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Humming noise without rotationNo noise or movement on start commandStarts but trips breaker immediately

Electric Motor Fails to Start

A step-by-step diagnostic guide for troubleshooting an electric motor that refuses to start or start up under load.

Primary Cause

Power supply failure, open control circuit, blown fuses, tripped overload relay, mechanical binding, or internal winding faults.

Resolution Summary

Systematically verify line voltage, test control circuits, check contactor coils, measure winding resistance, and verify shaft rotation freedom.

When an AC induction motor fails to start, the issue could reside in the electrical supply, the control circuit, or the mechanical load. Follow this diagnostic procedure to isolate the root cause.

Step-by-Step Diagnostic Procedure

1. Safety Checks First

Ensure the motor is isolated and lock-out tag-out (LOTO) procedures are followed before touching physical components or windings.

2. Verify Main Power Supply (Line Side)

  • Check Voltage: Use a digital multimeter to measure voltage at the motor starter/contactor line side terminals.
  • Check for Single Phasing: For three-phase motors, verify voltage between all phase combinations (L-L):
    • L1 to L2, L2 to L3, L3 to L1.
    • If any phase is missing (voltage is zero), check for a blown fuse or tripped circuit breaker upstream.

3. Check the Control Circuit

If the contactor does not pull in when the start button is pressed:

  • Overload Relay: Verify if the thermal or electronic overload relay has tripped. Press the Reset button.
  • Control Voltage: Test the voltage across the contactor coil. If there is voltage but the contactor doesn’t pull in, the coil is defective.
  • Interlocks and Sensors: Ensure emergency stop buttons, safety limits, and float/pressure switches are in their closed positions.

4. Evaluate the Motor Winding (Load Side)

If the contactor pulls in but the motor only hums or trips the breaker:

  • Measure Insulation Resistance (Megger Test): Test resistance between each winding and ground. A reading below 1 MΩ suggests moisture or degraded insulation.
  • Measure Coil Resistance: Check winding continuity and balance. Resistance between all three phases should be nearly identical.

5. Check Mechanical Binding

  • Free Rotation: With power isolated, manually turn the motor shaft or load coupling. If it cannot turn, the issue is locked bearings, a bound pump impeller, or gearbox issues.