Drive cooling fan
Common heat-related VFD and servo-drive failure point.
Yaskawa VFD Fault SC — What It Means
Yaskawa VFD fault SC (IGBT Short Circuit or Gate Drive Fault) indicates that the drive detected a short circuit condition on its output IGBT (insulated gate bipolar transistor) stage. The gate drive circuit monitors current through each IGBT switch, and when current spikes to a level indicating a short — either between two output phases, from an output phase to the DC bus, or through a failed IGBT itself — the gate drive fires a hardware overcurrent protection and shuts down the drive in microseconds. SC is a hard fault that cannot be cleared by software reset if the hardware fault condition still exists.
Common Causes
- Phase-to-phase short on output cables — The cable between the drive and motor develops a short between two conductors, creating a near-zero impedance path that immediately trips SC.
- Motor winding phase-to-phase short — Internal motor winding failure where two phases short together, presenting a near-zero load impedance to the drive output.
- Failed IGBT module — One or more IGBTs in the drive’s output power stage fail in a short-circuit mode (collector-emitter short). This is common after a previous overcurrent or overvoltage event that stressed the devices.
- Contactor or shorting device on output — A motor run/bypass contactor that closes while the drive is running can present a short to the drive’s output. Never close an output contactor while the drive is supplying voltage.
Step-by-Step Fix
- Disconnect the motor cable at the drive output (T1/T2/T3) — Remove all connections to the drive’s output terminals. Measure resistance between each pair of output terminals (T1-T2, T2-T3, T1-T3) with a multimeter. Any near-zero reading with the drive powered off and discharged indicates either a failed IGBT or the measurement includes a parallel path.
- Check motor winding resistance — With the motor cable disconnected from the drive, measure resistance between each motor terminal pair (U-V, V-W, U-W). All three readings should be equal within a few percent. A low or zero reading indicates a phase-to-phase short in the motor.
- Megger test motor and cable — Perform insulation resistance tests at 500V between phase conductors and between phases and ground.
- Test drive IGBT modules — With the drive fully de-energized and DC bus discharged (wait 10+ minutes), use a diode-test mode multimeter to check each IGBT. In one direction you should see a diode drop; in the other, near-infinite resistance. A short in both directions indicates a failed IGBT.
- Replace failed component — Replace the motor, cable, or drive IGBT module (or full drive power module) as indicated by the test results.
Parts Often Needed
| Part | Notes |
|---|---|
| Drive IGBT module / power board | Must match drive voltage/current rating; Yaskawa components are model-specific |
| Motor (replace or rewind) | If winding short is confirmed; rewinding larger motors is cost-effective |
| Output cable | Replace if insulation damage or conductor-to-conductor short found |
Technician Tips
When diagnosing VFD faults, always check the drive’s fault history before resetting. Modern drives store the last 5-10 faults with the drive state at the time (running, accelerating, decelerating, stopped). An overcurrent fault that occurs only during acceleration points to incorrect motor data or mechanical binding. The same fault during deceleration suggests regeneration issues. A ground fault that clears after disconnecting the motor and returns when reconnected means the motor or cable insulation has failed.
Prevention: Schedule annual infrared scanning of drive cabinets. Loose power connections and failing capacitors produce heat long before they cause faults. A 5-minute scan can prevent hours of unplanned downtime.
Need Replacement Parts?
Before ordering, match the drive part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
When to Call a Pro
SC faults involving failed IGBTs inside the drive require capacitor discharge verification, high-voltage PPE, and component-level knowledge. IGBT module replacement on industrial drives is typically performed by a Yaskawa service technician or certified drive repair center.