dknguyen

Yeah, but can does a given amount of current correspond to a given amount of torque regardless of the voltage being applied? HJames says it does.

Hero999

I both agree and disagree.

Let's ignore the hysteresis and eddy loses for the moment. This motor is perfect except for the copper losses and friction.

Stall
A huge current proportional to the applied voltage flows. The torque is directly proportional to the current which is directly proportional to the applied voltage up to the point where the core saturates.

Off load
Increasing the voltage increases the speed and therefore frictional losses which are effectively a load on our perfect motor; because this load has increased the effective torque has also increased causing a higher current draw. Note also that the increase in current doesn't obey ohm's law because the higher back EMF in the motor counteracts the increase.

In practice the eddy and hysteresis and eddy losses are pretty low and will just increase in a similar fashion to the frictional loss with increasing speed. For all intents and purposes the torque is proportional to the current draw.

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hjames

I was mostly trying to say that having another mosfet in series is a bad idea.

As for current/torque, I don't think anyone was saying that torque and current aren't proportional - just that as the motor spins at higher velocities, the torque required to maintain the steady-state velocity will increase, and will show up as an increase in no-load current.