Yaskawa A1000 OV Fault — What It Means
The OV fault on a Yaskawa A1000 drive means the internal DC bus voltage has exceeded the overvoltage detection level. For 200/230 V class drives this trip point is typically around 410 VDC, and for 400/460 V class drives it is around 820 VDC (or 740 VDC when parameter E1-01 is set below 400). This is a regeneration or power-supply problem, not a motor overload issue.
When the motor is driven faster than the commanded speed by the load, or when the drive tries to stop too quickly, energy flows backward into the DC bus capacitors. If the drive cannot dissipate that energy fast enough through a braking resistor or the incoming line, the bus voltage climbs until the OV trip level is reached and the drive faults out to protect itself.
Before You Replace Anything
Technicians often replace the drive itself when the real problem is a failed or undersized braking resistor or an open resistor connection. Always measure resistor continuity and verify braking circuit wiring before replacing the VFD.
Common Causes
- Overhauling or regenerating load (~40%) The load drives the motor faster than commanded speed (downhill conveyors, vertical motion, high inertia) and pushes energy back into the DC bus.
- Too-short deceleration time (~25%) Abrupt stops or aggressive decel ramps cannot dissipate regenerated energy fast enough, so bus voltage rises above the trip threshold.
- Failed or undersized braking resistor (~20%) An open, overheated, or incorrect-value braking resistor cannot absorb regeneration energy, or the braking transistor is damaged and not switching the resistor into the circuit.
- High incoming line voltage or supply transients (~10%) Sustained high line voltage, surges, unbalanced phases, or resonance with facility power-factor-correction capacitors can elevate the DC bus.
- Aging DC bus capacitors (~5%) Degraded electrolytic capacitors reduce filtering and make the drive more susceptible to voltage spikes.
Quick Diagnosis
Answer these to narrow it down fast.
Does the OV fault occur only during deceleration or stopping?
No: The fault may be incoming-line related. Measure line voltage and check for supply spikes or imbalance.
Is a braking resistor installed and is its wiring intact?
No: Install the correct braking resistor for your drive class and application, or repair the open wiring before resuming operation.
Is the incoming line voltage within the drive's rated input range?
No: Correct the supply voltage issue or install input voltage regulation before running the drive again.
Step-by-Step Fix
- Confirm the fault code. Read the exact alarm display and, if available, monitor the DC bus voltage value in the drive diagnostics to verify it exceeded the trip level.
- Check for regenerating or overhauling loads. Look for vertical motion, high inertia, downhill conveyors, or any condition where the load can drive the motor faster than the commanded speed.
- Increase the deceleration time. Lengthen the decel ramp parameter and, if the drive supports it, use a longer S-curve profile to reduce the rate at which energy is pushed back into the DC bus.
- Inspect the braking resistor circuit. Verify the braking resistor is installed, measure its resistance with an ohmmeter, check all wiring for continuity, and confirm the braking transistor is functioning.
- Measure the incoming line voltage. Use a multimeter or power analyzer to confirm the supply voltage is within the drive’s rated input range and look for sustained high voltage or transient spikes.
- Check for supply-quality issues. Inspect for phase imbalance, loose input terminations, distorted waveforms, and facility power-factor-correction capacitors that may resonate with the drive input.
- Add input mitigation if needed. Install a line reactor or DC link choke to reduce nuisance OV trips caused by supply transients or regeneration peaks, following the drive manufacturer’s recommendations.
- Evaluate drive hardware if the fault persists. Suspect degraded DC bus capacitors, faulty voltage sensing circuitry, or a damaged braking transistor. Replace the drive or the affected power section if those components test bad.
Parts Often Needed
| Part | Notes |
|---|---|
| Yaskawa A1000 braking resistor | Amazon | Matched to your drive class and power rating. Consult the A1000 manual or nameplate for the correct resistance and wattage. |
| Line reactor or DC link choke | Amazon | Reduces incoming transients and regeneration voltage spikes. Size per Yaskawa application guidance. |
When to Call a Pro
Call a qualified industrial electrician or drive technician if you are not trained to work with VFD high-voltage DC bus circuits. The DC bus can hold a lethal charge even after input power is disconnected. If increasing deceleration time and verifying the braking resistor do not clear the fault, or if you suspect failed DC bus capacitors or control board issues, professional diagnostics and repair are required. A technician will have the tools to safely measure DC bus voltage under load, evaluate capacitor ESR, test the braking transistor, and analyze supply power quality.
Rough cost: A pro service call runs about $150-500.