I am controlling the speed of DC motor having constant torque. How will I create constant torque/current to control speed because increase voltage automatically increase motor torque/current as well? Since they are interrelated controlling one keeping other constant

 

torque and speed are not necessarily related as much as you may think.

torque = motor current
voltage = motor speed

if you want to control torque then you limit or allow so much current. if you want to control the speed the you limit or allow so much voltage. sometimes you want to control both and you just have to figure out the application to determine who does what and when.

 

If the drive regulates speed, current is determined by the load. If the drive regulates current, speed is determined by the load. The drive can't regulate both speed and current at the same time.