What causes fault code VFD-THD1 (VFD Output Transistor Fault)?

IGBT module failure — end of life or surge damage. Output short circuit — motor cable pinched or shorted. Gate driver board failure — not controlling IGBTs properly. Power surge from lightning or switching transient. Motor with very low impedance — cable too short with large motor

How do I fix fault code VFD-THD1?

Power down and wait 5 minutes for capacitor discharge. Disconnect motor cables at drive output. Use IGBT tester or diode check on multimeter across each output phase to DC bus. Each IGBT should show ~0.4V in one direction, OL in other. If any IGBT reads shorted, drive needs repair or replacement. Check motor cables for shorts before reconnecting

VFD Output Transistor Fault

VFD-THD1

Drive output power transistor (IGBT) reported a desaturation or short-circuit fault. Output stage may be damaged.

critical vfd

vfd igbt transistor output short circuit

Probable Causes (ranked)

IGBT module failure — end of life or surge damage
Output short circuit — motor cable pinched or shorted
Gate driver board failure — not controlling IGBTs properly
Power surge from lightning or switching transient
Motor with very low impedance — cable too short with large motor

Fix Steps

1 Power down and wait 5 minutes for capacitor discharge
2 Disconnect motor cables at drive output
3 Use IGBT tester or diode check on multimeter across each output phase to DC bus
4 Each IGBT should show ~0.4V in one direction, OL in other
5 If any IGBT reads shorted, drive needs repair or replacement
6 Check motor cables for shorts before reconnecting

Field Trick

Diode-check from each output terminal to DC+ and DC-. You should see ~0.4V drop one way, OL the other. If any reads 0.0V both ways, that IGBT is shorted.

Suggested Replacement Parts

Parts matched to fault category: Vfd Drive

Allen-Bradley PowerFlex 525 AC Drive 1HP P/N: 25B-D2P3N114 1HP, 480V, 3-phase, built-in EtherNet/IP, EMC filter

AutomationDirect $395.00 Amazon $450.00 Grainger $520.00

ABB ACS580 General Purpose Drive 2HP P/N: ACS580-01-05A7-4 2HP, 480V, built-in safe torque off, adaptive programming

AutomationDirect $425.00 Amazon $480.00 Grainger $550.00

GS20 AC Drive 1HP (Budget Option) P/N: GS20-20P5 1HP, 230V, sensorless vector, compact size — good replacement for testing

AutomationDirect $149.00 Amazon $175.00

Prices are estimates. Verify part numbers match your equipment before ordering.

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