Abstract
A multiphase motor has several major advantages, such as high reliability, fault tolerance, and high power density. It is a critical issue to develop a reliable and efficient multiphase motor drive system. In this paper, a transformerless voltage source converter-based drive system for a medium-voltage (MV) multiphase motor is proposed. This drive converter employs cascaded H-bridge rectifiers loaded by H-bridge inverters as the interface between the grid and multiphase motor. The cascaded H-bridge rectifier technique makes the drive system able to be directly connected to the MV grid without the phase-shifting transformer because it can offset the voltage level gap between the MV grid and the semiconductor devices, provide near-sinusoidal AC terminal voltages without filters, and draw sinusoidal line current from the grid. Based on a digital signal processor (DSP), a complete improved Phase Disposition Pulse Width Modulation (PD-PWM) method is developed to ensure the individual DC-link capacitor voltage balancing for enhancing the controllability and limiting the voltage and power stress on the H-bridge cells. A downscaled prototype is designed and developed based on a nine-phase motor. The experimental results verify the excellent performances of the proposed drive system and control strategy in steady-state and variant-frequency startup operations.
Highlights
The origin of multiphase motor drives dates back to 1969 when a five-phase voltage source inverter-fed induction motor drive was proposed [1]
The proposed drive system can cover the main functions of the phase-shifting transformer in traditional multipulse MV drive systems: (1) harmonic cancellation provided by line-side cascaded H-bridges; (2) proper voltages for the motor fed by H-bridge inverters; (3) electrical isolation provided by the stator windings of the multiphase motor
It is expected that multiphase motor drive systems will be used widely in high-power MV and HV applications in the future
Summary
The origin of multiphase motor drives dates back to 1969 when a five-phase voltage source inverter-fed induction motor drive was proposed [1]. Unlike conventional MV drive systems using the phase-shifting transformer providing electrical isolation, the proposed multiphase motor drive system achieves electrical isolation by its independent and isolated stator windings because the 3k output voltages of the drive converter are not isolated electrically from each other. The voltage rating of each phase of the multiphase motor is only 1/k of the input grid voltage, the insulation should be designed carefully as different stator windings may work at different electric potentials which depend on the position of the basic power cells their connected. The proposed drive system can cover the main functions of the phase-shifting transformer in traditional multipulse MV drive systems: (1) harmonic cancellation provided by line-side cascaded H-bridges; (2) proper voltages for the motor fed by H-bridge inverters; (3) electrical isolation provided by the stator windings of the multiphase motor.
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