Wireman · Motor hums or won’t start: troubleshooting for electricians · 2023 NEC ·

Motor hums or won’t start

Capacitors, lost phases, low voltage, open windings and seized loads.

Before you start

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How it works

A motor needs a rotating magnetic field to start. Three-phase motors get one for free from three phases 120° apart. Single-phase motors fake it with a start winding and usually a capacitor. A motor that hums is getting power but can’t make that rotating field (a bad capacitor, a lost phase) or can’t turn the load (a seized bearing or pump).

Step by stepFull flowchart

Every question and answer in this guide. Tap “Step by step” to walk through it.

Is it a single-phase or three-phase motor?
- Single phaseWhat does it do when it’s supposed to start?
  - Hums but doesn’t turnTest: Test the capacitorExpect: Run capacitors within about ±6% of the label. Start capacitors within the printed range (like 88–108 µF).
    - Low, open or bulgingFailed capacitor
    - Within ratingWith power locked out, does the shaft turn freely by hand?
      - Stiff, rough or won’t turnMechanical: seized bearing or jammed load
      - Turns freelyTest: Measure voltage while it tries to startExpect: No lower than about 90% of nameplate during the start.
        Drops below 90%Voltage drops too far on start
        Holds steadyStart switch or start winding
    - No capacitor on this motorWith power locked out, does the shaft turn freely by hand?
      - Stiff, rough or won’t turnMechanical: seized bearing or jammed load
      - Turns freelyTest: Measure voltage while it tries to startExpect: No lower than about 90% of nameplate during the start.
        Drops below 90%Voltage drops too far on start
        Holds steadyStart switch or start winding
  - Nothing at all, no soundTest: Check for voltage at the motorExpect: The nameplate voltage, within 10%.
    - No voltage at the motorNo power reaching the motor
    - Full voltage, motor silentOpen winding or internal thermal protector
  - Starts, then trips or shuts offIt starts, so this is an overload problem
- Three phaseTest: Measure all three phasesExpect: Three readings equal within about 1–2% and within 10% of nameplate.
  - One reading low or missingSingle-phasing (one phase lost)
  - All three equal and normalWith power locked out, does the shaft turn freely?
    - No, it’s tight or seizedMechanical: seized bearing or jammed load
    - YesTest: Test the windingsExpect: The three resistances within about 5% of each other, and insulation well above 1 MΩ (IEEE 43 recommends at least 5 MΩ for motors under 1000 V).
      - One reading open or unbalancedDamaged winding
      - Low insulation resistanceWinding insulation breakdown
      - All goodCheck the starter under load