WiremanWireman · VFD ground fault: troubleshooting for electricians · 2023 NEC ·
VFD ground fault
Test the motor and leads with the drive disconnected, then look at cable length and wet conduit.
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How it works
A VFD’s ground fault detection compares the current going out on the three output leads. When they don’t add to nearly zero, some current is returning through ground, and the drive trips. A real insulation failure in the motor or leads is the most common cause. Long motor leads also leak some current to ground through cable capacitance because of the drive’s fast switching, which can cause nuisance trips on long runs.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- When does it fault?
- Every time it startsTest: Insulation-test the motor and leads with the drive disconnectedExpect: Well above 5 MΩ to ground on every lead.
- Motor reads lowHas the motor been wet, idle for a long time, or in a damp area?
- Yes, it could be wetMoisture in the motor
- No, it’s dryMotor winding insulation failure
- Leads read lowDamaged motor leads
- Everything reads highHow long are the motor leads, and what does the drive manual allow?
- Longer than the manual allowsLeakage from long motor cable
- Within the limitPossible drive problem
- Motor reads lowHas the motor been wet, idle for a long time, or in a damp area?
- After rain, washdown or humid weatherTest: Insulation-test the motor and leads with the drive disconnectedExpect: Well above 5 MΩ to ground on every lead.
- Motor reads lowHas the motor been wet, idle for a long time, or in a damp area?
- Yes, it could be wetMoisture in the motor
- No, it’s dryMotor winding insulation failure
- Leads read lowDamaged motor leads
- Everything reads highHow long are the motor leads, and what does the drive manual allow?
- Longer than the manual allowsLeakage from long motor cable
- Within the limitPossible drive problem
- Motor reads lowHas the motor been wet, idle for a long time, or in a damp area?
- Only at higher speeds, long motor leadsHow long are the motor leads, and what does the drive manual allow?
- Longer than the manual allowsLeakage from long motor cable
- Within the limitPossible drive problem
- Every time it startsTest: Insulation-test the motor and leads with the drive disconnectedExpect: Well above 5 MΩ to ground on every lead.