al 16 alarm in danfoss vfd

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Danfoss VFD (FC-102, FC-202, FC-302, FC-51, or VLT 2800), Alarm 16 (AL 16) stands for Short Circuit. This is a critical alarm that triggers when the drive detects a phase-to-phase short circuit. Unlike a ground fault (where power leaks to the earth), a short circuit occurs when two of the output phases (U, V, or W) connect directly to each other. To protect the sensitive IGBTs (internal power transistors), the drive trips instantly.

al 16 alarm in danfoss vfd

 

Common Causes

  1. Motor Winding Failure: The insulation inside the motor has melted or broken down, causing the copper windings of two phases to touch.

  2. Cable Damage: A short in the power cable between the drive and the motor (often caused by moisture, physical pinching, or heat).

  3. Loose Connections: Strands of wire at the motor terminals or the drive's output terminals are touching an adjacent phase.

  4. Internal Drive Failure: A short-circuited IGBT or power card inside the VFD itself.

Troubleshooting Steps

1. Visual Inspection (Power Off)

  • Turn off the main power and wait for the capacitors to discharge (approx. 5–20 minutes).

  • Check the output terminals on the VFD (U, V, W) for any stray wire strands or signs of arcing (burnt plastic/smell).

  • Check the motor terminal box for moisture, carbon soot, or loose connections.

2. The "Open Output" Test

This is the fastest way to see if the drive is broken or if the motor is the problem.

  • Disconnect the motor wires from the VFD terminals (U, V, W).

  • Try to start the drive with no motor attached.

  • Result A: If the drive runs (or gives a "Missing Motor Phase" alarm), the VFD is healthy. The short is in your cable or motor.

  • Result B: If Alarm 16 returns immediately with nothing connected, the internal VFD hardware has failed (likely a blown IGBT).

3. Test the Motor and Cable

If the drive passed the test above, you must find the short in the field:

  • Disconnect the motor from the cable so you can test them separately.

  • Resistance Test: Use a multimeter to check resistance between phases (U-V, V-W, W-U). They should be balanced and very low (e.g., 1–5 Ohms). If you get 0 Ohms, you have a dead short.

  • Insulation Test: Use a Megger to test between phases. Never Megger while the cable is still connected to the VFD.

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