Positioning accuracy improvement for a magnetically levitated system using zero-sequence current of a permanent magnet motor

We have proposed a driving method for both a three-phase permanent magnet motor drive and one-degree-of-freedom controlled magnetic suspension using only one three-phase inverter. The suspension winding is connected between the neutral-point of the Y-connected motor winding and the middle point of the voltage sources. Hence, the suspension force is actively controlled by the zero-sequence current. The experimental result showed that the vibration of the magnetic suspension increased under the loaded condition of the motor compared with that under no load. The vibration mainly includes the fundamental and 3rd components of the driving frequency, 1 f and 3 f. This paper focuses on the positioning accuracy improvement of this magnetic suspension system. We have found that the vibration 1 f is caused by the detection error of the current sensor and the three-phase unbalanced resistance and inductance values, and the vibration 3 f is caused by the PWM drive. To improve the positioning accuracy, we investigated both the current detection method and the observer-based voltage disturbance compensation. The experimental results demonstrated that these compensations improved the positioning accuracy of the magnetic suspension. © 2019 The Institute of Electrical Engineers of Japan.