VFD Overvoltage Trip: Causes & How to Fix It

A VFD overvoltage trip is most often caused by regenerative energy during deceleration: when a high-inertia load is slowed down quickly, the motor acts as a generator and pumps energy back into the drive, raising the DC bus voltage until the drive protects itself. The other common causes are supply voltage swells and grid transients. This guide explains what the fault code means, how to diagnose it in four steps, and which fix applies to each cause.

What "Overvoltage" Means Inside a VFD

The VFD converts AC to DC on the intermediate (DC bus) circuit and then back to adjustable AC at the output. The DC bus voltage is the drive's internal power rail – typically about 540 V DC for a 380 V drive and about 680 V DC for a 480 V drive. When the bus voltage exceeds the drive's protection threshold, usually around 120–130% of nominal, the drive trips and shows an overvoltage fault (often codes such as OV, E.OU, OU or similar). The drive does this on purpose: exceeding the threshold risks damaging the IGBT power stage and capacitors.

Cause 1: Regenerative Energy During Deceleration (Most Common)

When a motor with high inertia – a fan, centrifuge, conveyor with heavy load, or large flywheel – is decelerated, the motor briefly becomes a generator. Kinetic energy flows back into the DC bus and pushes the voltage up. If the deceleration ramp is shorter than the time the load needs to shed that energy, the bus overvoltages and the drive trips. This is why the fault usually appears "when I stop the machine", not during steady running.

Cause 2: Supply Voltage Swells and Grid Transients

Grid events can also push the DC bus above the threshold: short voltage swells, capacitor-bank switching on the same network, lightning surges, or a generator that overspeeds momentarily. These are usually intermittent – the fault appears at random times, not tied to starting or stopping. A long or poor-quality supply is often the background cause.

Cause 3: Faulty Braking or Regenerative Hardware

On drives with a braking chopper and resistor, an open braking resistor or a failed chopper leaves the regenerative energy nowhere to go, so the bus rises until the trip. Overload protection may not catch this because the resistor only conducts briefly during deceleration. If the drive trips on overvoltage while the brake resistor shows no activity, the braking circuit itself is suspect.

How to Diagnose: Four Steps

1. Note when the trip happens – during deceleration, at start-up, or randomly during running. This alone points to the cause in most cases.
2. Check the fault log and DC bus voltage reading at the trip (most drives store both).
3. Measure the supply voltage over a period (a few days if possible) to catch swells or unbalance.
4. Verify braking hardware: measure braking resistor continuity and check the chopper status.

Fault Code Quick Reference

Fixes, by Cause

• Longer deceleration ramp: the cheapest first step – extend the deceleration time so the load sheds energy gradually
• Braking resistor / chopper: add a braking resistor for frequent stops or high-inertia loads; verify existing hardware if fitted
• Dynamic braking unit: for large inertia or frequent cycles where a resistor alone cannot absorb the energy
• Input reactor or surge protection: for supply-side swells and transients; a line reactor also reduces harmonics
• Check the supply: work with the site electrical team if the mains is unstable or shared with capacitor banks

Frequently Asked Questions

Why does my VFD overvoltage only when stopping? That is the classic signature of regenerative energy during deceleration. Increase the deceleration time first; if the process requires a fast stop, add a braking resistor.

Is overvoltage damage dangerous? The drive trips to protect itself, so a single trip usually causes no damage. Repeated trips stress the capacitors and power stage, so the cause should be fixed rather than the trip cleared repeatedly. See our VFD FAQs for related questions.

Can I just increase the overvoltage level in parameters? Only as a temporary workaround within the manufacturer's range – it reduces the safety margin and does not remove the energy causing the problem. Fix the root cause instead. Our VFD programming guide explains where these parameters live.

If your drive trips on overvoltage and the fixes above do not resolve it, contact our support team – we can review the fault log remotely. For related reading, see how to troubleshoot IGBT faults, our application notes, and the VFD vs. soft starter guide.