Common Mode Current Measurements in Traction Systems for Electric Vehicles

The market for partly or fully electrified vehicles is expanding fast. The number of sources and loads that are connected to the vehicles traction voltage systems (TVS) increase and thus also the ElectroMagnetic Compatibility (EMC) requirements on these sources and loads. These requirements should make sure that neither function nor lifetime of any source or load is severely affected by another. The EMC requirements include both Common Mode (CM) and Differential Mode (DM) voltages and currents created by the modulation of the various power electronic converters involved as well as intentional and parasitic impedances of the TVS and reach up to at least 10 MHz for CM and 100 kHz for DM. This article presents a theory for the dimensioning of CM-capacitances in an Electric Machine Drive (EMD) that is confirmed by measurements on a commercial electrical Volvo truck. The conclusions point out a recommendation for selection of the CM-capacitances of the EMD vs. the CM capacitance of the Electrical traction Machine (EM). As a rule of thumb the recommendation is that the CM-capacitance of the EMD is around 50 times the CM-capacitance of the EM and mounted inside the EMD to avoid big loops of CM currents in the vehicle.