ram wrote:
> "the theory" is actually that torque in a DC motor is proportional to
> the applied voltage regardless of RPM.

This is just plain wrong.

Torque generated by a DC motor is proportional to current.  Of course the
torque delivered to the load is reduced by friction.  Some of the friction
is proportional to speed, so it can look as if torque decreases with speed.

Since motor windings are inductors, the current thru them decreases at high
speed when there is little time between reversals to build up full current.
This again makes torque look like it drops off at high speed, although it's
really the current that drops off which reduces the torque.

The current thru the motor is roughly proportional to the applied voltage
minus the back EMF, which is proportional to speed.  At a constant voltage,
torque therefore decreases reasonably linearly with speed until you get to
higher speeds where some of the other effects become significant too.


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