Effects of Multi-Level Converters on Common-Mode Voltages in Induction Motor Variable Frequency Drives

Multi-level power electronic converters (ML-PECs) are a new breed of PECs that are constructed using series-connected switching elements to increase their voltage and power ratings. These PECs can offer several advantages, among which is the inherent ability to reduce harmonic distortion at low switching frequencies. Such a feature of ML-PECs can have a direct impact on common-mode voltages (CMVs) and common-mode currents (CMCs), which are experienced by variable frequency motor drives (VFDs). This paper analyzes possible effects of ML-PECs on CMVs and CMCs experienced by induction motor VFDs. The presented analysis aims to relate CMVs and CMCs with the switching frequency and number of levels over a wide range speeds and load torques. The analysis of ML-PECs effects on CMVs and CMCs is conducted using a 50$hp$ induction motor VFD, when operated by 3, 5, and 7 levels diode-clamped ML-PECs. Analysis results show that the inherent abilities of ML-PECs to reduce harmonic distortion can significantly reduce CMVs and CMCs in induction motor VFDs.