A wound rotor motor is a variation of the three-phase induction motor, designed to provide high starting torque for loads with high inertia, while requiring very low current.
Wound rotor motors are also referred to as “slip ring motors.”
The stator of a wound rotor motor is the same as a typical induction motor, but the rotor has a three-phase winding, with each of the winding terminals connected to separate slip rings. In contrast, a traditional induction motor (aka “squirrel cage motor”) has windings that are permanently short-circuited by an end ring.
The slip rings on the wound rotor motor contain brushes that form an external, secondary circuit into which impedance (resistance) can be inserted. During starting, this resistance is placed in series with the rotor windings. This added resistance causes the rotor current to run more in phase with the stator current, which increases the torque that is developed. But added resistance also decreases the current in the secondary circuit, so a very high starting torque can be produced with low starting current.
Traditional squirrel cage induction motors can require anywhere from 400 to over 1000 percent of their full load current when starting.
If the full resistance is inserted into the secondary circuit when the motor is running, the rotor current decreases and the motor speed decreases. But, as the motor speed decreases, more voltage is induced in the rotor windings, and more current is produced to create the necessary torque at this reduced speed.
Gradually reducing the resistance allows the motor to come up to normal operating speed, providing smooth acceleration for the load. By keeping some resistance in the secondary circuit, speed can be controlled up to a point. But this method for regulating speed loses its effectiveness as the speed increases – up to about 50 percent of rated speed at full load. Once the resistance in the secondary circuit is completely shorted out, the motor then behaves electrically like a traditional squirrel cage motor.
The downsides of wound rotor motors are the complexity and maintenance added by the slip rings and brushes, as compared to traditional squirrel cage motors. However, wound rotor motors are beneficial in applications with high inertial loads – such as large fans, pumps, and grinding mills – because the wound rotor design allows the load to be accelerated gradually through speed and torque control. And they can develop very high starting torque at standstill with low inrush current. Although traditional induction motors with variable speed drives now dominate, wound rotor motors can also be used for adjustable-speed applications, as long as very precise speed control isn’t required.
Feature image credit: TECO-Westinghouse Motors, Inc.