VFD Fault Troubleshooting Guide
Drive fault names vary between brands, but most faults fall into the same few families. This guide explains each family, the usual causes and what to check first.
Safety first. Drives store dangerous voltage in their DC bus capacitors after power is removed. Lock out and tag out the supply, wait the discharge time printed on the drive, and measure to confirm the bus is discharged before touching any terminal. Only qualified persons should work on drives, and live work must follow your site procedures and electrical safety rules.
Start the same way every time
- Write down the fault code and text exactly as shown, before you reset anything.
- Note when it happened: at start, during acceleration, at steady speed, during deceleration or at stop. The timing narrows the cause more than anything else.
- Check the fault history in the drive. Repeated identical faults point to a real cause, while a one-off may be a power event.
- Look at the machine, not only the drive. Many drive faults are mechanical problems the drive correctly detected.
- Find the cause before you reset. Repeated resets into a short circuit or ground fault can damage the drive.
Fault families
| Fault | What the drive saw | Common causes | First checks |
|---|---|---|---|
| Overcurrent / short circuit | Output current exceeded the limit, often instantly | Shorted motor or cable, ground fault, acceleration set too fast, jammed load, wrong motor data | Inspect cable and motor insulation (with the drive disconnected), lengthen the acceleration time, check the load turns freely, confirm motor nameplate data is entered |
| Ground fault | Current leaking from an output phase to earth | Damaged or wet motor winding or cable, very long cable, a failed drive output stage | Insulation-resistance test of motor and cable with them disconnected from the drive, following the motor and drive maker's test voltage limits |
| Motor overload | Current above the set limit for too long | Mechanical overload, overload setting below the motor's current, low supply voltage, low-speed running of a self-cooled motor | Compare running current with the nameplate, check for binding or a dragging brake, verify the overload parameters |
| Overvoltage (DC bus) | The DC bus rose too high | Deceleration too fast so the motor feeds energy back, overhauling load, high line voltage | Lengthen the deceleration time, check the line voltage, ask whether the application needs a braking resistor or regenerative unit |
| Undervoltage (DC bus) | The DC bus fell too low | Line dips or outages, loose input connection, blown input fuse, a failing pre-charge circuit | Check input voltage on all phases, tighten and inspect input terminals, check input fuses and upstream protection |
| Overtemperature | Heatsink or internal temperature too high | Blocked fan or filter, high ambient temperature, high carrier frequency, running near the limit | Clean heatsink and filters, confirm fans run, check the cabinet temperature and clearances, review carrier frequency |
| Input phase loss | One or more input phases missing or unbalanced | Blown fuse, loose lug, upstream breaker problem | Measure each input phase and check fuses and connections |
| Output phase loss | One output phase not carrying current | Open motor lead, loose terminal, contactor opening or one pole failing | Check motor leads and terminals, check any output contactor |
| Communication loss | Network or control signal stopped | Cable or connector fault, wrong network settings, timeout set too short | See the network troubleshooting guide, check the drive's network parameters |
| External or safety fault | A wired input told the drive to stop | Emergency stop, safe-torque-off input not satisfied, interlock open, failed relay | Check the safety circuit and what each input is configured to do. Never bypass safety inputs. |
Quick checks that save time
- Does the motor turn freely? After lockout, turn the shaft by hand. A stiff or locked load explains many overcurrent faults.
- Is the load actually what you expect? Compare the running current with the motor nameplate. The motor current calculator gives a reference for a motor's expected full-load current.
- Does speed match the command? The motor speed calculator shows the synchronous speed and slip for a given frequency and pole count.
- Is the cable long? Very long motor cables can cause overcurrent and ground fault trips. Check the drive manual's maximum cable length and whether output filters are needed.
- Did anything change? New load, new cable, a new acceleration time, a power-up after maintenance or a recent storm all matter.
When to call for help
If a fault returns immediately after reset with the motor disconnected, the drive's output stage may be damaged. If you see burn marks, a swollen capacitor or a smell of burnt insulation, stop and get the drive manufacturer's support or a repair shop involved.
This is general guidance for training and reference. Fault code meanings and corrective actions are specific to each drive; follow the manufacturer's manual.