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From Open Electrical
The use of fast switching circuitry in variable speed (or frequency) drives result in output waveforms with higher levels of harmonic components. These harmonics can cause:
- High electromagnetic interference (EMI) - where the cable is the antenna and the radiated EMI from the cable can induce voltages and currents on nearby cables and electrical equipment. This can especially be a problem when EMI causes noise and crosstalk in control and instrumentation cables.
- High earth currents - due to harmonics causing unbalances in the three-phase output. A portion of the unbalanced currents return to the source (i.e. inverter) via the earth conductor.
Therefore, VSD cables often have the following special characteristics:
- Heavy duty screen - usually copper, applied over the entire cable bundle to reduce EMI
- Three earth conductors - located symmetrically in the cable cross-section so that the phase-to-earth distance is identical for each phase, and the cable is "electrically balanced". Sometimes you'll see the designation "3C + 3E" - this isn't a mistake!
- Larger earth conductors - to further reduce the impedance of the earth conductor return parth and therefore reduce earth currents
- Robust insulation grades - such as XLPE is commonly used over PVC, so that the cable can better withstand transient voltage spikes