An estimation of the rotor displacements of bearingless motors based on a high frequency equivalent circuits

Bearingless motors have been developed for canned pumps, blood pumps, spindle drives, and computer disk drives. The bearingless motor is a motor with magnetically integrated bearing functions. The rotor is suspended with no mechanical contact by magnetic force. The magnetic suspension of the bearingless motors, however, requires displacement sensors to detect the rotor displacements. A self-sensing bearingless motor is an effective solution to reduce cost and shorten a shaft length. In this paper, a self-sensing method based on the self-inductance variations is presented. This method is more suitable for PWM inverters. However, as the frequency is increased, the detected voltage, which includes the rotor displacement, is decreased. The frequency characteristics of the impedance are also discussed with a high-frequency equivalent circuit. It is round that the iron permeability reduction is a major cause of a decrease in the detected voltage at a low frequency. In addition, the iron loss resistance has a significant influence at a high frequency.