woundrotormotor

Wound rotor motor, also known as wound-rotor induction motor, is a type of three-phase AC induction motor in which the rotor windings are wound and connected to external circuits via slip rings and brushes. This construction contrasts with squirrel-cage rotors, where the rotor is a closed conductive cage. The external rotor circuit can include resistors or other apparatus to adjust torque characteristics.

During startup, the external resistors are inserted into the rotor circuit, increasing rotor resistance and producing a high starting torque while limiting stator current. As the motor accelerates and slip decreases, the external resistance is reduced or bypassed, allowing normal operation. In some designs, resistors can be gradually reduced to provide smooth acceleration. Dynamic braking can also be employed by routing rotor current through the external circuit to dissipate kinetic energy.

Advantages include high starting torque, strong low-speed torque, and controllable acceleration. Disadvantages include higher maintenance due to slip rings and brushes, energy losses in the external resistors, and typically lower efficiency at running speed compared with modern squirrel-cage motors. The need for external resistors and associated hardware makes the drive more complex and potentially more costly.

Applications are found in heavy starting-load scenarios such as cranes, hoists, rolling mills, and large conveyors, where high starting torque and torque control at low speeds are important. In recent decades, variable-frequency drives have largely supplanted wound-rotor solutions for new installations, but wound-rotor motors remain viable in very large or specialized applications where their torque characteristics are advantageous.