EXPERIMENT 8 Induction Motor - Purdue University Calumet

Purdue University Calumet
Department of Engineering Technology
Electrical Power and Machinery ECET-212
Experiment 8
THE INDUCTION MOTOR
OBJECTIVES
1. Establish the relationship between slip and the frequency of the rotor currents.
2. To study the torque-speed characteristics of a wound rotor induction motor.
EQUIPMENT AND SUPPLIES
1- Wound rotor induction motor
1- Prony brake
1- AC voltmeter
1- Oscilloscope
3- Ohm, 4.5 A max rheostats
2- AC Ammeters (2A) & (4A)
2- SPST Switches
1- Tachometer
EXPLANATION
I. RELATIONSHIP BETWEEN SLIP AND THE FREQUENCY OF THE ROTOR
CURRENTS
Application of a three phase voltage to the stator results in a rotating field within the motor. The
speed with which the field makes one revolution is dependent on the frequency and number of poles.
This speed is the synchronous speed and is found by the expression.
120 *f/p
In order for the induction motor to work, a current must be induced in the rotor. This can only happen
if the rotor is lagging or slipping behind the rotating field. Slip is then defined to be the percentage the
rotor speed is of synchronous speed. or
S = (ns-n)/ns
The frequency of the current in the rotor is a function of slip. If the rotor is at rest, the rotating field
moves past the rotor at the line frequency “f 0 ”. However, as the rotor rotates, the relative speed
between the rotating field and the rotor decreases and induces a frequency equal to the difference in
speed between the rotating field and the rotor. Therefore, the induced rotor frequency is
f 2 = S*f0

The voltage induced in the rotor at slip “S” is E 2 = S *E br . Where, E br is the open circuit voltage
induced in the rotor when at rest.
PROCEDURE:
1. Calculate the synchronous speed from the name plate data of your motor.
2. Set up the circuit of figure 1.
3. With the rotor circuit open, energize the stator of the motor.
4. Measure and record all the open circuit line to line “rotor” voltages.
E ab = E bc = E ca
5. Calculate the open circuit phase voltage E br induced in the rotor at rest.
6. Connect three 2 ohm resistors to the rotor terminal in a Y.
7. Make sure the prony brake is tightened so that the rotor will not turn. Energize the stator.
8. Quickly measure and record the voltage “E br ” across each resistor. Do not allow the motor to be
energized for long periods of time as the resistors will overheat.
9. Loosen the prony brake so that the rotor will rotate freely.
10. Connect an oscilloscope across one of the 2 ohm resistors.
11. Measure and record the frequency “f2” of the rotor current, the voltage induced in the rotor “E2”
and the rotor speed.
12. Load the motor with the prony brake until the speed drops to around 1000 rev/min.
13. Measure and record the frequency of the rotor current and the induced voltage.
14. Verify that f 2 =S*f 0 for the two rotor speed you measured.
15. Also, verify that E 2 =S*E br .
16. Turn off all power to the motor.
II SPEED-TORQUE CHARACTERISTICS OF A WOUND-ROTOR MOTOR
1. Make the rotor connection as shown in figure 2.
2. Energize the motor, with the external rheostat at minimum resistance position.
3. Perform a load test from no-load to approximately 34 oz load.
4. Record stator current, rotor current, speed and torque. Tabulate your result.
5. Monitor the rotor speed and gradually add torque until at some load the rotor speed will gradually
begin to decrease without loading it any heavier. That is, for every load there should be a unique
speed and the speed should stay relatively constant at that load. However there will be one value
of load where the speed will start to gradually decrease until the rotor stops. That speed is “n d ”
and its value should be recorded. Also, record the value of breakdown torque.
6. Increase the torque load to make the rotor firmly stall. Now very gradually decrease the torque
load until the rotor starts to accelerate. That value of torque where the rotor just begins to
accelerate is the starting torque. Record the value.
7. Repeat steps 3 through 6 with rheostat at mid resistance position and at maximum resistance.
8. Plot speed as ordinate versus torque as abscissa for each load test.

QUESTIONS
1. Why does a poly phase motor start without special starting winding?
2. What should happen if one phase was open-circuited while the 3-phase motor was running?
3. How can you reverse the direction of the motor? Why?