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Cabling best practice Part 4: Length

04 May, 2021

Long cables between a motor and a VSD are not generally a good idea, but are sometimes unavoidable. ABB's Martin Richardson explains the factors to consider.

When designing a VSD system, the motor cables should be kept as short as possible to ensure EMC (electromagnetic compatibility) compliance. But sometimes, long motor cables are inevitable – such as for pumps in hazardous areas, where the drive must be placed outside the high-risk environment, or borehole pumps, where the borehole depth and location may require long cables.

What constitutes a long cable? It is all relative to the size of the VSD and the EMC environment. For a large VSD, it may be 200m or more, but for a small VSD, perhaps just 10m. The VSD manufacturer should be able to advise on cable type and maximum cable length and will usually publish information on cable lengths at which the VSD has been tested.

One major advantage of using a LV AC drive is that the cable only needs to be sized for the motor full-load current plus overload, without allowing for harmonic content. To avoid over-dimensioning when using traditional cable-sizing tools, a resistive load should be selected, unless starting with a VSD is an option.

It is also necessary to work out what the volt drop will be with the planned cable length. A motor under VSD control will draw a current equal to the full-load current plus a possible margin due to any overload requirement, with an allowable volt drop of perhaps 4% through the VSD.

Consider, also, capacitance coupling, especially at high frequencies, which will build up between the conductor and the screen when using long motor cables. A cable acts as a distributed capacitor and the longer it is, the higher its capacitance. Capacitance coupling in a long cable can cause a substantial current in the earthed screen, which may result in a level that overloads the drive’s internal EMC filter components or chokes. The installation may still work, but not as expected.

The type of cable is also important. In an industrial environment, steel-wire-armoured cable is generally adequate to maintain the Faraday cage needed for EMC compliance. The cable needs to be correctly glanded and connected to the main earth point of the AC drive. Little difference is seen between three- and four-core cable up to 10mm2, or even 16mm2, for runs of up to 30 or 40m. Beyond this, however, a three-core cable with a separate potential equalising is recommended.

When designing for long lengths, the earthing of the system is possibly the most important point. The interference should return to its source directly, rather than via stray paths. Multiple earths are highly effective in restricting touch potentials for personnel protection, but less effective for reducing noise. In fact, the cable tray, trunking or conduit can also radiate and should be grounded to the drive’s PE terminal.

It is important to use filters to counteract the effect of the long cable’s capacitance – du/dt filters will reduce the rate of change of the voltage pulses and the effects of long cables, although there will still be a finite limit and, in some cases, the du/dt filter cable limit itself may restrict the cable length. Sine filters are designed to produce a completely sinusoidal waveform, with no distance limitation. However, they also add a significant extra volt drop – a further challenge to overcome.

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