Motor Fails to Start — No Rotation on Startup
Symptoms
Motor is energized (contactor pulls in, pilot light on) but shaft does not rotate. Humming or buzzing sound may be present. Overload trips within seconds. No visible smoke or burning smell initially.
Likely Cause
Most commonly: open circuit on one phase (single-phasing), seized bearings, mechanical jam in driven equipment, or failed start capacitor on single-phase motors. Less commonly: shorted rotor bars or stator winding faults.
Testing Procedure
1) Check incoming voltage on all three phases at MCC or starter with motor de-energized.
2) Attempt to rotate shaft by hand after lockout — resistance indicates mechanical jam or seized bearing.
3) For single-phase motors, test start and run capacitors with a capacitance meter.
4) Measure winding resistance phase-to-phase and phase-to-ground with a megohmmeter (>1 MΩ acceptable).
5) Check all fuses and overload elements for open condition.
6) Verify control circuit: check auxiliary contacts, timer relay, and interlock wiring.
Tools Required
Clamp meter, multimeter, megohmmeter, capacitance meter, tachometer, torque wrench, bearing puller (if needed)
Safety Precautions
Lock out and tag out before attempting to rotate shaft manually. Verify zero energy state with a multimeter before touching terminals. Do not bump-start motor repeatedly — thermal damage accumulates. Secure driven load before re-energizing after clearing mechanical jam.
Step-by-Step Solution
1. LOTO the motor and driven equipment.
2. Attempt to rotate shaft by hand — if locked, identify and clear mechanical jam or replace seized bearing before proceeding.
3. Check all three phase voltages at the starter terminals with motor disconnected.
4. Inspect and test fuses; replace any open fuses as matched sets.
5. For single-phase motors, discharge and test start capacitor — replace if capacitance is more than 10% out of rated spec.
6. Perform megger test; values below 1 MΩ indicate winding issue — escalate to rewind or replacement.
7. After clearing root cause, re-energize and verify rotation direction and starting current.
Prevention
Perform annual bearing inspection and lubrication per manufacturer schedule. Check phase balance monthly on critical motors. Install phase-loss relays on three-phase motors driving critical loads.
Engineering Notes
Single-phasing is a frequent root cause and often goes undetected — always check all three phases before condemning the motor mechanically. A seized bearing will often show amperage on two phases with zero RPM, mimicking a stator fault.