A practical guide to protecting motors and improving drive-system reliability
WHAT IT DOES
A load reactor, also called an output reactor, is installed between the VFD and motor. It helps reduce PWM-related voltage stress, motor heating and audible noise, particularly on long motor-cable runs.
Motor Lead Length
Typical Recommendation
0-50 ft | 0-15 m
Typically not required
50-150 ft | 15-45 m
Consider for older, non-inverter-duty or sensitive motors
150-300 ft | 45-90 m
Load reactor recommended
300-1000 ft | 90-300 m
Use a dV/dT filter
Over 1000 ft | over 300 m
Use a Sine-wave filter
Add or evaluate a load reactor when...
Long Motor Cables
Long leads increase reflected-wave voltage and cable-capacitance effects.
Older or Standard Motors
Motor insulation may not be designed for the fast rise times from an IGBT VFD.
Excess Heat or Noise
An output reactor can soften the PWM waveform and may reduce heating and audible whine.
Multiple Motors / Repeated Issues
Evaluate when one VFD feeds multiple motors, or where winding failures or nuisance trips occur.
Benefits
Protect
Reduces voltage spikes and insulation stress
Improve
Can reduce motor heating and audible noise
Extend
Supports longer motor life and system reliability
RULE OF THUMB: At about 150 ft (45 m), evaluate a load reactor. Beyond about 300 ft (90 m), evaluate a dV/dT filter instead.
Important: Lead-length limits vary by VFD voltage, carrier frequency, cable type and motor insulation. Always follow the drive and filter manufacturer's application guidance.
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