214|1.216|1.219|1.220|1.222|1.224|
50 |1.195|1.197|1.201|1.204|1.206|1.208|1.210|1.212|1.214|
60 |1.185|1.188|1.191|1.193|1.196|1.198|1.200|1.202|1.203|
70 |1.175|1.178|1.180|1.183|1.185|1.187|1.189|1.191|1.193|
80 |1.164|1.167|1.170|1.173|1.175|1.177|1.179|1.181|1.183|
90 |1.154|1.157|1.160|1.162|1.165|1.167|1.169|1.170|1.172|
100 |1.144|1.147|1.150|1.152|1.154|1.156|1.158|1.160|1.162|
110 |1.133|1.136|1.139|1.142|1.144|1.146|1.148|1.150|1.152|
120 |1.123|1.126|1.129|1.131|1.133|1.136|1.138|1.140|1.141|
130 |1.113|1.116|1.118|1.121|1.123|1.125|1.127|1.129|1.130|
140 |1.102|1.105|1.108|1.110|1.113|1.115|1.117|1.119|1.120|
150 |1.091|1.095|1.098|1.100|1.102|1.104|1.106|1.108|1.110|
160 |1.081|1.084|1.087|1.090|1.092|1.094|1.096|1.098|1.100|
170 |1.070|1.074|1.077|1.079|1.081|1.083|1.085|1.087|1.089|
180 |1.060|1.063|1.066|1.069|1.071|1.073|1.075|1.077|1.079|
190 |1.050|1.053|1.056|1.058|1.060|1.063|1.065|1.066|1.068|
200 |1.039|1.043|1.045|1.048|1.050|1.052|1.054|1.056|1.058|
210 |1.029|1.032|1.035|1.037|1.040|1.042|1.044|1.046|1.047|
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
Temp of | STEAM PRESSURE BY GAUGE |
feed water. +-----+-----+-----+-----+-----+-----+-----+-----+-----+
Deg. Fahr. | 140 | 150 | 160 | 170 | 180 | 190 | 200 | 210 | 220 |
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
32 |1.234|1.236|1.237|1.239|1.240|1.241|1.243|1.244|1.245|
40 |1.226|1.227|1.229|1.230|1.232|1.233|1.234|1.236|1.237|
50 |1.215|1.217|1.218|1.220|1.221|1.223|1.224|1.225|1.226|
60 |1.205|1.207|1.208|1.210|1.211|1.212|1.214|1.215|1.216|
70 |1.194|1.196|1.197|1.199|1.200|1.202|1.203|1.205|1.206|
80 |1.184|1.186|1.187|1.189|1.190|1.192|1.193|1.194|1.195|
90 |1.174|1.176|1.177|1.179|1.180|1.181|1.183|1.184|1.185|
100 |1.164|1.165|1.167|1.168|1.170|1.171|1.172|1.174|1.175|
110 |1.153|1.155|1.156|1.158|1.159|1.160|1.162|1.163|1.164|
120 |1.143|1.145|1.146|1.147|1.149|1.150|1.151|1.153|1.154|
130 |1.132|1.134|1.136|1.137|1.138|1.140|1.141|1.142|1.144|
140 |1.122|1.124|1.125|1.127|1.128|1.129|1.131|1.132|1.133|
150 |1.111|1.113|1.115|1.116|1.118|1.119|1.120|1.121|1.123|
160 |1.101|1.103|1.104|1.106|1.107|1.108|1.110|1.111|1.112|
170 |1.091|1.092|1.094|1.095|1.097|1.098|1.099|1.101|1.102|
180 |1.080|1.082|1.083|1.085|1.086|1.088|1.089|1.090|1.091|
190 |1.070|1.071|1.073|1.074|1.076|1.077|1.078|1.080|1.081|
200 |1.059|1.061|1.063|1.064|1.065|1.067|1.068|1.069|1.071|
210 |1.049|1.051|1.052|1.053|1.055|1.056|1.057|1.059|1.060|
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
Temp. of | STEAM PRESSURE BY GAUGE |
feed water. +-----+-----+-----+-----+-----+-----+-----+-----+-----+
Deg. Fahr. | 230 | 240 | 250 | 260 | 270 | 280 | 290 | 300 | |
-------------+-----+-----+-----+-----+-----+-----+-----+----+------+
32 |1.246|1.247|1.248|1.250|1.251|1.252|1.253|1.254|
40 |1.238|1.239|1.240|1.241|1.242|1.243|1.244|1.245|
50 |1.228|1.229|1.230|1.231|1.232|1.233|1.234|1.235|
60 |1.217|1.218|1.219|1.220|1.221|1.222|1.223|1.224|
70 |1.207|1.208|1.209|1.210|1.211|1.212|1.213|1.214|
80 |1.196|1.198|1.199|1.200|1.201|1.202|1.203|1.204|
90 |1.186|1.187|1.188|1.189|1.190|1.191|1.192|1.193|
100 |1.176|1.177|1.178|1.179|1.180|1.181|1.182|1.183|
110 |1.166|1.167|1.168|1.169|1.170|1.171|1.172|1.173|
120 |1.155|1.156|1.157|1.158|1.159|1.160|1.161|1.162|
130 |1.145|1.146|1.147|1.148|1.149|1.150|1.151|1.152|
140 |1.134|1.135|1.136|1.137|1.138|1.139|1.140|1.141|
150 |1.124|1.125|1.126|1.127|1.128|1.129|1.130|1.131|
160 |1.113|1.115|1.116|1.117|1.118|1.119|1.120|1.121|
170 |1.103|1.104|1.105|1.106|1.107|1.108|1.109|1.110|
180 |1.093|1.094|1.095|1.096|1.097|1.098|1.099|1.100|
190 |1.082|1.083|1.084|1.085|1.086|1.087|1.088|1.089|
200 |1.072|1.073|1.074|1.075|1.076|1.077|1.078|1.079|
210 |1.061|1.062|1.063|1.064|1.065|1.066|1.067|1.068|
-------------+-----+-----+-----+-----+-----+-----+-----+-----+
One boiler horse power being equal to _an evaporation of_ 34½
_lbs. of water from a feed water temperature of 212° Fahr., into
steam at the same temperature_, the boiler capacity is accordingly
148,105 ÷ 34.5 = 4,293 boiler horse power.
The rate of evaporation is given at 8 lbs. of water (from and at
212° Fahr.), for which the fuel required is
148,105 ÷ 8 = 18,513 lbs. of coal per hour.
For a rate of combustion of 15 lbs. of coal per hour per square
foot of grate,
grate area = 18,513 ÷ 15 = 1,234 sq. ft.
For stationary boilers the usual ratio of heating surface to
grate area is 35:1, accordingly the heating surface corresponding
to this ratio is
1,234 × 35 = 43,190 sq.ft.
The above calculation is based on a rate of evaporation of 8
lbs. of water per lb. of coal and a rate of combustion of 15 lbs.
of coal per sq. ft. of grate. For other rates the required grate
area may be obtained from the following table:
----------------------------------------------------------------------
GRATE SURFACE PER HORSE POWER (KENT)
------------+------+-----+--------------------------------------------
|Pounds| |
| of | Lbs.| Pounds of coal burned per square foot of
|water | of | grate per hour
| from | coal+----+----+----+----+----+----+----+----+----
|and at| per | | | | | | | | |
| 212° | h.p.| 8 | 10 | 12 | 15 | 20 | 25 | 30 | 35 | 40
| per | per | | | | | | | | |
|pound | hour+----+----+----+----+----+----+----+----+----
| of | | Square feet grate per horse power
| coal | |
------------+------+-----+----+----+----+----+----+----+----+----+----
Good coal | }10 | 3.45| .43|.35 | .28| .23| .17| .14| .11| .10| .09
and boiler | } 9 | 3.83| .48| .38| .32| .25| .19| .15| .13| .11| .10
| | | | | | | | | | |
Fair coal |{8.61 | 4. | .50| .40| .33| .26| .20| .16| .13| .12| .10
or boiler |{8 | 4.31| .54| .43| .36| .29| .22| .17| .14| .13| .11
|{7 | 4.93| .62| .49| .41| .33| .24| .20| .17| .14| .12
| | | | | | | | | | |
Poor coal |{6.9 | 5. | .63| .50| .42| .34| .25| .20| .17| .15| .13
or boiler |{6 | 5.75| .72| .58| .48| .38| .29| .23| .19| .17| .14
|{5 | 6.9 | .86| .69| .58| .46| .35| .28| .23| .22| .17
| | | | | | | | | | |
Lignite and|}3.45 |10. |1.25|1.00| .83| .67| .50| .40| .33| .29| .25
poor boiler|} | | | | | | | | | |
------------+------+-----+----+----+----+----+----+----+----+----+----
=General Arrangement of Station.=--In designing an electrical
station, it is preferable that whatever rooms or divisions of the
interior space are desired should determine the total outside
dimensions of the plant in the original plans of the building than
that these latter dimensions be fixed and the rooms, etc., be fitted
in afterward.
=SAVING DUE TO HEATING THE FEED WATER=
Table showing the percentage of saving for each degree of increase in
temperature of feed water heated by waste steam.
----------------------------------------------------------------------
Init| Pressure of steam in boiler, lbs. per sq. inch above atmosphere
temp|-----------------------------------------------------------------
of |
feed| 0 | 20 | 40 | 60 | 80 | 100 | 120 | 140 | 160 | 180 | 200
----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
32°|.0872|.0861|.0855|.0851|.0847|.0844|.0841|.0839|.0837|.0835|.0833
40 |.0878|.0867|.0861|.0856|.0853|.0850|.0847|.0845|.0843|.0841|.0839
50 |.0886|.0875|.0868|.0864|.0860|.0857|.0854|.0852|.0850|.0848|.0846
60 |.0894|.0883|.0876|.0872|.0867|.0864|.0862|.0859|.0856|.0855|.0853
70 |.0902|.0890|.0884|.0879|.0875|.0872|.0869|.0867|.0864|.0862|.0860
80 |.0910|.0898|.0891|.0887|.0883|.0879|.0877|.0874|.0872|.0870|.0868
90 |.0919|.0907|.0900|.0895|.0888|.0887|.0884|.0883|.0879|.0877|.0875
100 |.0927|.0915|.0908|.0903|.0899|.0895|.0892|.0890|.0887|.0885|.0883
110 |.0936|.0923|.0916|.0911|.0907|.0903|.0900|.0898|.0895|.0893|.0891
120 |.0945|.0932|.0925|.0919|.0915|.0911|.0908|.0906|.0903|.0901|.0899
130 |.0954|.0941|.0934|.0928|.0924|.0920|.0917|.0914|.0912|.0909|.0907
140 |.0963|.0950|.0943|.0937|.0932|.0929|.0925|.0923|.0920|.0918|.0916
150 |.0973|.0959|.0951|.0946|.0941|.0937|.0934|.0931|.0929|.0926|.0924
160 |.0982|.0968|.0961|.0955|.0950|.0946|.0943|.0940|.0937|.0935|.0933
170 |.0992|.0978|.0970|.0964|.0959|.0955|.0952|.0949|.0946|.0944|.0941
180 |.1002|.0988|.0981|.0973|.0969|.0965|.0961|.0958|.0955|.0953|.0951
190 |.1012|.0998|.0989|.0983|.0978|.0974|.0971|.0968|.0964|.0062|.0960
200 |.1022|.1008|.0999|.0993|.0988|.0984|.0980|.0977|.0974|.0972|.0969
210 |.1033|.1018|.1010|.1003|.0998|.0994|.0990|.0987|.0984|.0981|.0979
220 | -- |.1029|.1019|.1013|.1008|.1004|.1000|.0997|.0994|.0991|.0989
230 | -- |.1039|.1031|.1024|.1018|.1012|.1010|.1007|.1003|.1001|.0999
240 | -- |.1050|.1041|.1034|.1029|.1024|.1020|.1017|.1014|.1011|.1009
250 | -- |.1062|.1052|.1045|.1040|.1035|.1031|.1027|.1025|.1022|.1019
----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
NOTE.--An approximate rule for the conditions of ordinary
practice is a saving of 1 per cent. made by each increase of
11° in the temperature of the feed water. This corresponds to
.0909 per cent. per degree. The calculation of saving is made as
follows: Boiler pressure, 100 lbs. gauge; total heat in steam
above 32° = 1,185 B.T.U. feed water, original temperature 60°,
final temperature 209°F. Increase in heat units, 150. Heat units
above 32° in feed water of original temperature = 28. Heat units
in steam above that in cold feed water, 1,185-28 = 1,157. Saving
by the feed water heater = 150 ÷ 1,157 = 12.96 per cent. The
same result is obtained by the use of the table. Increase in
temperature 150° × tabular figure .0864 = 12.96 per cent. Let
total heat of 1 lb. of steam at the boiler pressure = H; total
heat of 1 lb. of feed water before entering the heater = _h'_, and
after passing through the heater = _h''_; then the saving made by
the heater is (_h''_-_h'_) ÷ (H-_h'_).
Under usual conditions the plans of an electrical station are readily
drawn, as they are generally of a simple nature. The engines and
generators will occupy the majority of the space, and these are
usually placed in one large room; in some stations, however, they are
located respectively in two adjacent rooms. The boilers are generally
located in a room apart from the engines and dynamos, and in some
cases a separate building is provided for them; the pumps, etc., must
be installed not far from the boilers, and space must also be allowed
near the boilers for coal and ashes.
[Illustration: FIG. 2,720.--Floor plan of an electrical station
having a belted drive with counter shaft.]
Fig. 2,720 shows the floor plan of an electrical station, in
which a countershaft and belted connections are used between
the engines and generators. Referring first to the plan of the
building itself, A represents the engine and dynamo room, B
denotes the boiler room, C the office, D the store room, and E the
chimney connected with the boilers by means of the uptake _w_.
Referring next to the apparatus installed, S, S, S, S represents
a battery of four boilers; these are connected by steam piping
VV to the two steam engines, M and M, which are belted to the
countershaft O. Belted to the countershaft are the generators,
T, T, T, T, the circuits from which are controlled on the
switchboard, H.
=Ques. What are the objections to the arrangement shown in
fig. 2,720.=?
Ans. The large space required by the belt drive especially in
locations where land is expensive. Another objection is the
frictional loss due to the belt drive with its countershaft, etc.
[Illustration: FIG. 2,721.--Elevation of station having a belted
drive with countershaft, as shown in plan in fig. 2,720.]
=Ques. What are the desirable features of the belt drive?=
Ans. High speed generators may be used, thus reducing the first
cost, and the multiplicity of speeds and flexibility of the system
resulting from the use of a friction clutch.
Thus in fig. 2,720, each pulley may be mounted on the counter
shaft O with a friction clutch. A jaw clutch may also be provided
at Z, thus permitting the shaft O to be divided into two sections.
It is therefore possible by this arrangement to cause either of
the engines to drive any one of the generators, or all of them, or
both of the engines to drive all of the generators simultaneously.
=Ques. Under what condition is the counter shaft belt drive
particularly valuable?=
Ans. In case of a break down of any one of the engines or generators,
and also when it becomes necessary to clean them without interrupting
the service.
[Illustration: FIG. 2,722.--Plan of station arranged for extension.
The space required for a central station depends upon the number and
kind of lights to be supplied, and upon the character and arrangement
of the machinery. In calculating the size of building required, two
things must be carefully considered: first, the building must be
adapted to the plant to be installed in the beginning; and second, it
must be arranged so that enlargement can be made without disarranging
or interfering with the plant already in existence. This is usually
best secured by providing for expansion in one or two definite
directions, the building being made large enough to accommodate
additional units that will be necessary at some future time because
of the growth of the community and consequent increased demand for
electric current.]
=Ques. How may the design in fig. 2,720 be modified for the
installation of a storage battery?=
Ans. If a storage battery be necessary, a partition may be
constructed across the room A, as indicated by the dotted lines, and
the battery installed in the room thus formed.
[Illustration: FIG. 2,723.--Interior of old Riverside station
showing at the right, seven 6,000 horse power alternators driven by
reciprocating engines, and at the left, a number of turbine units
aggregating 90,000 horse power.]
=Ques. Mention a few details in the general arrangement of the
building fig. 2,720.=
Ans. Two doors to the room A may conveniently be provided at K and L,
the former connecting with the boiler room B, and the latter serving
as the main entrance to the station. There is little that need be
added to what has already been stated regarding the boiler room B.
The door at F provides for the entrance of coal and the removal of
ashes, while at P, the pump and heaters may conveniently be located.
In the office C, visitors may be received, the station reports made
out, bulletins issued from time to time, and whatever engineering
problems arise may here be solved on paper by the engineer in
charge of the plant. The store room D will be found convenient for
various supplies, tools and appliances needed in the operation of
the station. These may here be kept under lock and key and the daily
waste and loss resulting from carelessness avoided.
=Ques. What important point should be noted in locating the engines
and boilers?=
Ans. They should be so placed that the piping between them will be as
short and direct as possible.
=Ques. Why?=
Ans. The steam pipe should be short to reduce the loss of heat
between engine and boiler to a minimum, and both short and direct to
avoid undue friction and consequent drop in pressure of the steam in
passing through the pipe to the engine.
Entirely too little attention is given to this matter on the
part of designers and it cannot be too strongly emphasized that,
for economy, the steam pipe between an engine and boiler should
be as short and direct as possible, having regard of course, for
proper piping methods.
=Ques. What should be provided for the steam pipe?=
Ans. A heavy covering of approved material should be placed around
the pipe to reduce the loss of heat by radiation. For this purpose
hair felt, mineral wool and asbestos are used.
[Illustration: FIG. 2,724.--View of engine and condenser, showing how
to arrange the piping to secure good vacuum. _Locate the condenser as
near the engine as possible_; =use easy bends= _instead of elbows;
place the pump_ =below= _bottom of condenser so the water will
drain to pump_. At A is a relief valve, for protection in case the
condenser become flooded through failure of the pump, and at B is
a gate valve to shut off condenser in case atmospheric exhaust is
desired to permit repairs to be made to condenser during operation.
=A water seal= should be maintained on the relief valve and =special
attention= _should be given to the stuffing box_ of the gate valve
=to prevent air leakage=. _The discharge valve of the pump should be
water sealed._]
=Ques. How should the piping be arranged between the engine and
condenser, and why?=
Ans. It should be as short and direct as possible; especially should
elbows be avoided so that the back pressure on the engine piston will
be reduced as near as can be to that of the condenser.
That is to say, in order to get nearly the full effect of the
vacuum in the condenser the frictional resistance of the piping
should be reduced to a minimum.
Where 90° turns are necessary, easy bends should be used instead
of sharp elbows. The force of this argument must be apparent by
noting the practice of steam turbine builders of placing the
turbine right up against the condenser, and remembering that a
high vacuum is necessary to the economical working of a turbine.
See fig. 1,445, page 1,182.
=Ques. What are the considerations respecting the number and type of
engine to be used?=
Ans. In the illustration fig. 2,720, two engines M and M' are
employed, one belted to each end of the countershaft O. These engines
should be of similar or identical pattern; for a small output
they may be either simple or compound, as the conditions of fuel
expenditure may dictate, but if the output be large, triple expansion
engines or turbines are advisable.
[Illustration: FIG. 2,725.--"Dry pipe" for horizontal boiler: it is
connected to the main outlet and its upper surface is perforated with
small holes, the far end being closed. With this arrangement steam is
taken from the boiler over a large area, so that it will contain very
little moisture. _All horizontal boilers without a dome should be
fitted with a dry pipe;_ most engineers do not realize the importance
of obtaining dry steam for engine operation.]
Corliss or similar slow speed engines may advantageously be used
in either case. In all cases the engine should be run condensing
unless the cost for circulating water is prohibitive; even in such
cases cooling towers may be installed and effect a saving.
In operation, during the greater part of the day, one engine
running two or perhaps three of the generators, will carry the
load, but when the load is particularly heavy, as in the morning
and evening, both engines and all the generators may be required
to meet the demands.
[Illustration: FIG. 2,726.--Method of connecting a header to a
battery of boilers. Where two or more boilers are connected to a
single header, the use of a reliable non-return boiler stop valve is
necessary, and in some countries their installation is compulsory. A
non-return boiler stop valve will instantly close should the pressure
in the boiler to which it is attached suddenly decrease below that
in the header, and thereby prevent the entrance of steam from the
other boilers of the battery. This sudden decrease in pressure may be
caused by a ruptured fitting or the blowing out of a tube, in which
event an ordinary stop valve taking the place of a non-return boiler
stop valve would be inadequate, as the loss of steam from the other
boilers of the battery would be tremendous before an ordinary valve
could be reached and closed, assuming that it would be possible to do
so, which in the majority of cases it is not. Should it be desired
to cut out a boiler for cleaning or repairs, the non-return boiler
stop valve will not permit steam to enter the boiler from the header,
even should the handwheel be operated for this purpose, as it cannot
be opened by hand, but can, however, be closed. A non-return boiler
stop valve should be attached to each boiler and connected to an
angle valve on the header. A pipe bend should be used for connecting
the valves, as this will allow for expansion and contraction. The
pipe should slope a trifle downward toward the header and a suitable
drain provided. This drain should be opened and all water permitted
to escape before the angle valve is opened, thereby preventing any
damage due to water hammer.]
By exercising a little ingenuity in shifting the load on
different machines at different times, both engines and dynamos,
may readily be cleaned and repaired without interrupting the
service.
=Ques. For economy what kind of steam should be used?=
Ans. Super-heated steam.
The saving due to the use of superheated steam is about 1% for
every ten degrees Fahr. of super-heat. It should be used in all
cases.
=Ques. How should the machines be located?=
Ans. Sufficient space should be allowed between them that cleaning
and repairing may be done easily, quickly and effectually.
[Illustration: FIGS. 2,727 and 2,728.--Method of preventing vibration
and of supporting pipes. The figures show top and side views of
a main header carried in suitable frames fitted with adjustable
roller. While the pipe is illustrated as resting on the adjustable
rollers, nevertheless the rollers may also be placed at the sides
or on top of the pipe to prevent vibration, or in cases where the
thrust from a horizontal or vertical branch has to be provided for.
This arrangement will take care of the vibration without in any way
preventing the free expansion and contraction of the pipe.]
=Ques. How should the switchboard be located?=
Ans. In fig. 2,720, the switchboard H is mounted against the wall
dividing the room A from the room B, and is in line with the machines.
The advantages arising from a switchboard thus installed are,
that the switchboard attendant working thereon can obtain at any
time an unobstructed view of the performance of each individual
machine, and he has in consequence a much better control of them;
then, too, while he is engaged at the engines or generators he
can also see the measuring instruments on the switchboard, and
ascertain approximately the readings upon them.
In cases of emergency it is sometimes necessary for the
engineer in charge of a plant to be in several places at the same
time in order to prevent an accident, and that this seemingly
impossibility may be approximated as nearly as possible, it is
essential that the controlling devices be located as closely
together as is consistent, and that no moving belt or pulley
intervene between them.
These conditions are well satisfied in fig. 2,720, and owing to
the short distances between the generators and the switchboard the
drop of voltage in each of the conducting wires between them will
be low.
This latter advantage is worthy of notice in a station
generating large currents at a low pressure. To offset the
advantages just mentioned, the location of the switchboard in
line with the machines introduces an element of danger to the
switchboard, its apparatus, and the attendant, on account of the
possible bursting of a flywheel or other parts of the machines
from centrifugal force.
[Illustration: FIGS. 2,729 and 2,730.--Points on placing stop valves.
The first and most important feature is to ascertain whether the
valve will act as a water trap for condensed steam. Fig. 2,729
illustrates a common error in the placing of valves, as this
arrangement permits of an accumulation of condensed steam above the
valve when closed, and should the engineer be careless and open the
valve suddenly, serious results might follow owing to water-hammer.
Fig. 2,730 illustrates the correct method of placing the valve. It
sometimes occurs, however, that it is not convenient to place the
valve as shown in fig. 2,730 and that fig. 2,729 is the only manner
in which the valve can be placed. In such cases, the valve should
have a drain, and this drain should always be opened before the large
valve is opened.]
If the switchboard be placed in the dotted position at H', or,
in fact, at the opposite end of the room A, the damage to life
and property that might result from the effects of centrifugal
force would be eliminated, but in place thereof would be the
disadvantages of an obstructed view of the machines from the
switchboard, an obstructed view of the switchboard from the
machines, inaccessibility between these two, and a greater drop
of voltage in the majority of the conducting wires between the
generators and the switchboard.
=Ques. Describe a second arrangement of station with belt drive and
compare it with the design shown in fig. 2,720.=
[Illustration: FIG. 2,731.--Plan of electrical station with belt
drive without counter shaft. The installation here represented
consists of two boilers, S, etc., and three sets of engines and
generators, T, M, etc. Sufficient allowance has been made in the
plans, however, for future increase of business, as additional space
has been provided for an extra engine and generator set, as indicated
by the dotted lines.]
Ans. A floor plan somewhat different from that presented in fig.
2,720 is shown in fig. 2,731. Here a belt drive is employed, but no
countershaft is used. Each generator, therefore, is dependent upon
its respective engine, and in consequence the flexibility obtained by
the use of a countershaft is lost. On the other hand, there is less
loss of mechanical power between the engines and generators in the
driving of the latter, and less floor space is necessary in the room
A. If, however, the floor area of this room be made the same as in
the previous arrangement and the same number of machines are to be
installed, they may be spaced further apart, affording in consequence
considerably more room for cleaning and repairing them.
In operation, the normal conditions should be such that any two
of the engine and generator sets may readily carry the average
load, the third set to be used only as a reserve either to aid the
other two when the load is unusually heavy or to replace one of
the other sets when it becomes necessary to clean or repair the
latter.
The switchboard may perhaps be best located at H, as a similar
position on the opposite side of the room A would bring it beneath
one or more of the steam pipes and thus endanger it should a
possible leakage occur from these pipes. If located at H, however,
it will be in line with the machines, and therefore will be
subject to the disadvantages previously mentioned for such cases;
consequently it might be as well to place it at the further end
of the room, either against the partition (shown dotted) of the
storage battery room if this be built, or else (if no storage
battery is to be installed), against the end wall itself. The
nearer end of the room A would not be very desirable for the
switchboard installation on account of being so far removed from
the machines, and therefore more or less inaccessible from them.
Outside of what has now been mentioned, the division of the floor
plan and the arrangement therein is practically the same as in
fig. 2,720, accordingly what has already been stated regarding the
former installation applies, therefore, with equal force to the
present installation.
=Ques. Describe a plant with direct drive.=
Ans. This type of drive is shown in fig. 2,732. Each engine is
directly connected to a generator, that is, the main shafts of both
are joined together in line so that the generator is driven without
the aid of a belt.
=Ques. What is the advantage of direct drive?=
Ans. The great saving in floor space, which is plainly shown in fig.
2,732, the portion A' representing the saving which results over the
installations previously illustrated in figs. 2,720 and 2,731.
=Ques. How could the floor space be further reduced?=
Ans. By employing vertical instead of horizontal engines.
=Ques. What should be done before drawing the plans for the station?=
Ans. The types of the various machines and apparatus to be installed
shoul
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