Modeling conducted emissions in servo drives

This paper presents a modeling approach to simulate conducted emissions in PWM inverter driven PMSM servo drives to help designers to get a deeper understanding of interaction of common-mode noise sources and various circuit elements in a servo drive system. Conducted emissions at the ac mains input are caused by coupling and leakage of high frequency common-mode currents through complex return paths in the system. Common-mode currents are caused by switching of PWM signals at high slew rates that charge and discharge various parasitic elements in a drive system such as motor cable capacitance, motor and other common-mode elements inside the drive hardware. Identifying dominant frequencies above the allowed limits per IEC61800-3 or CISPR 11 for Industrial equipment before a product is fully designed through simulation will help to reduce costly re-spins and identify mitigation techniques upfront. This paper describes how common-mode circuit elements are modeled in frequency domain through measurements and presents a simulation model for the commonmode circuit and concludes with validation of simulation results with experimental results obtained in an actual servo drive system.