QualifiedHigh risk

Motor hums but won't start to turn

On start the motor hums or buzzes but won't rotate — it may draw heavy current and trip protection. Often a sign it's only getting two phases or is mechanically locked.

Start fault-finding

Safety first

A humming, non-turning motor draws locked-rotor current and overheats fast. Don't leave it energised. A seized load can release suddenly — keep clear.

Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.

Full detail — causes, the why, and common mistakes.

Likely causes

Ranked from most to least likely.

1
Single-phasing (lost one phase)
Most likely
With only two phases, a three-phase motor can't develop starting torque — it hums and overheats.
2
Mechanically jammed or seized load
#2
A seized bearing or jammed driven machine stops the rotor turning even with full supply.
3
Failed start capacitor (single-phase motors)
#3
On single-phase motors, a dead start capacitor leaves the motor humming with no starting torque.
4
Motor winding fault
Least likely
An open or shorted winding can leave the motor unable to start while still drawing current.

Reports are saved on this device to reflect what you actually find.

Testing sequence

Work through one test at a time. Expected reading and what each result means.

Test 1 of 3

Isolate immediately if humming. Then, on a brief controlled attempt, check all three phase-to-phase voltages at the motor.

Expected reading

Three balanced phase-to-phase voltages at the motor.

Your actual reading

If it passes

All phases present — suspect a mechanical jam, capacitor, or winding.

If it fails

A missing phase explains the hum — chase the single-phasing.

Often leads toThree-phase equipment single-phasing (lost a phase)Motor overload keeps tripping

View all expected readings at once

1. Isolate immediately if humming. Then, on a brief controlled attempt, check all three phase-to-phase voltages at the motor.

Three balanced phase-to-phase voltages at the motor.

2. Isolate, lock off, uncouple if practical, and turn the shaft by hand.

Shaft turns freely.

3. For single-phase motors, test the start capacitor; for three-phase, test winding balance/insulation per procedure.

Healthy capacitor / balanced windings with good insulation.

Fault-finding flowchart

The same logic as a decision tree.

1
start
Motor hums, won't turn
→ step 2
2
decision
Are all three phases present at the motor?
Yes→ step 3No→ step 4
3
decision
Does the shaft turn freely by hand?
Yes→ step 5No→ step 6
4
result
Single-phasing — chase the lost phase.
5
result
Test start capacitor (1ph) or windings (3ph) — repair/replace.
6
result
Seized/jammed load — hand to mechanical.

Common mistakes apprentices make

Leaving the motor humming and letting it cook on locked-rotor current.
Not checking for a lost phase before blaming the motor.
Forgetting the start capacitor on single-phase motors.
Failing to uncouple the load to separate a jam from a motor fault.

When to stop & escalate

A confirmed single-phase supply fault goes upstream; a seized driven machine goes to mechanical; a faulty winding means a motor repair/rewind.

If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.

Related faults

General testing

Three-phase equipment single-phasing (lost a phase)

Three-phase equipment is misbehaving — motors humming, struggling, overheating, or tripping — because one phase has been lost somewhere between the supply and the load.

5 causes3 test steps

QualifiedHigh risk

Motors & overloads

Motor overload keeps tripping

The thermal/electronic overload trips repeatedly, either on start or after the motor has run for a while. Resetting only buys you a short run before it trips again.

5 causes4 test steps

QualifiedHigh risk

Motors & overloads