Cause : UnderVoltage Limit comes when the voltage level at the drive's input terminals is lower than the minimum acceptable operating range.

The A0503 alarm in Siemens Micromaster 420 (MM420) indicates an undervoltage limit alarm. This alarm occurs when the drive detects that the DC link voltage has fallen below the permissible limit. Here’s how to diagnose and address this issue:

1. Check Supply Voltage:

   • Verify the input supply voltage to the drive. Ensure that it is within the specified range. Low supply voltage can cause the DC link voltage to drop below acceptable limits.
   • Check for fluctuations or dips in the mains power supply that could lead to undervoltage conditions.

2. Inspect Power Connections:

   • Examine the power connections to the drive, including cables and terminals. Ensure that connections are secure and there are no loose or corroded connections that could cause voltage drops.

3. Review Drive Settings:

   • Verify the drive parameters related to voltage limits and fault thresholds. Key parameters to check include:
     • DC link voltage limit
     • Brake chopper activation threshold

4. Brake Chopper and Resistor:

   • Ensure that the drive has a functioning brake chopper and braking resistor (if applicable). These components help to dissipate excess energy and can affect DC link voltage stability.