Page 124 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

INDUCTION MOTORS

INDUCTION MOTORS 6.13

Variation of the Torque-Speed Characteristics
Figure 6.16 illustrates the variation of the torque-speed characteristic of a wound rotor induc-
tion motor. Recall that the resistance of the rotor circuit can be changed because the rotor
circuit is brought out to the stator through slip rings. As the rotor resistance increases, the pull-
out speed of the motor decreases, but the maximum torque remains constant. The advantage
of this characteristic of wound rotor induction motors is the ability to start very heavy loads.
The maximum torque can be adjusted to occur at starting conditions by inserting a high
resistance. Once the load starts to turn, the extra resistance can be removed from the circuit,
and the maximum torque will shift up to near synchronous speed for normal operation. If the
rotor is designed with high resistance, then the starting torque is high, but the slip is also high
during normal operation. However, the higher the slip during normal operation, the smaller the
fraction of power converted to mechanical power, and the lower the efficiency. A motor with
a high rotor resistance has a high starting torque and poor efficiency during normal operation.
If the rotor resistance is low, then the starting torque is low and the starting current is
high. However, the efficiency is high during normal operation. A compromise between
high starting torque and good efficiency is needed.
A wound rotor induction motor can be used to provide high starting torque during start-up
by inserting extra resistance. The extra resistance can be removed during normal operation
to increase the efficiency. However, wound rotor induction motors are more expensive,
require more maintenance, and have a more complex automatic control circuit than squirrel-
cage induction motors. Also, wound rotor induction motors cannot be used in hazardous
and explosive environments because completely sealed motors are needed. Figure 6.17
illustrates the desired motor characteristic.

FIGURE 6.16 The effect of varying rotor resistance on the torque-speed charac-
teristic of a wound-rotor induction motor.

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