What you are seeing
You hit start and one of a few things happens. The motor sits there dead and silent. It hums or growls but will not turn, and if you leave it there it heats up and trips. It starts but runs backward after a repair or a wiring change. Or it starts sometimes and not others. Each of those points somewhere slightly different, so the symptom is worth noting exactly.
The most useful single fact is whether the motor is getting a start command at all, and whether it has all three phases when it does. A motor that hums but will not spin is the classic sign of a lost phase, and that is where a lot of these calls end up.
The usual causes, roughly in order
Before you suspect the motor, confirm it is actually being told to run. A tripped overload that has not been reset, a control fuse, a bad start button or selector switch, a safety interlock or emergency stop that is open, or a control circuit fault can all leave a perfectly good motor sitting still. This is the most common and the most overlooked.
If the motor hums or growls but will not turn, and heats up if you leave it, it is very likely running on two phases instead of three. A blown fuse, a tripped single pole, a failed contactor pole, or a loose or burned connection in one phase will do it. Single phasing will cook a motor quickly, so this one you catch fast and shut down.
The contactor is what actually switches power to the motor. Burned or welded contacts, a bad coil, or a starter that will not pull in will stop the motor from starting even when everything upstream is fine. A buzzing or silent contactor when you call for start is a strong clue.
If the overload trips as soon as the motor tries to start, it is protecting against something real: a jammed or seized load, a bearing gone bad, a mechanical bind, or a motor that is drawing too much. Resetting into it repeatedly just heats things up.
A motor that starts but runs backward after a motor swap or a wiring change is almost always a phase order issue at the connections. It is a routine fix, verify and correct the phase connections and confirm rotation, but it needs to be caught before the driven equipment is damaged.
Only after the supply, controls, and connections check out do you land on the motor: a failed winding, a grounded winding, or a shot bearing. Testing the motor is how you confirm this instead of assuming it, so you do not replace a good motor over a bad fuse.
A safe field checklist
Safety first. Three-phase circuits are dangerous and the checks that involve live voltage or landing conductors are for qualified people only, with the right instruments and PPE. The items below start with what anyone can safely observe and note before a qualified person does the live testing.
- 1 Note the exact behavior: dead and silent, humming but not turning, running backward, or intermittent. That alone points at controls, a lost phase, rotation, or the motor.
- 2 Check that any overload or breaker has actually been reset, and that no emergency stop, guard interlock, or disconnect is left open. Motors sit dead over these constantly.
- 3 Listen at start. A hum or growl with no rotation strongly suggests a lost phase, and that motor should be shut off, not left to heat up.
- 4 Look and smell at the motor and starter for heat, a burning smell, or discoloration. Any of those means stop and get it tested, not restarted.
- 5 For a qualified person: verify all three phases are present and balanced at the starter, check the control circuit for the start signal, and check the contactor contacts and coil.
- 6 For a qualified person: with the motor isolated, test the windings and check the bearing before condemning the motor, so a good motor is not replaced over a supply or control fault.
When to call a pro
If the motor hums but will not turn, shut it down and call. That is the signature of a lost phase, and every minute it sits there single phasing is heating the windings toward a failure. The same goes for any burning smell, discoloration, or a breaker that trips the instant the motor tries to start.
Beyond the obvious, three-phase troubleshooting means live voltage, phase measurement, and winding tests with proper instruments, which is qualified work. When the safe checks above do not clear it, that is the point to bring someone in who can measure all three phases, test the controls, and test the motor, then fix the connection, the starter, or the motor that is actually at fault instead of guessing.
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