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 {#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 | Amazon | Must match drive voltage/current rating; Yaskawa components are model-specific |
| Motor (replace or rewind) | Amazon | If winding short is confirmed; rewinding larger motors is cost-effective |
| Output cable | Amazon | Replace if insulation damage or conductor-to-conductor short found |
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.