Danfoss VFD Alarm 29 or AL29 Heat Sink Temp Fault

Staring at Alarm 29 or AL29 on your Danfoss VLT drive? Learn why your heat sink is overheating and follow our step-by-step troubleshooting guide to restore cooling.

Danfoss VFD Alarm 29 or AL29 Heat Sink Temp Fault


In the world of industrial automation, heat is the ultimate enemy of electronics. If your Danfoss VLT drive (such as the FC302, FC102, or FC202) suddenly stops and displays Alarm 29: Heat Sink Temp, the drive has activated its thermal protection to prevent permanent damage to its internal power transistors (IGBTs).

Alarm 29 indicates that the temperature of the aluminum heatsink—the part responsible for dissipating heat—has exceeded its safety threshold. Unlike some faults that clear with a simple button press, this alarm requires the drive to physically cool down before it will allow a reset. Here is a human-made guide to diagnosing and fixing the cause of this overheat trip.

Why Does Alarm 29 Occur?

The "Trip" and "Reset" points for Alarm 29 are hardcoded into the drive’s firmware based on its power size. If the heatsink gets too hot, the drive shuts down. It will not allow you to restart until the temperature drops below a specific "Safe" level (hysteresis). Common causes include airflow blockages, mechanical fan failure, or installation errors.

Step-by-Step Troubleshooting Guide

Safety Note: Always disconnect power and wait for the DC bus capacitors to discharge (usually 15 minutes) before inspecting fans or cleaning the unit.

1. Inspect the Cooling Fan

The most common cause of Alarm 29 is a dead or dying fan. Fans are mechanical "wear items" that eventually fail.

  • Physical Test: With the power OFF, try to spin the heatsink fan manually using a thin non-conductive tool. It should spin freely. If it feels "crunchy," stiff, or doesn't move at all, the bearings have seized.
  • Electrical Test: Check if the fan is receiving power when the drive is running. If the fan is silent during operation, it likely needs replacement.

2. Clean the Heatsink Fins

Over time, dust, oil mist, and textile fibers can build up in the aluminum fins of the heatsink, acting like an insulating blanket.

  • The Fix: Use dry, oil-free compressed air to blow out the heatsink from the top and bottom. If the environment is greasy, you may need to use a specialized electronics-safe cleaning agent to remove the film.

3. Check for Proper Clearance

VFDs need to "breathe." If you mount them too close together or against a wall, they will suck in their own hot exhaust air.

  • Requirements: Ensure there is adequate clearance above and below the drive as specified in the manual (typically 100mm to 200mm).
  • Verify that no wire ducts or other components are blocking the air intake at the bottom or the exhaust at the top.

4. Analyze Ambient Temperature

The air entering the drive must be cool enough to take heat away. If your electrical cabinet is sealed and lacks ventilation, the internal air temperature will quickly exceed the drive's 40°C or 50°C rating.

  • Action: Check cabinet intake filters and exhaust fans. If the room is too hot, you may need to add an air conditioning unit to the cabinet.

5. Verify Motor Cable Length

This is a "hidden" cause that often surprises technicians. Extremely long motor cables have high capacitance. This creates "leakage" currents that cause the IGBTs to switch harder and generate significantly more heat than normal.

  • Check: If your cable exceeds the maximum length specified in the Danfoss design guide, you may need to install an Output Filter (LC or dV/dt filter) to reduce the thermal load on the drive.

Summary Checklist for Alarm 29

Checkpoint Desired State
Heatsink Fan Must rotate freely and be audible during run.
Heatsink Fins Must be clean and free of dust "mats."
Cabinet Temp Should be below 40°C (104°F) for most models.
Mounting Space Ensure no blockages at the top or bottom of the unit.
Cable Length Ensure cable is within manufacturer distance specs.

How to Reset the Alarm

Because Alarm 29 is a thermal trip, you cannot reset it immediately.

  1. Leave the control power on so the fan (if working) can continue to cool the unit.
  2. Wait 10–20 minutes for the temperature to drop below the reset threshold.
  3. Press the [RESET] key on the LCP or toggle the digital reset input.

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