EMC Question of the Week: January 7, 2019
In an electric vehicle, the three phase wires connecting the motor drive to the traction motor should
- be unshielded
- be individually shielded
- have a shield around all three wires
- both b. and c.
Answer
The best answer is "c." The high dV/dt and dI/dt of the waveforms on these wires require shielding to meet automotive EMC requirements. However, individually shielding each of the phase wires forces a large amount of high-frequency differential-mode current to return on the shields. This creates problems when attempting to terminate these shields at both ends. Vehicles with individually shielded phase wires require expensive shielded connectors and tend to have significant inter-system and intra-system EMC problems.
When all three wires are enclosed in the same shield, only the smaller common-mode currents return on the shield and the impedance of the shield termination is far less critical. Vehicles with all three phase wires contained within a single shield are much more likely to meet their EMC requirements without unnecessarily expensive connectors.
Of course, eliminating the phase wires all together, by packaging the motor drive within the motor housing, is the ideal situation from an EMC standpoint. In this case, it is important to monitor where high-frequency differential-mode currents will flow on the housing in order to ensure that these paths are relatively unimpeded.
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