VSD reports output phase loss / motor phase loss
The drive trips or warns of an output (motor) phase loss — it isn't seeing balanced current on all three output phases to the motor.
Safety first
Prove dead and discharge the DC bus before working on output terminals. A phase-loss can leave the motor struggling and overheating.
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.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
Broken output connection / motor lead
Most likelyAn open output conductor or loose terminal between the drive and motor loses one phase.
- 2
Motor winding open on one phase
#2An open winding presents as a missing output phase to the drive.
- 3
Loose terminal at drive or motor
#3A loose output terminal at either end intermittently loses a phase.
- 4
Detection sensitivity / no motor connected
Least likelyRunning with no/with a very small motor, or over-sensitive detection, can flag phase loss.
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.
Isolate, prove dead, discharge, and check continuity of all three output conductors drive-to-motor.
Continuity on all three output phases.
Wiring intact — check the motor windings.
Open conductor/connection — locate and repair.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
VSD output phase loss
→ step 2 - 2decision
Do all three output conductors have continuity drive-to-motor?
Yes→ step 3No→ step 4 - 3decision
Are the motor windings balanced/continuous?
Yes→ step 5No→ step 6 - 4result
Open conductor/connection — locate and repair.
- 5decision
Are terminals tight and detection appropriate?
Yes→ step 7No→ step 8 - 6result
Open/unbalanced winding — motor repair.
- 7result
All sound — monitor; may have been a transient.
- 8result
Loose terminal or over-sensitive detection — correct it.
Common mistakes apprentices make
- Not isolating/discharging before testing output terminals.
- Overlooking a loose output terminal that intermittently drops a phase.
- Assuming the drive when a motor lead is broken.
- Ignoring the motor winding as a cause.
When to stop & escalate
An open motor winding is a motor repair. Repeated output phase-loss with sound wiring/motor should go to the drive documentation with the exact code.
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.
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.
VSD trips on overcurrent / overload
The drive trips with an overcurrent or overload code — on start, on acceleration, or under running load. It may restart and trip again on the same point in the cycle.
Motor overload keeps tripping
The thermal/electronic overload trips repeatedly, either on start or after the motor has run for a while. Resetting only buys you a short run before it trips again.