Reduced Axial Length Pole-Specific Rotor for Bearingless Induction Machines

The induction machine has proven to be one of the most difficult motor technologies to transform into a bearingless motor due to currents that are induced in the squirrel cage rotor by the suspension field. This paper develops a new rotor for use in a bearingless induction motor to solve this problem. The rotor is described as 'pole-specific' because its winding couples to the motor field but not the suspension field. This new structure uses a common end-ring to reduce the rotor's axial length, solving rotor dynamic challenges present in previous efforts to develop a pole-specific rotor. This paper presents the new rotor concept and leverages traditional cage rotor design theory to develop equivalent circuit models and a theoretical framework that reveals which combinations of poles and slots can be used to implement the new rotor. Experimental results validate the new rotor concept and demonstrate its performance improvement over the classical squirrel cage rotor. A case study is presented of a high performance 50 kW, 30,000 r/min design for an industrial compressor with a modeled efficiency of 96.8%.