A New Energy Recovery Scheme for Doubly Fed, Variable Speed Induction Motor Drives

Speed control of wound rotor induction motors with the slip-power feedback to the supply utilizing a converter cascade is used in many industrial applications requiring high power and a relatively narrow speed range. As the rotor voltage is typically lower than the supply voltage due to the narrow range of slip frequency, a step-up transformer must be inserted between the inverter and supply in order to improve power factor and reduce line harmonics. In this paper, a new concept is presented that eliminates the need for a transformer by feeding back slip power to the stator winding directly. The selection of feedback point on the stator winding, the equivalent circuit, and the main parameters of this system are discussed. Test results on a wound rotor induction motor that indicate that this system also has good performance in addition to its poten- tial for lower cost are shown.