CALL FOR BEST PRICE
+1-800-724-4209
Common VFD Faults: Overvoltage, Overcurrent, Undervoltage and Overload (Causes and Fixes)
Most VFD trips fall into a handful of categories. The fault code names differ between brands, so check your drive manual for the exact code, but the causes and fixes are much the same everywhere. Here is what each one usually means.
Safety first: a drive's DC bus capacitors stay charged after power is removed. Lock out, wait the time stated on the drive, and verify zero voltage before opening it or touching motor terminals.
Quick reference
| Fault | Most common causes | First things to try |
|---|---|---|
| Overvoltage | Decelerating too fast; overhauling load; high supply voltage | Longer decel time; enable bus regulation; add braking resistor |
| Overcurrent | Accelerating too fast; short or ground fault in motor or cable; jammed load | Megger motor and cable; longer accel; check mechanics |
| Undervoltage | Supply dips or loss; loose input connection; blown input fuse | Check supply and connections; enable ride-through if available |
| Motor overload | Load too heavy for too long; wrong motor FLA set; drive or motor undersized | Verify parameters; measure current; check load |
| Overtemperature | Blocked or failed fan; dirty heatsink; hot enclosure | Clean, check fan, improve ventilation |
| Ground fault | Damaged motor insulation or cable | Insulation test motor and cable, drive disconnected |
| Phase loss | Missing input phase or open motor lead | Check fuses, contactors and terminals |
Overvoltage
The DC bus voltage rose above the drive's limit. The usual cause is deceleration: when the drive slows a motor faster than the load would coast, the motor acts as a generator and pushes energy back into the drive. High-inertia loads (fans, flywheels, centrifuges) and overhauling loads (lowering hoists, downhill conveyors) are the usual suspects.
- Increase the deceleration time, or set stop mode to coast where that is acceptable.
- Enable the drive's bus regulator or overvoltage control, which extends deceleration automatically.
- Add a braking resistor if stops must be fast.
- If it trips when the motor is not decelerating, measure the supply voltage; it may be high, or have spikes from capacitor switching on the utility side.
Overcurrent
Output current exceeded the drive's limit, often instantly.
- Trips immediately on start, even unloaded: suspect a short or ground fault. Disconnect the motor at the drive and test the motor and cable insulation with a megohmmeter. Never megger with the drive connected.
- Trips during acceleration: increase the accel time, check the torque boost setting, and make sure the motor nameplate data is entered correctly.
- Trips while running: look for a mechanical jam, a failing bearing, or a load beyond the drive's rating.
- Opening a contactor between the drive and motor while running can also cause overcurrent trips and damage.
Undervoltage
The DC bus dropped too low, usually because the supply dipped or was lost. Check for loose input terminals, a blown fuse or a missing phase, and look at what else starts on the same supply. Some drives offer power-loss ride-through to keep running through short dips.
Overload
The drive's electronic motor overload (an I²t calculation) estimates the motor is overheating. Confirm the motor FLA and service factor are entered correctly, measure actual current, and check whether the load or duty cycle has changed. A motor running continuously at low speed and high torque may also need extra cooling.
Overtemperature
Clean the heatsink and filters, check the cooling fan runs, and measure the enclosure temperature. Most drives are rated for around 40 °C ambient without derating. Leave the clearances above and below the drive that the manual requires.
A general troubleshooting routine
- Record the fault code, and the drive's fault log with time stamps if available.
- Note what the machine was doing: starting, running, stopping, or idle.
- Check parameters against the motor nameplate and application.
- Inspect wiring, terminals and cooling.
- Test the motor and cable insulation with the drive disconnected.
- Reset and run unloaded, then loaded, watching current on the drive display.
When the drive itself has failed
Repeated hardware faults with a healthy motor and supply, visible damage, or a drive that will not power up usually mean replacement. Size the replacement correctly; see how to size a VFD. We stock Unitronics UMI drives and new surplus Allen-Bradley PowerFlex drives. Call 1-800-724-4209 or request a quote.