VFD Motor Lead-Length Filter Guide
See whether a long drive-to-motor cable typically calls for a load reactor, dv/dt filter or sine-wave filter, and how the worst-case peak compares with the motor’s insulation rating.
Inputs
Results
| Approx. DC bus voltage | – |
| Worst-case reflected peak at motor | – |
| Motor insulation peak rating | – |
| Typical guidance | – |
DC bus ≈ 1.35 × VLL. With full reflection on a long cable, the peak at the motor can approach 2 × the DC bus. MG1 Part 30 (general purpose, ≤ 600 V): 1,000 V peak with ≥ 2 µs rise time. MG1 Part 31 (inverter duty, ≤ 600 V): 3.1 × rated voltage with ≥ 0.1 µs rise time.
How the guidance works
Typical thresholds at 460 V are: for a general-purpose or unknown motor, no output filter up to about 50 ft, a load reactor up to about 200 ft, a dv/dt filter up to about 600 ft, and a sine-wave filter beyond that. For an inverter-duty motor: often no filter up to about 300 ft, a load reactor or dv/dt filter to about 600 ft, a dv/dt filter to about 1,000 ft, and a sine-wave filter beyond. Thresholds are scaled down at higher voltages and tightened for carrier frequencies above 4 kHz.
Drive manufacturers publish their own maximum cable lengths, which depend on the drive frame, cable type and shielding, and those limits always take priority. Lowering the carrier frequency is a no-cost first step. Read more on dv/dt and power quality and load reactors.
General guidance only. Follow the drive and motor manufacturers’ installation requirements; the filter selection should be confirmed by a qualified engineer and the filter manufacturer.