The motor hums, vibrates or trips the breaker but the shaft does not turn — a classic single-phase or locked-rotor symptom that is almost always electrical or a hard bind, not a dead winding.
These are the failure paths we see most often on a motor for a hums but won't start condition. A humming motor has current but no rotation torque, which points to a start aid, a missing phase or a mechanical block. Likelihood reflects field frequency; First Check is what you can do remotely before sharing readings.
| Cause | Likelihood | First Check |
|---|---|---|
| Failed start / run capacitor (single-phase) | High | Meter the capacitor; a swollen or out-of-range cap is a strong clue |
| Single-phasing on a three-phase supply | Medium | Measure the three phases at the terminals; one missing means hum, no start |
| Mechanical seizure / load lock | Medium | Turn the shaft by hand with power off; a stuck shaft will not start |
| Low or unbalanced supply voltage | Low–Medium | Measure the voltage at the terminals under start |
Work through these in order. Each step is something you can perform and report back on without opening the enclosure or swapping parts.
With power off, rotate the shaft by hand. If it will not move, the load or a bearing is seized — that is a mechanical lock, not an electrical fault, and we address it differently.
Meter the start/run capacitor. A value far from rating, or a swollen can, means it cannot build the phase shift a single-phase motor needs to start. Fitting is on your side; we guide the value.
For a three-phase motor, confirm all three lines are present and balanced. A missing phase makes the motor hum and sit still — single-phasing also overheats it within minutes.
Check the supply voltage under start. A sag or an unbalanced supply prevents starting and we address it upstream of the motor.
If it trips immediately on start, that points to a short or a hard lock. Capture the trip behavior so we can separate an electrical fault from a seized load.
A motor that hums but will not start is almost always electrical — a failed capacitor, a missing phase or a hard lock — not a dead winding. The shaft test and the phase check split it: a free shaft plus a missing phase or a bad cap is electrical, a stuck shaft is mechanical. Cross-reference the broader collections: Motor won't start · Motor & starters.
When to escalate: if the shaft turns free and the phases and capacitor are good, the start winding or centrifugal switch may be at fault — still something we address from the readings, not a rewind. A seized shaft means the load or bearing, and we point you there rather than at the motor.
Usually not. Hum-with-no-start is capacitor, phase or a lock; we address those first. A winding is only suspected if it reads open or short, and we confirm that from the meter.
Because it has current but no rotation torque — a failed capacitor on single-phase or single-phasing on three-phase. The shaft and phase checks show which one.
Yes. A motor on two phases draws high current on those phases and overheats within minutes. We check phase balance before anything else.
If it trips, no. A hard lock or a short needs the cause addressed first. Repeated starts only add heat and risk the winding.
Send the capacitor reading and the phase voltages. We split capacitor, phase and lock remotely so you avoid an unnecessary rewind.
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The load/supply sibling of a start failure.
A drive fault tied to the same motor.
Another VFD/motor current fault with a similar remote path.
Full pumps, motors & drives troubleshooting collection.