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Clarence Edward Clewell.

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ture RI drop is large, that is, when the output current is large,
the tendency of the terminal voltage to decrease is compensated
for by the additional field magnetism produced by the series
winding, which causes a larger voltage to be induced in the arma-
ture.

Obviously, by changing the number of series turns or by vary-
ing the proportion of the full load current which flows through
the series winding, the degree of this compensating effect may
be correspondingly changed. In that case where the full load
and no-load voltages are the same in value, the machine is said to



DIRECT CURRENT 41

be flat compounded. Where the full load voltage is greater than
the no-load voltage, the machine is said to be over-compounded.

In Experiment 9 it was shown that the terminal voltage of the
shunt generator is changed by variations in the field current. As
the load on a shunt generator increases, the field current might
be increased sufficiently for each increase in output current to
maintain a constant terminal voltage throughout the range from
no-load to full load, provided there was enough margin in the
field rheostat resistance to permit of the necessary increases in
the field current. As shown in the foregoing, this same effect
may be produced by means of a series winding placed on the mag-
nets in addition to the shunt winding, the necessary increases in
field magnetism being produced by the current output which
flows through the series turns.

The ampere-turn (one ampere flowing through one turn of
wire) is a common unit of magnetizing effect. Thus, if 2 am-
peres flow through 5,000 turns of wire, the magnetizing effect is
equivalent to 1 ampere through 10,000 turns. In the latter part
of this experiment, the generator is operated as a shunt machine
at normal voltage and speed (speed maintained constant through-
out). Starting with no load, the current output is increased in
steps, and the field rheostat is adjusted at each observation for
the initial terminal voltage value, in other words, the voltage is
maintained constant throughout by the hand manipulation of the
field rheostat.

As an illustration, if the shunt field current required to pro-
duce the initial voltage, at full load, is 1.5 amperes, while at no-
load the field current required is 1.0 ampere, this increase of 0.5
ampere required at full load, multiplied by the number of shunt
field turns (assumed as 10,000 this product is 0.5X10,000=5,000
ampere- turns) represents the additional field excitation required
at full load in order to maintain constant terminal voltage.

By the aid of a series winding through which the full load cur-
rent flows (assumed to be 100 amperes) this same magnetizing
effect, that is, the additional 5,000 ampere-turns, may be secured
by the use of 50 series turns. Hence, the effect at full load is
the same whether 0.5 ampere is added to the no-load value of
shunt field current through 10,000 shunt turns, or whether the
full load current of 100 amperes flows through 50 turns wound
on the magnets as a series winding in addition to the shunt wind-



42



LABORATORY MANUAL



ing. Obviously the series turns produce no effect on the excita-
tion at no-load if connected as shown in Fig. 14.

Current Supply. From the Compound Generator assigned.

Apparatus Kequired. (1) Compound generator driven by a
variable speed motor; (2) field rheostat; (3) speed indicator;
(4) double-pole single-throw switch; (5) low resistance to be used



Voltmeter



Shunt Field
Rheostat



Ammeter




Series Field ' <j



00000



Lamp Bank Used as Load

Fig. 14. Compound generator, using the short shunt connection.
Note that no current flows through the series winding when the main
switch is open. (Note: This diagram shows the general scheme of
connections. As indicated by the windings, the machine would be
differentially compounded, while in practice it is ordinarily arranged
for cumulative compounding, that is, the series winding would be the
reverse of that shown.)

as a shunt around the series field; (6) lamp bank to be used as
a load; (7) two ammeters; and (8) a voltmeter.

Order of Work. 1. With the generator arranged for compound
operation, connect the lamp bank through the double-pole single-
throw switch and an ammeter to the machine terminals, as shown
in Fig. 14. With the switch open and with normal speed, adjust
the voltage to its normal value by means of the shunt field rheo-
stat. Turn off all the lamps, close the switch, and then turn on
enough lamps to load the machine to about 50 per cent, of its ca-
pacity (see name plate on the machine). Keeping the speed at
its normal value throughout, record the terminal volts and load
current before and after throwing on the lamps. The field rheo-
stat is to be untouched after the initial adjustment.

2. Same as 1, except that full load current is to be used.



DIRECT CURRENT



43



3. Same as 2, except that the low resistance shunt is to be con-
nected to the terminals of the series winding, and its resistance
varied until the terminal voltage is 5 per cent, lower at full load
than in the observations of item 2.

4. Same as 1 and 2, except that the machine is to be run 10
per cent, below rated speed throughout, and the shunt current
adjusted to give normal no-load voltage, that is, the same value
as used in items 1 and 2. (Note : Due care must be taken to in-
dicate on the data sheet, to which speed the observations of items
1, 2 and 4 refer, in each case.)

5. The generator assigned is to be arranged for shunt opera-
tion, that is, the series winding is to be. disconnected, and the
machine is to be connected to the lamp bank as shown in that






Terminal Volts
(Constant)


Field Amperes


Output Amperes


Speed (Constant)


1






(Zero Load)




2






(Quarter Load)




3






(Half Load)















Form 9.

portion of Fig. 13 involving self excitation, that is, omit the two
terminals on the left of the field switch shown in this illustration.

6. Starting at no load (load switch open) adjust the voltage to
its normal value at normal speed and observe the terminal volts,
field current, output current (=zero) and speed. Use Form 9.

7. Turn off all the lamps, close the switch, and then turn on
enough lamps to equal ^ of the rated capacity of the machine,
adjusting the field rheostat until the terminal voltage is the
same as in 6 and keeping the speed constant. Observe the ter-
minal volts, field current, output current and speed as in Form
9.

8. Same as 7, for %, %, full load and y overload in turn,
maintaining the speed constant and adjusting the field rheostat
for constant terminal voltage in each case.

9. Ascertain and record the number of shunt field turns on
the generator.



44 LABORATORY MANUAL

Written Report. 1. In item 1, Order of Work, what causes the
voltage to be maintained, notwithstanding the effect of RI drop
in the armature? How does the terminal voltage, under load,
compare in items 1 and 2, Order of Work, for half and full load
current output ?

2. What effect does the shunt, around the series field terminals,
have on the terminal voltage for a given load output ?

3. What would the effect be of increasing the number of series
field turns, on the full load voltage ? On the no-load voltage ?

4. When the compound generator is run 10 per cent, below
normal speed in item 4, Order of Work, with the shunt current
adjusted to give normal no-load voltage, how does the compound-
ing compare with that at normal speed ? Explain.

5. Plot a curve using the field current as ordinates and out-
put current as abscisses, from the observations in items 6, 7 and
8, Order of Work.

6. Calculate the number of series turns necessary for flat com-
pounding.

7. If the speed had fallen off 10 per cent, between a set of ob-
servations, what effect would this have had on the necessary in-
crease of field current to bring the voltage up to normal value
before taking the next set of observations in items 6, 7 and 8,
Order of Work?

8. If it was desired to calculate the series turns necessary to
over-compound this machine by 5 per cent, at full load, what ad-
ditional observations would have been necessary in the experi-
ment in items 6, 7 and 8, Order of Work?

9. Why may it be an advantage to have the terminal voltage
of a compound generator higher at full load than at no-load in
some cases ? Explain,

10. From the observations in this experiment, what are your
conclusions as to the adaptability of the compound generator for
electric lighting and power service? Are generators usually
shunt or compound ?

EXPERIMENT 12.
Study of the Storage Battery.

The object of this experiment is to gain a working knowledge
of the practical construction and operation of the storage bat-
tery. While a knowledge of the principles of construction and of



DIRECT CURRENT 45

the theory of operation is an advantage, in this case it is possibly
more important to understand something of the practical opera-
tion and, hence, this feature will be emphasized somewhat to the
exclusion of the more theoretical items.

Theory. The storage cell is commonly known as a secondary
cell on account of the necessity of charging before taking current
from a battery made up of a number of such cells. The term
secondary further distinguishes the storage battery from the pri-
mary cell where the electromotive force and current are the re-
sult of direct chemical action on the elements which make up the
battery.

There are certain practical points regarding the operation of
storage batteries which are referred to in the text book, but the
student should send a post card to the manufacturer of the bat-
tery examined, with a request for the circular dealing with the
operation of the given battery. It is suggested that this circu-
lar be attached to the Written Report for future reference.

Apparatus Required. (1) Several grids from a storage bat-
tery; (2) a regular storage battery equipment (found in most
laboratories); (3) foot rule; (4) voltmeter; and (5) a hydro-
meter for measuring the specific gravity of the electrolyte.

Order of Work. 1. Make an inspection of the storage battery
grids available, sketching same with dimensions. Describe briefly
the construction of these plates on the data sheet.

2. Inspect the storage battery equipment in the Laboratory,
measuring approximately the size of the grids and the size of the
glass jars. Note the height of the electrolyte in the jars, and
the general method of supporting the jars and of making connec-
tions between the cells. Make sketches or describe these various
items briefly on the data sheet.

3. Measure the voltage of one or two of the cells separately
and of the entire battery, recording these observations.

4. Measure the voltage of the battery when delivering various
currents and when receiving various currents.

5. Count and record the number of cells in the battery, and
measure the temperature and the density of the electrolyte.

6. Secure the name and address of the manufacturer of the
battery examined.



46 LABORATORY MANUAL

Written Report. 1. What is the active material in the grids
inspected in item 1, Order of Work? How is this active material
deposited and held on the plates in the manufacture of the bat-
tery ? Of what metal are the supporting plates constructed ?

2. How are the jars of the battery mounted, and what precau-
tion is taken in the battery room in case of leakage ?

3. What is the average electromotive force per cell of the bat-
tery inspected? What is the average rate of discharge in am-
peres allowable based on an 8-hour rate? (See Article 200 in
text book.)

4. What is the effect on the battery terminal voltage when de-
livering various currents ?

5. Same, when receiving various currents?

6. What is the general procedure in charging a battery ?

7. What precautions must be observed when a battery is to be
unused for several months?

8. How can it be determined when a battery requires charging
and when it is sufficiently charged?

9. What results if a battery is discharged or charged at too
rapid a rate ?

(It is suggested that items 6, 7, 8 and 9, of Written Report,
be answered by the aid of the pamphlet secured by the student
from the manufacturer of some standard battery equipment, pre-
ferably the one inspected.)



EXPERIMENT 13.
Study of the Wiring in the Laboratory.

In this Experiment the opportunity is given for gaining in-
formation in regard to methods of distributing the electric cur-
rent for lighting and power throughout a given building. The
general items connected with such work are here given preference
over details and such few details as are desired will be mentioned
in the following directions under appropriate heads.

Theory. Interior wiring for the supply of electric lamps is ar-
ranged for by the constant voltage system of distribution. This
means that each lamp must receive approximately the same volt-
age no matter how many are turned on and, hence, the size of



DIRECT CURRENT 47

wires must be large enough in central portions of the building
where heavy currents flow, as to cause only a low voltage (RI)
drop, while in more remote portions of the building the size of
wires may be smaller. Power mains must also follow this general
scheme.

An ordinary method used for lighting circuits is to run heavy
wires from the main switch (or bus bars) to panel (switch) boxes
on the various floors, and from these panel boxes individual cir-
cuits of 660 watts maximum, are run to the various rooms. The
660 watts limit is fixed by the National Board of Fire Under-
writers as a safeguard against fire risk. Thus, if 100 watt tung-
sten lamps are used, each circuit is limited to 6 lamps.

Order of Work. 1. In the room assigned, observe and record
the number, size and type of lamps and reflectors used ; the area
of the floor space in square feet; the mounting height and spac-
ing of the lamps ; and the arrangements for other uses of current,

2. Kecord by sketches and explanation the method of mount-
ing the lamps at the ceiling ; the switching arrangement ; and the
number of lamps per switch.

3. Note the general arrangement of conduit or wiring to this
room from the panel box, and inspect the panel box, making a
sketch and giving dimensions on the drawing; note the fuses and
circuit breakers; also the kind of insulation and the size of the
wire used.

4. Note how the conduit is run to the panel box from the main
switch board, recording such data as may be necessary for an ex-
planation in the Written Report.

5. Note and record the number of switches on the main switch
board which supply lighting and service outlets throughout the
building.

6. From plans or measurements determine and record the ap-
proximate number of square feet area of the floors in the build-
ing.

Written Report. 1. How many watts per square foot are used
for lighting in the room assigned? What is the ratio of spacing
to mounting height of the lamps, and what effect does this have
on the distribution of the light throughout such a room ?



48, LABORATORY MANUAL

2. Describe briefly the general scheme used in running the
wires from the main switch board to the lamps in the building
inspected.

3. Same, for the service outlets.

4. Based on the watts per square foot used for lighting in the
room inspected, and on the total floor area in the building, how
many watts are required as a maximum for lighting the Labora-
tory? Express this result also in kilowatts.

5. Does the generator which supplies the lamps need to have a
capacity equal to this maximum requirement? (Suggestion:
Give due weight in answering this question to the average num-
ber of lamps which may be used at any one time in such a build-
ing.) What size of generator would you consider about proper
for such a lighting load?



EXPERIMENT 14.
Shunt Motor Speed Features.

See Articles 129, 132, 133, 135, 139 and 145 in the text book,
also the Theory under Experiments 4 and 6 in the Manual.

The object of this experiment is (a) to make a study of the
factors by which the speed of the shunt motor may be varied or
controlled; (b) to observe the tendency of the speed to decrease
with increasing load; and (c) to take the observations for the
calculation of the so-called regulation of the machine.

Theory. By the term speed control is meant the changing of
conditions exterior to the motor for the purpose of obtaining de-
sired speed values. Thus, changing the field rheostat (by hand),
the speed of the machine may be varied over quite a wide range ;
or changing the supply voltage across the armature terminals
(by means of a rheostat in series with the armature or other-
wise), the speed may be varied.

On the other hand, as the load supplied by the motor is in-
creased, for example, when an added load is placed on a hoist be-
ing raised by the machine, the motor slows down and thus per-
mits an increased current to flow into the armature necessary for
developing the additional mechanical force (or torque). The



DIRECT CURRENT



49



field winding being connected to constant voltage supply mains,
receives the same current as before in this case. This drop in
speed depends on inherent properties of the motor, chief of
which is the resistance of the armature winding. Changes which
occur in the speed due to inherent properties, are referred to as
speed regulation to distinguish them from changes which are
made by varying conditions exterior to the machine, and referred
to as speed control.

Among the principal means for controlling the speed of a





Supply Mains (110 Volts D C.)






fl









Starting




Fig. 15. Shunt motor. The "adjustable resistance" is not ordinarily
required in addition to the starting box, but is here used merely for
convenience in varying the voltage across the armature terminals.

shunt motor is by the hand manipulation of the field rheostat re-
sistance; and that by changing the voltage across the armature
terminals of the machine by a rheostat in series with the arma-
ture only.

The main items which govern inherent changes in the sp>eed
(that is, the regulation) are the resistance of the armature wind-
ing together with slight magnetic reactions in the armature which
tend to modify the effective magnetic field produced by the field
winding.

The percentage regulation of the speed is defined as the differ-
ence between full load and no-load speed divided by the full
load speed, supply voltage and field current remaining un-
changed. ( Obviously this result must be multiplied by 100 to ex-



50



LABORATORY MANUAL



press it as a percentage.) Thus, if the full load and no-load
speeds are 1,500 and 1,650 revolutions per minute respectively,
the regulation is equal to 10 per cent. If the speed at full load
falls off more than indicated in this case, the numerical value of
the regulation will be greater and, hence, will indicate a certain
inferiority in the construction of the machine.

Current Supply. 110 or 220 volts Direct Current.

Apparatus Required. (1) Shunt motor; (2) starting box; (3)
field rheostat; (4) 'armature rheostat ; (5) speed indicator; (6)
brake to be used for loading the motor; (7) ammeter for the field
circuit; (8) ammeter for the armature circuit; and (9) volt-
meter.



No.


Position of
Field Rheostat


Speed


Armature
Amperes


Position of
Armature Rheostat


Speed


Armature
Amperes


Armature Amperes


Speed


1


1 (All Out)




(For No
Load)


(Out)




(Zero
Load)


(Zero Load)




2


2




1


(In)







(Quarter Load)




3


3







(Out)




(Full
Load)


(Half Load)




4


4







(In)




-

























Form 10.

Order of Work. 1. Connect the motor to the supply mains as
shown in Fig. 15. With the field rheostat all cut out, start the
motor by means of the starting box and cut out all the armature
rheostat resistance. With the motor unloaded, observe and re-
cord the speed for this and for 4 other positions of the field
rheostat, gradually increasing its resistance until the motor speed
is considerably above normal. Use Form 10.

2. With the motor unloaded, adjust the field rheostat for nor-
mal speed, and observe the speed for no resistance and for a
fairly high resistance inserted in the armature circuit, leaving
the field rheostat untouched after the initial adjustment.

3. Same as 2 except that the motor is to be loaded by the brake
until the armature current is a fair proportion of the full rated
current. (Note : To be most instructive the resistance inserted in
the armature circuit for the second observation in items 2 and 3
should be the same value in each case.)



DIRECT CURRENT 51

4. Adjust the motor for normal speed at no load by the field
rheostat after reducing the armature rheostat resistance to zero,
and observe and record the speed for zero, %, %, % and full
load armature currents in turn (assume full load armature cur-
rent as equal to the current rating on the name plate of the ma-
chine). Leave the field rheostat untouched throughout these
observations after the initial adjustment.

Written Report. 1. In item 1, Order of Work, why does the
speed increase as the field rheostat resistance is increased ? "What
is the range of speed control by means of the field rheostat
method as observed ?

2. In items 2 and 3, Order of Work, why is the speed affected
more by the armature rheostat when the motor is loaded than
when unloaded?

3. From the observations in item 4, Order of Work, plot a
curve using speed as ordinates and armature current as abscissas.

4. Calculate the percentage speed regulation of the motor.

5. The torque of a motor depends on the field magnetism ef-
fective in the armature and on the armature current. When the
field magnetism was weakened in item 1, Order of Work, the
motor speeded up. Should not a weakened field cause less torque
and, hence, less rather than more speed? Explain. (See Ar-
ticle 135 in the text book.)

6. To what is the decrease in speed with increased load due as
observed in item 4, Order of Work?



EXPERIMENT 15.
Efficiency of a Shunt Motor by the Brake Method.

See Article 158 in the text book.

The object of this experiment is to determine the efficiency
(output divided by input) of a shunt motor by measuring the
mechanical output and the electrical input at various loads.

Theory. The efficiency of the electric motor like that of other
machines is defined as the ratio of output to input. In a deter-
mination of the efficiency, therefore, it is necessary to measure
the mechanical output as indicated by a brake attached to the



52 LABORATORY MANUAL

pulley, and for each value of output thus observed to measure
the electrical input.

In the actual calculation of the efficiency, the mechanical out-
put in horse-power may readily be transformed by multiplying
the horse-power by 746, the number of watts in one horse-power,
and the output is thus expressed in the same units as the input.

Since the losses in a motor are partly constant (or nearly con-
stant) and partly variable, and since the variable losses (RP) in
the armature winding vary as the square of the current, it will
be obvious first, that the constant losses play a much larger part
for low loads, thus reducing the efficiency at low loads as com-
pared to full load, and second, that the RP loss in the armature,
increasing as the square of the current, becomes quite large in
proportion for high current values and, hence, the efficiency tends
to fall off after a certain maximum value has been reached near
full load.

Current Supply. 110 or 220 volts Direct Current.

Apparatus Required. (1) Shunt motor; (2) starting box; (3)
field rheostat; (4) speed indicator; (5) brake to be used for load-
ing the motor; (6) ammeter; and (7) voltmeter.

Order of Work. 1. Connect the motor to the supply mains in
the usual manner, arranging the ammeter to measure the entire
current input to both field and armature.

2. Start the motor and bring it to normal speed at no load, that
is, with the brake detached completely from the pulley. Ob-
serve and record the input (volts and amperes).

3. Attach the brake and tighten until the ammeter indicates
% the full rated current (see name plate on the machine). Ob-
serve and record the torque exerted at the pulley and the volts
and amperes input.

4. Same as 3, for y 2 , %, full load, and 1*4 and, if practicable,
P/2 of full load current in turn.

Written Report. 1. Calculate the efficiency of the motor in per


1 2 4 6 7 8

Online LibraryClarence Edward ClewellLaboratory manual. Direct and alternating current → online text (page 4 of 8)