Design and Modeling of the Bearingless Induction Motor

The bearingless version of the induction motor (IM) has seen little consideration as a high performance, medium to high power motor. This paper investigates and develops design and performance evaluation techniques that are necessary to systematically study and design the bearingless IM. Key differences in the design considerations of the bearingless IM from that of the classical line-fed or inverter-fed IM are first explored and a candidate design (4 poles, 15,000 rpm, 50 kW) is created using a modified analytic design approach. Finite element analysis (FEA) is used to explore the impact of the rotor topology (squirrel cage or pole-specific rotor winding) on the machine performance. It is shown that for low speed designs, the traditionally-used cage rotor can yield acceptable performance. However, the cage rotor is found to not be a workable solution for high speed designs (an example design operated at 500 Hz requires 20% of rated stator current to merely support the shaft weight). Finally, several FEA techniques are proposed and evaluated based on their ability to rapidly and accurately evaluate the bearingless IM design performance. This investigation will provide the necessary foundation for future efforts in designing and optimizing the bearingless IM.