EMC in Electric Vehicle Traction Systems

Due to the powertrain electrification in electric and hybrid vehicles, enormous challenges arise considering the vehicle’s electromagnetic compatibility (EMC). Compared to vehicles with internal combustion engine in electric vehicles power electronic traction systems in the 100kW range using battery voltages of up to 800V are implemented. These drive systems produce high broadband electromagnetic emissions (EMI), affecting the vehicle’s inner EMC. In order to provide a safe and stable parallel operation of the 12V wiring system and the high volt (HV) traction system, the HV system is built up completely isolated from the 12V System (IT-Network) and thoroughly shielded. Unfortunately, as the motor’s shaft, which is directly connected to the 12V ground is not integrated in the shielding concept, it represents a weak point. The broadband emissions generated by the drive inverter flow via the shaft into the gearbox and further into e.g. wheel sensors or other parts of the 12V system (see Fig.1). Another component, which is directly connected to the 12V side of the inverter, is the resolver, which placed directly at the shaft. Thus, the high frequent components of the motor current, can directly couple into the inverter’s 12V side by the resolver cable.

Fig. 1 Electric motor with internal coupling paths for high frequency disturbance currents

Furthermore, current measures to increase the efficiency of electric traction systems like increasing the voltage of the traction battery or decreasing the switching time of the power transistors result in an increase on the system’s EMI. Thus, additional shielding and filtering measures have to be realized to fulfil the EMC requirements.

In order to meet the challenges considering the electric vehicles EMC currently the new components, new measurement methods and new requirements considering the EMI and the immunity of the HV components are added to the automotive standards.

One of the setups presented in the standards is for EMI measurements on electric vehicle drive inverters. As presented in Fig.2, beside connecting a filtered supply voltage at the HV DC and the 12V port of the inverter, a three phase load has to be connected on the motor side. Currently, the original motor is connected to realize a normal operation mode during EMI measurements. To provide other operating point then no load operation during the test, a load machine, producing broadband EMI as well, has to be connected to the motor. As the test setup is placed in a shielded environment and there should be no impact of the load machine on the measurement, it is placed outside the shielded room. To transmit mechanical power a shaft feedthrough with a special bushing and filtering has to be realised, so that the shielding properties of the shielded environment is not degraded. Realising such a filtered feedthrough for mechanical power of 100 kW and rotating speeds of 15000 rpm is complicated to realize. To present an alternative, a passive load, modelling the high frequency impedance of the original motor, was developed. Using such a passive impedance network the complete measurement setup can be placed inside the shielded chamber. As all components are passive the load has no impact on the EMI measurement and in addition no rotating set of machine is needed. The fact that the complete setup can be placed inside the shielded chamber increases the flexibility of this concept.

Fig. 2 Setup for EMI measurements on electric vehicle drive inverters

Author of the article

Dr.-Ing. Sebastian Jeschke
Project Management Electric Mobility