The Wound-Rotor Induction Motor Part (2)

Objective:

   · To observe the characteristics of the wound-rotor induction motor at no-load and full-load.

   · To observe speed control using an external variable resistance.

Discussion:

The three ends of the three-phase rotor windings are brought out to three slip rings mounted on the rotor shaft. The brushes bearing on the slip rings play an important role in realizing maximum advantage from the wound-rotor motor. By connecting the brushes through rheostats, it becomes possible to develop a higher starting torque than is possible with a squirrel-cage motor. On starting, the full resistance of the rheostats is maintained in the rotor circuit, thus providing the very maximum starting torque.

As the motor approaches normal operating speed, the rheostat resistance is gradually reduced until it is out of the circuit entirely at full speed. Although the starting torque of the wound-rotor motor is higher, it is not as efficient as the squirrel-cage motor at full speed, because the resistance of the rotor windings is always more than that of a squirrel-cage motor.

A special feature of the wound-rotor motor is its variable speed capability. By varying the rheostat resistance, it is possible to vary the percentage of slip and thus, vary the motor speed. In such cases, below full speed operation means the motor is running at reduced efficiency and mechanical output power. In addition, because of a high rotor resistance, the motor is made more susceptible to variation in speed as the load changes.

Equipment Required:

• Three-Phase Wound-Rotor Induction Motor
• Electrodynamometer
• Single-Phase Power Factor Meter
• Three-Phase Rheostat
• Power Supply
• AC Ammeter
• AC Voltmeter
• Wires

Procedure:

CAUTION!!!

High voltages are Present In the Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement!!!

1.  a. Examine the construction of the Three-Phase Rheostat, paying particular attention to the circuit schematic diagramed on the face of the module.

b. Note that the arms of the three rheostats are separately brought out to terminals 1, 2 and 3. The remaining ends of the rheostats are wired together internally and brought out to the N terminal.

c. Note that the three rheostats are ganged together and that their individual resistances can be varied simultaneously by turning the single control knob.

d. When the control Knob is fully CCW the resistance of each rheostat is 75 Ω.

2. Using your Three-Phase Wound-Rotor Induction Motor, Electrodynamometer, Single-Phase Power Factor Meter, Three-Phase Rheostat, Power Supply, AC Ammeter and AC Voltmeter, connect the circuit shown in Figure – 1. Do not couple the motor to the electrodynamometer at this time!

3.   a. Set the speed control rheostat knob at its full CCW position for zero resistance.

b. Turn on the power supply and adjust E1 to 380 Vac. The motor should be running.

c. Measure and record in Table – 1, the three line currents, the two power factor meter indications (remember, to observe the polarities) and the motor speed.

d. Return the voltage to zero and turn off the power supply.

4.   a. Couple the motor to the electrodynamometer with the timing belt.

b. Set the dynamometer control knob at its full CCW position.

c. Repeat procedures 3 for each of the torques listed in Table – 1, maintaining the input voltage at 380 Vac.

d. Return the voltage to zero and turn off the power supply.

5.     a. Set the speed control rheostat knob at its full CCW position for maximum resistance.

b. Uncouple the motor from the electrodynamometer.

6.   a. Turn on the power supply and adjust E1 to 380 V ac. The motor should be running.

b. Measure and record in Table – 2, the three line currents, the two power factor meter indications and the motor speed.

c. Return the voltage to zero and turn off the power supply.

7.   a. Couple the motor to the electrodynamometer with the timing belt.

b. Set dynamometer control knob at its full CCW position.

c. Repeat procedure 6 for each of the torques listed in Table – 2, maintaining the input voltage at 380 V ac.

d. with a developed torque of 0.9 N-m, rotate the speed control rheostat knob from full CW to full CCW.

e. Does the motor speed change? Yes

f. Does the developed torque change? Yes

       g. Return the voltage to zero and turn off the power supply.