Lab Information
EXPERIMENT NO. 14
STARTING CHARACTERISTICS AND DIRECTION OF ROTATION
OF A THREE-PHASE INDUCTION MOTOR
PURPOSE:
To determine the starting torque and the direction in which a three-phase induction motor will
rotate.
APPARATUS REQUIRED:
1. One IM-100 Induction motor.
2. One PB-100 Prony Brake
3. Two Hampden AC Ammeters
4. Two Hampden AC Wattmeters
5. One Hampden AC Voltmeter
6. One 0 to 208V-30 Power Supply
PROCEDURE:
1. Couple the Induction Motor to the Prony Brake and make the connections shown in Figure 28.
Adjust the belt of the brake tight against the pulley so that the motor will not rotate when
switched on.
2. Have the instructor check your connections and brake hookup.
3. With the output at its zero setting, switch on the 0 to 208V-30 power supply. Slowly adjust the
voltage to 104V. Quickly record the torque, amps, and watts.
4. Assuming that the starting current is directly proportional to the voltage applied, predict the
starting current at full line voltage.
5. Assuming that the starting torque varies directly with the square of the applied voltage (i.e., T
= KV2), predict the starting torque at full line voltage.
6. Uncouple the motor from the Prony Brake and make the connections shown in Figure 29. Start
the motor at full line voltage and note the direction of rotation from the outboard end (right side
when looking at front side of motor's panel).
7. Shut the motor off. Swap any two stator leads. Restart the motor and again note the direction
of rotation.
REPORT:
1. Show data obtained and calculations performed.
2. Draw a phasor diagram of your measured (reduced) starting volts and amps.
3. What factors determine the direction of rotation of a three-phase motor?
FIGURE 28
FIGURE 29