Definition and acquisition of CM and DM EMI noise for general-purpose adjustable speed motor drives

Separating conducted EMI noise into different modes, common mode (CM) and differential mode (DM), is important to the appropriate application of emission reduction techniques. While the CM/DM separation is well defined and understood for the single-phase or DC system, the same cannot be said for three-phase converter systems, common for general-purpose adjustable speed drives (ASD). Based on the study of CM and DM propagation characteristics of a three-phase diode-front converter, this paper identify different noise modes for different front-diode conducting patterns. The impact on EMI filter components by these noise modes is summarized. Finally, a time-domain based method is proposed to separate and acquire CM and DM noise components for the diode-front three-phase systems. Simulation and experimental verifications are presented.