Current Regulation in Parallel Combined Winding Bearingless Motors

To improve power density and motor performance, new bearingless motor topologies combine torque and magnetic suspension coils into a single winding. Of these topologies, the parallel dual-purpose no-voltage winding is advantageous from the bearingless drive perspective because it requires the least amount of hardware. However, this topology can result in undesirable current controller performance from cross-coupling effects between the suspension and torque operation. This paper investigates these cross-coupling effects using rotating reference frame theory to derive relevant system disturbance transfer functions. The nature of this coupling is explained in relation to the machine and control parameters (inductances, resistances, controller gains) to provide insights for bearingless machine and control designers. The paper proposes and simulates different compensation techniques to minimize or eliminate the cross-coupling. It is shown that with careful machine design or with proper feedback control compensation, the motor controller can be implemented as a conventional motor drive, without knowledge of the magnetic suspension system. Finally, experimental validation is provided via two prototype machines.