Machine and Process Management

4 tips on how to limit cable-length related electric motor damage

Your electric motors may be ageing prematurely.  How variable speed drives (VSDs) are cabled to electric motors can have a major impact on performance. Let’s look at the potential issues and remedies.

Consequences of overvoltage and overcurrent conditions

For motors connected to VSDs, a combination of fast-switching transistors and long motor cables can cause a temporary overvoltage at the motor terminal connection. Caused by overcurrent, the main risk to VSDs is a short circuit fault. Disturbances from long cable and motor interactions can also disrupt equipment connected to the same network.

Then there’s the motor – overvoltage can occur between two motor windings, which can wear out the winding insulation and lead to motor failure.  You also need to watch for motor bearing degradation, caused by common-mode voltage generated by the VSD inverter.

Motor cable length has an effect – the longer the motor cable, the higher the overvoltage. For applications with motors running in parallel, the appropriate cable length is the sum of all the cables – for example, when each of three motors connects to a single VSD with a 20-meter (60-foot) cable, total length is 60 meters (197 feet). Calculating the cable length correctly helps protect the VSD from any unexpected tripping.

4 preventive measures to safeguard equipment

By observing a few planning steps and best practices, you can avoid the harmful side effects and headaches:

  1. VSDs are available with preconfigured software for more dependable operation.
  2. Output reactors limit starting current and oppose rapid current changes by reducing peak voltage and dV/dt voltage (rise time at the drive output).
  3. Output dV/dt filters minimize overvoltage and capacitive leakage between phases and phases to ground.
  4. Differential sinus filters suppress the overvoltage effect and reduce electromagnetic compatibility disturbance.

For applications that involve long cables, unknown motor insulation levels, or nonstandard motors, a dV/dt output filter or sinus filter is the best preventive measure.

Selecting the right preventive measures depends on motor characteristics and cable length.

Selecting the right preventive measures depends on motor characteristics and cable length.

Best practices include:

  • Specify a motor designed for speed drive applications
  • Specify VSDs that integrate voltage reflection superimposition software suppression
  • Minimize the distance between the motor and VSD
  • Use unshielded cables if possible – shielded cables calculate as twice the length
  • Reduce the VSD switching frequency

You can read a full discussion about selecting the best solution for connecting motors to VSDs in a white paper that I co-authored with my colleague, Heu Vang.

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