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Clarence Edward Clewell.

Laboratory manual. Direct and alternating current online

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edge of the practical features of construction and operation of
the important electric lamps in common use; and, second, from
this study of lamps to secure a definite idea of the voltage re-
quirements of electric circuits which supply power for the opera-
tion of lamps in practice.



28 LABORATORY MANUAL

Theory. The lamps in residence and commercial lighting are
nearly always operated from constant voltage electric supply
mains. At the end of the Table on page 378 of the text book,
a summary is given of the effects produced on carbon incan-
descent lamps by a change of 1 per cent, in the voltage. For ex-
ample, a fall of 1 per cent, in the voltage causes a loss of 5 per
cent, in the candle-power, hence, the importance of maintaining
constant voltage at the terminals of incandescent lamps.

In the operation of a shunt generator, the voltage at the
terminals of the machine falls off somewhat as the machine is
loaded. If this drop in the voltage is great enough to pro-
duce much decrease in the candle-power of the lamps supplied
by the generator, either the field current or the speed may be
varied as the load changes, thus maintaining a constant or fairly
constant terminal voltage ; or a series field winding may be added
to the machine, thus making it a compound generator, in which
the tendency of the voltage to decrease is offset by the strength-
ening of the field due to the current in the series winding.

Several of the experiments on the electrical features of genera-
tors will emphasize this tendency of the voltage to fall as the
load is increased, and it will be well in making these subsequent
observations to keep in mind why constant terminal voltage is a
most important consideration in the distribution of electric power
for lighting.

Among the electric lamps in common use may be mentioned
the Tungsten (or Mazda) incandescent lamp; the Mercury Va-
por (or Cooper Hewitt) lamp; the carbon filament incandescent
lamp; and the arc lamp. Under arc lamps, may be mentioned
the Flaming Carbon arc used in some cases for street lighting
and for commercial or factory lighting; the Metallic Flame or
Magnetite arc lamp used mainly for street lighting; and the
older Enclosed Carbon arc lamp.

Current Supply. 110 volts Direct Current.

Apparatus Required. (1) A board on which are mounted the
parts of a carbon filament incandescent lamp; (2) a Tungsten
(or Mazda) lamp; (3) Focusing, Intensive and Extensive Holo-
phane prismatic reflectors with prints of the respective distribu-
tion of light curves; (4) a form "0" and form "H" shade
holder; (5) a Mercury Vapor (or Cooper Hewitt) lamp with a



DIRECT CURRENT 29

diagram of the internal electrical connections; (6) one or more
of the principal types of arc lamps with diagram of the internal
electrical connections.

Order of Work. 1. Make a sketch of the various parts (label-
ling each) involved in the manufacture of carbon incandescent
lamps.

2. Make a simple diagram of a tungsten lamp showing the
method employed in mounting the filament.

3. Make a simple sketch of the Focusing, Intensive and Ex-
tensive Holophane Prismatic reflectors, showing the general
shape of each, also the approximate distribution curves of each.

4. Make a sketch of the form "0" and of the form "H" shade
holder giving dimensions on the sketches.

5. Make a diagram of the electrical connections of the Mer-
cury Vapor lamp, recording the rating of the lamp in volts, am-
peres and watts.

6. Connect the Mercury Vapor lamp through a suitable rheo-
stat to the supply mains and observe its action at starting.

7. Make a diagram of the electrical connections of the arc
lamp (or lamps) available, recording the rating in volts, amperes
and watts.

8. Connect the arc lamp (or lamps) through a suitable rheo-
stat to the supply mains and observe the action at starting, also
the method of feeding the carbons as they are consumed.

Written Report. 1. Describe briefly the principal items in the
manufacture of carbon incandescent lamps.

2. What is the function of the vacuum ?

3. Why is a reflector a necessary auxiliary with the Tungsten
lamp for its most economical operation ?

4. For what purposes are each of the Holophane reflectors best
suited ?

5. Aside from supporting the reflector, what other function
has the shade holder in connection with Tungsten lamps ?

6. Describe briefly the method of operation of the Mercury
Vapor lamp.

7. Same for the arc lamp (or lamps).

8. Between the generator in the electric power station and dis-
tant lamps there is apt to be an appreciable voltage drop which
depends on the current flowing through the lines. If the attend-



30 LABORATORY MANUAL

ant in the power station regulates the voltage of the generator
by a voltmeter which indicates the voltage in the station, how
is constant or fairly constant voltage assured at the distant lamps
in residence lighting work ?



EXPERIMENT 8.

Building Up of Voltage in Shunt Generator.
See Articles 117, 118, 119 and 120 in the text book.

In many of the laboratory experiments as well as in practical
generator operation, it is necessary to understand the conditions
which must be met in order that a self-excited generator may
build up to its normal voltage at starting. The object of this ex-
periment is to gain a working knowledge of these conditions and
how to meet them.

Theory. Electric generators may be classed as to the produc-
tion of the necessary magnetic field under the head either of sepa-
rate or self excitation. For separate excitation, the field winding
is connected directly to the supply mains as shown in Fig. 10
so that the field current is derived from a generator already in
operation, and the voltage of the machine rises to its normal value
as soon as the armature is brought up to its normal speed because
the magnetism is at its full value at the start. In this case the
field magnetism is practically independent of the operation of
the machine, being dependent on the voltage of the supply mains
and on the hand manipulation of the field rheostat.

For the self excitation of a shunt generator, the field winding
is connected to the armature terminals through a field rheostat
as shown in Fig. 10, and the voltage of the machine even after
the armature is brought up to normal speed may sometimes
amount only to the several volts due to residual magnetism in
the field poles, that is, to the small amount of magnetism left
over from the last time the machine was in operation. Since
the field current and, hence, the magnetism depends on the volt-
age induced in the armature in this case, and since the voltage
itself is dependent on the field magnetism produced by the field
current, it is obvious that the generation of electromotive force
must be cumulative, starting from the few volts produced by



DIRECT CURRENT



31



residual magnetism and rising to the normal voltage of the ma-
chine.

It will further be obvious that no electromotive force can be
induced in a self-excited generator (connected for self excita-
tion) unless there be a small amount of residual magnetism in
the field poles to begin with. When machines are in regular
operation with an occasional shut-down, there is usually sufficient



Field Rheostat




Separate Excitation



Self Excitation



Fig. 10. Two methods of exciting the field of a shunt generator:
(a) separate excitation is shown to the left; and (b) self excitation to
the right.

residual magnetism in the fields for the machine to build up each
time it is started. In practice, direct current generators are
usually operated with self excitation.

The principal condition to be met for building up is that the
electromotive force produced at starting by the rotation of the
armature in the weak residual magnetism be such that the cur-
rent it produces in the field winding shall aid or increase the re-
sidual magnetism. If this condition is fulfilled, the increased
magnetism produces an increased electromotive force which, in
turn, produces an increased field and the electromotive force soon
rises to its full normal value being limited by the saturation of
the field magnet iron.

If the machine fails to build up, the necessary favorable con-
ditions can usually be secured either by reversing the connection



32



LABORATORY MANUAL



of field winding to armature terminals or by running the arma-
ture in the opposite direction.

Other causes which sometimes affect the building up of a ma-
chine may be due to poor contact of the brushes on the commuta-
tor, to the fact that the field rheostat resistance is all cut in, or
to too little residual magnetism.

Current Supply. 110 volts Direct Current.

Apparatus Required. (1) A shunt generator; (2) reversing
switch for readily interchanging the connections of field winding
to armature terminals; and (3) a voltmeter with a range slightly
above the rating of the generator.



Voltmeter




Switch


P f .


^


s





Fig. 11. The reversing switch is a convenient means for reversing
the terminals of the field winding as connected to the armature termi-
nals. (The student should trace the circuit for the two positions of
the reversing switch to determine how the connections are reversed.)

Order of Work. Make a diagram of the exact connections for
the following items labelling each instrument with its laboratory
number on the drawing.

If the machine fails to build up in all of the following cases
on first or second trial, temporarily disconnect the field from the
reversing switch and connect it to the supply .mains for a short
time so a-s to insure the necessary residual magnetism in the
field magnets. Then proceed as directed under the following
heads :

1. Arrange the assigned shunt generator for self excitation as
shown in Fig. 11. The reversing switch inserted between the
field winding and the armature terminals provides a convenient
means for reversing the connections of field to armature.



DIRECT CURRENT



33



2. "With the field disconnected (reversing switch open) drive
the machine at normal speed and observe the electromotive force
at the armature terminals produced by residual magnetism. Re-
cord the electromotive force (volts) and the speed as in Form 6,

3. Throw in all the field rheostat resistance, close the reversing
switch to the arbitrary position "A" (this letter should be
marked in chalk on one end of the switch for reference) and with
the armature rotating at normal speed in a forward 1 direction,
gradually cut out the field rheostat. Observe the initial and final
values of electromotive force produced, that is, before and after



Open



A"



Forward



Out



Form 6.

cutting out the field rheostat resistance, the speed, and the posi-
tion of the reversing switch as in Form 6.

4. Same, throwing the reversing switch in the opposite direc-
tion (mark this second position on the other end of the switch
"B"). Note that this second position of the reversing switch
changes the connections of field to armature.

5. Same as 3 with armature rotating in a backward direction.

6. Same as 4 with armature rotating in a backward direction.

7. Note carefully what effect is produced on the building up in
those cases where the conditions are favorable for building up,
except that the field rheostat is all cut in.

Written Report. 1. To what is the electromotive force as ob-
served in item 2, Order of Work, due?

iThe terms forward and backward as referred to direction of rotation
are of course arbitrary.

4



34 LABORATORY MANUAL

2. In that case, under items 3 and 4, Order of Work, where the
machine built up to its normal voltage, explain briefly to what
the increase of voltage was due. In the other case, what pre-
vented the machine from building up ?

3. Where a shunt generator fails to build up, why should re-
versing the direction of armature rotation be favorable to build-
ing up ?

4. Why should excessive brush resistance tend to prevent
building up even when the connections of field to armature or the
direction of armature rotation are favorable?

5. As a summary of the observations, explain briefly the vari-
ous steps which should be taken if a shunt generator fails to build
up. If the fault is due to a lack of, or insufficient residual mag-
netism, how could this lack be determined by a simple test ob-
servation? What would happen if one of the field coils be re-
versed ?



EXPERIMENT 9.
Electrical Features of the Shunt Generator.

See Article 121 in the text book, also the Theory under Experi-
ment 7 in the Manual.

The object of this experiment is (a) to make a study of the fac-
tors by means of which the voltage of a generator may be varied
or controlled; (b) to observe the tendency of the terminal voltage
to decrease with increasing output; and (c) to take the observa-
tions for the calculation of the so-called regulation of the ma-
chine.

Theory. By the term voltage control is meant the changing
of conditions exterior to the generator for the purpose of main-
taining some given value of voltage at the terminals of the ma-
chine at various loads. Thus, by changing the field rheostat re-
sistance (by hand), the voltage of the machine may be varied
over quite a wide range; or by changing the speed of the ma-
chine (by changing the speed of the driving engine) the volt-
age may be varied.

On the other hand, as the output (load) of the shunt genera-
tor increases, for example, by turning on more of the lamps it
supplies, the voltage (RI) drop in the armature increases (be-



DIRECT CURRENT 35

cause 7 increases) and the terminal voltage falls off. This drop
in voltage obviously depends on an inherent property of the ma-
chine, namely, the amount of fixed resistance in the armature
winding. Changes which occur in the terminal voltage of a gen-
erator, due to inherent properties, are referred to as voltage
regulation to distinguish them from changes which are made by
varying conditions exterior to the machine, and referred to as
voltage control.

The principal means for controlling the voltage of a shunt
generator are hand variations of the field rheostat resistance, and
changes in the speed by variations in the speed of the driving
engine.

The main items which govern inherent changes in the voltage
(that is, the regulation) are the resistance of the armature wind-
ing, together with slight magnetic reactions in the armature
which tend to decrease the effective magnetic field and, hence, the
voltage.

The percentage regulation of the generator is defined as the
difference between the full load and the no-load voltage divided
by the full load voltage at constant speed (obviously this result
must be multiplied by 100 to express it as a percentage). Thus,
if the full load and the no-load voltages are, 100 and 110 respect-
ively, the regulation is equal to 10 per cent. If, in this case,
the numerical value of regulation be greater, indicating a larger
drop in voltage at full load, it is apparent that the numerically
greater value of regulation indicates a certain inferiority in the
construction of the machine.

Current Supply. From the Shunt Generator assigned.

Apparatus Required. (1) Shunt generator driven by a vari-
able speed motor; (2) field rheostat; (3) speed indicator; (4)
double-pole single-throw switch; (5) lamp bank to be used as a
load; (6) an ammeter; and (7) a voltmeter.

Order of Work. 1. Connect the field rheostat between the field
winding and the armature terminals for shunt (self excitation)
operation. Drive the generator at its normal speed, and main-
taining constant speed, observe and record the terminal voltage
at no-load for 5 different positions of the field rheostat handle,
starting with all the resistance cut in and gradually decreasing



36



LABORATORY MANUAL



the resistance. Use Form 7. (Note: Although not shown in Fig.
12, it will be an advantage to connect an ammeter in the field
circuit as a guide to the changes which take place in the field
current in item 1 as the field rheostat is adjusted; to insure in
item 2 that the field current remains constant ; and as a guide to
the changes in the field current in items 3 and 4.)



^


Constant Speed


Constant Field Current





Constant Speed


Position of
Field Rheostat


Volts


Speed


Volts


Output Amperes


Volts


1


1 (All In)








1


(Zero Load)




2


2








2


(Half Load)




3


3








3


(Zero Load)





















Form 7.

2. Adjust the terminal voltage by means of the field rheostat
for its normal value at normal speed and, leaving the field rheo-
stat untouched, reduce the speed to a value 20 per cent, below
normal, and observe and record the terminal voltage at no-load
for the low speed and for 4 other values of speed, gradually in-
creasing it until somewhat above normal.



Field Rheostat



Ammeter




66666



Lamp Bank Used as Load



Fig. 12. Diagram for loading a shunt generator. The lamps may
conveniently be disconnected from the machine by the main switch. A
voltmeter is to be connected to the armature terminals.

3. Connect the lamp bank through the double-pole single-
throw^ switch and an ammeter to the armature terminals as shown
in Fig. 12. With the switch open and with normal speed adjust
the voltage to its normal value by means of the field rheostat,
after which the field rheostat is to be left untouched. Turn off



DIRECT CURRENT 37

all the lamps, close the switch, and then turn on enough lamps
to load the machine to about 50 per cent, of its capacity (see
name plate on the machine). Keeping the speed at its normal
value throughout, observe and record the terminal volts before
and after throwing on the lamps. Use Form 7.

4. Same as 3 except that full load current is to be used.

Written Report. 1. In item 1, Order of Work, why do the
changes of the field rheostat change the terminal voltage ? What
is the range in the control of voltage by this means as observed?

2. Same, for speed change in item 2, Order of Work.

3. In item 3, Order of Work, what causes the voltage to drop
as the load is thrown on the machine? How does this drop in
voltage for half and for full load compare ?

4. From the observations under item 4, Order of Work, cal-
culate the percentage regulation of the machine.

5. From the observations in this experiment, what are your
conclusions as to the adaptability of the shunt generator for elec-
tric lighting?

EXPERIMENT 10.

Shunt and Separate Field Excitation Compared.
See the Theory under Experiment 8 in the Manual.

The object of this experiment is to afford an opportunity for a
study of the factors entering into the changes of voltage due to
loading a shunt wound generator both for shunt (self) and sepa-
rate field excitation.

Theory. In the shunt wound generator connected for self ex-
citation (shown in Fig. 10), as the load is increased the volts
(RI) drop in the armature increases and, hence, the terminal
voltage decreases. With each decrease in terminal voltage the
shunt field current, equal to E/R, falls off, and as a consequence
the field magnetism and the induced armature voltage in turn
are reduced. Hence, a decrease in terminal voltage produces
what may be termed a cumulative reduction of terminal voltage
in the shunt self-excited machine.

In the shunt wound generator connected for separate excita-
tion (shown in Fig. 10), as the load is increased, the volts (RI)
drop in the armature increases and, hence, the terminal voltage



38



LABORATORY MANUAL



decreases, however, with the following difference: The field cur-
rent in this case is independent of the terminal voltage of the
armature, being connected to the constant voltage supply mains,
and, hence, any decrease in the terminal voltage of the armature
has no effect on the field current. For this reason, the decrease
in terminal voltage is less for a given load current in the case of
separate than for shunt (or self) excitation.

Current Supply. 110 volts Direct Current. '

Apparatus Required. (1) Shunt generator; (2) double-pole
double-throw switch for readily connecting the field winding




able' |-42_2H

T. r



00000



Lamp Bank Used us Load



Field Rheostat



Fig. 13. A study of self and separate excitation. The double-pole
double-throw switch is a convenient means for quickly connecting the
field winding either to the armature terminals or the supply mains.

either to the supply mains or the armature terminals ; (3) double-
pole single-throw switch for connecting the armature terminals
to the load of lamps ; (4) field rheostat ; (5) speed indicator ; (6)
lamp bank to be used as the load; (7) ammeter for the field cir-
cuit; (8) ammeter for the main circuit; and (9) voltmeter.

Order of Work. 1. Connect the field winding through the
double-pole double-throw switch and an ammeter, as shown in
Fig. 13, for self or separate excitation. Connect the armature
terminals through the double-pole single-throw switch and an
ammeter to the lamp bank. "With the load switch open, and the
field winding connected to the armature terminals, drive the



DIRECT CURRENT



39



machine at normal speed and see that it builds up to normal
voltage, making the required adjustments by means of the field
rheostat. Throw the field switch for separate excitation and see
that the instruments read in the same direction for both self and
separate excitation. If not, reverse one set of connections.

2. Throw the field switch for self excitation, and with normal
speed and normal no-load voltage, throw on 14 full load current,
keeping the speed constant and leaving the field rheostat un-
touched after the preliminary adjustment for normal no-load
voltage. Observe and record the terminal voltage, field current,
load current and speed before and after throwing on the *4 load
current. Use Form 8.

(Speed Constant Throughout)



JZ5


Self Excitation


Separate Excitation


Volts


Field Amperes


Load Amperes


Speed


Volts


Field Amperes


Load Amperes


Speed


1






(Zero Load)












2






(Quarter Load)












3






(Zero Load)












4






(Half Load)













Form 8.

3. With the load switch open throw the field switch for sepa-
rate excitation and repeat the observations outlined under 2.

4. Same as 2 and 3, except that %, % and full load current
are to be used in turn.

Written Report. 1. Why does the voltage fall off more for
self than for separate excitation in items 2, 3, and 4, Order of
Work.

2. Plot a curve using volts as ordinates and load current as
abscisses for zero, %, %, %, and full load current, both for self
and for separate excitation.

3. Why do the values of field current change in the case of
self excitation, but not for separate excitation (assuming con-
stant voltage supply mains) ?

4. Is there more likelihood of a self-excited shunt generator
having its polarity reversed on starting up than a separately ex-
cited machine ? Why ? (See Article 118 in the text book.) Give



40 LABORATORY MANUAL

a case where such a reversal of polarity might be a serious disad-
vantage, and explain briefly.

5. If the speed, electromotive force and current to a lamp bank
are the same, what are the relative field currents by self and sepa-
rate excitation ?

EXPERIMENT 11.
Electrical Features of the Compound Generator.

See Article 122 and the example at the end of Article 122 in
the text book, also the Theory under Experiment 9 in the Manual.

The object of this experiment is (a) to observe the tendency of
the series field winding in a compound generator to offset the de-
crease of terminal voltage due to armature volts drop (RI) ; (b)
to note the effect on the terminal voltage produced by changing
the series field current (by means of a shunt) for a given output
current; (c) to note the effect on the compounding by a change
in the running speed of the machine, the initial no-load voltage
being the same as in (a) ; and (d) to take the necessary observa-
tions on the generator operated as a shunt machine (the series
winding disconnected) for a determination of the number of
series turns required for compounding.

Theory. Like the shunt generator, as the output of the com-
pound generator increases, the terminal voltage tends to decrease
due to the volts (RI) drop in the armature winding. The series
winding, however, on the compound generator through which all
or most of the output current flows, being wound on the field mag-
nets with the shunt windings, produces a magnetic field which is
nearly proportional to the load current. Hence, when the arma-


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Online LibraryClarence Edward ClewellLaboratory manual. Direct and alternating current → online text (page 3 of 8)