Best practices for EMC cables and glands used with variable speed drives
Author: John Smith Date Posted:2 May 2017
Variable speed drives (VSD) are used extensively in modern times, not only for speed control of motors, but also for efficiency. The downside is that the square wave DC output of a VSD causes electromagnetic interference (EMI) to occur. This occurrence can cause electrical and electronic devices nearby to malfunction or fail.
Screened cables are considered the best method of controlling the electromagnetic interference associated with VSD’s – RFI filters are generally insufficient, especially when cable runs are long.
However, screened cables need to be installed correctly to minimise the risk of EMI problems.
We’ve put together a list of installation tips to help you ensure your VSD setup is compliant and trouble free.
- Braided copper screens are most effective type of motor cables screen, but they must have screen coverage of at least 80%.
- Motor cable screens must be terminated at both ends, and must be continuous from the motor to the VSD.
- Screens should be terminated with 360° contact with the gland. The gland should be circumferentially earthed to the gland plate (for example with a barbed lock nut).
- Ensure equipment, screens and enclosures are connected to the system earth.
- When securing earths and screens, use star washers and barbed locknuts to ensure low impedance connections.
- Symmetrical cables with split earths should be used where possible.
- If conduit is used, use conduit with a plastic covering to ensure the right path to earth rather than through other metal items (trays, etc.) that come into contact.
- Communications cables nearby should be twisted pair, shielded, with the shield terminated one end.
- Communications cables and motor cables must be segregated; where crossing is unavoidable this must be at an angle of 90°.
- Communications cables and motor cables should be routed on separate trays.