Troubleshooting VFD Overcurrent Faults: Why It Trips and How to Fix It

If there is one fault code that every maintenance technician sees in their nightmares, it’s Overcurrent.

Whether it shows up as "OC," "F1," "F12," or "OverCurrent," the result is always the same: The machine stops dead, the silence is deafening, and you have to figure out why the drive thinks the motor is pulling more amps than it can handle.

 

Unlike a simple warning, an Overcurrent fault is the drive trying to save its own life (and your motor's life). It detects a spike in amperage that exceeds the hardware limit, and it cuts power instantly to prevent an explosion.

So, how do you find the root cause? Is it the drive, the motor, or the machine? Let’s troubleshoot this like a pro.

The Big Question: When Does It Trip?

Before you open your toolbox, look at when the fault happens. This is your biggest clue.

1. Does it trip immediately upon hitting "Start"? (This usually points to electrical shorts or hardware failure).

2. Does it trip while ramping up speed? (This points to inertia or programming).

3. Does it trip while running at full speed? (This usually points to a mechanical jam or sudden load change).

Common Cause #1: Acceleration Time is Too Short

This is the "rookie mistake" (we’ve all done it).

If you ask a heavy fan or a loaded conveyor to go from 0 to 60 Hz in 1 second, the motor has to pull a massive amount of inrush current to overcome that inertia. If that inrush exceeds the drive's trip point, you get an OC fault.

The Fix:
Go into your parameters and find Accel Time (Acceleration Time). Double it. If it’s set to 5 seconds, try 10. If the fault goes away, your ramp was just too aggressive for the load.

Common Cause #2: Mechanical Binding (Something is Stuck)

If the ramp time is fine, look at the machine itself. A VFD is just a sensor; if the motor physically cannot turn because something is blocking it, the current will skyrocket.

The Fix:

• Safety First: Lock out/Tag out the power.

• The Hand Spin: Can you turn the motor shaft or the load by hand?

• Check for seized bearings, a jammed gearbox, or debris caught in a pump impeller. If the mechanical side is tight, the electrical side will trip.

Common Cause #3: Short Circuit or Ground Fault

If the drive trips the millisecond you hit the start button, you might have a short in the motor or the cables.

The Fix:

1. Disconnect the Motor: Power down, verify zero energy, and remove the U, V, and W motor leads from the bottom of the drive.

2. Run the Drive Empty: Reset the fault and try to run the VFD without the motor connected.

   • If the VFD trips with no wires attached: Your VFD is blown. The internal IGBTs (transistors) are likely shorted. It’s time to buy a new drive.

   • If the VFD runs fine empty: The problem is "downstream"—it’s either your cable or your motor.

3. Megger the Motor: Use an insulation tester (Megger) to check the motor windings for a short to ground. Also, check phase-to-phase resistance with a quality multimeter.

Common Cause #4: The "Flying Start"

Are you trying to start a fan that is already spinning backward due to a draft?
If a motor is spinning one way, and the VFD tries to force it the other way instantly, you will get an overcurrent trip.

The Fix:
Enable the "Flying Start" or "Catch on the Fly" parameter in your VFD settings. This allows the drive to read the motor's current speed and direction before taking control, rather than slamming it with power.

Summary Checklist

An Overcurrent fault feels scary, but it’s logical. Just follow the path of the current:

1. Check the Load: Is the machine jammed?

2. Check the Settings: Is the Accel time too fast?

3. Check the Hardware: Disconnect the motor leads. If the drive still trips, the drive is dead. If it doesn't, check your motor and cables.