Three-phase motor single-phasing while running
A running three-phase motor loses one phase. It keeps turning but loses torque, draws heavy unbalanced current, gets hot and noisy, and often trips the overload — but won't restart if stopped.
Safety first
Single-phasing overheats windings fast and can be a fire risk. Stop the motor rather than leaving it struggling. Treat all three phases as live and confirm isolation before opening terminals.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Apprentice mode
Full walk-through — the why, what readings mean, mistakes & safety.
Likely causes
Ranked from most to least likely.
- 1
Blown fuse or open contactor pole on one phase
Most likelyOne HRC fuse blown, a burnt contactor pole, or an open isolator contact removes a phase, leaving the motor on two and drawing badly unbalanced current.
- 2
Loose or burnt connection in one phase
#2A high-resistance joint at a terminal, lug, or isolator can open under heat/load, dropping that phase while running.
- 3
Supply-side phase loss
#3A lost phase from the supply/distribution affects the whole board, not just one motor — check whether others are affected.
- 4
Open winding or motor terminal fault
Least likelyAn internal winding break or a failed motor terminal connection removes a phase at the motor itself.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
On the tools? Job Mode shows one test at a time with thumb-sized buttons — your place and readings carry straight over.
Enter Job ModeStop the motor. With a clamp meter, measure current in all three phases during a controlled run (if safe) — or measure phase-to-phase voltages at the starter.
Three balanced currents/voltages.
All three phases present and balanced — the single-phasing may be intermittent; check connections.
One phase missing/low pinpoints which phase is lost — trace that phase back through the starter.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Motor single-phasing while running
→ step 2 - 2decision
Are all three phases present and balanced at the starter?
Yes→ step 3No→ step 4 - 3result
May be intermittent — inspect connections under load for a dropping phase.
- 4decision
Is a fuse/contactor pole/isolator open on the missing phase?
Yes→ step 5No→ step 6 - 5result
Open protection/pole on that phase — replace and find why it failed.
- 6decision
Is the phase loss isolated to this motor (others unaffected)?
Yes→ step 7No→ step 8 - 7result
Local fault — repair the connection or test the motor windings/terminals.
- 8result
Supply-side phase loss — escalate to the board/distribution.
Common mistakes apprentices make
- Leaving the motor running on two phases 'until it can be looked at'.
- Replacing a blown fuse without finding why that phase opened.
- Not checking whether the phase loss is supply-side and affecting other gear.
- Assuming the motor is fine because it was still turning.
When to stop & escalate
If the phase loss is supply-side or in the distribution, escalate to the board/supply rather than the motor. A motor that's been single-phasing may have heat-damaged windings — have it tested before returning to service.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
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Related faults
Three-phase equipment single-phasing (lost a phase)
Three-phase equipment is misbehaving — motors humming, struggling, overheating, or tripping — because one phase has been lost somewhere between the supply and the load.
Motor overload trips after running for a while
The motor starts and runs fine, then the thermal overload trips minutes or hours later. Reset it and it runs again before tripping once more — a heat/loading pattern, not a start fault.
Phase-failure / monitoring relay has tripped the circuit
A phase-failure or phase-sequence monitoring relay has dropped out and is holding the control circuit off, stopping the equipment — even though the panel looks powered.