This is because your SCR controlled DC will control the tension and current in a loop.
The preset torque will be constant over the entire speed range. It can be set up to maximum value at the base speed or can be set lower then that. If the load torque demand is higher then the preset limit, the speed will go down until 0, keeping the same torque output. If the load torque demand is lower then preset torque, the speed will go up to the max speed.
Regards
marie65