Cause : Excess temperature difference comes when the temperature variation between individual IGBTs is exceeding the acceptable limit.

The A4B1 warning in your ABB ACS560 drive indicates an excess temperature difference warning. This means that the temperature within the drive is not uniformly distributed, with a significant difference between the temperatures of different Internal Gate Bipolar Transistors (IGBTs).

1. High temperature difference between the IGBTs of different phases.
2. Check the motor cabling.
3. Check cooling of drive module(s).