Multilevel inverter modulation schemes to eliminate common-mode voltages

TLDR

Conventional two‑level PWM inverters and commonly used multilevel inverter modulation schemes generate high‑frequency common‑mode voltages that can drive motor shaft voltages, bearing currents, and conducted EMI, leading to premature bearing failures and electronic equipment malfunctions. This paper proposes methods to eliminate common‑mode voltage in multilevel PWM inverters. The authors compare sinusoidal PWM and space‑vector modulation schemes, presenting detailed simulation results that contrast conventional and novel modulation strategies. Inverters with an odd number of levels produce zero common‑mode voltage by switching among specific states, thereby eliminating bearing currents and reducing EMI, as experimentally confirmed with a novel SVM approach on a conventional multilevel inverter.

Abstract

It is well known that conventional two-level pulsewidth modulated (PWM) inverters generate high-frequency common-mode voltages with high dv/dt. Similarly, commonly used multilevel inverter modulation schemes generate common-mode voltages. Common-mode voltages may cause motor shaft voltages and bearing currents and conducted electromagnetic interference (EMI). Premature motor bearing failures and electronic equipment malfunctions have been reported to be directly related to bearing currents and EMI. In this paper, approaches to eliminating common-mode voltage when using multilevel PWM inverters are presented. It is shown that inverters, which have an odd number of levels, will generate zero common-mode voltage by switching among certain states. Therefore, motor bearing currents will be eliminated and conducted EMI will be reduced. Both sinusoidal PWM and space-vector modulation (SVM) schemes are discussed and detailed comparative simulation results between conventional and novel modulation schemes are provided. The value of the proposed technique is demonstrated experimentally by applying the novel SVM approach to a conventional multilevel inverter.