.
Bulletin 100AC-EX Experiment Manual for AC Motors
EXPERIMENT NO. 15
SPEED CONTROL OF INDUCTION MOTORS
PURPOSE:
To discover what effect changing applied voltage and frequency have on the speed of a
three-phase squirrel-cage induction motor.
BRIEFING:
An induction motor is inherently a constant speed machine. Not that its speed doesn’t
change at all. It changes a little from no load to full load. Its speed, however, is tied to the
frequency of the incoming power. This determines the speed of the revolving field-synchronous speed. Synchronous speed can be computed from frequency with the equation:
Synchronous Speed = No Af&es * f
.
From this equation you can see that you can change the synchronous speed of a motor by
changing the frequency of the incoming power. This is not often done in industry, however.
It requires an extra device to vary the frequency. It may be cheaper to use a DC motor, whose
speed can be easily changed.
You can also get a very limited speed control by varying the applied voltage.
The motor’s torque, remember, is proportional to the applied voltage squared. To illustrate,
suppose you have a given load coupled to a motor. The motor is delivering a certain torque to
drive the load at a certain speed. If the applied voltage is less, the torque output of the motor
is less. The motor will therefore drive the same load at a slower speed.
Within a narrow range of speed (about 5%) this is accomplished without an increase in motor
current. The lower applied voltage counteracts the tendency for the current to increase due to
the slower rotation of the rotor. Beyond that range, current begins to rise rapidly.
In this experiment you will first investigate the limited speed control of a loaded motor
through voltage variation. Then you will investigate the wide range of speed control available
through frequency variation.
15-l
Bulletin 100AC-EX Experiment Manual for AC Motors
PERFORMANCE OBJECTIVES:
Upon successful completion of this experiment, the student will be able to:
1. Explain the constant speed characteristics of three-phase induction motors.
2. Make the correct connections for a narrow range speed control from applied volt-
age.
MACHINES REQUIRED:
DM-1OOA DC Machine
SM-100-3A Synchronous Machine
IM-100 Induction Motor
POWER REQUIRED:
0 - 125 volt Variable DC, 5 amps
0 - 150 volt Variable DC, 1 amp
Variable 3+ AC Supply
0 - 150 volt DC Voltmeter
0 - 300 volt AC Voltmeter
0 - 4 Amp AC Ammeter
ADDITIONAL MATERIAL REQUIRED:
MGB-100-DG Bedplate
SLA-1OOD Strobe-Tachometer
RL-1OOA Resistance Load Bank
PROGRAM PLAN:
A. SPEED CONTROL FROM VOLTAGE
step 1.
Place the DM-1OOA on the right side of the bedplate. Place the IM-100 on the left
side of the bedplate. Couple and clamp the machines securely. Install guards.
15-2
Bulletin IOOAC-EX Experiment Manual for AC Motors
step 2.
VARIABLE
30 AC
SUPPLY
Connect the DM-1OOA as shown in Figure 15-l. Note that this is a separately
excited shunt generator connection. Be sure that the switches on the RL-1OOA are
all in the downward (OFF) position.
STATOR
4A
6
L2 0
Ll /J-J?
0 I O
Ab
Bb
DC GENERATOR
INDUCTION MOTOR
T2
Tl
%kJNT
I
M
‘\
A
Figure 15-1
\
0-150V
DC
6’
L -- f
km
IA
H
step 3.
Connect the IM-100 Induction Motor as shown in Figure 15-l. Do not turn power
On yet.
Step 4.
Have someone check your connections to be sure they are correct. Adjust the field
rheostat on the DM-1OOA to its maximum resistance, fully clockwise, position.
step 5.
With the motor switch OFF, turn ON the main AC, the variable AC, and the 0 150 volt DC current breakers. Adjust the AC supply to 208 volts. Adjust the excitation supply to 115 volts.
Step 6.
Put a temporary jumper across the terminals of the AC Ammeter to protect its
movement from the large inrush of starting current.
step 7.
Turn the motor ON. Remove the jumper from the Ammeter terminals.
Step 8.
Load the generator by turning ON twelve steps of resistance on the RL-1OOA.
step 9.
Use the generator’s field rheostat, and if necessary, the excitation supply to produce a terminal generator voltage of 100 volts DC.
step 10/ . Measure the motor voltage, current, and speed and record these value8 in TEST
RESULTS.
step 11
Slowly reduce the motor’s applied voltage while watching the AC Ammeter, also
keeping the load voltage at 100 volts by means of the generator’s field rheostat or
excitation supply. Current will begin to decrease, then increase. Stop when the
current returns to the value read in Step 10.
15-3
Bulletin 100AC-EX Experiment Manual for AC Motors
Step 12. Measure the motor voltage and speed and record these values in TEST RESULTS.
Step 13. Turn off all switches. Disconnect all leads.
B. SPEED CONTROL FROM FREQUENCY
Step 14. Place the DM-1OOA on the left side of the bedplate; the SM-lOO-3A on the right
side; and the IM-100 in a spot where there will be no chance of touching or having
wires touch the spinning rotor.
DC MOTOR
+d
I
I
Ss A L T E R N A T O R
ARMATURE
A
v1
DAMPER
-
I
2
r
RHEO.
4,X6
0-? 25VDC
SUPPLY
I
START
-
’
I
3
----a470 !-o-
3
2
START /
SW.
i
1A
TAZH
l--l
4 TACHI
+
0-1SOvDC
SUPPLY
kA
b
c
ARMATURE
Figure 15-2
Step 15. Connect the DM-1OOA as shown in Figure 15-2. Note that this is a self-excited
shunt motor connection.
Step 16. Connect the SM-lOO-3A as an alternator as shown in Figure 15-2. Note that the
three-phase AC generated by the alternator is used to run the induction motor. Be
sure the alternator switch is in the SYNC RUN position.
Step 17. Have someone check your connections to be sure they are correct. Turn the
motor’s field rheostat fully counterclockwise to its minimum resistance position.
15-4
Bulletin 100AC-EX Experiment Manual for AC Motors
step 18. Turn the voltage control knob of the 0 125 volt supply to zero. Then turn ON the
following: the main AC, the 0 - 125V DC, and the DC motor.
step 19. Slowly increase the motor’s voltage to 125V DC. Turn the field rheostat knob
clockwise until the DC motor is running at 1900 rpm.
step 20. Turn on the 0 - 150V DC Excitation supply and increase its output until the excitation current reaches 1.0 amp. (At this time the induction motor should start to
turn, slowly at first and gradually pick up speed.)
step 21. After the induction motor has reached full speed, readjust the DC motor’s speed
with its field rheostat to 1900 RPM.
step 22. Readjust the alternator’s excitation, if necessary, to maintain 1 amp excitation current.
Step 23. Read the induction motor’s speed and record in TEST RESULTS (B).
Step 24. Turn the motor’s field rheostat knob clockwise until the motor runs at 2100 RPM.
step 25. Repeat Steps 22 and 23.
Step 26. Turn OFF all circuit breaker switches. Disconnect all leads.
TEST RESULTS:
A. SPEED CONTROL FROM VOLTAGE
Step 10 readings:
volts
amps
Step 12 r e a d i n g s : volts
rpm.
B. SPEED CONTROL FROM FREQUENCY
Step 24 readings:
rpm
Step 25 readings:
rpm
15-5
rpm.
Bulletin 100AC-EX Experiment Manual for AC Motors
1.
Compute the percentage drop in voltage that was possible before current started to
%
increase [(Vl - V2)/Vl] x 100 =
2.
Why did the current start to increase when the voltage dropped off?
3.
Compute the percentage change in speed that occurred from full voltage to the re%
duced voltage [(Sl - S2)/‘Sl] x 100 =
4.
Compute the frequency of the voltage generated by the alternator when being driven
at 1900 rpm. This is a four pole alternator.
f = (speed x poles)/l20. =
5.
HZ
Compute the synchronous speed of a four-pole induction motor running at the frerpm. Allowing for slip, does this agree with your
quency computed in No. 4
speed measurement?
QUICK QUIZ:
1.
If there is a drop in power line voltage, an induction motor will:
a) Speed up.
b) Slow down.
c) Continue to
2.
at the same speed.
During emergencies, power companies often reduce voltage. If they drop 10%, motor
current will:
a) Remain the same.
b) Go down.
c) Go up.
15-6
Bulletin 100AC-EX Experiment Manual for AC Motors
3.
S p e e d ( control from voltage is not necessarily an accurate control because:
a) The synchronous speed keeps changing anyway.
b) The speed changes when load changes anyway.
c) There are wide swings in powerline frequency anyway.
4.
A higher frequency voltage will make an induction motor:
a) Run
faster.
b) Run slower.
c) Keep running at the same speed.
5.
The synchronous speed of an induction motor depends on:
a) The number of poles and the frequency of the applied voltage.
b) Applied voltage times motor current.
c) The torque required to drive the load.
15-7