Improving the reliability and EMC performance in speed regulating system of asynchronous motor

This paper proposes a strategy based on three-phase four-leg structure that can improve either reliability or EMC performance in speed regulating system of asynchronous motor. In the strategy, if the system is working orderly, the fourth leg functions in balancing the output voltage so as to reduce the common mode (CM) voltage of the system. During this process, the use of state exchange method not only breaks through the limitation of SPWM modulation index to avoid the extremely low utilization ratio of direct current (De) voltage, but also reduces the total harmonic distortion factor (THD) of the output voltage. If an inverter leg of system is in failure, the fourth leg acts as the redundant leg to take the place of that one. The transitory open-loop control is adopted to reduce the overshoot current and to switch leg smoothly. Thus the system can run uninterruptedly in the fault tolerant operation mode and the holistic reliability can be improved markedly. The effectiveness of the strategy is proved by simulation and experiment results.