Common Mode and Differential Mode Current Repartition in Inverter-fed Motor Drives

N. Idir, J.J Franchaud, and R. Bausière (France)

Keywords

Inverterfed Motor Drives, Power Switching, ElectroMagnetic Interference (EMI), Common Mode Current, Differential Mode Current, Spreading Impedance.

Abstract

The use of high frequency switching power devices in PWM inverter induces high voltage variations (dv/dt) that result in exciting parasitic elements into the power circuit, leading to conducted and radiated emissions at high frequencies. Thus the advent of these devices has generated several unexpected problems, such as premature deterioration of ball bearings and high levels of electromagnetic emissions, caused by circulation of parasitic currents. The conducted emissions are propagated in Common Mode (CM) and Differential Mode (DM). The disturbing behaviour of the inverter can be reduced to 2 commutations modes: commutation from diode to transistor associated with diode reverse recovery, and commutation from transistor to diode where highest dv/dt may occur. Figure 1 : PWM inverter-motor drive structure The value of voltage rate-of-rise during the commutations of the inverter depend on the kind of commutation: thus the spectral contents of the disturbing currents will be different. The DM current flowing between the inverter and the motor depends on the following elements: impedance of the cable between inverter and motor, and differential impedance of the asynchronous motor. As CM current paths cross the input rectifier, path impedances will thus depend on the state of the rectifier diodes: when a diode is in the on-state, it behaves as a low resistance and when it is in the off-state, its stray capacitance lets the CM current flow. It thus appears difficult to determine the relative contribution to disturbances of CM and DM currents generated by power converter commutations [2] [4]. In this paper, the repartition of conducted emissions into CM and DM currents is analysed, showing that both propagation modes may interfere.

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