What is maximum length of cable run allowed between variable speed drives and motors?

Long cables between the motor and the drive should generally be avoided, but sometimes lengthy cable runs are unavoidable. When using a long motor cable, it is important to size it correctly, to allow for the volt drop and to consider the impact on EMC performance.

 

When designing a drive system, the motor cables should ideally be kept as short as possible. But sometimes, long motor cables are inevitable, typically when the motor-driven application, is located in an environment where it is not possible to place a drive. This includes applications such as pumps and fans in hazardous areas, where the drive has to be placed outside the hazardous area. Long cables are also common with borehole pumps, as well as with the fin fans used in power station cooling towers. In this case, the drive is located in a clean area some distance away.

So what constitutes a long cable? It’s all relative to the size of the drive, and to some extent the EMC levels required. For a large drive, it may be 200 or 300 metres, but for a small drive, perhaps just 5 or 10 metres.

“We once installed a 3 km long “downhole” motor cable for electro submersible pumps at the Wytch Farm oilfield in Dorset,” recalls Geoff Brown, senior drives consultant at ABB.

The drive manufacturer should be able to advise on maximum cable length and will usually publish information on the cable lengths to which the drive has been tested.

For help with correctly sizing your cable run send application details to sales@quantum-controls.co.uk

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