WiremanWireman · Noisy or vibrating motor: troubleshooting for electricians · 2023 NEC ·
Noisy or vibrating motor
Separate electrical noise from mechanical with a coast-down test, then find the cause.
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How it works
Motor noise is either magnetic or mechanical. Magnetic noise comes from the windings and core and depends on voltage and current: a hum at twice line frequency is normal, while a loud growl can mean unbalanced voltage or a winding or rotor problem. Mechanical noise comes from bearings, the fan, misalignment, a loose base or the driven load. The quickest way to tell them apart is to cut the power and listen as the motor coasts.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- With the motor running, cut the power and listen as it coasts down. What happens to the noise?
- Stops the instant power is cutTest: Measure the voltage at the motorExpect: All three within about 1% of each other, and within 10% of nameplate.
- Unbalanced, missing or highVoltage problem at the motor
- Balanced and normalIs the motor on a VFD?
- Yes, a steady high-pitched whineDrive carrier noise
- No, or the noise is a growl or pulsing humWinding, rotor or air gap problem
- Keeps going as it coastsUncouple the load (belt off or coupling apart) and run the motor alone. Is the noise still there?
- Gone, the motor runs quiet aloneThe driven equipment
- Still thereTest: Feel the shaft by handExpect: Smooth, even rotation with no side play.
- Rough, gritty or has playWorn bearings
- Smooth, but rubs at one spotSomething is rubbing
- Feels fineMounting or balance
- Stops the instant power is cutTest: Measure the voltage at the motorExpect: All three within about 1% of each other, and within 10% of nameplate.