Three-phase motor won't start: a fault-finding order

It's Monday, 6am. The line's back after the weekend shut. Everything starts except the infeed conveyor. Operator presses start. Nothing.

By 6:20 someone has decided the motor's gone. By 6:40 there's a call out to the motor shop for a spare. At 7:15 a sparky turns up, opens the panel, and finds a tripped overload that nobody reset after Friday's clean.

Most "dead motors" aren't dead. The motor is the last thing in a long chain, and the most expensive thing to swap. So it gets blamed first and checked last. Turn that round.

Listen before you open anything

Press start and watch what happens. The motor's behaviour tells you which half of the circuit to look at.

• Nothing at all, no click and no hum? The contactor isn't pulling in, so look at the control circuit.
• Contactor clicks but the motor's silent? The control side's working. Power isn't reaching the motor, so look at the supply side.
• Motor hums but won't turn? Power's getting there, but either one phase is missing or the shaft can't move.
• Starts, then trips? It's running into something: load, a winding fault, or a protection setting. Our guide to a motor tripping on overload picks up from here.

If it's humming, stop it. A motor stalled on two phases or a seized shaft draws heavy current and cooks its windings fast. Don't let anyone hold the start button to "see if it catches".

Nothing at all: the control circuit

The contactor coil has to see its supply. Something in the chain is stopping it. Work through:

1. The overload. Has it tripped? Is it set to manual reset? A tripped overload opens its auxiliary contact and the coil drops out.
2. The control fuse or MCB. A blown fuse on the control transformer kills everything quietly.
3. E-stops, pull wires and guard switches. One left pressed or one door open is enough. Our guide to e-stop circuits that won't reset covers the chain in more detail.
4. The PLC output, if there is one. Is the output on? Is its LED lit? Is the voltage there at the terminal?
5. The coil itself. An open coil won't pull in however good the supply is.

On a long chain, don't work from one end. Test in the middle and halve it. Our guide to half-split fault-finding explains why that's so much faster.

Contactor in, motor silent: the supply side

Now the problem is between the incoming supply and the motor terminals. Check the isolator is on, then the main fuses or the motor circuit breaker. Then look at the contactor's main contacts. Pitted or burnt contacts can pull in and still not pass current on one pole.

Measure phase to phase on the outgoing side of the contactor with it pulled in. You want three readings that match. Two good and one low tells you which pole to chase.

This is live testing. HSE's guidance on safe working practices is clear that electrical accidents often happen during fault-finding after a breakdown, when the pressure to repair leads to risks being taken (HSG85, paragraph 38)*. Plan it. Use the right meter and fused leads, and the right person.

Hums but won't turn: phase or mechanics

A hum with no rotation usually means one of two things.

The first is a lost phase. A three-phase motor won't start on two. It just sits there and hums. Chase it as above: fuses, contactor poles, then the terminal box. Loose connections in the terminal box are common on motors that vibrate.

The second is a shaft that can't turn. A seized bearing, a gearbox that's locked up, a brake that hasn't released, or a jammed load. To find out which, isolate and decouple. If the motor shaft turns freely by hand and the load doesn't, the motor's fine.

Watch the brake. A brake motor on a hoist, a lift table or an incline holds that load up. Release the brake without the load secured and it comes down. Support it or lower it first.

Prove it dead before you open the terminal box

Before anyone lifts the terminal box lid, isolate, lock off and prove dead. Our guide to safe isolation covers the full sequence.

Two points catch people on motors. First, a three-phase motor has three supply conductors. HSE's guidance says that on a three-phase system, you prove all supply conductors are dead (HSG85, paragraph 53). One good reading isn't enough.

Second, use the right tester. The same guidance says multimeters, which can be set to the wrong function, are not recommended for proving dead on low-voltage systems. Nor are non-contact "volt sticks" (HSG85, paragraph 54). Use a two-pole voltage detector and check it works before and after on a proving unit (HSG85, paragraph 55).

And don't use the e-stop as your isolation. The PUWER code of practice says emergency stops and interlocked guards are not suitable as a means of isolation (L22, paragraph 195).

If the motor runs from a drive, wait for the DC bus to discharge before you go near the output terminals. HSE's guidance says equipment that stores charge, such as capacitors, should be safely discharged before work starts (HSG85, paragraph 48).

Testing the motor itself

Only now does the motor get the blame. With it isolated and the cables off:

• Check winding resistance across all three windings. They should be close to each other. One open or one way out means a winding fault.
• Do an insulation resistance test, winding to earth and between windings. Our guide to insulation resistance testing on motors covers what the readings mean.
• If the motor runs from a drive, disconnect the drive before you insulation test. Test voltage into the drive output can damage it.
• Turn the shaft by hand. Feel for rough bearings or rubbing.

If the windings are balanced, the insulation's good and the shaft turns freely, the motor's probably not your problem. Go back up the chain.

The cost of the wrong call

Back to Monday morning. The motor shop was called before anyone checked the overload. If the spare had been fitted, the line would have been down for hours to swap a good motor. The real fault was a reset button.

That's what having the wrong person on a fault costs you. A good mechanical fitter can change a motor all day. Finding why it won't start is electrical fault-finding, and it needs someone confident in a live panel. If you've worked out what an hour of downtime costs, you'll know the difference between a 15-minute find and a three-hour swap.

Here's our view. No motor gets changed on your site until someone has checked the control circuit, the supply on all three phases and the windings. Write that down. It'll save you a motor or two a year.

What to do on Monday

1. Check which motors on your critical lines have manual-reset overloads, and make "check the overloads" part of every restart after a shut.
2. Put a two-pole voltage detector and a proving unit in every sparky's kit. Take multimeters off the proving-dead job.
3. Agree a rule: no motor swap until the control and supply sides have been tested and written down.
4. Make sure the night shift has an electrical engineer they can call when the motor won't start and nobody on site is confident in the panel.

* Paragraph numbers correct as of 1 September 2026 and refer to HSG85, third edition (2013), and L22, fourth edition (2014, amended 2018). Check HSE's HSG85 page for the latest.