Voltage Sensorless Control of Split Capacitor for Three-Phase Four-Wire Motor System With Zero-Sequence Suspension Winding

This article aimed to simplify magnetically levitated motor system by implementing the voltage sensorless control of the split capacitor potential for a three-phase four-wire motor system with a zero-sequence suspension winding. In the system, the suspension winding for magnetic suspension is connected between the neutral point of the
Y
-connected three-phase motor winding and the midpoint of two capacitors of direct-current (dc) link, thereby enabling simultaneous control of a three-phase interior permanent magnet synchronous motor and an axial magnetic bearing (iron-ball magnetic suspension, in this article), with only six power switches. However, the split capacitor potential is unbalanced by the dc component of the zero-sequence current, owing to the offset detection error of the current sensor and the bias current of the magnetic suspension. For suppressing the imbalance, we propose a voltage sensorless control. In the proposed method, the split capacitor potential is controlled without voltage detection by utilizing the zero-sequence voltage reference in the controller. The experimental results demonstrated that the split capacitor potential was successfully balanced while the iron-ball magnetic suspension and motor drive were performed. For further improvement of voltage sensorless control, a simplified control system was configurated with a filter controller based on an equivalent block diagram. The results indicated that the use of the filter controller improved the positioning accuracy of the magnetic suspension for the sensorless control system.