SCR DC Motor Controllers... Need Advice.

I need some advice about SCR drives. I need to know if they provide
constant torque at all speeds. For example if the controller is
set to half speed, is the torque also reduced to half or does it
remain constant? And finally, will there be any difference in
performance in a motor running from a 90v SCR drive set at full
speed, and a motor wired directly to a 90v source? Thanks in advance!

Mark