동기 PWM을 적용한 인버터의 선형 영역 확장을 위한 정적 과변조 기법

초록

When AC motors used in applications such as railway vehicles and vacuum cleaners operate in the high-speed region, the fundamental frequency is relatively high compared to the switching frequency, resulting in a low frequency modulation index. Under these conditions, if asynchronous PWM with a fixed switching frequency is employed for voltage modulation, the switching frequency cannot be synchronized to an integer multiple of the fundamental frequency. As a result, subharmonics appear in the inverter output voltages and currents, leading to torque ripple, increased losses, and noise in AC motors. To address these issues, synchronous PWM is required to synchronize the switching frequency with an integer multiple of the fundamental frequency. In this case, an overmodulation method is essential to drive the inverter up to six-step mode, leading to enhanced output torque capability of AC motors. However, conventional overmodulation methods developed for asynchronous PWM are not suitable for use in synchronous PWM. Therefore, this paper proposes a steady-state overmodulation method applicable to synchronous PWM. The proposed method can extend the linear region of the inverter by achieving a unit voltage gain up to the six-step mode, thereby enhancing the output torque capability compared with conventional overmodulation methods. Its effectiveness was verified through computer simulations and experiments on a PMSM (Permanent Magnet Synchronous Motor).

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