electrical troubleshooting
Motor won't start / hums and trips
Common symptoms: motor hums; motor won't start; fan won't start; compressor hums; locked rotor; starts and trips breaker
Stop and call a pro if:
- lockout-tagout-required
- capacitor-stored-energy
Step-by-step diagnostic flow
Step 1
Before working on the motor or its capacitor, establish an electrically safe work condition. Capacitors store dangerous energy even after power is removed.
Step 2
With power off, can you rotate the motor shaft by hand? Does it spin freely, or does it bind?
A motor that hums and trips is either mechanically seized (bearings, foreign object) or has lost a start path (start capacitor, start winding, centrifugal switch).
Step 3
Does this motor have a visible capacitor (a cylindrical or rectangular can with two or three terminals, usually wired into the motor housing)?
Step 4
Power back on (carefully). At the motor's supply terminals, measure voltage with a meter. Is it within ±10% of the motor's nameplate voltage?
Motors that try to start under low voltage will draw locked-rotor current and trip the breaker. Long runs of undersized wire are a frequent cause.
Possible outcomes
Stop — establish electrically safe condition and discharge capacitors first
high confidenceNFPA 70E §120.5 verification, plus capacitor discharge. Motor capacitors can hold 300 V+ for hours.
- Turn off the motor disconnect and lock it
- Verify zero energy at the motor terminals
- Discharge any capacitor across an insulated 5–20 kΩ resistor before touching its leads
Likely mechanical seizure — bearings, foreign object, or pump head
high confidenceIf the shaft won't turn by hand, the motor is asked to start against a frozen load and stalls — humming and tripping on locked-rotor current.
- With power off, separate the motor from the load (belt, coupling, impeller)
- If the motor spins free without the load, the load is the problem (e.g. seized pump)
- If the motor itself binds, bearings have failed — replace or rebuild
- Motor nameplate (HP, RPM, frame)
- Whether the load is direct-drive, belted, or coupled
Likely failed start or run capacitor
high confidenceSingle-phase motors use a capacitor for starting torque. A bulged, leaking, or open capacitor leaves the motor unable to generate the phase shift needed to start — it hums and trips.
- De-energize and discharge the capacitor (across an insulated resistor, NOT a screwdriver short)
- Inspect for bulging top, leaking electrolyte, or scorching
- Test capacitance with a multimeter — should be within ±10% of nameplate µF
- Replace with an identical µF and equal-or-higher voltage rating
- Capacitor µF and voltage from the can
- Motor brand/model
Likely supply voltage out of range
medium confidenceMotor inrush is 5–8× run current; if supply voltage is already low (long wire run, loose connection, undersized conductor), the motor stalls during start.
- Check supply voltage at the panel and at the motor terminals — compare
- If voltage drops sharply when starting, conductor sizing or a loose connection is the cause
- Tighten lugs at the breaker and at the disconnect
- Voltage at panel vs at motor, both static and during start attempt
- Wire run length and gauge
Likely internal winding fault — pro service
medium confidenceShaft turns freely, no capacitor, voltage is correct, yet the motor stalls — usually a shorted start winding, open centrifugal switch, or thermal overload that needs a service technician.
- Leave the motor de-energized
- Document model and symptom timeline
- Motor nameplate (HP, V, A, RPM)
- Whether it ever started normally and when it changed
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