Why VFDs Damage Motors and Practical Protection Solutions

Introduction

VFDs (Variable Frequency Drives) are standard industrial equipment for motor speed control and energy saving, widely used on fans, pumps, conveyors and processing machinery.

However, factory maintenance teams often face a frustrating issue: ordinary motors driven by VFDs wear out far faster than grid-powered motors. Sudden winding breakdown or bearing burnout leads to unplanned downtime, high replacement costs and production losses.

This article breaks down the hidden hazards of PWM output from drives and shares easy-to-implement protective measures.

1. Core Root Cause: PWM Pulse Voltage

Mains power supplies smooth sine wave voltage to motors. VFDs operate differently:

1. Convert AC to DC via rectifiers

2. Generate high-frequency PWM pulse signals through inverters to adjust motor speed

This pulse waveform is the source of all motor damage, triggering two typical failure modes: winding insulation aging and bearing electrical corrosion.

2. Failure Mode 1: Stator Winding Insulation Breakdown

How damage happens

When PWM pulses travel through connecting cables, impedance mismatch creates reflected voltage waves.Superimposed incident and reflected waves produce severe voltage spikes, reaching nearly 3 times the rated input voltage.

• High-voltage surges repeatedly strike winding insulation

• Higher carrier frequency generates more spikes per minute

• Extra heat from high-frequency components accelerates insulation fatigue

Visible consequences

Insulation cracks, short circuits between coils, sudden motor tripping during operation.

3. Failure Mode 2: Electric Erosion of Motor Bearings

How damage happens

Asymmetric magnetic fields from PWM induce 10–30V shaft voltage on the motor rotor.Once shaft voltage breaks down the insulating oil film inside bearings, tiny electric arcs form constantly.

• Arcs burn micro pits on bearing balls and races

• Pits expand into grooves under long-term vibration

• Obvious abnormal noise, vibration and eventual bearing seizure

Critical reminder

The lower the motor running frequency, the higher the shaft voltage and the more severe bearing damage.

4. Targeted Protection Solutions (Sorted by Cost & Effect)

① Output Reactor — Low-cost basic solution

Suitable: Cable length ≤30mAdvantage: Affordable, simple installationFunction: Suppress partial voltage spikes, slow insulation aging

② dv/dt Filter — Mid-range comprehensive protection

Suitable: Cable length ≤300mAdvantage: Far stronger spike suppression than reactorsFunction: Limit steep voltage changes from PWM pulses

③ Sine Wave Filter — Premium full protection

Suitable: Any long cable distance, high-standard production linesAdvantage: Convert PWM pulses back to clean sine waveFunction: Eliminate voltage spikes entirely; motors run same as grid power

④ Peak Voltage Absorber — Compact all-in-one protector

Suitable: High-power heavy-duty motorsAdvantage: Small footprint, low voltage dropFunction: Protect both windings and bearings at the same time

Extra Tip for New Projects

Select dedicated inverter-duty motors with reinforced winding insulation from the start to reduce later modification costs.

5. Conclusion

VFDs deliver outstanding energy efficiency and flexible speed control, yet built-in PWM voltage brings unavoidable risks to standard motors.

By matching protective components based on cable length, motor power and working conditions, factories can effectively avoid premature motor damage, extend equipment service life and guarantee stable continuous production.