How to Fix IGBT Short Circuit (SCF) Faults in Schneider Altivar Drives

Staring at SCF4 or SCF1 on your Schneider Altivar VFD? Learn how to diagnose IGBT short circuits, test your motor, and clear the fault safely.

 

Solving the Schneider Altivar Short Circuit Fault: SCF1, SCF3, and SCF4

If you are working with a Schneider Altivar drive and the screen suddenly turns red with an SCF fault, your first instinct might be to panic. These are "Hard Faults," meaning the drive has detected a massive rush of current and shut down in milliseconds to prevent a fire or total hardware failure.

On the Schneider platform, these codes break down as follows:

• SCF1: Motor short circuit.
• SCF3: Ground fault (current leaking to earth).
• SCF4: IGBT short circuit (Internal power component failure).
• SCF5: Short circuit detected during motor start-up.

What is an IGBT and Why Does it Trip?

The IGBT (Insulated-Gate Bipolar Transistor) is the high-speed switch inside the drive that creates the variable frequency for your motor. If an IGBT "shorts," it creates a direct path for electricity to flow unchecked. This is usually caused by heat, aging, or a severe external short circuit in the motor cables.

Step-by-Step Troubleshooting Guide

1. The "Isolation Test" (The Most Critical Step)

We need to know if the "short" is inside the drive or out in the field.

• Power down the drive and wait for the DC bus to discharge.
• Disconnect the motor wires from terminals U, V, and W.
• Ensure the disconnected wires are safe and not touching anything.
• Power the drive back up and try to "Start" it at a low frequency.
• If the SCF fault returns immediately: The drive's internal IGBTs are damaged. You likely need a new VFD.
• If the drive runs fine: The VFD is healthy! The problem is in your motor or the cables.

2. Inspect the Motor and Cables

If the drive passed the isolation test, it's time to find the external short:

• Check the motor terminal box. Look for moisture, charred wires, or conductive dust.
• Check the motor cable for any nicks or melt marks where it might be touching a metal conduit.
• Use a Megger (Insulation Tester) to check the motor windings to the ground.
  Important: Always disconnect the cable from the VFD before Meggering, or you will destroy the drive!

3. Check the Switching Frequency (SFr)

Sometimes, an IGBT trips because it is getting too hot.

• Check Parameter SFr (Switching Frequency).
• If the frequency is set very high (e.g., 12kHz or 16kHz) to reduce motor noise, it puts a massive thermal load on the IGBTs.
• Try lowering the switching frequency to 2kHz or 4kHz and see if the fault clears.

4. Look for "Phase-to-Phase" Shorts

Use your multimeter (in Ohms mode) to check the resistance between the motor phases (U-V, V-W, U-W). They should be very low but balanced. If one pair shows 0.0 Ohms, you have a direct copper-to-copper short circuit in the motor.

How to Reset an SCF Fault

1. Correct the external wiring or replace the faulty motor.
2. For "Hard Faults" like SCF4, you often cannot reset the drive with the keypad alone. You must Cycle the Power (turn it off, wait for the screen to go dark, and turn it back on).
3. If the fault is SCF4 and it returns with nothing connected, the drive's power bridge has failed and the unit must be replaced.

Summary

A Schneider drive short circuit fault is a protective measure. While SCF4 often signals the end of a drive's life, SCF1 and SCF3 are usually solvable issues in the wiring or motor. Always perform the Isolation Test first—it’s the fastest way to know if you’re buying a new motor or a new VFD.