DC Motor running as a Generator

[IrisLab]

Can someone explain to me the effects/consequences of running a DC motor as a DC generator?

In particular, suppose we have a DC motor where we attach a variable resistor R across the leads of the motor. This R is our load for our generator. Now, suppose we turn the shaft of the motor (generator?) with a torque T and angular velocity w. This would induce a voltage v across R and a current i through R.

I'm assuming (neglecting losses) that Tw = vi = (v^2)/R = R i^2.

Suppose that I have can turn the shaft with a constant T and that w is free to seek whatever angular velocity. Now, if I vary the R, I would change the current draw and thus vi would vary. With T constant, w would have to vary to compensate, right?

Any help on this matter would be greatly appreciated! Thanks.