How to Troubleshoot Yaskawa Drive Error CoF (Current Offset Fault)?

Getting the "CoF" Current Offset Fault on your Yaskawa drive? This indicates an internal failure in the current detection circuit. Learn steps for a quick recovery or hardware replacement.

 

If your Yaskawa drive stops and displays the CoF (Current Offset Fault), the drive’s self-diagnostic system has detected an error in its internal current detection circuitry. Essentially, the "eyes" the drive uses to measure how much current is going to the motor have drifted out of calibration, or have experienced a hardware failure.

Because accurate current measurement is required for the drive to safely control the motor and protect itself from overcurrent, it shuts down immediately upon detecting this offset. Here is how to diagnose and potentially resolve this fault.

What Triggers the CoF Fault?

The CoF fault points to an internal issue within the drive's control loop rather than an external motor or cabling fault. Common triggers include:

• Electronic Drift: The internal sensing components (like the A/D converter or Hall Effect sensors) have drifted due to extreme temperature changes or aging.
• Electrical Noise: High-frequency electromagnetic interference (EMI) has flooded the current sensing circuit, causing an invalid offset.
• Hardware Failure: A physical component failure on the control board or power board.
• Power Surge: A recent voltage spike may have damaged the sensitive current-monitoring electronics.

Step-by-Step Troubleshooting Guide

Safety Warning: Always disconnect main power and wait for the DC bus capacitors to discharge (refer to your manual for time) before opening the drive or performing any internal inspections.

1. The "Cold Boot" Power Cycle

Because the CoF fault is often related to the current sensor’s "zero-point" calibration, a full power cycle can sometimes force the drive to re-calibrate.

• Turn off all power to the drive (including any external 24V DC control power).
• Wait at least 15 minutes. This is vital to allow the internal processor and sensing circuits to fully discharge and reset.
• Reapply power. If the CoF fault is gone, the drive has successfully re-calibrated.

2. Check for External Noise

If the fault re-appears, ensure that external noise isn't tricking the sensors:

• Ensure all signal cables (I/O, analog) are shielded, and the shields are grounded at the cabinet entry.
• Verify that motor power cables are not running parallel to or touching signal/control cables.
• Check the drive’s grounding. A poor or high-resistance ground connection can lead to unstable readings in the current detection circuit.

3. Inspect the Drive Environment

• Check for moisture or condensation inside the drive. Moisture on the control board can bridge connections and create false "offsets" in the current sensing signals.
• Check for extreme dust or metallic conductive debris. A conductive "dust bridge" across the board can cause signal offsets. Clean the drive using dry, oil-free compressed air if necessary.

4. Hardware Replacement

If the CoF fault triggers immediately every time you power up the drive after a long discharge period, the internal current sensing hardware is permanently damaged.

• The CoF fault is typically not field-repairable. It indicates that the current detection circuit (often integrated into the power card or main control board) has failed.
• In this scenario, the drive unit must be replaced, or the power/control board assembly must be sent to an authorized Yaskawa service center for professional repair.

Final Recommendation

Do not attempt to operate the drive with a persistent CoF fault. If the drive cannot measure current accurately, it cannot protect the motor or itself from an overcurrent event. Continuing to run the drive in this state could result in the destruction of the output transistors or motor failure.