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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-

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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