Design of dual purpose no voltage combined windings for bearingless motors

A winding design approach is proposed to create a single motor winding which is able to produce both radial force and torque. This approach can be used to design new bearingless motors as well as to transform conventional motor designs into bearingless motors by simply modifying the winding end-connections. The resulting winding has two sets of terminal connections: one for torque and one for suspension. The suspension terminals experience no motional-EMF when the rotor is centered, which means that the suspension drive can have a low voltage rating and that rotor vibrations can be passively dampened by simply short-circuiting the suspension terminals. Bearingless motors that use these so-called 'dual purpose no voltage windings' can be designed to have higher torque density and lower losses associated with the magnetic suspension operation than traditional bearingless motors which utilize separate torque and suspension windings. It will be shown that many popular winding designs, including fractional-slot and concentrated windings, can be realized as dual purpose no voltage windings. The proposed approach applies to traditional p± 1 polepair bearingless motors as well as the bearingless consequent-pole and ac homopolar motors. Fractional-slot motor winding theory is used to derive the new winding requirements and a generalized design procedure; example designs are explored through finite element analysis and experimental results from a hardware prototype of a bearingless ac homopolar motor.