Improved overmodulation technique for enhancing torque capability of inverter-driven AC motors

Abstract

The overmodulation operation of a three-phase inverter can enhance the output torque capacity of the AC motors. In this case, the enhancement varies depending on the overmodulation schemes applied for the overmodulation operation. Steady-state overmodulation schemes are usually employed to give the unit voltage gain up to the six-step mode of an inverter. However, these schemes lead to a slow transient response of the output torque and require a complex algorithm for implementation. On the other hand, dynamic overmodulation schemes can achieve a rapid transient response of the output torque and are simple to implement. However, inverters using these schemes cannot work up to the six-step mode since the voltage gain of the inverter is less than one in the overmodulation region. In this paper, a technique is proposed that can increase the DC voltage utilization of an inverter by improving the voltage gain in dynamic overmodulation operation. An inverter with dynamic overmodulation using the proposed technique can work up to a six-step mode. Hence, AC motors can obtain enhanced torque capacity during flux-weakening operation. Consequently, an improved transient torque response can be achieved in the high-speed region. From simulation and experimental results, it was validated that the proposed technique can be used for dynamic overmodulation operation for not only the continuous PWM but also the DPWMs (discontinuous pulse width modulations).