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Clarence Edward Clewell.

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cent, from the observations in items 2, 3 and 4, Order of Work.

2. Plot a curve, using the efficiency as ordinates, and the input
current as abscissas.



DIRECT CURRENT 53

3. Explain briefly the general shape of this curve, that is, why
it follows the form taken.

4. Name the constant and variable losses in a generator or
motor. (See Article 155 in the text book.)

5. Why should the constant losses in a motor cause the effici-
ency to be low at low loads ?

6. Why does the RP loss in the armature cause a reduction of
the efficiency after a certain maximum value near full load ?

7. How could the armature current be found mathematically
for a given machine at which the efficiency is a maximum ?



EXPERIMENT 16.

Series Motor Speed Features.

See Articles 140, 141, 142, 143, 144 and 145 in the text book.

The object of this experiment is to make a study of the speed
of a series motor as affected by the load.

Theory. The torque of a series motor is proportional to the
field magnetism and to the armature current. Since the field
winding and the armature are connected in series and, hence,
the same current flows through each, the torque is roughly pro-
portional to the square of the armature current.

Under a light load, the series motor takes but little current to
produce the torque required and, hence, the resistance in series
with the motor on starting is made large enough to allow only a
small current to pass through the motor. The motor under
these conditions speeds up rapidly and the greater the speed the
more counter electromotive force induced and, hence, the less the
current through the machine until the resistance is cut out. As
shown with the shunt motor (See Experiment 14), to weaken
the field increases the speed and, hence, the speed of an unloaded
series motor becomes excessive and would damage the machine if
allowed to run under this condition. The series motor must,
therefore, alw r ays be connected to its load, as in street cars where
the motor is geared or mechanically connected to the load. As a
precaution, therefore, always see that a load is connected or
coupled to the series motor before connecting it to the supply
mains.



54 LABORATORY MANUAL

Under load, the operation of the series motor is somewhat dif-
ferent from that of the shunt motor. For example, if the load
on a series motor be doubled, the field current as well as the ar-
mature current is increased, so that the speed is reduced much
more than in the shunt motor where the field current is practi-
cally constant at all loads. Again, if the armature current be
doubled in a series motor, the torque is increased nearly four



Supply Mains


(110 Volts D. C.)




A

-M f I





Ammeter




Pig. 16. Series motor. The load, which must be connected to the
motor throughout the experiment, is not shown in this diagram. The'
adjustable resistance represents the starting controller used with series
motors.

times, since both armature and field current are doubled, while
to double the armature current in the shunt motor merely doubles
the torque (the shunt field current remaining constant).

Hence, under heavy loads, due to the fact that the large start-
ing armature current flows through the series field in the series
motor, it has a greater starting torque than the shunt motor
where the field current remains sensibly constant irrespective of
the value of the armature current.

The series motor is, therefore, well adapted to those cases where
a large starting torque is desirable and where, under heavy loads,



DIRECT CURRENT



55



the speed should fall in order that the power requirements and,
hence, the current may not be excessive as in street car opera-
tion. Where constant speed at all loads is a necessary require-
ment, the series motor is not adapted, as its speed variation be-
tween small and heavy loads is very large as compared with that
of the shunt motor. Note that a given value of current through
the series motor produces the same torque whatever the speed.

Current Supply. 110 or 220 volts Direct Current.

Apparatus Required. (1) Series motor; (2) starting resist-
ance, (some form of rheostat having a fairly large current carry-



Lever Arm =


No.


Force at End
of Brake Lever


Amperes


Volts


Speed


Shunt


1










Without Shunt
Around Series Winding


f>










3










4










5










6










"5 G

^ "7)















Form 11.

ing capacity); (3) speed indicator; (4) brake to be used for
loading the motor ; (5) ammeter; and (6) voltmeter.

Order of Work. 1. Connect the motor to the supply mains as
shown in Fig. 16, and arrange to have the brake permanently at-
tached to the motor pulley throughout the experiment.

2. With the starting resistance all in, and the brake mode-
rately tight, throw in the main switch and gradually increase the
speed by cutting out the starting resistance. Tighten the brake
until the current input equals % more than the current rating
on the name plate of the machine. Observe and record the
torque (the tangential force at the rim of the pulley multiplied
by the radius of the pulley), current, volts at terminals of motor,
and speed. Use Form 11.



56 LABORATORY MANUAL

3. Reduce the load until the input current equals the full load
rating of the machine and repeat the observations of item 2.

4. Same for %, %, and 14 load current values in turn.

5. Place a low resistance around the series field terminals as
a shunt. Operate the machine at its full load current value and
observe the torque, current, volts and speed.

Written Report. 1. Explain briefly why the load must be per-
manently connected to a series motor.

2. Plot a curve using speed as ordinates and torque as ab-
scissas from the observations of items 2, 3 and 4, Order of Work.

3. Explain the decrease in speed with increasing load as shown
by this curve.

4. What changes were observed in the speed and torque in
item 5 as compared with the corresponding observations in item
3, Order of Work? Explain.

5. A street car equipped with series motors, running at con-
stant speed on the level, approaches an up grade. Explain the
action of the motors in propelling the car up the grade, as re-
gards speed, torque and current in-take, assuming that the motor-
man leaves the controller untouched.

6. Sometimes on climbing a steep grade a motorman throws
the controller to the series notch. What is the "series notch",
and why should this be an advantage under the circumstances ?



EXPERIMENT 17.
Efficiency ; Stray Power Test ; Brake Test.

See Articles 155, 156 and 158 in the text book, also the Theory
under Experiment 15 in the Manual.

Theory. The losses in a generator or motor may be classed
under the head either of losses which may readily be calculated,
or of losses which are not subject to calculation. Thus the resist-
ance losses (RP) may easily be calculated after measuring the
resistance of field and armature, and from the currents involved.
The friction losses in the bearings and at the brushes of the ma-
chine and in windage and the losses in the iron of the machine
(usually called hysteresis and eddy current losses) cannot easily
be calculated and are referred to as stray power loss. That is,



DIRECT CURRENT 57

the stray power loss includes all the losses in a generator or motor
except the RP or resistance losses, and is practically constant
independent of the load.

A simple experiment to determine the stray power loss in a
generator or motor is to drive the machine as an unloaded motor
and to measure the power input under this condition. Obviously
this input is all loss, since there is no useful power being deliv-
ered at the pulley. If, from this input at no load, the RP losses in
both field and armature be subtracted, the remainder represents
the stray power loss in watts.

Inasmuch as the stray power loss is sensibly constant at all
loads, the efficiency of a generator, for example, may be calcu-
lated when the stray power loss is known for any assumed load
by the use of the equation for effiicency :

Efficiency = Out P"L_

Output -f alJ losses

Output



Output -j- stray power loss -{- RI 2 losses

Suppose, for example, it was desired to calculate the efficiency
at half load. If the generator is rated at 10 kilowatts, and the
stray power loss is found by experiment to be 500 watts, the ef-
ficiency may be calculated by a substitution in the equation as
follows :

. 5000

5000 + 500 + RP loss in field and armature

The RP loss in the field and armature are easily calculated from
the resistance of the two windings, the terminal voltage and the
armature current corresponding to the assumed load. Thus the
RJ 2 loss in the field is equal to R l X(E/R l ) 2 , and the RP loss
in the armature is the resistance of the winding (R 2 ) multiplied
by the square of half the full load current as indicated on the
name plate of the machine.

(Note : While the stray power loss varies slightly for different
loads and speeds, it is treated as constant in this experiment for
simplicity. )

Current Supply. 110 or 220 volts Direct Current.

Apparatus Required. (1) Shunt machine; (2) starting box;
(3) field rheostat ; (4) speed indicator ; (5) ammeter for the field



58 LABORATORY MANUAL

circuit; (6) ammeter for the armature circuit; and (7) volt-
meter.

Order of Work. 1. Connect the machine to the supply mains
through the starting box, arranging an ammeter in both field and
armature circuits.

2. Run the machine as a motor at normal speed and, with no
load, observe and record the amperes to both field and armature
and the volts at the motor terminals. (Note : Since the starting
current of an unloaded motor is apt to be larger than its nor-
mal running current after starting, be sure to close the short cir-
cuiting switch about the armature ammeter before starting the
motor. This makes possible the use of an instrument of low
range for observing the rather small currents of the motor while
in operation at no load.)

3. Record the full rated current and voltage of the machine
as indicated on the name plate.

4. Shut down the machine and measure the resistance of the
armature by the voltmeter-ammeter method as described in Ex-
periment 2.

5. Arrange to load the machine by a brake, and observe and
record the torque, speed, current in field and armature, and ter-
minal volts at % % an d full rated load in turn.



Written Report. 1. From the observations in items 2 and 4,
Order of Work, calculate the stray power loss, and the field and
armature resistance of the machine used.

2. From item 5, Order of Work, calculate the output and in-
put to the motor in watts, and calculate the efficiency of the
motor by the brake method for these observations.

3. Calculate the efficiency of the motor for 14, l /2, % and full
load in turn by the stray power method, using the stray power
loss as found in item 1, Written Report, and using the terminal
volts and input current at full load as given on the name plate
of the machine. Compare these calculations of efficiency with
those found by direct measurement in item 2, Written Report.

4. Calculate the efficiency of the machine by the stray power
method if run as a generator at full load, assuming the terminal
volts and output current at full load to be the value given on the
name plate of the machine.



DIRECT CURRENT



59



EXPERIMENT 18.
Static Torque Test on a Motor.

See Article 125 in the text book, also the Theory under Ex-
periment 4 in the Manual.

The object of this experiment is to make a study of the torque
of a motor in terms of the field and armature current, while the
machine is at rest.





Supply Mains (110 Volts D. C.)






-4


A

1 -S f


T




A

Uii t


i






Fig. 17. Study of the torque produced in an armature for various
values of field and armature currents. Note that the currents in field
and armature can be adjusted independently.

Theory. The mechanical force which turns an electric motor
is produced by the action of the magnetic field on the current in
the armature conductors. The simplicity of the elements which
produce motion in the motor are sometimes lost sight of on ac-
count of the conditions which determine the armature current,
such as load, speed and the like. In this experiment, these sec-
ondary conditions are eliminated by taking the observations on
the motor when at rest, and the definite relation of field and
armature current to the torque produced is thus emphasized.

The magnetic field is not directly proportional to the current
which produces it, because as the field magnets become saturated,
the magnetism ceases to increase in direct proportion to the cur-



60 LABORATORY MANUAL

rent (see Article 98 in the text book). Hence, in this experi-
ment, if the torque is not found to vary directly with the field
current throughout the observations, it must be remembered that
the torque is varying directly with the magnetic field, but the
field is not varying directly with the field current due to satura-
tion of the iron. Obviously the saturation effect will not be very
noticeable for small values of the field current.

Current Supply. 110 or 220 volts Direct Current.

Apparatus Required. (1) Shunt motor; (2) resistances for
both field and armature circuits ; (3) Prony brake; (4) ammeter
for field circuit; (5) ammeter for armature circuit; and (6)
voltmeter.

Order of Work. 1. Connect the field winding through a field
rheostat, which should possess a wide range of adjustment, to
the supply mains ; also, the armature through a suitable rheostat
to the supply mains; each to have its own switch as shown in
Fig. 17.

2. Clamp the brake tightly to the pulley of the motor and with
all the field and armature resistance cut in, throw in first the
field arid then the armature switch. See that the torque acts
against the opposition of the brake, and that the armature does
not rotate.

3. With a constant field current, that is, with normal volts
across the field terminals, adjust the armature current to % full
load value and observe and record the field and armature cur-
rent and the torque produced.

4. Same as 3, using %, %, full load and l 1 /^ load currents
through the armature in turn.

5. With, say, full load armature current, reduce the field cur-
rent to % its normal value, and observe and record the field and
armature currents and the torque produced.

6. Keeping the armature current constant, repeat the obser-
vations of item 5 for %, normal and l 1 ^ normal values of the
field current.

7. Maintaining constant field current and constant torque, de-
crease the resistance in series with the armature and allow the
motor to run. Find and record the relation between armature
current and torque for a number of different speeds; also the
relation between the speed and the volts across the armature
terminals for each of these values of speed.



DIRECT CURRENT 61

Written Report. 1. From the observations in items 3 and 4,
Order of Work, how does the torque (or mechanical force) at
the pulley vary with the armature current for a constant value
of field current f

2. From items 5 and 6, Order of Work, how does the torque
vary with the field current for a constant value of armature cur-
rent?

3. If the torque did not vary directly with the field current in
the experiment, to what is such irregularity due ?

4. From the general observations of this experiment, explain
why a shunt motor must slow down when an added load is thrown
on its pulley if it is to carry this added load ?

5. From item 7, Order of Work, what is the relation between
armature current and torque at different speeds; and what is
the relation between speed and volts across the armature in each
of these cases ? Explain briefly.



EXPERIMENT 19.
Shunt Generators in Parallel.

The object of this experiment is to observe the factors which
enter into the operation of shunt generators in parallel, first, as
regards the necessary conditions for throwing one generator in
parallel with another machine, and second, as to the items which
are involved in the equal or proportionate sharing of the total
output of a power station by the various generators connected
in parallel for supplying this total output.

Theory. In many electric stations it is the practice to supply
power from bus bars to which are connected a number of genera-
tors, each delivering its share of the total load supplied from the
common bus bars. In this way, when the load requirements are
low, say during the day in a lighting station, a few of the gen-
erators may be operated at or near full load and, hence, at high
efficiency, and as the total output of the station increases, one
after another of the remaining machines may be connected to the
bus bars in parallel with those already in operation. Obviously
the positive terminal of each machine must be connected to the
positive terminal of the bus bars.



62 LABORATORY MANUAL

If before connecting a machine to the bus bars its voltage is
just equal to that of the bus bars, no current will flow. If the
voltage induced in the armature be slightly higher, a current will
flow. Suppose the machine, when carrying no load, has a voltage
3 per cent, higher than the -bus bars and that before it is con-
nected to the bus bars it is loaded until the terminal electromo-
tive force decreases (due principally to the RI drop in the arma-
ture) and becomes equal to that on the bus bars. If now the load
be thrown off quickly and the machine be connected to -the bus
bars, it is in condition to continue delivering the same current to
the bus bars that it formerly delivered to its independent load.

Hence, if the bus bar voltage be much below that of the volt-
age induced in the armature connected to it, the current supplied
by the armature will rise when the two are connected, until the
volts (RI) drop in its armature winding and the leads from the
armature terminals to the bus bars equals the difference between
the bus bar and the induced armature voltage. If this difference
be zero, no current will flow from the machine; while if the dif-
ference be such that the accompanying RI drop in the leads and
armature involves a current greater than normal for the machine,
the generator will, of course, be overloaded.

As the induced voltage of a generator depends on the field
magnet strength for constant speed conditions, the load may be
increased or decreased on a given machine connected in parallel
with others, by the simple variation of its field rheostat resist-
ance, assuming that the driving engine delivers a corresponding
increased or decreased load.

Where two similar machines of the same capacity are arranged
for parallel operation, the output from each should equal one-
half of the total power supplied by the bus bars. If it should
be desirable, however, to reduce the load on one of the machines
and yet maintain the total output constant at a constant bus
bar voltage, it would be necessary to increase the field resistance
of the one generator to lower its part of the total load, and to
reduce the field resistance of the other generator to increase its
part of the total load, thus maintaining the voltage and the total
output at a constant value. In this way the total load may be
shifted from one machine to another.

Since the terminal voltage of a shunt generator varies with the
load (see Experiment 9), and further, since this change of volt-



DIRECT CURRENT



age with load is not apt to be exactly the same with any two ma-
chines, even after two or more shunt generators are adjusted to
give their share of the total load, they may not continue to share
the total load in this exact proportion for all bus bar or total
loads, on account of this variation in the voltage changes for dif-
ferent machines. This will give rise to slight fluctuations in the
sharing of the loads, which, if sufficiently noticeable, can be off-
set by hand regulation of the field rheostats when necessary.



Voltmeter



Flexible Leads




J




ri <><)<)(j


3




j


Lamp Bank Used as Load


1






Bus Bars










L


1


}




L


A

b : i


*





Ammeter





Fig. 18. Study of the parallel operation of shunt generators. A
voltmeter, not shown in the diagram, is to be available for measuring
the voltage of the individual machines.

Current Supply. From the Shunt Generators assigned.

Apparatus Required. (1) Two shunt generators; (2) lamp
banks to be used as a common load supplied from bus bars; (3)
field rheostats for each machine; (4) a double-pole single-throw
switch for each machine and for the total load (3 in all) ; (5)
three ammeters, one for each machine and one for the total out-
put current from the bus bars; (6) two voltmeters, one for the
bus bars and one for the on-coming machine.



64 LABORATORY MANUAL

Order of Work. 1. Arrange the connections of the two assigned
shunt generators as shown in Fig. 18.

2. With all switches open, start up the two generators and ad-
just the voltage of each to its normal value.

3. Connect one of the generators ("A") to the bus bars (two
lengths of wire) and turn on enough lamps to load the machine
to its full capacity.

4. Adjust the voltage of the other generator ("B") to the
same value as the bus bar voltage and connect it to the bus bars,
being sure that the positive terminal of the machine is connected
to the positive terminal of the bus bars.

5. Vary the field rheostat of machine "B" until the current
in the two machines has the same value, at the same time adjust-
ing the field rheostat of machine "A" so that the bus bar voltage
remains constant. Then turn on enough lamps to load each of
the machines to its full rated capacity.

6. With both machines fully loaded, observe and record the
bus bar voltage, current delivered by each machine and total
current taken from the bus bars.

7. Leaving the field rheostats untouched, repeat the observa-
tions of item 6 for %, % and % of the total load current and
for zero current from the bus bars in turn.

8. Turn on the lamps and vary the field rheostats and lamps
until each machine delivers one-half of its rated load. Adjust
the field rheostat of the two machines until the machine "B" is
delivering all the current and machine "A" is unloaded, main-
taining constant voltage at the bus bars throughout this adjust-
ment. Now disconnect machine " A " from the bus bars.

Written Report. 1. What would be the result if the negative
terminal of machine "B" was connected by mistake to the posi-
tive terminal of the bus bars in item 4, Order of Work ?

2. From the observations in items 6 and 7, in which of the two
machines is the armature voltage reduced the more as the total
load is decreased?

3. In item 8, why must all the load be shifted to machine " B "
before machine "A" is disconnected from the bus bars?

4. What would have been the result if in item 4, Order of
Work, machine "B" had been connected to the bus bars when



DIRECT CURRENT 65

its induced voltage was much above that of the bus bars? If it
was lower ?

5. Explain any inequalities observed in the sharing of the total
bus bar load by the two machines as the total output was reduced
in item 7. If any inequalities existed, to what were they due?
Explain.

EXPERIMENT 20.
Compound Generators in Parallel.

See Article 122 in the text book, also the Theory under Experi-
ment 19 in the Manual.

The object of this experiment is to observe the factors involved
in the parallel operation of compound generators, first, in regard
to the conditions necessary before throwing one generator in
parallel with another machine, and second, in regard to the items
which influence the equal or proportionate sharing of the total
output of a power station by the various generators connected in
parallel for supplying this total station output.

Theory. As stated in experiment 19, the general practice in
electric stations is to supply power from bus bars to which are
connected a number of generators in parallel with each other,
each delivering its share of the toal load supplied from the com-
mon bus bars. In this way it is possible to operate the machines
at a relatively high efficiency even when the station output is low,
by disconnecting certain machines and thus keeping the remain-
ing machines in operation at or near full load. In practice the
generators thus used are compound wound machines.

As in the parallel operation of shunt generators, where the cur-
rent delivered by a given machine depends on the difference be-


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Online LibraryClarence Edward ClewellLaboratory manual. Direct and alternating current → online text (page 5 of 8)