Have you ever had issues with variable speed drives (VSDs) creating interference to other equipment?

If you've ever had electronic equipment in close proximity to VSDs fail, one of the possible causes could be radio frequency interference (RFI). Because of the high frequency switching of the IGBTs in the VSD's inverter section, VSDs generate RFI. It is therefore critical to use the correct type of motor cable and to earth the motor cable shield at both the VSD and motor end.

The motor cable shield on VSD circuits should be earthed at both the VSD and the motor end, which may surprise you. This is because on control circuits or communication networks, you only earth the shield at one end to prevent circulating earth currents from corrupting the control signal.

On VSD motor cable circuits, however, you want to provide electrical noise with a low impedance path to earth to prevent RFI emissions, and the fact that you get circulating earth currents is irrelevant because we are dealing with power, not control signals.

The next step in avoiding RFI is to use the proper type of motor cable, which ABB recommends: three phase conductors and a concentric or otherwise symmetrically constructed PE conductor, and a shield.

ABB recommends earthing the motor cable shield by twisting together shield wires into a bundle (the bundle length must be less than five times its width) and connecting it to the terminal marked earth on the VSD next to where the motor cables are connected.

The motor cable shield must be earthed 360 degrees with an EMC cable gland at the motor end, or the shield wires must be twisted together.