Common-mode current attenuation techniques for use with PWM drives

Insulated gate bipolar transistors (IGBTs) have become the unanimous choice for power semiconductor switches in adjustable speed drives (ASDs), also known as variable frequency drives (VFDs). The most common modulation strategy adopted for motor control in ASDs is pulse width modulation (PWM). In general, the output is modulated at a carrier frequency ranging from 1 kHz to 20 kHz. Recent experience with PWM drives have shown that there exists two distinct application issues: (a) common-mode current and shaft voltage due to common-mode voltage; and (b) line-line overvoltage across stator windings in cases where the distance between motor and the inverter is larger than the critical lead length. The high carrier frequency particularly in small sized VFDs (up to 75 kW), along with fast rise and fall time of the IGBTs employed results in steep fronted common-mode voltage which causes nontrivial common-mode or ground currents to flow. When the distance between the motor and the VFD is long and there exists a mismatch in the cable and motor surge impedance, there is voltage amplification at the motor terminals. This paper focuses on the issue relating to common-mode noise. A new scheme to attenuate the common-mode current is presented. Experimental results showing the effectiveness of the proposed solution is presented.