StF is an Excessive Starting Time fault. If the starting time exceeds the value of tLS, the soft starter will not run and displays the StF error code. The conditions for the end of starting are: line voltage applied to the motor (min. firing angle) and motor current less than 1.3 In.
Check the following:
Check that tLS (Max start time) is bigger than ACC (Acceleration time). See SET menu
Check the mechanism (wear, mechanical play, lubrication, blockages, etc.)
Check the sizing of the soft starter motor in relation to the mechanical requirement
Check ILt value: if the value is too low, the motor may not reach acceleration and full speed.