A wound rotor induction motor has its rotor windings linked to slip rings that are fitted with brushes which enable external resistance to be applied to the motor. This unique property makes these kinds of motors able to vary their speed and torque.
A wound rotor induction motor possesses speed control properties. This arises due to the application of external resistance. Higher resistance reduces the rotor current, which consequently reduces the speed of the motor. Its velocity, altered as desired, leads to improved energy consumption.
Wound rotor induction motors are most effective in cases of very heavy loads that need smooth and steady acceleration. The high torque they offer at the start makes them suitable for weightier loads, and their speed control mechanism ensures steady acceleration. A typical application of these motors would be in elevators and conveyor belts.
During start up, wound rotor induction motors draw huge amounts of current, from 500 to over 1000 percent in large motors. In some cases, these starting currents can be overwhelming to the system and applying resistance not only reduces it but it also increases the starting torque of the motor. The resistance can then be effectively reduced and completely shorted out at maximum speed.