WiremanWireman · VFD overcurrent fault: troubleshooting for electricians · 2023 NEC ·
VFD overcurrent fault
When the fault happens tells you whether it’s the ramp, the load, or a short on the drive output.
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How it works
A VFD turns incoming AC into DC on an internal bus, then switches that DC with transistors (IGBTs) to build a variable-frequency output for the motor. The transistors can only survive a limited current for a very short time, so the drive watches output current closely and trips fast. An overcurrent fault means the motor current spiked past the drive’s limit: the drive asked for too much too fast, the load jammed, or there’s a short or ground on the output.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- When does the overcurrent fault happen?
- The instant it gets a run commandTest: Insulation-test the motor and leads, with the drive disconnectedExpect: Insulation well above 5 MΩ to ground, and three equal lead-to-lead resistances.
- Low to ground or lead to leadShort or ground fault in the motor or output leads
- Motor and leads test goodPossible failed drive output section
- While speeding upHow is the acceleration time and boost set, compared with the load?
- Short ramp or high-inertia loadAccel ramp too fast for the load
- Motor data not entered or doesn’t matchMotor data not set in the drive
- Settings look reasonableTest: Compare running current to the motor and drive ratingsExpect: At or below motor FLA, balanced between the three leads, and within the drive’s output rating.
- Near or above the drive’s ratingDrive too small for the motor or duty
- Spikes when the machine binds or loads upMechanical overload or jam
- Unbalanced between leadsTest: Insulation-test the motor and leads, with the drive disconnectedExpect: Insulation well above 5 MΩ to ground, and three equal lead-to-lead resistances.
- Low to ground or lead to leadShort or ground fault in the motor or output leads
- Motor and leads test goodPossible failed drive output section
- While running at speedTest: Compare running current to the motor and drive ratingsExpect: At or below motor FLA, balanced between the three leads, and within the drive’s output rating.
- Near or above the drive’s ratingDrive too small for the motor or duty
- Spikes when the machine binds or loads upMechanical overload or jam
- Unbalanced between leadsTest: Insulation-test the motor and leads, with the drive disconnectedExpect: Insulation well above 5 MΩ to ground, and three equal lead-to-lead resistances.
- Low to ground or lead to leadShort or ground fault in the motor or output leads
- Motor and leads test goodPossible failed drive output section
- When a contactor or disconnect on the output opens or closesSwitching between the drive and the motor while running
- The instant it gets a run commandTest: Insulation-test the motor and leads, with the drive disconnectedExpect: Insulation well above 5 MΩ to ground, and three equal lead-to-lead resistances.