Control of Variable Frequency Drive PWM to Mitigate Motor Overvoltage Due to Double Pulsing in Reflected Wave Phenomenon

摘要

It is well understood that variable frequency drives (VFDs) can cause overvoltage up to twice the DC bus voltage (2 pu, where 1 per unit is the DC bus voltage) at the machine winding terminals, especially if long power cables are used and/or fast rise and fall times are implemented. If the voltage oscillation is not fully decayed when the next pulse arrives, double pulsing occurs and the overvoltage can go up to 4 pu in one switching event. To prevent the damage of the 4 pu overvoltage to the electric machine insulation system, filters are commonly used between the VFD and the machine. The filters, however, add significant cost and size to the system. The higher dv/dt associated with the increasingly popular wide-bandgap devices requires even larger and more expensive filters to be installed. This paper proposes a new PWM control method, the double pulsing offset technique (DPOT), to restrain the overvoltage peak-to-peaks within 2 pu regardless of the switching frequency or dV/dt of the power semiconductor devices. The need to install filters, therefore, is potentially eliminated with the implementation of DPOT. This control scheme was validated using an accurate reflected wave phenomenon simulation model using finite element analysis (FEA). It was also fully implemented online and offline using a DSP and a SiC-MOSFET -based inverter. The experimental results with various operating conditions of the inverter are presented and analyzed. The results show that DPOT is successful in eliminating double pulsing even in harsh operating conditions such as high dV/dt, long cable length, and frequent polarity reversals.