Clarence Edward Clewell.

# Laboratory manual. Direct and alternating current online

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Font size The six terminals of a three-phase armature may either be con-
nected for the so-called Y (or Star) or the Delta connection, and
similarly the three component parts of the receiving circuit for
the three-phase line may either be connected for Y or for Delta
operation.

From Fig. 406, page 512 of the text book, showing a T con-
nected system, it can readily be seen that the electromotive force
between any two of the outside line wires, X, Y and Z (the wire

ALTERNATING CURRENT

81

connected to the middle point is generally omitted in practice)
is the vector sum of the electromotive forces induced in the two
coils connected in series between these two line wires. Further,
as the electromotive forces in series are 60 apart in phase, the
vector sum of the two is equal to the V3 (=1.73) times that in
one, since the electromotive forces are the same in each of the
three armature coils, and as explained in the text book. The cur-

Fig. 24. Study of the voltage and current relations in a three-phase
"Delta" connected receiving circuit.

rent in any one of the outside line wires, however, is supplied by
one armature coil only, as will be seen by an inspection of the
illustration. Hence, the current in the line wire and in the arma-
ture coil to which it is connected, is the same in value.

From Fig. 408, page 513 of the text book, showing a Delta
connected system, it will be seen that the electromotive force be-
tween any two of the line wires, X, Y and Z, is equal to the elec-
tromotive force induced in the armature coil, shown in the upper
part of the figure, connected between the terminals of the two line
wires. Further, since any one of the line wires receives current

7

82

LABORATORY MANUAL

from the two coils terminating at its connection to the armature
circuit, the current in any one line wire is the vector sum of the
currents in the two coils or the \/3 times the current in one coil
(since the current has the same value in each of the three arma-
ture coils).

Hence, if E and 7 be the voltage and current respectively in a
single armature coil of a Y connected generator, the voltage be-
tween any two line wires is V3#, while the current per line is
simply I. "With a Delta connection, this same machine would
produce a voltage E between any two line wires and a current
per line equal to \/3L Thus a given number of turns on the ar-
mature of a three-phase generator results in a higher line volt-

No

Connection Used

Amperes

Volts

Lamp Bank
"1"

Lamp Bank
2"

Lamp Bank
"3"

Line

Lamp Bank
"1"

Lamp Bank

"2"

Lamp Bank
"3"

Line

1

"Y"

2

,, Y ,

3

"Delta*

Form 13.

age if Y than if Delta connected. Note that the total power is
the same for the given machine no matter which way it is con-
nected, since it is expressed by ^3EI in each case (assuming
unity power factor).

Current Supply. Three-phase Alternating Current.

Apparatus Required. (1) Three similar receiving circuits
(lamp banks) ; (2) four ammeters, one for each receiving cir-
cuit and one for a single line; (3) voltmeter.

Order of Work. 1. Arrange the three lamp banks as a Y con-
nected receiving circuit to be supplied by a three-phase generator
as shown in Fig. 23. Adjust the current so as to be equal in each
of the three lamp banks, and observe and record the current in
each of the three lamp banks and in one of the lines, also the
voltage between any two line w^ires and across each of the three
lamp banks. Use Form 13,

ALTERNATING CURRENT 83

2. Same, for twice the current value in each of the lamp banks.

3. Arrange the three lamp banks as a Delta connected receiv-
ing circuit, as shown in Fig. 24, and repeat the observations
called for in items 1 and 2.

4. Ascertain whether the generator armature windings are
connected Y or Delta.

5. Observe the number of terminals in one of the three-phase
induction motors in the laboratory.

Written Report. 1. From the observations in items 1 and 2,
Order of Work, calculate the numerical relation between line
and individual receiving circuit voltage and current. Record
these calculations in the Form on the data sheet.

2. Same, for item 3, Order of Work.

3. When the armature terminals of a three-phase generator
are arranged for the Y connection, where are the connections
made in the machine ? Explain.

4. Same, for Delta connected armature winding. Explain.

5. In Fig. 406, page 512 of the text book, a fourth wire is
shown tapped to the common intersection of each of the Y con-
nected armature windings. Why can this fourth wire usually
be omitted in practical cases ?

6. In the three-phase Delta connected armature winding, why
is it that there is no circulation of current about the three wind-
ings which are in reality connected in series as a closed loop ?

7. Why is the current per line in a Delta connected circuit
equal to the \/3 times the current in one armature winding?

EXPERIMENT 24.
Study of the Transformer.

See pages 268, 269, 270, 271 and Articles 161, 164 and 165 in
the text book.

The object of this experiment is a study of the principles of
construction and operation of the transformer.

Theory. If two electrically separate coils of wire be wound on
the same iron core, one of which is connected to alternating cur-
rent supply mains, the rapid reversals of magnetism produced
by the alternating current in the one coil (the primary) set up

84.

LABORATORY MANUAL

an induced alternating electromotive force in the other coil (the
secondary) .

The reversals of magnetism induce an electromotive force in
the primary as well as in the secondary, and this induced elec-
tromotive force (sometimes called counter electromotive force)
opposes the impressed electromotive force, thus keeping the pri-
mary current down to a low value when no current is being de-
livered by the secondary coil. If the second coil delivers current
to lamps, the magnetic field produced by it passes through the
primary coil and the counter electromotive force of the primary
coil is reduced, thus permitting the necessary increase in primary
current to maintain the load on the secondary. Note that the

No.

Primary

Secondary

No of Coils

Series or Parallel

Ei

Ii

No. of Coils

Series or Parallel

E 2

Ij

1

2

3

Form 14.

primary current, with no load on the secondary, is merely that
required to magnetize the iron core.

Since the magnetic field in the primary coil is roughly pro-
portional to the number of turns of wire for a given current,
and since the electromotive force induced in the secondary coil
by the given rapidly changing magnetic field is proportional to
the number of turns of wire in the coil, the induced electromotive
force in the secondary winding (E 2 ) is related to the impressed
electromotive force (E^ in the primary directly as the number
of turns of wire in the two coils. Obviously the power delivered
by the secondary winding (E 2 I 2 ) is equal to the power received
by the primary winding (EJ.J (ignoring the slight losses in the
transformer), hence, the current in the secondary (7 2 ) is related
to the current in the primary (7J inversely as the number of
turns of wire in the two coils.

ALTERNATING CURRENT 85

Current Supply. 110 volts Alternating Current.

Apparatus Required. (1) An iron core with a permanently
wound fine wire coil in four equal parts with taps to be used as
a primary, and four rather coarse insulated wire coils, to be used
as a secondary, wound with the same number of turns each as
the primary coils, and with taps; (2) an uninsulated copper
ring to fit loosely over a reactance coil with an iron core ; ( 3 ) two
ammeters; and (4) voltmeter.

Order of Work. 1. Using one of the heavy wire coils, connect
one of the primary coils to the supply mains, and observe and re-
cord the number of turns in each coil, the primary volts (E^,
and secondary volts (E 2 ). Use Form 14.

2. Same, with 2, 3 and 4 primary coils in series, in turn.

3. With the four primary coils in series, use two of the sec-
ondary coils in series and repeat the observations called for in
item 1.

4. Same, for 3 and 4 secondary coils in series, in turn.

5. With the four primary coils in series and one of the sec-
ondary coils, connect the secondary terminals to a lamp bank and
observe and record E lf E 2 , 1^ and 7 2 .

6. Same, using 2, 3, and 4 secondary coils in series and 2 and
4 secondary coils in parallel, in turn.

7. With the reactance coil connected to the supply mains, place
the copper ring over the iron core, holding it with a pair of pliers.

8. Inspect the windings, the iron core and the case of a com-
mercial transformer, making a rough sketch with dimensions.

Written Report. 1. Calculate the relation of primary to sec-
ondary turns, and E^ to E 2 in item 1, Order of Work. Explain.

2. Same, for items 2, 3 and 4, Order of Work. Explain.

3. Calculate the relation of primary to secondary turns, E^ to
E 2 , and I to 7 2 in items 5 and 6, Order of Work. Explain.

4. Explain the phenomena observed in item 7, Order of Work.

5. In a step-down transformer (E l high and E 2 low) why is
the primary coil wound with fine wire, and the secondary with
heavy wire ?

6. Why is the iron core of a transformer laminated ?

7. Has the nature of the space between the coils and the outer
case, and the construction and size of the outer case anything to
do with the power rating of a transformer ? Explain.

86 LABORATORY MANUAL

EXPERIMENT 25.

Electrical Features of the Transformer and the Transmission of

Power.

See Article 69 in the text book, also the Theory under Experi-
ment 3 in the Manual.

The object of this experiment is (a) to gain facility in the
handling of a commercial transformer; and (b) to study the
effect of the voltage value on the transmission of power.

Theory. The coils of a commercial transformer usually end in
a terminal board which is accompanied by a diagram of connec-
tions for the various available voltage combinations. While these
connections are ordinarily shown on this diagram, it is advisable
to know that in connecting the two secondary coils in series, the
two coil terminals connected together must be of opposite po-
larity at the same instant ; also in connecting the two secondary
coils in parallel, the terminal of one coil must be connected to the
terminal of the second coil possessing the same polarity at each
instant. (This also applies to the two primary coils.)

In the transmission of a given amount of power (El) over long
distances, the volts (RI) drop and the watts (RP) loss in the
transmission lines obviously depend on the resistance of the lines
and on the current. Hence, for a given size and weight of wire
(that is, for a given resistance of the lines) the losses of trans-
mission are less the smaller the current. Since El represents the
power, the transmission losses are, therefore, reduced for a given
amount of power transmitted, by using a high voltage (E) and a
small current (I). The use of high voltages for transmission is
made possible by the transformer, which is arranged to step-up
the voltage of the generator at the beginning of a long line, and a
second transformer at the end of the line steps down the voltage
to a practical working value, thus securing all the advantages of
high voltage (that is low current) throughout the length of the
transmission wires.

Current Supply. 110 volts Alternating Current.

Apparatus Required. (1) Two commercial transformers with
four coils each (that is, two primary and two secondary coils
each), and preferably with a one to one ratio; (2) lamp bank

ALTERNATING CURRENT

87

to be used as a receiving circuit; (3) three ammeters; and (4)
voltmeter.

Order of Work. 1. Connect the primary of one of the trans-
formers to the supply mains (with the two primary and the two
secondary coils each in parallel), and connect the secondary to
the line. Arrange the second transformer in the same way as re-
gards the connection of its coils, attaching the terminals at the
end of the line to its primary and the lamps to its secondary as
shown in Fig. 25.

Supply Mains (110 Volts 60 Cycles A. C.)

Supply Transformer
Usually Step Up Transformation)

Fig. 25. Study of the transformer and the transmission of power at
different voltages. The short transmission line should be of such
length and size as to give a voltage and line loss equal to, say, ten per
cent, of the power transmitted in the first case.

2. Observe and record E lf E 2 , 7 X and 7 2 in each of the two
transformers.

3. Connect the two transformers so that the line voltage is
double the value used in items 1 and 2, recording a diagram of
the transformer coil connections, and with the same load on the
secondary of the receiving transformer, observe and record E lf
E 2t I I and I 2 in each transformer.

88 LABORATORY MANUAL

Written Report. 1. Calculate the voltage and power loss in the
two lines from the observations in items 1 and 2, Order of Work.

2. Same, for item 3, Order of Work.

3. How do the voltage and power losses compare with the two
different line voltages ? Explain.

4. Explain, using diagrams, what transformer connections
were used to secure the double line voltage in item 3, Order of
Work.

5. Why must the two terminals, connected together in the case
of a two-coil secondary, be opposite in polarity at each instant
for the series connection of the two coils ? Explain. What would
result if these two terminals were of the same polarity at each
instant ?

6. Why must the terminal of one coil be connected to a ter-
minal of the second coil having the same polarity at each instant
in a two-coil secondary for parallel connection of the two coils?
Explain. What would result if these two terminals were of oppo-
site polarity at each instant?

7. From the observations in this experiment, explain why high
voltage increases the efficiency of transmission of electric power
over long distances.

EXPERIMENT 26.
Study of the Induction Motor.
See Fig. 385a, page 497 in the text book.

The object of this experiment is to gain a working knowledge
of the construction, and to observe the principles involved in
the production of motion in the induction motor.

Theory. If the field poles of a direct current motor were ro-
tated about the armature, induced electromotive forces would be
set up in the armature conductors. If, further, the armature
conductors were so connected that the induced electromotive
forces thus set up produced a flow of current, the armature wires
carrying current would be acted on by the moving magnetic field
thus producing a mechanical force or torque tending to cause
the armature to rotate.

In the induction motor a rotating magnetic field is produced
by alternating current supplied to the stationary field windings

ALTERNATING CURRENT 89

(called the stator) and this rotating field acts on short circuited
conductors thus inducing currents. This, obviously, fulfills the
condition for motor action, namely, wires carrying current in a
magnetic field. It will be noted that, while the magnetism is ro-
tating, the field windings are themselves stationary, the rotating
field effect being made possible by the rapidly changing and re-
versing alternating current in the stator. The production of this
rotating magnetic field by the alternating current is one of the
objects of study in this experiment.

Current Supply. 110 volts Direct Current, a low frequency
Alternating Current and Alternating Current of the same fre-
quency as the motor assigned.

Apparatus Required. (1) The field (stator) of a three-phase
induction motor with the rotor removed; and (2) a simple iron
device corresponding to a compass needle, pivoted in the position
of the rotor to indicate the poles of the stator; (3) brake to be
(5) voltmeter; and (6) ammeter. (If a two-phase induction
motor is used, the same instructions apply with obvious modifica-
tions.)

Order of Work. 1. Mark or label the three terminals of the
stator winding as " 1," "2" and "3," in consecutive order, and
connect the direct current supply mains through a suitable pro-
tective resistance to the terminals 1 and 2, then to 2 and 3, then
to 3 and 1 in turn, going through this succession several times.
Make a sketch showing the succcessive positions of the pointer as
indicating the magnetic field rotation. Observe the number of
poles of the stator.

2. Connect the stator to a low frequency supply and observe
the action on the pivoted pointer.

3. Make a sketch of the arrangement of several coils in the
stator winding as related to each other, that is, showing the an-
gular displacement of the various coils.

4. Make a diagram of the rotor winding showing the details of
the end connections of the rotor conductors.

5. Place the rotor in position in its bearings and connect the
stator to the rated supply mains (alternating current) allowing
the machine to come up to its normal speed. Observe and re-

90 LABORATORY MANUAL

cord the speed in revolutions per minute and note the frequency
of the circuit used.

6. With normal voltage across the motor terminals, load by
means of a brake, and observe and record the torque, current,
and slip with increasing load, that is, for zero, 14, %, % and

7. Same as item 6, except that % of the normal voltage value
is to be used.

Written Report. 1. Explain just how a three-phase current
produces a rotating field effect similar to that produced in item
1, Order of Work.

2. What determines the number of poles in the stator where
the windings are flush with the iron ?

3. What is the object in short circuiting the rotor conductors?

4. Prom the observation of the number of poles in 1 and the
frequency in item 5, Order of Work, calculate the number of
revolutions per minute of the rotating field.

5. What is the numerical difference between the number of
revolutions per minute of the rotor as observed in item 5, Order
of Work, and that of the stator as calculated in item 4, Written
Report? (Note that the difference between these speeds is the
slip of the motor. )

6. Plot two curves on the same sheet, using the speed of the
rotor (found from the observations of stator magnetism speed
and the slip) as abscissas and torque as ordinates, from items 6
and 7, Order of Work.

EXPERIMENT 27.

Electrical Features of the Induction Motor.
See the Theory under Experiment 26 in the Manual.

The object of this experiment is to observe the conditions which
affect the speed and torque of the induction motor.

Theory. The induction motor may roughly be considered a
constant speed machine under constant load conditions. How-
ever, as the load increases, the rotor speed falls and the difference
between the speed of the rotating field and that of the rotor, name-

ALTERNATING CURRENT 91

ly the slip (see item 5 under the heading, Written Report, in Ex-
periment 26) increases.

Note that the magnetic field rotates at a speed which is pro-
portional to the frequency of the supply current. Thus, if the
frequency be reduced to half its original value the magnetic field
rotates at half the original speed. The rotor speed depends at no-
load on this rotational speed of the magnetic field and, hence, for
half value.

On the other hand, the torque of the motor depends on the
magnetic field (which is proportional to the impressed voltage
E) and on the rotor current (proportional to the magnetic field
and, hence, to the impressed voltage). The torque may, there-
fore, be said to depend on the square of the impressed voltage
(E 2 ).

Hence, if the frequency be maintained constant and the im-
pressed voltage reduced to half value, the no-load speed of the
motor will remain sensibly the same as before (since the magnetic
field rotates as before and the torque required at no-load is negli-
gible). If, however, when the motor is loaded, the impressed
voltage be reduced to half value, the frequency being maintained
constant, the slip will be increased because the available torque
is reduced by the reduction of E and this must be made up by the
larger rotor currents set up in this case by the higher induced
electromotive force in the rotor due to the greater relative mo-
tion between rotor and field or, in other words, to the greater
slip.

Note, also, that the frequency of the rotor currents is highest
when the rotor is at rest, that is, at starting, hence, the react-
ance of the rotor winding is greatest at this time, and the power
factor of the rotor at its lowest. This means that the magnetic
field and the rotor currents (the two factors which produce the
motion) are at their maximum phase difference when the motor
is starting. Since the most advantageous condition for the torque
is when the field and rotor currents are in phase with each other
(or at their least phase difference) it is obvious that the torque
at starting is lower than at any other point of the operation of
the motor due to the low power factor.

The principal means for improving the power factor at start-
ing and thus improving the starting torque, is to insert an auxili-

92 LABORATORY MANUAL

ary resistance in series with the rotor, which is all cut in at start-
ing and gradually cut out as the rotor comes up to normal speed.

Current Supply. Three-phase Alternating Current.

Apparatus Required. (1) Three-phase induction motor with
auxiliary resistance arranged in the rotor circuit; (2) brake to
be used for loading the motor ; (3) device for measuring the slip ;
(4) ammeter; and (5) voltmeter.

Order of Work. 1. Connect the ammeter in one of the leads
of the stator circuit and arrange to drive the motor from supply
mains with its normal voltage and frequency.

2. Bun the motor unloaded, and observe and record the slip
for normal and for % voltage.

3. Starting at zero load with all the auxiliary resistance cut
out, and at %, %, %, and full load in turn, observe and record
the slip and the torque for normal voltage throughout.

4. Same as item 3, for % voltage, as far as the load can be car-
ried.

5. Same as item 3 7 with % the auxiliary resistance cut in.

6. Same as item 3, with all the auxiliary resistance cut in.

7. Tighten the brake, and measure the current in one line and
the torque at the pulley at starting, with zero auxiliary resistance
in the rotor circuit and using % voltage to prevent the flow of an
excessive current.

8. Same as item 7, with % the auxiliary resistance in the ro-
tor circuit and using % voltage.

9. Same as item 7, using all the auxiliary resistance in the ro-
tor circuit and % voltage.

10. With all the auxiliary resistance in the rotor circuit, re-
peat the observations called for in item 7, first, at % voltage, and
second, at normal voltage.

Written Report. 1. How does the slip compare for the two
voltages in item 2, Order of Work? Explain.

2. Plot four curves on the same sheet from the observations
of items 3, 4, 5, and 6, Order of Work, and explain from these
curves how the speed and torque vary with the amount of auxili-
ary resistance in the rotor circuit.

3. What effect on the speed and torque is produced in item 4,
Order of Work, by the reduced voltage? Explain.

ALTERNATING CURRENT 93

4. Compare the starting torque for items 7, 8 and 9. How
does the auxiliary resistance in the rotor circuit affect the start-
ing torque ? Explain.

5. How does the slip at normal, and at % voltage, in item 2,
Order of Work, compare with the corresponding values in items
3 and 4, Order of Work, at full load? Explain.

6. In item 10, how does the starting torque compare at */>, and
at normal voltage ? Explain.

EXPERIMENT 28.
Study of the Synchronous Motor.
See Fig. 411, page 515 in the text book.

The object of this experient is to secure an idea as to the gen-
eral operation of the alternator when run as a motor (usually re-
ferred to as the synchronous motor) .

Theory. In an alternating current generator (commonly
called an alternator) the magnetic field is produced by direct
current from a separate source of supply, while the current in
the armature conductors is, of course, alternating current. If
the alternator is used as a motor, the field winding is supplied
by direct current as before, and, as alternating current is sup-

Online LibraryClarence Edward ClewellLaboratory manual. Direct and alternating current → online text (page 7 of 8)