We have used 25 HP ATS 48 Soft Starter for Sugar Grinding Mill Application. The motor rating is 15 HP. Current on Full Load is 21A. The Motor Thermal Protection has been set to Class 20 (Severe Application).

The motor starts normally from a cold start. But after running for over 2 hrs at constant load, the soft starter trips on OLF Fault ( Motor Thermal Fault). During this time, the Motor Thermal State display shows continues rise in % Temp even if the current drawn is 21A only.

Is Soft Starter suitable for Sugar Grinding Mill Application? Or is there a parameter setting problem.

Hello viral

Welcome to the forum.

If you set the overload protection to 21 Amps, and run at 21 Amps, then I would expect that the modeled temperature would be close to tripping.

If you have a transient load where the average current is 21 Amps, but the current is peaking at say 50% overload and 50% load, then although the average current is still 21 Amps, the modeled temperature will go high the the overload could trip.
This is because the heating in the stator winding is due to the copper loss which is a function of the power dissipated in the stator windings. Copper loss is a current squared function and the temperature is dependent on the average of the current squared.
If the current is 50% for half the time, and 150% for half the time, then the average current is 100%.
The average current squared (temperature) is 125% and the starter will trip. If you ran the motor continuously under these conditions, the motor would fail due to overheating!!
I suspect that the protection is responding to transient loading.

Make sure that the protection current is set to the motor rating, not the average current. If the overload continues to trip, then you should consider reducing the load on the motor, or increasing the motor size.

Best regards,