Interleaving angle variation analysis for variable frequency PWM drives

Generally, drive manufacturers include input filters and/or output filters to meet system requirements. Depending on damping, these filters have the potential to introduce resonant conditions. Often the switching frequency changes with the rotational speed of the drive machine leading to the possibility that switching harmonics align with the resonant peak of the filters. As the frequency of the drive decreases, the number of pulses per half cycle of the output fundamental can increase to maintain a reasonably constant switching frequency. For drives that utilize interleaving, another possibility investigated in this paper is to dynamically alter the phase angle between interleaved converters. This allows selective elimination of harmonics near the filter resonance while reducing losses due to a lower switching frequency. Typically, the phase shift is based on the number of system inverters and is used to increase the effective switching frequency. The phase-shifting strategy in this paper differs from the traditional method by targeting specific harmonics of interest. This paper¹ describes the details of implementing strategies to dynamically alter the phase shift between interleaved converters to eliminate harmonics.