A motor that won’t start stops production, and the cause is often simple: a tripped breaker, a missing phase or a stuck contactor. The key is to work in order, from the easy checks to the deeper tests, and to listen to what the motor is telling you. This guide gives you the eight most common causes and a checklist you can follow on the job.
Safety first: Lock out the power before touching terminals, connections or the shaft. Voltage measurements on live circuits must only be done by a qualified person, with suitable instruments and protective equipment, following local rules. Check the equipment manual for your motor and starter.
First, listen to the motor
What you hear and see narrows down the search quickly:
- Nothing at all (no sound, contactor stays open): look at the supply, the protection devices and the control circuit (causes 1 and 2).
- Contactor closes, motor hums but doesn’t turn: suspect phase loss, low voltage, a mechanical jam or a failed capacitor (causes 4 to 7).
- Motor starts, then trips quickly: this is usually an overload or a mechanical or electrical fault. See our guide on overload relays tripping.
8 common causes
1. No power or tripped protection
A tripped breaker, a blown fuse, an open disconnect or an overload relay that hasn’t been reset will keep the motor from starting. An engaged emergency stop or an open door interlock does the same.Fix: With the power locked out, check each protective device and reset or replace it. Don’t just reset a breaker repeatedly. Find out why it tripped.
2. A fault in the control circuit
The motor can’t start if the contactor coil doesn’t get power. Possible culprits are a blown control fuse, a failed control transformer, a faulty start or stop button, an open safety interlock, a safety relay that isn’t reset, or a PLC output that isn’t energized.Fix: Follow the control circuit step by step. A qualified person can measure the voltage at the coil terminals while a start is requested.
3. The contactor doesn’t pull in or its contacts are worn
If the coil has power but the contactor doesn’t close, the coil may be damaged. If it closes but the motor doesn’t start, the contacts may be pitted, burnt or dirty, and may not pass current on all three phases.Fix: Inspect the coil and contacts with the power locked out. If the contactor buzzes or chatters, see our guide on contactors humming or chattering.
4. Phase loss (single phasing)
Fix: A qualified person measures the voltage between all three phases at the motor terminals. Find the missing phase and repair the cause. Don’t keep trying to start the motor, because it can overheat quickly.
5. Low voltage or voltage drop
A weak supply, long or undersized cables or loose connections can drop the voltage during starting. The motor then lacks the torque to get the load moving.Fix: Measure the voltage at the motor terminals while it tries to start, and compare it with the nameplate. Fix the supply, the cables or the connections.
6. Mechanical jam or a brake that doesn’t release
A seized bearing, a jammed conveyor, a blocked gearbox or a damaged coupling can stop a healthy motor from turning. On motors with a brake, a brake that doesn’t release keeps the shaft locked.Fix: With the power locked out, turn the shaft by hand if it is safe to do so. It should turn freely. If the motor has a brake, check that its supply and release mechanism work. Look at the load for blockages.
7. Motor fault: windings or capacitor
Open or burnt windings, a ground fault, or damaged insulation will prevent a motor from starting. A single-phase motor often has a start or run capacitor, and when it fails, the motor hums without starting.Fix: With the motor disconnected and locked out, measure the winding resistance and the insulation resistance (with a megohmmeter) and compare them with the manufacturer’s values. Replace a failed capacitor with the exact rating.
8. A drive or soft starter fault
If a variable frequency drive (VFD) or a soft starter controls the motor, the problem may be in the drive. Typical causes are an active fault code, a missing run command or enable signal, an open safety input, or wrong parameters after a replacement.Fix: Read the fault code on the drive, check the run and enable signals, and look up the code in the manufacturer’s manual. Reset only after you understand the fault.
Step-by-step troubleshooting
- Note what happens when you press start: silence, hum or quick trip.
- Check the obvious: emergency stop, disconnect, breaker, fuses and overload reset.
- Check the control voltage and the start circuit (qualified person).
- See whether the contactor pulls in. If not, look at the coil circuit and the coil.
- If it does, measure all three phases at the motor terminals (qualified person).
- With the power locked out, turn the shaft by hand and check the brake and the load.
- Test the motor windings, the insulation, the capacitor (single-phase) and the drive.
Mistakes to avoid
- Repeated start attempts: each attempt heats the windings. Wait and find the cause first.
- Bypassing safety devices (interlocks, overload relays, emergency stops) to “see if it runs”. This can destroy equipment and cause serious injury.
- Resetting without investigating. A tripped device is a symptom, not the problem.
Quick reference
| What you notice | Likely cause |
|---|---|
| No sound, contactor stays open | Tripped protection, control circuit fault |
| Contactor closes, motor hums, doesn't turn | Phase loss, low voltage, jam, failed capacitor |
| Motor starts, then trips fast | Overload, mechanical or winding fault |
| Drive shows a fault code | Drive fault, missing run or enable signal |
| Shaft won't turn by hand | Seized bearing, jam, brake not released |
When to call a specialist
If you find burnt insulation, a failed winding test, or a fault you can’t explain, stop and call a qualified electrician or a motor repair shop. Replacing a motor is far cheaper than a fire or an accident.
Have you found a surprising reason why a motor wouldn’t start? Share it in the comments below.
Print this checklist and keep it in your toolbox or your panel.
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