Wound Rotor motors are sometimes referred to as "Secondary Resistance Starting". The stator windings of a wound-rotor motor are identical to those of a standard squirrel-cage motor. A wound-rotor motor does fall into the category of induction motors, however, the conductor bars of the squirrel-cage rotor are replaced with three phase windings wound with the same amount of poles as the stator.

 

The wound rotor windings terminate in slip rings mounted on the rotor shaft. Carbon brushes ride on the slip rings, and during start-up they are externally connected in series to a three-phase resistor bank. (One resistor for each phase, wye connected).

 

Each set of external resistors are shorted out simultaneously in one or more steps as the motor comes up to speed. Wound-rotor motors were one of the first types of motors to allow variable-speed operation. By placing high wattage variable resistors in series with the rotor windings, you could effectively control the speed of the motor.

 

Wound-rotor motors are especially useful because they are able to deliver high starting torque without overloading the electrical supply system.