VFD Fault Codes Explained: How to Troubleshoot a Variable Frequency Drive

A variable frequency drive trips, the motor coasts to a stop, and the display shows a code. That code is the drive telling you what it saw in the last few milliseconds before it protected itself. Read it correctly and you will usually find the problem is outside the drive. Here is how to work through the common VFD fault codes in order, and how to know when the drive itself needs repair.

Start Here: What a Fault Code Is Actually Telling You

Every VFD manufacturer uses its own code numbers, but the underlying faults are the same across Allen-Bradley PowerFlex, Siemens SINAMICS, ABB, Yaskawa, Danfoss, Schneider and the rest. The drive monitors DC bus voltage, output current, heatsink temperature and a handful of internal signals, and trips when one leaves its window.

Before you touch anything:

• Write down the code and the drive's fault history. Most drives log the last several faults with the bus voltage, output current and frequency at the moment of the trip. That snapshot is your best evidence.
• Note what the motor was doing. Accelerating, running at speed, decelerating or sitting at zero speed each point to different causes.
• Do not reset and run repeatedly. Clearing a fault five times in a row turns a cheap problem into an expensive one.

Overcurrent (OC) Faults

An overcurrent fault means output current spiked past the drive's limit. It is the most common VFD fault code and the one most often blamed on the drive when the drive is fine.

Check, in order:

• Acceleration time. Too short a ramp on a high-inertia load demands more current than the drive can supply. Lengthen the ramp and retest.
• Mechanical load. A seized bearing, jammed conveyor or failed gearbox pulls the motor into overload. Turn the motor shaft by hand with power locked out.
• Motor and cable insulation. A phase-to-phase or phase-to-ground short in the motor leads or windings trips OC instantly on start. Megger the motor and cable with the drive disconnected. Our guide on how to test an electric motor covers the steps.
• Output contactor timing. Opening or closing a contactor between drive and motor while the drive is running will trip OC every time.
• Motor nameplate parameters. Wrong rated current, voltage or slip values in the drive make its current limit calculations wrong.

If the drive trips OC with the motor leads disconnected, the output transistors (IGBTs) or current sensors are suspect and the drive needs bench repair.

Overvoltage (OV) Faults

Overvoltage is almost always a DC bus problem, and the DC bus rises for two reasons:

• Regeneration during deceleration. A load with momentum drives the motor as a generator and pumps energy back into the bus. Fix: lengthen the deceleration ramp, enable the drive's overvoltage suppression or add a dynamic braking resistor.
• High incoming line voltage. Check incoming voltage at the drive terminals under load, phase to phase. Transients from utility switching or nearby power factor correction capacitors also show up here; a line reactor absorbs them.

Undervoltage (UV) and DC Bus Faults

Undervoltage means the bus sagged. Look for:

• Loose or corroded incoming power connections. Check terminal torque and look for discoloration.
• A lost input phase. Single-phasing a three-phase drive drops the bus and overworks the remaining phases. Measure all three incoming phases.
• Sagging supply. A large motor starting across the line elsewhere in the plant can pull the bus down momentarily.
• Aging DC bus capacitors. Capacitors lose capacity over years of heat exposure. A drive that trips UV on load steps that it used to handle is a candidate for capacitor replacement, which is a routine repair-shop job.

Overtemperature (OH) Faults

The drive's heatsink or internal ambient exceeded its limit. This is a slow-moving fault, so the cause is usually visible:

• Clogged heatsink or cabinet filters. Blow out the heatsink fins and replace filters.
• Failed cooling fan. Drive fans have a finite life. Listen for it; if it is not turning, replace it before it takes the power section with it.
• High ambient temperature. Check the enclosure temperature against the drive's rating. Panels in direct sun or next to ovens need ventilation or cooling.
• Derating. A drive running above its rated carrier frequency or at altitude is a smaller drive than its label says.

Ground Fault (GF) Codes

The drive detected current leaking to ground. Lock out and megger the motor and the motor cable separately. Long cable runs with high carrier frequency can also produce enough capacitive leakage to trip sensitive drives; an output reactor or dV/dt filter solves it. If the motor and cable test clean and the fault persists with leads disconnected, the fault is in the drive's output section.

Motor Overload (OL) Faults

The drive calculated that the motor has been drawing more than rated current for too long. Verify the motor nameplate data in the drive, then look at the mechanical load and the duty cycle. A motor that is now too small for a process that has grown over the years will trip OL honestly. If the motor itself is hot and drawing high current with no mechanical cause, it may need rewinding; see our motor repair services.

Communication and External Faults

Fieldbus timeouts, lost run-enable signals and external fault inputs are wiring and network problems far more often than drive problems. Check the control wiring, terminal torque, shield grounding and the PLC side of the link. The PLC troubleshooting field guide covers the controller end.

When the Fault Is in the Drive

After the checks above, these symptoms point to the drive hardware:

• Trips OC or GF with the motor leads disconnected
• Display blank or scrambled with correct incoming power
• Burnt smell, discolored board or visible capacitor bulging
• Faults that follow the drive when it is swapped to a known-good motor
• Repeated UV trips with verified clean incoming power

A drive in this condition is repairable far more often than it is scrap. Power sections, IGBT modules, bus capacitors, gate driver boards, control boards, cooling fans and current sensors are all replaceable at the component level. For larger or discontinued drives, repair is usually the faster and less expensive path, and it avoids re-engineering parameters and wiring for a different model.

VFD Preventive Maintenance That Prevents Most of These Codes

• Quarterly: Clean heatsinks and filters, check fan operation, inspect for discoloration.
• Annually: Torque power and control terminals, record DC bus voltage and output current at known load, back up parameters to a file.
• Every few years: Plan bus capacitor and cooling fan replacement based on run hours and ambient temperature rather than waiting for a failure.
• Always: Keep a current parameter backup off the drive. A repaired or replacement drive is useless until it is configured.

VFD Repair From IVS Incorporated

IVS Incorporated repairs variable frequency drives, DC drives and servo drives from Allen-Bradley, Siemens, ABB, Yaskawa, Danfoss and other manufacturers at our Plymouth, Michigan facility. Every drive is diagnosed at the component level and tested under load before it ships back. If your drive is throwing a code you cannot clear, send us the model and the fault history and we will tell you whether it is a panel problem or a drive problem. See our drive repair services or request a quote.

FAQ

What is the most common VFD fault code?

Overcurrent. It is also the one most often caused by something other than the drive: a short acceleration ramp, a mechanical overload or a motor insulation fault.

Can I reset a VFD fault and keep running?

Once, to see whether it was a transient. If the fault returns, stop and diagnose. Repeated resets on an overcurrent or ground fault can destroy the output transistors.

How do I know if the VFD or the motor is the problem?

Disconnect the motor leads and run the drive. If it faults with no load connected, suspect the drive. If it runs clean, megger the motor and cable.

Is it worth repairing a VFD?

For most drives above a small horsepower range, and for any discontinued model, yes. Component-level repair restores the exact unit, so parameters and wiring stay the same.

How long do VFDs last?

It depends mostly on heat and cleanliness. Cooling fans and bus capacitors are the wear items; replacing them on a schedule extends drive life considerably.