May 2021

Drives & Controls & BACK TO BASICS n SPONSORED BY 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. W hen 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 10mm 2 , or even 16mm 2 , 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. n Cabling best practice Part 4: Length COMMENT n WILL OPTIMISM BE CRUSHED BY REALITY? Spring may have taken its time to arrive this year, but that doesn’t seem to be dampening a sense of hyper-optimism that appears to be breaking out across many parts of the UK’s manufacturing and automation sector. No doubt, the rapid rollout of the vaccination programme in the UK – especially compared to many other advanced countries – has contributed to the feeling that things can only get better from now on. This probably goes some way to explain the remarkable finding in a recent Confederation of British Industry survey that UK manufacturers are the most optimistic that they have been for almost 50 years (see page 6). The survey of 288 manufacturers found that many are planning to increase investments in plant, machinery and r&d, take on new employees and spend money on training. There were more positive signs in last month’s IHS Markit/CIPS PMI (Purchasing Managers Index) which hit a 121-month high and found optimism in the manufacturing sector to be at its highest for seven years (not quite half-a-century, though). So, are we seeing the start of a real recovery? Certainly there is pent-up demand that is now being released, as projects that had been put on hold during the pandemic are finally given the go-ahead. But there are still some nagging doubts about how rosy the future will really be. The vital automotive sector will take time to recover from its doldrums, and we have yet to see the full impact on the automation sector of the global semiconductor shortage. Some manufacturers, such as theWelsh VSD manufacturer Invertek, seem to have coped well with the shortage but some semiconductor suppliers are now quoting lead times of well over a year. There is also the possibility that prices might have to rise (some already have) as a result of the soaring costs of all sorts of materials, from copper and steel, to plastics and even cardboard, which is the victim of our increased reliance on Amazon and other online delivery services during lockdown. If prices do rise across the board, how will this affect the pace of recovery? Another imponderable is the Brexit effect. Are the difficulties experienced so far by some exporters and importers really just “teething problems”and will trade with the EU settle down to a new normal? Or will the increased bureaucracy, paperwork and other obstacles, make some UK companies decide to abandon doing business with Europe – or to set up operations in the EU with a consequent loss of jobs in the UK? If our emergence from the pandemic continues to go smoothly, the manufacturing sector’s optimismmay be borne out in reality. Let’s hope so. We could all do with some good news. Tony Sacks, Editor