Ensuring a Smooth Transition from Linear Modulation, through Overmodulation, and into Six-Step

Six-step operation of a motor drive system can increase overall efficiency and maximize peak power output. It is generally achieved as an extension of an overmodulation algorithm. However, due to the nature of sampling and execution processes in a digital signal processor, if fully relying on existing state-of-the-art overmodulation techniques, then undesirable voltage imbalance and current oscillation can be introduced when transitioning into six-step operation. This is especially true during low pulse-ratio operation. This paper presents a method to achieve a smooth transition from linear modulation, through overmodulation, and into six-step for a 3-phase motor drive system. It is proposed that an averaged ideal volt-second solution coupled with manipulation of the pulse-width-modulation carrier waveform can be used to ensure a smooth transition.