The ATS48 could be used with Slip Ring motor, but due to the fact a Slip Ring motor is designed for high torque applications the most efficient answer would be to use an ATV71 constant torque variable speed drive.
If a Soft Start Unit is to be used to start a Slip Ring Motor there are a number of things to be considered.
The resistors are to be removed and the Rota shorted out, but engineers agree that a small amount of resistance should be maintained. This is usually achieved in the cables.
This has in theory turned the motor into a Squirrel cage motor with HIGH TORQUE starting. To use a soft start on a high torque application the current limit must be set to maximum this would mean the motor would be able to pull high current to get it away. The average starting current for a Slip Ring Motor is 250% of its Full Load Current where as if using a Soft Start it would be 700% of Full Load Current. Using a 55Kw motor as an example at 400V
FLC x 250 = 100 x 2.5 = 250A
FLC x 700 = 100 x 7 = 700A
As can be seen from the above calculations all the wiring and circuit equipment will have to be changed to be able to cope with the higher currents
So as previously stated the best option if you are replacing a Slip Ring Starter is to use an Altivar Variable Speed Drive from the ATV71 range