Techniques for driving inductive load, e.g DC motor, with PWM and constant current
Torque is directly proportional to current. So if you want to control the torque, you need to close the current-loop.

Higher than the rated voltage is fine, as long as you don't exceed the breakdown voltage (determined by the insulation). I typically drive motors from a 160 VDC bus, regardless of their rating. I just need to be sure I don't exceed the current limits (peak and continuous) or temperature limit.

A higher voltage allows for better speed control, as you can easily counter the BEMF at maximum rated motor speed.