Engineering

VFD Overvoltage Fault and Braking Resistor Sizing

A VFD overvoltage fault is not always a resistor problem. The first clue is when the trip appears: at power-up, at steady speed, during a fast stop or only after repeated cycles. This page gives a practical order for checking the fault before parts are ordered.
VFD fault
DC bus
deceleration
brake chopper
resistor duty
First decision If the drive trips only while slowing a high-inertia load, regeneration is likely. If it trips while idle or running steadily, check the supply and load condition before treating it as a braking resistor issue.
VFD drive and braking resistor inside a control cabinet for overvoltage fault diagnosis

Start With the Moment of the Fault

The same fault code can point to different causes depending on timing.
When the trip appearsMost likely directionFirst useful checkTypical next step
At power-up or idleIncoming supply, DC bus threshold, drive settings or line disturbance.Measure supply voltage and compare the fault history with machine state.Correct supply issue, transformer tap or drive parameter before adding brake hardware.
At steady speedOverhauling load, unstable supply, line regeneration or a load pushing the motor.Check whether the load can drive the motor mechanically.Review load behaviour and drive mode; braking parts may not be the first fix.
During decelerationRegenerated energy raises the DC bus during a fast stop.Increase decel time temporarily and watch whether the trip disappears.If the stop time cannot be extended, check brake chopper and resistor sizing.
During emergency or quick stopStop demand is higher than the drive and existing braking path can absorb.Compare required stop profile with the drive manual and safety design.Use suitable braking hardware or a different stopping method.
After a resistor was fittedWrong resistance, wrong duty rating, missing chopper, thermal trip or wiring issue.Check resistance against the drive minimum and check resistor temperature protection.Correct the package instead of simply fitting a larger-looking resistor.

What Happens During a Fast Stop

During deceleration the load can feed energy back into the drive.

A VFD normally rectifies the incoming AC supply into a DC bus, then uses the inverter section to control the motor. During a hard stop, a motor connected to a heavy or overhauling load can act like a generator. The returned energy raises the DC bus voltage.

If the drive cannot absorb or remove that energy quickly enough, it trips on overvoltage to protect itself. Extending the deceleration time often proves the point: the same load stops more slowly and the bus has less energy to absorb at once.

This is why the first repair should not be guesswork. A braking resistor may be correct for a fast repeated stop, but it is not the answer to every overvoltage code.

Fast deceleration energy returning from motor to VFD DC bus
Bus evidence
When possible, use the drive fault history or DC bus monitor to see whether the voltage rise matches the stop command.
Brake chopper switching regenerated energy from the VFD into a braking resistor
Plain rule
The brake chopper is the switch. The resistor is the heat path. A resistor alone does not create braking if the drive has no braking transistor or external braking unit.

Brake Chopper and Resistor Are Not the Same Part

This distinction prevents many bad fixes.

The resistor is only the place where braking energy becomes heat. The chopper circuit, braking transistor or external braking unit decides when that path is connected to the DC bus.

Before sizing the resistor, confirm whether the drive has an internal brake transistor, whether it needs an external braking unit, and what minimum resistance the manufacturer allows. A resistor below that minimum can pull too much current through the braking device.

The power rating is a separate question. It depends on how much energy is produced during each stop and how often that stop repeats. A resistor that survives one stop may still overheat in a short cycle.

Quick Braking Resistor Screening

This is a first-pass estimate, not a substitute for the drive manual.

The calculator below is deliberately conservative and simple. It helps decide whether the job is light, moderate or likely to need a proper braking package. Final resistance, wattage, thermal protection and mounting must come from the drive manual and resistor data.

Use it when the fault happens during deceleration and the machine cannot simply stop more slowly. For hoists, centrifuges, downhill conveyors, high-inertia fans or safety-related stops, treat the result as a reason to do a formal check, not as a purchase specification.

Do not skip
Check the drive minimum brake resistance before ordering. The calculated resistance window is useless if the chosen resistor is below the braking transistor limit.

Screening calculator

Enter the motor and stop cycle, then calculate. The result gives a screening estimate for peak braking power, average heat load and the resistance window to check against the drive manual.
Braking resistor selection checks for resistance duty cycle power rating and mounting clearance
Selection check
Resistance protects the braking transistor. Power and energy rating protect the resistor. Both checks are needed.

What Must Be Checked Before Ordering

A braking resistor is not selected from motor kW alone.
Drive minimum resistanceThe selected resistor must not be below the drive or braking module limit.
Peak braking powerThe resistor and chopper must handle the short burst created by the stop.
Average powerRepeated stops create heat over time, even if each single stop looks acceptable.
Duty cycleShort cycle times need more thermal capacity than occasional stops.
Mounting and airflowThe resistor is a heater. It needs space, protection and suitable cable routing.
Thermal protectionA resistor fault should not silently become a cabinet fire risk.

When a Braking Resistor Is Not the First Answer

The useful fix depends on why the DC bus is rising.
The fault occurs while idleStart with incoming voltage, drive settings, supply quality and fault history. A resistor does not correct a high supply.
The load regenerates continuouslyA downhill conveyor, lowering axis or overhauling load may need a regenerative unit rather than a heat-dumping resistor.
The drive has no chopperAn external resistor will not work unless there is a braking transistor or braking unit to switch it.
The stop time can be extendedIf production allows a longer ramp, parameter correction may be cleaner than adding heat to the cabinet area.
The trip follows a retrofitCheck motor data, ramp settings, load changes and wiring before assuming the resistor became too small.
The area is hot or dustyA resistor can solve the drive trip and create a thermal problem elsewhere if mounting is poor.
Decision order

A Clean Troubleshooting Sequence

  1. Record when the overvoltage fault appears.
  2. Check incoming supply and drive fault history.
  3. Temporarily increase deceleration time if the fault appears during stopping.
  4. Confirm whether the drive has a braking transistor or needs an external braking unit.
  5. Check minimum brake resistance in the drive manual.
  6. Estimate peak and average braking load for the stop cycle.
  7. Review mounting, airflow, cable route and thermal protection before installation.

The Goal Is a Reliable Stop, Not Just a Cleared Alarm

A hidden braking problem often returns after the first successful test.

After a resistor is fitted, the job is not finished when the first stop works. The drive should be checked through the real machine cycle, including repeated stops, warm cabinet conditions and the fastest stop that production actually uses.

Look for heat rise around the resistor, nuisance thermal trips, braking unit alarms and any change in stop distance. If the resistor is mounted inside or near a cabinet, make sure its heat does not create another control problem.

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Practical Checklist

Do not start with the resistorStart with the fault timing and supply condition.
Use decel time as a testIf a longer ramp clears the fault, regenerated energy is likely.
Check the chopperMake sure the drive can actually switch a braking resistor.
Respect minimum ohmsNever choose a resistor below the drive limit.
Size for heatRepeated braking is a thermal problem, not just an electrical one.
Protect the installationUse suitable mounting, airflow, cables and thermal protection.

Common Questions

Why does a VFD trip on overvoltage during deceleration?

A fast stop can make the motor act as a generator. Energy returns to the DC bus faster than the drive can absorb it, so the DC bus voltage rises and the drive trips to protect itself.

Should I always add a braking resistor for an overvoltage fault?

No. First check when the fault happens. If it appears at idle or at constant speed, the cause may be supply voltage, load behaviour or wiring rather than deceleration energy.

What does the braking resistor actually do?

The resistor gives regenerated energy a place to become heat. The brake chopper or braking unit switches that path on when the DC bus rises.

Why is minimum brake resistance important?

The drive or braking module has a current limit. A resistor below the allowed minimum can overload the braking transistor, so the drive manual must be checked before selection.

What rating matters most for a braking resistor?

Resistance, peak energy and average power all matter. The duty cycle decides how much heat the resistor must survive over repeated stops.

When is a regenerative unit better than a braking resistor?

If energy is returned often or continuously, a regenerative unit may be better because it returns energy to the supply instead of turning it into heat inside or near the machine.