Drives&Controls & BACK TO BASICS n SPONSORED BY Mitigating long cable distances Cable runs in motor-drive applications should ideally be kept as short as possible, but in some situations long cable distances are unavoidable. Liam Blackshaw, ABB’s UK product manager for LV drives, discusses some of the mitigations that can be used to ensure that both drive and motor function optimally. In general, the less cabling you have between a motor and its drive, the better. But in hazardous environments, or in mining, pumping or offshore applications where the drive has to be situated away from the motor, long cable runs are sometimes unavoidable. The longer the cable, the higher its capacitance and impedance. There is also the impedance of the motor’s windings to consider – smaller motors tend to have higher impedances than larger ones. A very long cable paired with a small motor will lead to an impedance mismatch, which creates reflected waves in the cable. This can then lead to a voltage overshoot at the windings, and a brief voltage spike. This can be further compounded by the rise time of the IGBTs in the drive – a faster rise time will lead to a bigger overshoot. If the voltage overshoot exceeds the insulation rating of the windings then, over time, the insulation will start to break down, eventually resulting in motor failure. When it comes to cabling, how long is too long? That depends on the rating and frame size of the motor and drive, the length and type of the cabling, the application and the environment. For lower power applications the maximum can be a few metres. For larger applications, it can be hundreds of metres. The motor and/or drive manufacturer will specify the maximum cable length so, if in any doubt, contact them to make sure. Bear in mind also that multiple motors being operated from a single drive count towards the same cable length allowance, so two 50m cables in a multi-motor arrangement are equivalent to one 100m cable for a single motor. There are several ways to mitigate long cabling. The most common is to add an output filtering device between the drive and motor. This can be an output reactor, a dV/dt filter, or a sinewave filter. These all work slightly differently, but in general they work to filter out any high-frequency voltage spikes, reduce reflected waves, and eliminate voltage peaks. This may however have knock-on effects for motor operation – a lower voltage to the motor means higher current, and a hotter motor, so this also needs to be taken into account to ensure maximum system efficiency. If in any doubt, contact your motor and/or drive manufacturer, and they will be able to advise the most appropriate solution for your assets.
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