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Effect of Higher-Order VFD Harmonics on Motor Life

Effect of Higher-Order VFD Harmonics on Motor Life 

Read time: ~4 minutes

Executive summary

Higher-order PWM harmonics from VFDs raise winding temperature, stress insulation, and cause bearing EDM—quietly shortening motor life. Risk jumps with high switching frequency, long motor leads, poor grounding, and non-inverter-duty motors. A simple stack—filters, proper cable/terminations, shaft grounding, and spec discipline—restores reliability fast.

What’s on a VFD output (and why it matters)

A VFD synthesizes a sine via high-frequency switching. Besides the fundamental, you get:

  • Higher-order components (carrier + sidebands)
  • Fast dv/dt edges that reflect on long cables
    These create extra heating, torque ripple, over-voltage at terminals, and common-mode currents.


Where harmonics hurt motors

  • Thermal: I²R at harmonic freq + iron/stray losses → hotter windings → faster insulation aging.
  • Mechanical: Voltage harmonics → torque pulsations → vibration/noise and fatigue.
  • Insulation: High dv/dt + long leads → spikes and partial discharge risk.
  • Bearings: Common-mode voltage drives shaft currents → EDM fluting and grease breakdown.

When problems spike

  • High switching frequency (set “high” by habit)
  • Long motor leads or multiple motors per drive
  • Asymmetric/loose EMC terminations, poor earthing paths
  • Motors not rated “inverter-duty”

Measure first (acceptance checklist)

  • THDu at motor terminals, THDi at drive input
  • Shaft/common-mode voltage (peak, repetitive)
  • Winding temperature rise, vibration (mm/s)
    Capture a commissioning baseline and trend quarterly.

Fix what matters (mitigation ladder)

  1. Specify right hardware: Inverter-duty motor (IEC 60034-17/-25; NEMA MG1 Pt 31).
  2. Tune the drive: Use sensible switching frequency; for long leads, don’t default to max.
  3. Condition the output: Output reactor → dv/dt filter → sine filter (pick by lead length/risk).
  4. Cable & terminations: EMC-rated cable, 360° glands both ends, symmetrical grounds.
  5. Bearing protection: Shaft grounding ring (DE) + insulated NDE bearing.
  6. Earthing/bonding: Low-impedance PE back to the VFD, avoid daisy-chain earths.
  7. Verify: Re-measure THDu, shaft V, temp, vibration.

30-day action plan

Week 1: List top VFD-driven motors; measure THDu, shaft V, temps, vibration; note cable lengths/grounding.

Week 2: Lower switching freq where safe; add output reactors on noisy feeders; fix EMC terminations/PE paths.

Week 3: Add shaft grounding + insulated NDE bearing; for long leads, add dv/dt or sine filter.

Week 4: Re-measure, compare to baseline; set quarterly trending.

Business impact

By cutting thermal stress and bearing EDM, you extend MTBF, reduce change-outs, and stabilize uptime. On critical assets, payback for filters + bearing kits is commonly within a year through avoided failures and downtime.

CTA: Want this tailored to a specific line (pump, fan, compressor, spindle)? Share motor kW, cable length, switching freq, and any bearing/temperature issues—I’ll return a one-page, phased fix list.


Author: Mahesh A. Toraskar