WiremanWireman · VFD overvoltage or undervoltage fault: troubleshooting for electricians · 2023 NEC ·
VFD overvoltage or undervoltage fault
DC bus too high is usually deceleration; too low is usually the supply.
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How it works
Inside a VFD, the incoming AC is rectified onto a DC bus held up by capacitors. On a healthy supply the bus sits near 1.35 to 1.41 times the line voltage: about 650–680 V DC on a 480 V drive, about 325–340 V DC on a 240 V drive. When a motor is slowed faster than it would coast, it acts as a generator and pushes energy back onto the bus, raising it until the drive trips on overvoltage. When the supply sags or loses a phase, the bus falls and the drive trips on undervoltage.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- Which fault, and when?
- Overvoltage while slowing down or stoppingDoes the load have a lot of inertia, or does it overhaul the motor?
- Heavy or overhauling loadRegenerated energy on deceleration
- Light load, short decel timeRegenerated energy on deceleration
- Decel is long and it still tripsTest: Measure the drive’s input voltageExpect: All three equal within about 1–2%, within the drive’s rated input range, and steady.
- One phase missing or lowLost phase on the drive input
- All low, or sags when loads startSupply voltage sag
- High, or spikes recordedHigh line voltage or transients
- Normal and steadyTest: Check the DC bus readingExpect: About 1.35 to 1.41 times the measured line voltage, steady at idle.
- Bus low or unstable with good inputPossible internal drive problem
- Bus normalBrief supply events
- Overvoltage at idle or steady speedTest: Measure the drive’s input voltageExpect: All three equal within about 1–2%, within the drive’s rated input range, and steady.
- One phase missing or lowLost phase on the drive input
- All low, or sags when loads startSupply voltage sag
- High, or spikes recordedHigh line voltage or transients
- Normal and steadyTest: Check the DC bus readingExpect: About 1.35 to 1.41 times the measured line voltage, steady at idle.
- Bus low or unstable with good inputPossible internal drive problem
- Bus normalBrief supply events
- UndervoltageTest: Measure the drive’s input voltageExpect: All three equal within about 1–2%, within the drive’s rated input range, and steady.
- One phase missing or lowLost phase on the drive input
- All low, or sags when loads startSupply voltage sag
- High, or spikes recordedHigh line voltage or transients
- Normal and steadyTest: Check the DC bus readingExpect: About 1.35 to 1.41 times the measured line voltage, steady at idle.
- Bus low or unstable with good inputPossible internal drive problem
- Bus normalBrief supply events
- Overvoltage while slowing down or stoppingDoes the load have a lot of inertia, or does it overhaul the motor?