VFD Faults on Overcurrent at Startup
Symptoms
Variable frequency drive displays OC or OCA fault immediately or within the first few seconds of start command. Motor does not reach target speed. Fault clears on reset but recurs on next start attempt. No trip under manual bypass (if fitted).
Likely Cause
Most commonly: acceleration ramp time set too short for load inertia, motor parameters not programmed correctly (incorrect FLA, motor poles, or voltage), loose or corroded motor cable connection causing intermittent phase fault, or damaged IGBT in the drive output stage.
Testing Procedure
1) Review drive fault log — note if fault occurs at 0 Hz (immediately) or during ramp-up.
2) Check programmed acceleration time against load inertia — consult load data sheet.
3) Verify motor nameplate parameters match drive programming (voltage, FLA, frequency, poles).
4) Inspect motor cable terminals at drive output and motor junction box for corrosion or loose lugs.
5) Measure motor winding resistance and insulation resistance (megger) before reconnecting to drive.
6) Check DC bus voltage and output current waveform with a power quality analyzer during start attempt.
Tools Required
Laptop with VFD configuration software, multimeter, megohmmeter, clamp meter, power quality analyzer, torque screwdriver
Safety Precautions
VFD DC bus retains lethal voltage for up to 5 minutes after power removal — always wait and verify with a multimeter before opening drive enclosure. Do not megger motor windings while connected to VFD output — damage to IGBTs will result. Discharge bus capacitors before any internal drive work.
Step-by-Step Solution
1. Access drive parameters and increase acceleration time by 50% — attempt restart and observe fault behavior.
2. Run motor parameter auto-tune routine if available — this sets FLA, resistance, and inductance from measured values.
3. LOTO drive, wait 5 minutes for DC bus discharge, then inspect all output cable terminations — re-torque to specification.
4. Disconnect motor cables and megger motor windings (phase-to-phase and phase-to-ground at 500 V) — values below 1 MΩ require motor repair before reconnecting to drive.
5. If fault persists with all parameters correct and motor healthy, perform output IGBT test per drive manufacturer service manual — arrange drive repair or swap if IGBT failure confirmed.
6. After resolution, document final acceleration ramp settings and motor parameters in equipment record.
Prevention
Record and store final VFD parameter set after commissioning. Perform annual thermal imaging of drive enclosure under load. Clean drive cooling filters per manufacturer interval (typically every 3-6 months in industrial environments).
Engineering Notes
Overcurrent at 0 Hz (before any rotation) almost always indicates a wiring fault or IGBT failure — not a ramp setting issue. Overcurrent during ramp is almost always a parameter or inertia mismatch. This distinction narrows troubleshooting significantly and should be checked in the fault log first.