Nihon Kikai Gakkai ronbunshu (Oct 2020)
DC current control of a zero-sequence load using a diode rectifier for a three-phase four-wire motor drive system
Abstract
This paper describes the active DC current control of a zero-sequence load using a diode rectifier for a three-phase four-wire motor drive system, intended for application to (for example) a three-phase motor with an axial magnetic bearing. This motor drive system requires a three-phase inverter for rotation and a single-phase H-bridge inverter for magnetic suspension. To reduce the number of system components, the authors propose a unique current control method, where the suspension winding is connected between the neutral point of the Y-connected three-phase motor winding and the middle point of two split capacitors of the inverter. Such a wiring connection is called a three-phase four-wire system, and the current flowing in this suspension winding is defined as a zero-sequence current herein. Hence, only one three-phase inverter is required for the control of the three-phase motor current and single-phase magnetic suspension. However, the split capacitor voltages are unbalanced when DC current is provided to the zero-sequence load of the suspension winding because the capacitor voltage is proportional to the integral of the zero-sequence current. To supply DC current to the suspension winding and balance the split capacitor voltages, a novel circuit topology is proposed, where the diode rectifier circuit with suspension winding is connected to a zero-sequence load. When an AC current is provided to the zero-sequence load, a rectified current with a DC component is provided to the suspension winding, whereas the AC current flows in the split capacitors, thereby balancing the capacitor voltages. To verify the proposed DC current control, circuit simulations and experiments were performed. The experimental setup comprised a three-phase permanent magnet synchronous motor and a one-degree-of-freedom magnetic levitation. The test results show that the proposed method can provide a DC current to the suspension winding and control the magnetic levitation with balanced split capacitor voltages.
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