A guide to diagnosing Alarm 16 (AL16) on Danfoss drives. Learn how to identify if the short circuit is in the motor, the cable, or the internal drive hardware.

Troubleshooting Danfoss Alarm 16: Short Circuit

Danfoss Alarm 16 indicates short circuit condition. This means the drive has detected a massive surge of current between two output phases.

Danfoss VLT Troubleshooting: AL16

ALARM 16
Description: Short Circuit in Motor or Inverter

On a Danfoss FC51 Micro Drive, Alarm 16 is a critical safety trip. It indicates that the drive has detected a massive surge of current between two output phases. This is intended to protect the internal IGBTs from exploding when a short circuit occurs downstream.

⚠️ SAFETY FIRST: High voltage remains in the DC Link capacitors even after power-off. Wait at least 4 minutes (for drives up to 7.5kW) before touching motor terminals.

Common Root Causes

Phase-to-Phase Short: Two of the motor wires are touching each other (due to insulation failure or water in the terminal box).
Motor Winding Failure: The internal insulation of the motor has burned out.
IGBT Failure: The drive’s internal power module is shorted.
Cable Damage: The motor cable has been pinched, cut, or damaged in a conduit.

Troubleshooting Workflow

Step 1: The “Isolation” Test
Power down the drive and disconnect the motor cables from the terminals U, V, and W. Power the drive back up and try to start it.
Result: If AL16 persists without the motor connected, the drive’s internal IGBT is dead. If the alarm clears, the problem is in the cable or motor.
Step 2: Check the Motor Terminal Box
Open the motor connection box. Look for signs of carbon tracking (black soot), moisture, or loose connections that might be bridging phases.
Step 3: Insulation Resistance (Megger) Test
With the cable disconnected from the drive, use a Megger to test between:
- Phase U and Phase V
- Phase V and Phase W
- Phase U and Phase W
Values should be high (typically >100 MΩ). A low reading confirms a short.
Step 4: Check Cable Length
The FC51 is designed for shorter cable runs. If you have an unshielded cable longer than 50m or a shielded cable longer than 15m without an output filter, the capacitive charging current can occasionally trigger a false AL16.

Summary of Components to Check

Component	Check For
Motor Cable	Abrasions, moisture, or tight bends in conduit.
Motor Windings	Burnt smell or unbalanced resistance between phases.
Drive Terminals	Ensure no stray wire strands are touching adjacent terminals.
IGBTs	Use a multimeter (Diode test) to check for shorts across DC- to U,V,W.