Simple current control of permanent magnet synchronous machines for railway traction operating in six‐steps

Abstract

Due to the voltage limitation of the inverter DC‐link of railway traction systems, six‐step operations are widely used in the high‐speed region to improve the voltage utilization and increase the maximum fundamental frequency. The magnitude of the output voltage fundamental harmonic reaches the maximum for six‐step operations, while only the phase angle can be adjusted. This limitation makes it challenging to control the instantaneous current of permanent magnet synchronous motors (PMSM). This paper proposes a single d‐axis current regulator flux‐weakening control scheme to improve the current control over six‐step operations of PMSM without changing the inverter output voltage modulation ratio by controlling only one degree of freedom. The voltage in six‐step operations is generated by synchronous space vector pulse width modulation based on basic bus clamping strategy with an effective limitation of the switching frequency in the full‐speed region. A simplified model of the current regulator is developed for the analysis and design of the controller parameters. This paper presents clear rules for entering and quitting six‐step operations effectively to achieve a smooth transition between double current regulators and the single current regulator control. The numerical results are verified by experimental measurements on a 7.5 kW PMSM drive.