WiremanWireman · Motor runs slow or won’t reach speed: troubleshooting for electricians · 2023 NEC ·
Motor runs slow or won’t reach speed
Low voltage, a lost phase, a wrong connection, overload or a weak capacitor.
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How it works
An induction motor runs a little below its synchronous speed, which is set by its poles and the line frequency: 3600, 1800, 1200 or 900 rpm at 60 Hz. The difference, called slip, grows with load, so an 1800 rpm motor’s nameplate might say 1750. A motor that won’t come up to speed is short on torque (low voltage, a lost phase, a wrong connection, a weak capacitor) or is carrying more load than it can handle.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- When is it slow?
- Takes a long time to come up, or never doesTest: Measure voltage at the motorExpect: Within 10% of nameplate, and balanced between phases.
- Low, more than 10% under nameplateLow voltage at the motor
- One phase missing or very lowSingle-phasing
- NormalIs it a dual-voltage motor? Compare the connection to the nameplate diagram.
- Connected for the wrong voltageMotor connected for the wrong voltage
- Connected correctly, or single voltageIs it a single-phase motor?
- Yes, single phaseWeak capacitor or start switch
- No, three phasePossible rotor problem
- Comes up, then slows under loadTest: Measure current under loadExpect: At or below FLA.
- Above FLAToo much load
- At or below FLABelt or coupling slip
- It’s on a VFDDrive settings or speed reference
- Takes a long time to come up, or never doesTest: Measure voltage at the motorExpect: Within 10% of nameplate, and balanced between phases.