Woundrotoren

Woundrotoren, commonly referred to as wound-rotor motors, are a class of induction motors in which the rotor carries windings that are connected to the external circuit via slip rings and brushes. This arrangement contrasts with squirrel-cage rotors, where the rotor winding is short-circuited within the rotor. The external connections allow the rotor circuit to be inserted with resistors or power electronics for control.

Construction and operation

The stator of a woundrotor motor creates a rotating magnetic field when powered. The rotor consists of laminated iron cores with windings that are connected to external terminals through slip rings. By inserting resistors in series with the rotor windings, engineers can shape the rotor current, achieving higher starting torque and a wider range of speed control. As the motor accelerates, resistance can be reduced (or bypassed) to improve efficiency and allow near-synchronous running.

Control methods and braking

Rheostatic starting is the traditional method, using external resistors to limit starting current and boost starting torque. Part-winding starting uses two rotor winding groups to reduce peak current. Dynamic braking can be accomplished by shorting the rotor windings through braking resistors when the motor is running or stopping.

Advantages and limitations

Woundrotoren provide high starting torque and adjustable speed, useful for heavy-load applications. They allow smooth acceleration and precise torque control. However, they require more maintenance due to brushes and slip rings, have lower efficiency because energy is wasted in resistors, and involve a more complex drive system compared with squirrel-cage motors.

Applications

Historically used in cranes, hoists, hoisting gear, conveyors, rolling mills, and other heavy-load drives where high starting torque is essential. With modern power electronics, many new installations favor alternative methods, but woundrotoren remain in use in certain high-torque, variable-speed applications.