Shielded cable and good grounding give the high-frequency current generated by the inverter a controlled return path. The inverter's fast switching can turn an unshielded motor cable into an antenna that emits interference. The result can be faults in sensors and communication lines and unwanted tripping of residual current devices. Without a proper return path, this current finds its way through the machine frame and the bearings.

Points to observe in practice:

  • Use a shielded and, if possible, symmetrical motor cable between inverter and motor.
  • Bond the shield all around over a wide area with a gland or clamp at both the inverter and motor ends; do not leave long twisted pigtails.
  • Make sure the protective earth (PE) connection is continuous and low-impedance.
  • Route the motor cable in a separate duct from signal and sensor cables.
  • Choose the cable gland to suit the cable diameter and IP class, and close unused entries with blanking plugs.

In an inverter installation, too, the cable cross-section is chosen by rated current and line length. For checks before the first start, the article on motor commissioning and first start-up checks is a useful guide.