Troubleshooting Auxiliary Fan Faults

Is your VFD tripping on an Auxiliary Fan Fault ? Learn how to identify failures in external cooling fans and control unit ventilation.

 

Auxiliary Fan Fault: Protecting the "Brain" of the Drive

When we talk about VFD cooling, most people think of the massive fans blowing air through the power heat sink. But many high-performance drives, feature a secondary cooling system. When you see an Auxiliary Fan Fault, the drive is warning you that its "internal climate control" has failed.

While the main fan cools the IGBTs, the auxiliary fan is responsible for cooling the Control Unit (CU), internal capacitors, and localized electronics. If this fan stops, the "brains" of the drive can overheat even if the "muscles" (IGBTs) are still cool.

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Auxiliary Fan vs. Main Fan: What's the Difference?

• Main Fan: Large, high-CFM fan located at the base or top of the Power Module. It handles the bulk of the thermal load from the motor current.

• Auxiliary Fan: Smaller, often located inside the Control Unit housing or near the internal DC link capacitors. It ensures airflow over the microprocessors and sensitive logic boards.

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Common Causes of an Auxiliary Fan Fault

1. Dust Accumulation in the Logic Section

Because auxiliary fans are smaller, they have less torque. Even a small amount of "sticky" dust or oily residue can be enough to prevent the blades from starting up.

• The Fix: Use a can of electronics-grade cleaner or dry air to clear the area around the Control Unit.

2. Connector Issues (Vibration)

Auxiliary fans often connect to the main board via tiny, multi-pin plastic headers. In high-vibration applications (like rock crushers or large fans), these connectors can vibrate loose.

• The Fix: Reseat the fan's small white plastic connector on the internal PCB.

3. Software Configuration Mismatch

If you have recently replaced a Control Unit or Power Module, the software might be looking for an auxiliary fan that doesn't exist on that specific frame size, or vice versa.

• The Fix: Check parameter (Operating hours counter for fan) and Configurable fan monitoring.

4. Blown Internal Fuse

Some drives use a small surface-mount fuse to protect the 12V/24V auxiliary fan circuit. If the fan stalled and drew too much current, it might have popped this fuse.

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Troubleshooting Step-by-Step

Step	Action	Logic
1	Check Airflow	Place a thin piece of paper near the Control Unit vents. If it doesn't move, the fan is dead.
2	Listen for Chirping	A "chirping" or "whining" sound indicates the fan motor is trying to start but the bearings are seized.
3	Check p0251	View the fan operating hours. If it's over 30,000 hours, the fan has likely reached its mechanical end-of-life.
4	Test Voltage	Measure the voltage at the auxiliary fan pins (usually 12V or 24V DC). No voltage means a board-level failure.

An Auxiliary Fan Fault might seem like a "nuisance" trip, but it protects the most expensive part of the drive's logic. Operating a Control Unit without its dedicated fan can lead to processor "throttling," communication errors, and eventually, a total logic board failure.