What is a Wound Round Induction Motor (WRIM)?

Wound Round Induction Motor on Iron Ore Mine

Wound Rotor Induction Motor (WRIM)

 

Wound Rotor Motors consist of three-phase rotor windings. The ends are brought out to external terminals through a set of slip rings and an arrangement of carbon brushes. These motors are also called Slip Ring Motors due to this construction.

 

This type of motor is different from squirrel cage induction motors where rotor conductors are shorted on the rotor itself. Both motors are called Asynchronous motors due to the fact that slip is required to generate torque. 

Wound rotor induction motors have some attractive features compared to cage induction motors such as:

 

  • Starting characteristics for slip ring motors can easily be controlled by a variable external resistor connected to the rotor circuit.

  • A low starting current can be achieved while maintaining high starting torque to overcome load torque or for faster acceleration of high inertial loads (Low starting currents are essential for sites with weaker power systems such as remote mine site to avoid undesirable voltage drops in the power system whilst starting large motors)

  • The speed of motor can be controlled via an external resistor connected to the motor by adjusting the slip of the motor.

 

This can also be achieved by Slip Energy Recovery Drives (SER). SER drives are also known as Slip Power Recovery Drive (SPR) or Scherbius drives. With these drives, energy that would have been lost in the external rotor connected resistor can be recovered, allowing an energy efficient operation in a speed controlled wound rotor motor.