motor torque and speed curve question

[david90]

The image above shows the torque vs speed relationship, and I can understand why it's like that (more speed = more cemf = less current = less torque).

On the contrary, my SCR controlled DC drive manual said that a DC drive could control a DC motor in a way that give constant torque from 0 rpm to base speed.

How is it possible to have constant torque when you increase the motor rpm? It's also not consistent with the torque curve shown above.

 

[marie65]

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