Cause : Inverter I2T comes when the motor current has been too high for too long, exceeding the thermal capacity of the motor or the drive.

The A0505 alarm in Siemens Micromaster 420 (MM420) indicates an inverter I2t alarm. This alarm is triggered when the drive detects that the cumulative thermal stress (I2t) on the power semiconductors within the inverter section has exceeded the permissible limit. Here’s how to diagnose and address this issue:

1. Check Motor Load and Operation:

   • Verify that the motor load is within the rated capacity of the drive. Excessive motor load or operating conditions (such as frequent starts/stops or rapid accelerations/decelerations) can increase thermal stress on the drive's semiconductors.

2. Inspect Cooling and Ventilation:

   • Ensure that the drive has adequate cooling and ventilation. Overheating can occur if there is insufficient airflow around the drive or if cooling fans are not functioning properly.
   • Check for obstructions in the cooling vents and ensure that the drive is installed in a well-ventilated location.

3. Environmental Conditions:

   • Verify that the ambient temperature around the drive is within the specified operating range. High ambient temperatures can exacerbate thermal stress on the drive's components.

4. Drive Parameters and Settings:

   • Review the drive parameters related to thermal protection and current limits. Check parameters such as:
     • Maximum allowable temperature
     • I2t time constant
   • Ensure that these parameters are correctly set according to the drive's specifications.

5. Cooling System Inspection:

   • If the drive is equipped with optional cooling accessories (such as additional fans or external cooling units), ensure that they are operational and properly connected.