Journeyman refresher Refresher 4 of 10 · about 10 min
Motor troubleshooting flowcharts
What you’ll learn
- A fixed order that works for any motor complaint: supply, control, motor, load.
- A flowchart each for won’t start, trips on start, runs hot, and noisy or vibrating.
- The readings that split the problem in half fastest.
- The two safety steps that protect you and the drive: lockout, and lifting leads before insulation tests.
Before this: The troubleshooting method
Won’t start
- Is there voltage on all three lines at the line side of the starter? No — go upstream: disconnect, fuses, breaker, supply. Yes — next.
- Press Start. Does the contactor pull in? No — control problem: control fuse, control transformer, overload contact tripped, stop button or interlock open. Follow the control-circuit method. Yes — next.
- Voltage on all three load-side terminals with the contactor in? No — a burnt main contact or a loose lug on one phase. Yes — next.
- Voltage on all three leads at the motor box? No — open conductor or connection between starter and motor. Yes — next.
- Lock out. Does the shaft turn freely by hand, uncoupled? No — seized bearing or motor; or, if it turns uncoupled, a jammed load. Yes — test the windings: equal resistance on all three, insulation test to ground (leads lifted from any drive).
Trips on start
- Instant trip of the breaker or fuses? Yes — a short or ground fault: lock out, insulation-test the motor and cable separately. Do not keep re-closing. No, it is the overload after a few seconds — next.
- Does it hum without turning? Yes — single-phasing or locked rotor: check the three phases during the start and whether the load is jammed. No — next.
- Does the voltage at the motor sag hard during the start? Yes — weak supply or long run; the motor accelerates too slowly. No — next.
- Is the load high-inertia (fan, flywheel, loaded conveyor)? Yes — the acceleration may be outlasting the overload class; that is a question for the setting, the starter and the book, not for resetting again. No — check the overload setting against the nameplate, then the motor itself.
Runs hot
- Measure the three line voltages under load. Low, or more than a percent or two unbalanced? Yes — fix the supply first (work out the unbalance). No — next.
- Measure the three line currents. Balanced but above nameplate FLA? Yes — the load is asking too much: uncouple and compare. Unbalanced? — a supply or winding problem.
- Is the cooling working? Blocked fan cover, clogged fins, hot room, or a TEFC motor run slowly on a drive with no auxiliary blower.
- Duty cycle. Too many starts an hour heats a motor even when every run is normal.
Noisy or vibrating
- Does the noise stop the instant power is removed? Yes — electrical: unbalance, single-phasing, a winding fault, or drive carrier noise. No, it coasts down noisy — mechanical: next.
- Uncoupled, is it still noisy? Yes — motor bearings, a bent shaft, loose mounting or rotor rub. No — alignment, belt tension, coupling wear or the driven machine.
- On a drive? Pitting (fluting) on bearing races can come from shaft currents; the drive and motor makers say how to prevent it.
Worked check: is it the motor or the supply?
- A 460 V motor runs hot. Line voltages: 460, 468, 447 V. Average = 1375 ÷ 3 = 458.3 V.
- Largest deviation = 458.3 − 447 = 11.3 V. Unbalance = 11.3 ÷ 458.3 × 100 = 2.5%.
- Line currents: 26, 31, 22 A. Average 26.3 A; largest deviation 4.7 A → 17.7% current unbalance — about seven times the voltage figure.
- Roll the leads one position at the motor (all three, so rotation is unchanged). If the high current follows the supply line, it is the supply; if it stays on the same motor lead, it is the motor.
On the job
Tips
- Write down all three voltages and all three currents before you change anything — it is the baseline for everything after.
- Rolling the leads (all three, one position) separates a supply problem from a winding problem without a second meter.
- An overload trip is the motor telling you something. Find out what before you reset.
Common mistakes
- Measuring one phase and calling it good.
- Resetting the overload three times in a row “to see”.
- Meggering a motor with the drive still connected.
- Replacing the motor when the supply or the load was the problem — and burning out the new one.
Check yourself
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1.A three-phase motor hums but will not turn when started, and trips its overload. What is the first likely cause to check?
A three-phase motor at standstill on only two phases has no rotating field, so it hums and draws locked-rotor current until the overload trips. Measure all three line voltages at the motor side of the starter.
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2.A motor runs but draws about the same high current on all three legs and runs hot. Supply voltage is balanced and correct. Where do you look next?
Balanced high current with a good supply means the motor is being asked for more torque than it should give. Uncouple it: if the no-load current is normal, the problem is on the driven side.
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3.Line voltages at a motor read 460, 468 and 447 V. Why does that matter for a motor that is running hot?
Average 458.3 V; largest deviation 11.3 V (the 447 reading) → 11.3 ÷ 458.3 = 2.5%. A few percent of voltage unbalance drives a much larger current unbalance and extra heating. Find and fix the supply problem before blaming the motor.
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4.You are about to insulation-test (megger) a motor fed from a VFD. What must you do first?
An insulation tester can destroy a drive’s output transistors. Lock out, prove dead, lift the motor leads at the drive, and test the motor and cable on their own.
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5.A motor trips its overload a few seconds into every start, but runs fine once it gets going on a second try. Which is most likely?
If the start takes longer than the overload class allows — a big flywheel, a loaded conveyor, or voltage dipping during the start — it trips before the motor reaches speed. Measure the voltage at the motor during the start and time the acceleration.
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6.After a motor swap the new motor runs backwards. The correct fix is…
Swapping any two phases reverses the rotating field. Rolling all three one position keeps the same sequence and the same rotation.
Questions people ask
Should I megger a motor that tripped?
Often, yes — once it is locked out and proven dead. An insulation resistance test tells you whether the windings are going to ground. Disconnect it from any drive or soft starter first, test the motor and the cable separately if you can, and compare with earlier readings. What value is acceptable comes from the motor maker and your site’s own standards, not from this page.
How do I tell a motor problem from a load problem?
Uncouple it. If the motor runs smoothly and draws normal no-load current on its own, the motor is probably fine and the fault is in the driven machine — bearings, belts, a jammed pump or conveyor. If it still misbehaves uncoupled, the motor or its supply is the problem.
Why check the supply first?
Because it is quick, it is the most common cause, and it is the one that will burn out a replacement motor too. A lost phase, low voltage or a big unbalance will make a perfectly good motor overheat or trip.
Related
- The troubleshooting method — the loop these charts follow
- How an AC induction motor works
- Troubleshooting industrial controls
- Troubleshooting a control circuit (lab)
- The overload relay
- Voltage unbalance calculator
- Reading a motor nameplate
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