Scope of this technical record
Output phase loss, motor phase missing, phase-current imbalance or no-torque symptoms on one VFD output phase.
Do not run a loaded motor with a suspected output phase loss. Motor damage, contactor damage and power-stage stress can escalate quickly.
Why output phase loss is not only a wire problem
An output phase-loss indication can be caused by a loose terminal, bad contactor, motor lead, cable, current sensor, driver channel or IGBT/IPM phase leg. The useful split is whether the condition follows the motor/cable or stays with the drive output phase.
A repair page should connect this symptom to overcurrent and repeated-module-failure pages because a phase loss after repair may be a gate-drive or feedback problem rather than a missing wire.
Phase-loss route
The triage table keeps the investigation on the safest external evidence first, then moves inward to current feedback and driver hardware.
Output phase-loss evidence
| Evidence | Likely boundary | Next decision |
|---|---|---|
| One motor lead open | External motor/cable | Repair cable or terminal before drive testing |
| Output contactor drops one phase | Contactor / interlock | Inspect contactor contacts and control sequence |
| Current reading missing but torque present | Current feedback | Review sensor/channel evidence |
| No voltage/torque on one leg after module repair | Gate driver / phase leg | Check driver command and module connection |
| Trip occurs only under load | Load / motor current imbalance | Review machine and motor condition |
Field record checklist
- Which output phase is indicated
- Whether symptom follows motor/cable
- Output contactor and terminal evidence
- Current feedback readings
- Previous power-stage repair
- Static phase-leg checks
Technical basis and reference documents
This is an independent editorial technical reference. Original manufacturer documentation remains controlling for installation, repair and commissioning decisions.
Related workflow when phase loss escalates into overcurrent.
Diagnostic workflow
Overcurrent trip appears immediately at run command, during acceleration or before stable speed is reached.
Replacement IGBT, IPM or output module fails again during power-up, enable or early load testing.
Turn this record into a qualified service request
A repair decision is much more reliable when the request includes the exact identity of the drive, the first fault evidence and the machine condition when the symptom appeared.
- Complete drive type code / MLFB or nameplate model
- Fault code, fault value and first event before reset
- When the event appears: power-up, enable, ramp, run, decel or stop
- Motor/cable connected or isolated during the symptom
- Visible board, option-card, module and connector identifiers
- Previous repair history, replacement parts and repeat-failure pattern