Why does my motor hum but not turn?

A motor that hums but doesn't turn is usually receiving power but can't develop enough torque to start — most commonly from single-phasing (one of three phases missing or weak), a mechanical bind in the motor or driven load, or a run capacitor failure on a single-phase motor. Check line-to-line voltage on all phases first, then check whether the shaft turns freely by hand when de-energized.

Why does my motor trip the overload immediately every time I try to start it?

An immediate overload trip on starting usually means the motor is drawing locked-rotor current for longer than the overload relay's trip curve allows — from a mechanical bind, a load that's too heavy for the motor, single-phasing, or an overload relay set too low for the motor's actual full-load current. Confirm the overload setting matches nameplate FLC before assuming a mechanical fault.

Can a motor fail from running single-phased even briefly?

Yes. A three-phase motor running on two phases draws significantly higher current in the remaining windings to compensate, generating excess heat that can degrade winding insulation within minutes, especially on a motor already near full load. This is why phase-loss protection on overload relays matters — basic thermal overloads may not react fast enough.

How do I know if a motor problem is electrical or mechanical?

De-energize and lock out the motor, then try turning the shaft by hand. If it turns freely with no roughness or resistance, the problem is very likely electrical (power, control circuit, or starter). If it won't turn, or turns with grinding or heavy resistance, the problem is mechanical — a seized bearing or a bound driven load.

Why did resetting the overload relay not fix the problem?

An overload relay trips because of a real condition — too much current for too long. Resetting it without finding and fixing that cause (single-phasing, a mechanical bind, an undersized overload setting, or a genuine overload on the driven equipment) just delays a repeat trip, and running the motor again under the same fault condition risks further damage.

Electrical·9 min read·July 22, 2026

🔧 Motor Won't Start: Troubleshooting Guide for Electricians

A systematic diagnostic guide for a motor that fails to start — no power at the motor, control circuit faults, single-phasing, bad contactors, mechanical binds, and thermal overload trips, with the correct multimeter test sequence.

📖Illustrated Guide to NEC 2026 Electrical Inspection (Full Access)

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Start With the Right Question, Not the Right Part

"Motor won't start" is a symptom with a long list of possible causes, and the biggest time-waster in the field is jumping straight to replacing the motor before ruling out everything upstream of it. The overwhelming majority of "dead motor" calls are not actually motor failures — they're a missing supply, a control circuit that never energizes the starter coil, or a mechanical bind that the motor can't overcome. A disciplined diagnostic sequence gets you to the real cause in minutes instead of hours.

Topics covered

motor wont startmotor troubleshootingmotor starter faultsingle phasing motormotor overload tripthree phase motor problems