A DC bus overvoltage fault occurs when the VFD detects excessive voltage on its internal DC bus and trips to protect the drive’s components. The cause is often related to incoming power or regenerative energy returned from the motor, although braking equipment, electrical-system conditions and internal VFD hardware can also contribute.
One of the most useful troubleshooting clues is when the overvoltage fault occurs. A trip during deceleration can point toward very different causes than a fault occurring during startup or while the motor is running at a steady speed.
Common Causes of DC Bus Overvoltage
- Incoming line voltage that is too high
- Rapid deceleration of a high-inertia load
- Overhauling or regenerative loads such as certain conveyors or hoisting applications
- Incorrect deceleration parameters
- Braking resistor, braking chopper or braking-circuit problems where used
- Power factor correction capacitor switching or other power-system disturbances
- Transformer tap or incoming power problems
- Loose or damaged incoming power connections
- Drive sensing, braking or internal hardware problems
Overvoltage During Deceleration or Stopping
During deceleration, the motor can act as a generator and return energy to the VFD’s DC bus. This is particularly important with high-inertia or overhauling loads. If regenerative energy enters the DC bus faster than the drive can dissipate or manage it, the DC bus voltage rises until the VFD trips on overvoltage.
Depending on the application and VFD, troubleshooting may include reviewing the deceleration ramp, checking whether a manufacturer-provided bus-voltage regulation function is appropriate, and inspecting the dynamic braking system where one is installed. A braking resistor, braking transistor or chopper that is open, damaged or incorrectly sized may prevent regenerative energy from being dissipated properly.
Overvoltage During Startup or Acceleration
If the fault occurs at startup or during acceleration, the incoming electrical supply deserves close attention. Excessive line voltage, improperly selected transformer taps, utility fluctuations or power factor correction capacitor switching can contribute to elevated DC bus voltage.
Qualified personnel can verify that incoming AC voltage is appropriate for the VFD and evaluate whether the supply is fluctuating or experiencing transient conditions. In some applications, appropriately selected line reactors or DC chokes may be part of the solution for reducing the effect of supply disturbances, but the electrical system and VFD manufacturer’s recommendations should be evaluated before adding equipment.
Overvoltage During Steady Running or Idle
Overvoltage faults that occur while the equipment is running steadily or sitting idle may point toward incoming power-quality issues rather than deceleration energy. Possible contributors include utility disturbances, phase imbalance, loose or deteriorated connections, capacitor-bank switching and interaction with external harmonic-filter components.
Incoming power terminals and associated electrical connections should be inspected by qualified personnel for looseness, pitting, corrosion or other signs of deterioration. When the cause is not obvious, a power-quality evaluation may help identify transient voltage events or other electrical-system conditions associated with the fault.
Use the Timing of the Fault to Narrow the Search
Before resetting the drive, record the fault information and operating condition. Useful questions include: Did the fault occur during startup, acceleration, constant speed, deceleration or while idle? What was the incoming line voltage? Is a dynamic braking resistor or braking unit installed? Has anything recently changed in the facility’s electrical system or driven process?
These details can significantly shorten the troubleshooting process and help determine whether the problem is associated with the application, incoming power, braking system or the VFD itself. Our VFD preventive maintenance and free training program can help facility personnel become more familiar with recording drive conditions and recognizing developing issues.
Do Not Simply Increase Settings
Fault thresholds, deceleration settings and braking parameters are manufacturer- and application-specific. The underlying motor, load and electrical conditions should be understood before changing VFD parameters. Increasing a setting without identifying the root cause may hide a developing problem rather than correct it.
When to Investigate the VFD
If incoming voltage, motor/load conditions, braking equipment and application parameters have been verified but overvoltage faults continue, the VFD may need evaluation for DC bus sensing, braking circuitry, power components or other internal hardware problems. If repair viability is uncertain, see our VFD repair vs. replacement guide.
Related VFD Fault
If the drive is tripping because of excessive output current rather than DC bus voltage, see our VFD overcurrent fault troubleshooting guide.
Need Help With a DC Bus Overvoltage Fault?
Request IPS VFD field-service assistance for on-site troubleshooting or send the VFD to IPS for evaluation. Providing the drive manufacturer and model number, incoming voltage, fault history, application information and the point in the operating cycle when the fault occurs can help our technicians begin narrowing down the cause.