Variable Frequency Drive used for Speed Control and Slip Power Recovery.
A WRIM is a machine with a 3-phase wound stator that is usually connected directly to the power system. The rotor also has a 3-phase winding, usually connected in a wye (or star) circuit. The three terminals of the rotor winding are connected to separate slip rings, which are normally connected to a variable resistance such as a liquid rheostat or resistor bank, used for starting the motor. The speed of the motor can be changed by changing the rotor resistance, which changes the slip, rotor current, torque, and the speed.
In previous systems the power in the resistor was lost as heat. Later, speed control of wound rotor motors employed slip power recovery (SPR) controls for cost and energy efficiency reasons. These older implementations of SPR technology saved energy, but had disadvantages of low power factor operation and creating torque pulsations.
The use of state-of-the-art low voltage pulse-width modulated converters eliminates these disadvantages while retaining all the energy savings. This new implementation builds on standard variable frequency motor drives used in industry, with hardware which is very reliable and well supported, and is appropriate for new or existing motors.