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EXPERIMENT NO. 18
Bulletin 100AC-EX Experiment Manual for AC Motors
EXPERIMENT NO. 18
SPEED CONTROL OF WOUND-ROTOR INDUCTION MOTORS
PURPOSE:
To discover the effect that rotor resistance has on the speed of a wound-rotor induction
motor.
At
the instant of start, resistance in the rotor circuit prevents a large current inrush. The
price you pay for that is a reduced starting torque. This is compensated for by the fact that
you get more torque per ampere of starting current. Resistance accomplishes this by making
the rotor field closer to being in-phase with the stator field. In other words, it improves the
power factor of the rotor.
The reason that the rotor has such a poor power factor at start is that the induced rotor
frequency is at a maximum--equal to the frequency of the incoming power. Once the rotor
starts turning, however, this rotor frequency starts going down. With a wound rotor motor
running without load, the induced rotor frequency may be only 5 hertz or so.
At that fre-
quency, the rotor windings have practically no inductive reactance. If you had resistance in
series with the rotor windings, it would not improve rotor power factor. All it would do is
increase the losses in the rotor circuit.
Here is what happens: The motor, itself, automatically picks out the amount of slip it needs
to produce the rotor current that will drive the load at that speed. Now you add rotor resis-
tance Now that the rotor is losing some of its power to the resistance, it needs more slip so it
can produce more current from a higher induced rotor voltage.
That makes the rotor slow down. But the load hasnt changed. Therefore, the rotor draws
enough current so it can produce extra torque. The load, remember, is proportional to torque
times speed. If the speed goes down, the torque has to go up since the load is constant.
Rotor resistance, then, can provide speed control of a wound rotor motor. It is not absolutely
accurate, however, because speed changes with load. Whats more, there are bigger changes in
speed with load if there is resistance in the rotor circuit.
18-1 Bulletin 100AC-EX Experiment Manual for
AC
Motors
PERFORMANCE OBJECTIVES:
Upon successful completion of this experiment the student will be able to:
1. Explain control systems which cut out starting rotor resistance as the motor gains
speed.
2.
Explain how rotor resistance controls speed of a wound rotor motor.
WRM-100 Wound Rotor Motor
DYN-100.DM Dynamometer
POWER REQUIRED:
Fixed
AC
Supply
0
- 150
volts Variable DC, 1 amp
0 - 2
amp AC Ammeter
0.5 amp AC Ammeter
Two
(2) 0 -
150 volt DC Voltmeters
0
- 300
Volt AC Voltmeter
WRSC-100 Wound Rotor Speed Controller
MGB-100-DG Bedplate
RL-1OOA Resistance Load
HT-1OOJ Hand Tachometer-Generator
step 1. Place the
two
machine8 on the bedplate; motor on the left; dynamometer on the
right. Couple and clamp the machines securely. In&all guards.
step
2.
Connect the Wound Rotor Motor as shown in Figure 18-l.
step 3. Connect the dynamometer as shown in Figure 18-1.
18-2 Bulletin
100AC-EX Experiment Manual
for AC Motors
step
4. Have
someone check your connections to be sure they are correct. Be sure all of
the load switches on the RL-lOOA are in the downward (OFF) position. Adjust the
field rheostat on the dynamometer to its maximum resistance position, fully clock-
wise.
ROTOR SPEED CON TROLLER
VARIABLE
A C
INPUT
step 5.
Step
6.
step 7.
Step 8.
step 9.
step
10.
RL-1 OOA
DYNAMOMETER
Figure 18-1
With the WRSC-100 in its maximum resistance (start) position. Turn ON the main
AC and the motors circuit breaker.
Turn the knob on the WRSC-100 fully clockwise to its zero resistance (RUN) posi-
tion.
Turn ON the 0
-
150 volt supply and adjust the excitation to 115 volts.
Use the dynamometers field rheostat to adjust its terminal voltage to 100 volts
DC .
Zero the dynamometer scale by positioning the weight on the rear.
Load the dynamometer by turning ON resistance steps 1 through 8 on the RL-
1OOA.
18-3 Bulletin 1OOAC-EX Experiment Manual for
AC
Motors
step 11.
step 12.
NOTE:
step
13.
step
14.
step 15.
Step
16.
step
17.
step 18.
step 19.
step
20.
,
step
21.
Readjust the dynamometers field rheostat or the excitation supply, as required, to
maintain a terminal voltage of 100 volts.
Read torque, stator current, speed, and rotor current and record in TABLE 18-l of
TEST RESULTS.
You will SEE a slight vibration of the ammeter needle measuring rotor current.
This is due to the low frequency of the induced rotor voltage. As rotor speed in-
creases, the frequency decreases. Read the ammeter at the midpoint between nee-
dle extremes.
Turn the WRCS-100 counterclockwise one resistance tap.
Repeat Steps 11 and 12.
Repeat Steps 13 and 14 for each resistance tap until you read maximum resistance.
With the WRCS-100 still set at its maximum resistance point,
TABLE 18-i.
Remove load resistance No. 8 from the RL-lOOA circuit by pushing
downward.
Record motor speed in TABLE 18-2.
Turn the WRSC-IOO to its zero resistance position and repeat Step
Put load resistance No. 8 back in the circuit and repeat Step 18.
Turn OFF all circuit breakers. Disconnect all leads.
record speed in
its toggle switch
18
18-4 Bulletin
100AC-EX Experiment Manual
for AC
Motors
TEST RESULTS:
NO
RES. TAP
2 TAP 3 TAP 4 TAP 5 MAX
RES.
SPEED
TORQUE
ROTOR CURRENT
STATOR CURRENT
TABLE
1 8 - 1
SPEED - RPM
MAX. RES
ZERO RES.
LOADS 1-8
LOADS 1-7
CHANGE
l
TABLE
18-2
DE-BRIEFING:
1.
As you added resistance externally to the rotor circuit, while the load remained con-
stant, what happened to rotor speed?
why?
2
As
rotor speed decreased, what happened to the torque output of the motor?
why?
18-5 Bulletin
100AC-EX
Experiment Manual for AC Motors
3
As
rotor speed decreased, what happened to the current being induced into the rotor
circuit?
4
Compute the change in speed (in TABLE 18-2) due to the change in one load step. Do
this for both the maximum resistance and zero resistance positions of the WRSC-100.
5
Was there a larger change in speed with the resistance in
or out of the rotor circuit?
why?
QUICK QUIZ:
1
An
increase in the external rotor r&stance:
a)
b)
c)
2
When
a)
b)
c)
Increase wound-rotor motor speed.
Decrease wound-rotor motor speed.
Has no effect on wound-rotor motor speed.
the rotor slows down, the rotor frequency:
Goes up.
Goes down.
Remains the same.
18-6 3. With resistance in the rotor circuit, as load changes:
a) There is no change in speed at all.
b) There is a slight change in speed.
c) There is a noticeable change in speed.
4 . The larger the rotor resistance:
a) The less the rotor current.
b) The greater the rotor current.
c) No effect on rotor current.
5
Wound rotor motors are usually:
a) Started without external resistance in the rotor circuit, then cut in as the
motor speeds up.
b) Started with some external resistance in the rotor circuit, then cut in mor
e as
the motor speeds up.
c) Starte d with maximum external resistance in the rotor circuit, then cut out as
the motor speeds up.
18-7