Modelling and Torque Estimation of an Inverse Connected Cascaded WRIM in Rotor Reference Frame Operated with a Constant V/f Ratio

Wound-Rotor Induction Machines (WRIM) are extremely popular for high-power applications both as industrial motor drives and generators for harvesting wind-power energy. However, these machines suffer from a major drawback due to the use of slip-rings which require frequent replacements because of mechanical damage. Cascaded WRIMs are slowly becoming popular as an alternative for the conventional WRIM drives. In the inverse connected cascaded configuration, the rotors of two WRIMs are both mechanically coupled and electrically connected in reverse phase-sequence. This completely eliminates the need to use slip-rings as the rotors are not required to make electrical connections with any stationary circuit. However, both the modelling and control complexity of such a machine increases considerably. This paper aims to provide a simple model for an inverse connected cascaded wound rotor induction machine (ICC-WRIM) in the rotor frame of reference (FOR). The choice of the reference frame is justified in the paper. Further, a torque estimation technique using only the sensed stator currents is proposed. The validity of the developed model and torque estimation technique is established experimentally on a 7.4 kW laboratory prototype of an ICC-WRIM.