VFD Overvoltage Fault: Causes, Diagnosis, and Fixes

What is an overvoltage fault?

A VFD overvoltage fault (fault code varies by manufacturer — "OV", "DC BUS OV", "E-07", "F005" etc.) trips the drive and removes power from the motor when the DC bus voltage exceeds a protection threshold. The typical DC bus overvoltage trip point is approximately:

• 400 V AC supply: ~800 V DC trip threshold (some drives trip at 780 V, others at 820 V — check your drive's technical manual)
• 480 V AC supply: ~900 V DC trip threshold
• 240 V AC supply: ~400 V DC trip threshold

For context: a 400 V AC supply produces approximately 400 × √2 = 565 V DC on the rectified DC bus under normal operating conditions. The overvoltage trip at ~800 V DC allows about 235 V of headroom above normal bus voltage for transient events.

The DC bus and why voltage rises

A VFD has three stages: a rectifier (AC to DC), a DC bus (capacitor bank), and an inverter (DC to variable AC). The DC bus capacitors are sized to smooth the rectified DC and provide energy storage for transient load demands. The capacitors cannot absorb unlimited energy, and they have no way to return energy to the AC supply — a standard diode-bridge rectifier only conducts in one direction.

When an AC induction motor decelerates, its mechanical inertia keeps the rotor spinning faster than the inverter's commanded frequency. In this condition, the motor operates as a generator: the rotating magnetic field in the rotor induces voltages in the stator that are higher than the inverter output. Current flows back through the inverter's freewheeling diodes into the DC bus, charging the capacitors and raising the bus voltage.

If the rate of deceleration is too fast, or the mechanical load is high-inertia (flywheels, fans, centrifuges), the regenerated energy raises the bus voltage faster than the internal bus bleeder resistors (on drives that have them) or the braking chopper can dissipate it. When the voltage hits the trip threshold, the drive faults.

Root causes of VFD overvoltage faults

1. Deceleration too fast (most common)

The deceleration ramp time is set too short for the motor and load inertia. The drive tries to slow the motor faster than the kinetic energy of the rotating load can be dissipated naturally. Most common cause of OV faults, especially after a drive or parameter change.

2. High-inertia load

Some loads have high moment of inertia — flywheels, large fans, centrifuges, and large pumps. Even a slow deceleration ramp may still result in too much regenerated energy if the load inertia is very high. This is normal physics: the kinetic energy of a rotating load is E = ½ × J × ω², so a large flywheel at high speed contains a lot of energy that must go somewhere during stopping.

3. Overhauling load

An overhauling load is one that drives the motor rather than being driven by it. Examples: lowering a crane or hoist (gravity drives the load), a downhill conveyor, a compressor being driven by pipeline pressure. In these applications, the load can cause regeneration even at steady speed, not just during deceleration.

4. Mains voltage high

If the AC supply voltage is at the high end of its range (e.g., 420 V on a 400 V nominal supply), the DC bus rests at a higher voltage (~594 V DC). Less headroom remains before the overvoltage trip. During a deceleration that previously did not cause a fault, the higher starting bus voltage may now trigger an OV trip. Check the supply voltage with a power quality analyser.

5. Voltage spikes on the supply

Some drives trip on OV due to voltage transients on the incoming supply (switching of capacitor banks, nearby lightning events, utility switching) rather than regeneration. These trips typically appear at random times rather than consistently during deceleration.

How to diagnose which cause

Before changing any settings or adding hardware, record the following from the drive's fault log (most drives store the last several faults with timestamps and diagnostic data):

1. When does the fault occur? Always during deceleration → regeneration. At steady speed → overhauling load or supply transient. Randomly → supply voltage issue.
2. What is the DC bus voltage at the time of fault? The drive's fault log should include the bus voltage at trip. If the bus was ~600 V (normal for 400 V supply) just before the trip and then jumped to 800 V+, regeneration is confirmed.
3. What changed recently? Did someone change the decel ramp time? Was the load changed? Has a new piece of equipment been installed on the same supply circuit?
4. Is the fault consistent or intermittent? Consistent on every stop = decel ramp. Intermittent = supply voltage or overhauling load with variable demand.

Fix 1: Extend the deceleration ramp time

This is the simplest fix, costs nothing, and should always be tried first. Increasing the decel ramp time reduces the rate at which the drive commands speed reduction, which reduces the regenerated power at any instant. The motor takes longer to stop, but if the application can tolerate this, it eliminates the fault.

A rough guideline: if the drive is currently tripping on OV with a 5-second decel ramp, try 10 seconds and retest. Increase in steps until the fault no longer occurs. If you need the motor to stop in less time than the minimum required to avoid regeneration, you need a braking solution (below).

Some drives also have a parameter called "overvoltage stall prevention" or "kinetic energy buffering." When enabled, the drive automatically extends the decel ramp in real time if the DC bus voltage rises toward the trip threshold. This is a good default setting to enable for drives with variable loads.

Fix 2: Add a dynamic braking resistor

A dynamic braking resistor (DBR) dissipates the regenerated energy as heat when the DC bus voltage rises above the braking threshold (typically around 120–130% of nominal bus voltage, set by the drive). A braking chopper (IGBT transistor) switches the resistor across the DC bus; the chopper modulates the duty cycle to keep the bus voltage within limits.

Most drives in the 0.75–75 kW range have a built-in braking chopper. You add an external resistor to the chopper terminals (typically labelled DB, BR, P/BR, or similar — check your drive's wiring diagram). Larger drives may require an external chopper module as well.

Sizing the braking resistor

The braking resistor must meet two constraints:

1. Minimum resistance: Too low a resistance forces too much current through the chopper IGBT, which may damage it. The minimum resistance is set by the peak chopper current rating and the DC bus voltage: R_min = V_DC² / P_regen. The drive's technical manual will specify the minimum permissible resistance value — never go below this.
2. Power rating: The resistor must dissipate the average power based on the peak regenerated power and the braking duty cycle. Under-sized resistors overheat and fail.

Use the VFD Braking Resistor Calculator to calculate the minimum resistance and required power rating. Always verify against the drive manufacturer's application note — specific drives have specific minimum R values that override the general formula.

Braking resistor installation requirements

• Mount the resistor outside the VFD enclosure — it operates at high temperature (200–300 °C surface temperature is typical under load) and will overheat the drive if mounted inside.
• Ensure adequate clearance above and below the resistor for convection cooling.
• Connect the resistor's integral thermal protection switch (normally-open thermal contact) to a digital input on the drive to trip on thermal overload.
• Use correctly rated cable — check the drive manual for minimum conductor cross-section and maximum cable length for the braking resistor connection.

Fix 3: Regenerative front-end drive (active front end)

A regenerative drive replaces the standard diode bridge rectifier with a bidirectional active front end (AFE) consisting of IGBTs in a full-bridge configuration. The AFE can push current back into the AC supply when the motor regenerates — turning the regenerated energy into grid power rather than heat.

Regenerative drives are the correct solution when:

• The duty cycle of braking is very high (the resistor would need to be very large)
• Energy recovery is valuable (high-power applications, applications that brake frequently)
• An overhauling load requires sustained power to hold speed against the load (cranes, downhill conveyors)
• The thermal management constraints of a braking resistor cannot be met

Regenerative drives cost approximately 2–3× more than equivalent standard drives. For intermittent braking at low power, a braking resistor is almost always more cost-effective.

Mains overvoltage trips

If the drive faults on OV at random times not correlated with motor deceleration, the root cause may be supply voltage transients rather than regeneration. Diagnose with a power quality analyser on the incoming supply. Solutions include:

• Line reactor: An AC line reactor (2–5% impedance) on the incoming supply reduces the impact of voltage transients and also reduces harmonic currents drawn by the drive rectifier.
• Input EMC filter: Helps with high-frequency conducted disturbances from the supply.
• Move to a cleaner supply circuit: If the drive and a large transformer-switched load share a supply circuit, relocating the drive to a different circuit may resolve intermittent OV faults.

Which fix to choose?

Situation	Recommended fix
Process can tolerate slower stopping	Extend decel ramp (try this first — it's free)
Fast stopping required, braking <40% duty cycle	Dynamic braking resistor
High-duty braking, overhauling load, or energy recovery desired	Regenerative front-end drive
Fault occurs at random, not during deceleration	Supply analysis + line reactor