Yaskawa A1000 oU Fault - Causes & Fix

Yaskawa A1000 oU Fault — What It Means

The oU fault on a Yaskawa A1000 VFD indicates that the voltage on the internal DC bus has exceeded the drive’s overvoltage protection threshold. When this happens, the drive shuts down immediately to protect the power components (capacitors and IGBTs) from damage. The DC bus voltage rises when energy flows back into the drive faster than it can be dissipated. This fault is distinct from input power issues and specifically signals an internal voltage spike within the drive’s DC link circuit.

Before You Replace Anything

Technicians sometimes replace the control board or capacitors when the real cause is simply a deceleration time (C1-04) set too short for the load inertia. Check and extend decel time first before replacing any hardware.

Jump to Fix

Common Causes

• Regenerative braking energy (~50%) When the motor decelerates quickly or lowers a heavy load, it generates electrical energy that flows back into the DC bus and raises the voltage above the threshold.
• Deceleration time too short (~25%) Setting parameter C1-04 (deceleration time) too fast for the mechanical inertia of the load causes a rapid voltage spike during stopping.
• Input power supply spikes (~10%) Sudden surges or fluctuations in the incoming AC line voltage from utility switching, lightning, or unstable power can push the DC bus voltage over the limit.
• Failed DC bus capacitors (~8%) Degraded or dried-out electrolytic capacitors lose their ability to absorb voltage spikes and allow the DC bus to rise above the protection threshold.
• Defective chopper or braking resistor circuit (~5%) If the drive has a dynamic braking option, a faulty chopper transistor or disconnected braking resistor prevents regenerative energy from being dissipated.
• Loose power terminal connections (~2%) Poor connections at the input power terminals cause inductive voltage spikes that elevate the DC bus voltage momentarily.

Quick Diagnosis

Answer these to narrow it down fast.

Step-by-Step Fix

1. Stop the drive immediately and record the fault history by checking parameter U2-02 to see if oU is a recurring fault or a one-time event.
2. Measure the incoming AC line voltage with a calibrated voltmeter and verify it is within the drive’s rated range (200V or 400V class depending on your model).
3. Increase the deceleration time by adjusting parameter C1-04 to a longer duration if the fault occurs during motor stopping or braking, allowing more time for the mechanical load to coast down.
4. Inspect all input power terminals (L1, L2, L3) for tightness and clean any corrosion, since loose connections can cause inductive voltage spikes.
5. Install a braking resistor and chopper option if the application involves high inertia loads, downward motion, or frequent stopping, to dissipate regenerative energy safely.
6. Check the DC bus capacitors (if accessible) by measuring capacitance and looking for physical signs of bulging, leakage, or drying out, and replace the capacitor bank if degraded.
7. Install a line reactor or surge suppressor on the input power if voltage spikes or utility disturbances are present, to smooth the DC bus voltage and protect the drive.

Parts Often Needed

When to Call a Pro

Call a qualified drive technician or electrician if you are not comfortable working with high-voltage equipment or if basic parameter adjustments (deceleration time) do not resolve the fault. Internal repairs such as replacing DC bus capacitors, testing the chopper circuit, or diagnosing voltage detection faults require specialized tools, knowledge of VFD architecture, and adherence to electrical safety protocols. If the fault recurs after adjusting decel time and installing a line reactor, the drive may have component failure that only a trained professional should address.

Rough cost: A pro service call runs about $150-500.