Delta VFD "oc" (Over Current) Fault

Getting the "oc" overcurrent fault on your Delta VFD? Learn the top 5 causes, from wiring shorts to acceleration timing, and follow our step-by-step repair guide.

 

If you are using a Delta AC drive (such as the M, MS300, or C2000 series) and the display suddenly trips with the "oc" error code, your system has detected an Over Current condition. This is a critical protective function—the drive has shut down its output to prevent the internal power transistors (IGBTs) from burning out.

While an "oc" fault can be alarming, it doesn't always mean the drive is broken. Often, it is a signal that there is an issue with the motor, the wiring, or the way the drive is programmed. Here is a human-made, step-by-step guide to diagnosing and fixing the Delta "oc" fault.

What Triggers the "oc" Fault?

The "oc" fault appears when the output current exceeds approximately 200% of the drive's rated current. Depending on when the trip occurs, you might see variations like ocA (during acceleration), ocd (during deceleration), or ocn (during constant speed). However, the general "oc" code usually points to a sudden, massive spike.

Step-by-Step Troubleshooting and Remedies

1. Check Motor and Drive Compatibility

The first rule of VFD installation is sizing. If your motor is significantly larger than the drive, the "oc" fault is inevitable.

• Verification: Compare the horsepower (HP) or kilowatts (kW) on the motor nameplate with the output rating of the Delta drive.
• The Fix: Ensure the drive's output power is equal to or greater than the motor's power requirement. If the motor is too large, you must upgrade to a larger VFD.

2. Inspect for Short Circuits

A physical short circuit is the most dangerous cause of an overcurrent trip.

• Action: Check the power wiring between the AC drive (terminals U, V, W) and the motor. Look for nicks in the cable, moisture in the conduit, or loose wire strands in the motor terminal box.
• Testing: Disconnect the motor from the drive and use a multimeter to check for continuity between phases and from each phase to ground. Any reading indicates a short.

3. Increase Acceleration Time

If the "oc" fault happens exactly when the motor starts to spin, your "ramp" might be too aggressive. Trying to force a heavy load to full speed in 1 second requires a massive current burst.

• The Fix: Navigate to your Acceleration Time parameter (usually Pr.01-12 or Pr.01-09 depending on the model).
• Action: Increase the time (e.g., change 5.0s to 10.0s). A longer ramp allows the motor to build inertia without hitting the current trip limit.

4. Analyze Mechanical Loading

Is the machine physically jammed? If the motor shaft cannot turn, it will draw "locked rotor" current.

• Action: With the power OFF, try to rotate the motor shaft by hand. If it is stiff or seized, check for broken gearboxes, jammed conveyor belts, or seized bearings in the pump/fan.
• Remedy: Clear the mechanical obstruction and ensure the load is within the motor's capacity.

5. Is the Drive Damaged?

If you have disconnected the motor leads entirely, but the "oc" fault returns the moment you press "Start," the internal components of the drive are likely failed.

• Hardware Check: If the "oc" persists after the short circuit has been removed and the motor is disconnected, the internal IGBT module is likely shorted.
• The Fix: In this case, the drive must be sent back to the manufacturer or an authorized repair center for service.

Summary Checklist

Checkpoint	Action Required
Sizing	Ensure Motor HP ≤ Drive HP.
Wiring	Megger test cables and check U/V/W terminals for shorts.
Parameters	Increase Accel Time in Group 01.
Load	Check for mechanical jams or seized bearings.
Hardware	If "oc" occurs with no motor connected, replace the drive.