how to solve if MMI display Drive AC890 error STAC ?

Good Morning Faisal,

A STAC error appears on the keypad when the drive is unable to distinguish between an overcurrent/desat or overvoltage trip. When does this alarm appear on the keypad? Is it when the drive turns on, a particular point in your process, or is it when a run/jog command is given to the motor? 

Some initial checks to match would be the wiring, make sure that everything securely connected? Another thing I would check is the motor/motor leads, make sure that there are no shorts in the motor (you can check the resistance between each phase, as well as each phase and ground)? If the motor has feedback (Encoder, Resolver) I would check to make sure that the wiring is secure and well as it is counting properly and in the right direction.