Bulletin 100AC-EX Experiment Manual for AC Motors
EXPERIMENT NO. 14
RUNNING CHARACTERISTICS
PURPOSE:
To discover how a three-phase squirrel-cage induction motor behaves under load.
BRIEFING:
The speed at which the stator’s field revolves is the synchronous speed. As this field is cut by
the bars of the squirrel-cage rotor winding, current is induced into the bars. The rotor’s magnetic field (caused by this current) interacts with the stator’s field to produce torque on the
rotor.
The torque is directly proportional to rotor current, IR, and the cosine of the phase angle
between the rotor and stator fields (cos 0) . Another way of expressing this relationship is that
torque is directly proportional to the in-phase component of rotor current, 1~ cos 8.
At the instant of start, 1~ is high but the in-phase components is low because of the poor
power factor (cos 0). As the rotor picks up speed, both the induced rotor voltage and the
inductive reactance decrease. Basically IR is going down while cos 8 is going up.
Look at a trig table, or even at a cosine curve like this one:
1
/
//
i2
CD
/--
//
/
0
/
0
r
I
0
I
‘1
\
9o”
/
9
1
‘\\
.
---
//
/
/
/
/
/
/
.
0
/’
You can see that there is not very much difference in the value of cos 8 when 8 is 00 (cos 8
=l) and when 8 is 20” (cos 8 = 0.94).
Therefore, over the operating range of the motor, the rotor power factor does not play an
important part in the torque output. More important is rotor current. Rotor current falls off
sharply as the rotor approaches synchronous speed (i.e., slip approaches zero). Speed doesn’t
have to drop back very much to increase the rotor current, stator power factor, and torque.
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Bulletin 100AC-E)( Experiment Manualfor AC Motors
When you are running an induction motor without load, it draws almost as much current as it
does fully loaded. This no-load current, however, is made up of two components. The inphase component supplies electrical and mechanical losses. The quadrature (90 degrees out of
phase) component is the magnetizing current. It is quite large in comparison with the inphase part. As the motor is loaded, it is like putting a resistive load on the secondary of a
transformer. The in-phase component gets larger. The stator’s power factor improves accordingly. The increased rotor current does not necessarily add to the total current being drawn
by the motor. It simply uses more of that current for useful work.
In this experiment we will be using the two-wattmeter method of measuring power input. At
no-load, power factor is less than 0.5. That means that one wattmeter must be connected
with its voltage coils reversed and its reading subtracted from the other one. As the motor is
loaded, the power factor improves. When it reaches 0.5, the potential coil connections must be
connected normally, its reading added to the other one.
PERFORMANCE OBJECTIVES:
Upon successful completion of this experiment, the student will be able to:
1. Explain and predict the changes in speed and current that will occur as a squirrelcage induction motor is loaded.
2. Perform load tests on three-phase motors.
MACHINES REQUIRED:
IM-100 Induction Motor
DYN-100-DM Dynamometer
MATERIAL REQUIRED:
Fixed 3# AC Supply
0 - 150 volts, Variable DC Supply
000(2)
0-
150 volt DC Voltmeter
300 volt AC Voltmeter
4 amp
AC ammeter
0 - 600 watt AC Wattmeters
2 . 5 amp
DC Ammeter
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Bulletin 100AC-EX Experiment Manual for AC Motors
ADDITIONAL MATERIAL REQUIRED:
MGB-100-DG Bedplate
SLA-IOOD Strobe Tachometer
RL-1OOA Resistance Load
PROGRAM PLAN:
step 1.
Place the two machines on tne bedplate. Motor on the left; dynamometer on the
right. Couple and clamp the machines securely. Install guards.
step 2.
Connect the dynamometer as shown in Figure 14-1. Note that this is a separately
excited shunt generator connection. Be sure the rotor locking device has been removed from the dynamometer shaft.
INDUCTiON M O T O R
3c3 A C
INPUT
600W
GENERATOR
RL-1 OOA
STATOR
Bb
Ab
cm
J
Figure 14-1
step 3.
Connect the dynamometer as shown in Figure 14-1. 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 dynamometer to its maximum resistance position, fully clockwise.
step 5.
With the motor switch OFF turn on the main AC and the 0-15OV DC circuit breakers. Adjust the excitation supply to 115 volts.
Step 6.
Put the temporary jumpers across the terminals of the AC Ammeter and the current coils of the wattmeter to protect the meter movements from the large inrush
of starting current.
step 7.
Turn the motor ON. Remove the jumpers from the AC Ammeter and Wattmeters.
Step 8.
Use the dynamometer’s field rheostat to adjust its terminal voltage to 120V DC.
14-3
Bulletin 100AC-EX Experimen
t Manual for AC Motors
step 9.
Zero the dynamometer scale by positioning the weight on the rear.
Step 10. Record in TABLE 14-1 of TEST RESULTS, under “NO LOAD”, the values of line
& load amps, W#l, & W#2, torque, and speed.
Step 11. Turn ON load steps 1, 2, and 3, on the RL-1OOA Resistance Load Bank.
Step 12. Readjust the dynamometer’s field rheostat or the excitation supply, as required, to
maintain a terminal voltage of 120 volts. Then repeat Step 10.
Step 13. Turn ON load steps 4, 5, and 6. Repeat Step 12.
Step 14. Turn ON load steps 7, 8, and 9. Repeat Step 12.
Step 15. Turn OFF all circuit breaker switches. Disconnect all leads.
TEST RESULTS:
NO LOAD
S T E P S 1-3 / STEPS 1-6
STEPS 1-9
LOAD CURRENT
1 LINE VOLTAGE
LINE CURRENT
WA-ITS #1
I
WATTS #2
,
TOTAL WATTS
TORQUE
I
SPEED
1
I
I
P.F. (W/VA)
TABLE 14-1
14-4
I
I
Bulletin 100AC-EX Experiment Manual for AC Motors
1.
Combine the two wattmeter readings (subtract at no and low loads, add after wattmeter needle changed directions) and record under TOTAL WATT
S in TABLE 14-1.
2.
The equation for computing total apparent power input in voltamperes is:
VA = Line Volt x Line Amps x 1.73
Since the input voltage is 208 and does not change, it can be multiplied by 1.73 to
produce a new constant.
VA = 360 x Line Amps
Compute the volt amperes for each of the load steps and record under VA in TABLE
14-1.
3.
The motor power factor is the ratio of the true power (watts) to the apparent power
(volt-amperes). Perform this division for each of the load steps and recordin TABLE
14-1.
4.
Using the data you have compiled in TABLE 14-1, plot three curves on the graphs
provided.
a) Show how motor current changes as the torque output of the motor increases.
b) Show how speed changes as the torque output of the motor increases.
c) Show how the power factor changes as the torque of the motor increases.
5.
Someone suggests that you buy a motor rated for twice the torque you need so as to be
sure you are not working the motor too hard. Discuss why you think this is or is not a
good idea.
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Bulletin 100AC-EX Experiment Manualr fo AC Motors
QUICK QUIZ:
1.
Over the operating range of an induction motor (no load to full load):
a) There is a large variation in speed.
b) There is absolutely no variation in speed.
c) There is a small variation in speed.
2.
From no load to full load, there is:
a) Considerable change in power factor.
b) A small change in power factor.
c) Absolutely no change in power factor.
3.
At no load, poor motor power factor is due to:
a) The high frequency of induced rotor voltage.
b) The rotor power factor.
c) The quadrature stator magnetizing current.
4.
The mechanical power of the rotor is supplied by:
a) Active Power input to the stator.
b) Reactive Power input to the stator.
c) Apparent Power input to the stator.
5.
The frequency and value of induced rotor voltage depends on:
a) The rotor speed only.
b) The difference between rotor speed and the speed of the revolving stator field.
d Synchronous speed only.
14-6