Symptom
The drive displays an overvoltage or DC bus overvoltage fault and stops. It happens on deceleration, on a speed reduction, at random while running, or on power-up, and the fault log shows the bus voltage at the trip.
The short answer
Drive Faults on Overvoltage
A drive trips on overvoltage when its DC bus, which sits at about 1.35 times the supply line voltage, rises above the limit the capacitors and switches can stand, roughly 800 V on a 480 V drive. The bus rises either because energy comes back from the motor, when the load drives the motor faster than the drive is commanding, which happens on fast deceleration and on pumps with a driving fluid column, or because the supply itself is high or transient, from a lightly loaded feeder, a capacitor bank switching, or lightning. The fault log timing separates them: on deceleration is regeneration; at random or at power-up with no load is the supply.
Key points
- The DC bus is about 1.35 times the line voltage: near 650 V on a 480 V supply, tripping around 800 V.
- On deceleration it is regeneration: the load is driving the motor. Lengthen the ramp, enable overvoltage control, or add braking.
- At random while running or at power-up it is the supply: a high line voltage, capacitor switching, or a transient.
- Measure the line voltage at the drive over a day. A supply at the top of its tolerance leaves no margin.
- A line reactor absorbs transients; a braking resistor absorbs regeneration. They fix different problems.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Deceleration ramp too short | The trip is on stop or on a large speed reduction, at a consistent point; the bus voltage in the log is high. Lengthen the deceleration time. |
| Load driving the motor | A pump with a fluid column that keeps it turning, a fan with a wind load; the drive commands a lower speed and the load resists. Overvoltage control, or coast to stop. |
| Supply voltage high | Line voltage at the top of the utility tolerance, a lightly loaded feeder at night, a transformer tap set high. Measure over 24 hours. |
| Utility capacitor bank switching | A transient at the same time each day when the capacitor bank switches; a line reactor at the drive input. |
| Lightning or switching surge | Random trips during storms or utility operations; surge protection at the service and a line reactor. |
| Phase imbalance or a lost phase with high voltage on the others | The bus ripple increases and the peaks trip; measure all three phases. |
| Braking resistor or chopper failed | On a drive fitted with braking, the resistor open or the chopper not firing; the bus rises during deceleration despite the resistor. |
| Overvoltage controller disabled | A drive parameter that lets the drive extend the deceleration automatically to hold the bus below the limit; disabled, the drive trips instead. |
| Generator supply | A generator with a poorly regulated voltage or a light load; the voltage rises when the drive decelerates and regenerates into a source that cannot absorb it. |
| Drive capacitor or sensing fault | The measured bus voltage is wrong, or the capacitors have degraded; the drive trips at a normal line voltage. |
Where the bus voltage comes from
A drive rectifies the incoming three-phase supply and filters it on a capacitor bank, producing a DC bus at roughly 1.35 times the line-to-line RMS voltage: about 650 V from a 480 V supply, about 310 V from 230 V. The inverter draws from that bus to make the motor voltage. The bus can only rise above its rectified value in two ways: the supply rises, or energy comes into the bus from the motor side. Energy comes from the motor when the motor is turning faster than the drive is commanding, which makes the motor a generator, and that happens whenever the drive decelerates a load with inertia, or whenever the load itself is driving the shaft. The drive trips when the bus exceeds the limit its capacitors and switches are rated for, commonly around 800 to 820 V on a 480 V drive.
Read the fault log
| When it trips | Bus voltage | Points toward |
|---|---|---|
| On stop or a speed reduction, at a consistent point | Rising through the deceleration | Regeneration: deceleration too fast, overvoltage control off, no braking |
| At steady speed, at the same time each day | A step | A utility capacitor bank or a scheduled load change on the feeder |
| At random, in storms or during utility work | A spike | A transient from lightning or switching |
| At power-up with the motor stopped | High before the drive runs | Supply voltage high; transformer tap; light feeder load |
| At night or weekends | High while running | Lightly loaded feeder; line voltage at the top of tolerance |
| With a generator supplying the site | High during deceleration | The generator cannot absorb the regeneration |
| At normal measured line voltage | Reported high, not measured high | Drive sensing or capacitor fault |
Diagnostic procedure
- 1
Record the fault data
Fault code, bus voltage at trip, frequency, whether the drive was decelerating, and the time. The pattern across several trips is the diagnosis.
- 2
Measure the supply
Line-to-line voltage on all three phases at the drive input, at the time of day the trips occur and over a day if possible with a logging meter. Compare with the drive input rating and the expected bus voltage. A 480 V drive on a 505 V supply has a bus near 680 V and little margin.
- 3
Check the deceleration
If the trips are on stop or on speed reductions: the deceleration time, the overvoltage control parameter, and any braking resistor. Lengthen the ramp as a test; if the trip disappears, it was regeneration.
- 4
Check the load
A pump on a force main or a fan in a duct can be driven by the fluid. A drive commanding a fast speed reduction on a driving load regenerates whatever the ramp says. Overvoltage control lets the drive slow only as fast as the bus allows; coast to stop removes the problem for a pump that can coast.
- 5
Check the braking hardware
Where fitted, the resistor continuity and the chopper operation; a resistor that has opened leaves the drive with no path for the energy.
- 6
Check for transients
A line reactor of 3 to 5 percent impedance at the drive input reduces the transient the drive sees from capacitor switching and surges, and it is the standard fix where the supply is the cause. Surge protection at the service entrance where lightning is the source.
- 7
Check the drive
If the measured line voltage is normal, the drive is not decelerating, and the trips persist, the bus voltage sensing or the capacitors are suspect. Compare the drive display of the bus voltage with 1.35 times the measured line voltage.
- 8
Fix and confirm
Trend the bus voltage from the drive over a day and through several stops.
Common mistakes
- Treating a supply overvoltage as a drive fault and replacing the drive; the new drive trips too.
- Fitting a braking resistor to a drive that trips on capacitor switching; the resistor does nothing for a supply transient.
- Shortening the deceleration to make the pump stop before the check valve slams, then trading the slam for an overvoltage trip. A slow-closing check valve is the fix for the slam.
- Ignoring a supply at 500 V on a 480 V drive because it is within the utility tolerance. Ask the utility about the tap, or specify a drive input range that covers it.
Frequently asked questions
- The drive trips on overvoltage every time the pump stops. What do I change?
- The deceleration: lengthen the ramp, enable the overvoltage controller, or set the stop mode to coast. On a pump, a coast is usually acceptable and it removes the regeneration entirely. Check that the check valve closes gently with the new stop.
- The trips happen at 2 p.m. every day. Why?
- Something on the utility feeder switches at that time, most often a capacitor bank for power factor correction, and the transient reaches the drive. A line reactor at the drive input is the fix; the utility may also be willing to adjust the switching.
- Can a drive run on a generator without overvoltage trips?
- Yes, with a longer deceleration or the overvoltage controller enabled, because a generator cannot absorb regenerated energy the way the utility can. Generator voltage regulation also matters; a set with a poor regulator swings the bus.
- Should I add a line reactor to every drive?
- A 3 percent line reactor or a DC bus choke is standard practice on drives above a few horsepower: it reduces harmonics, protects against transients, and extends the drive life. Many drives include a DC choke; check before adding a reactor that duplicates it.
Related topics
- Drive Faults on OvercurrentAn overcurrent trip on a variable frequency drive: what the drive measured and when, the difference between a trip at start, during acceleration, at steady speed, and at deceleration, the motor and cable faults behind each, the parameter settings that cause nuisance trips, and the tests that separate them.
- Variable Frequency Drives in Pump PanelsWhat a drive adds to a pump panel and what it costs: speed control, soft start, reduced inrush, and diagnostics against heat, harmonics, cable and motor stress, bypass, and the minimum speed below which a pump does no work.
- Lift Station Generator OperationStandby power at a lift station: what the transfer switch does, how the controls ride through the transfer, staggered pump restarts, what SCADA should see, load testing, and the portable generator connection.
- Surge Protection for Signal CircuitsProtecting 4-20 mA loops, discrete inputs, network cables, and radio feeds from lightning and switching surges: where surges enter, the device types and how they clamp, the grounding that makes them work, the resistance and capacitance they add, and where to put them at a remote site.
- Motor Will Not Reach SpeedA drive that runs but never gets the motor to the commanded speed: the reference and its scaling, the maximum frequency and speed limits, current limit holding the ramp, a load beyond the motor, a wrong motor parameter set, and a low supply, with the drive readouts that separate them.
- Drive Causes Instrument NoiseWhen many instruments became noisy after a drive was installed, the drive installation is the place to look: the checklist for motor cable, shielding, grounding, filters, and carrier frequency that decides how much noise a drive puts into the plant.
Direct contact
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