ac servo winding constant tension

I read somewhere that the way was constant HP but this makes no sense as current will be very very high at slower speeds and tested out the same, when slowing down the process speed, the motor pulled extremely hard.

I have also been trying to use the tension * radius = torque equation but can not get this to work either as test results show varying tension as the role gets bigger and if speed is changed.

I need to know how to calculate my current so that tension remains constant with increase in diameter and speed/rpm. any insight here would be greatly appreciated
Thanks