[Joe Johnson]

I think there is a better way than to arbitrarily limit the voltage to the motor.

If Mabuchi says 12V is okay, I believe what they are saying is that the armature can stand the 24,000 RPM without "bird caging"*

Given that, I think what is really needed is a way to prevent the build up of heat inside the motor. This can be done more elegantly than simply to limit the voltage of the motor (which is to restrictive at times and not restrictive enough at others).

Here is what I propose to those who want to get the most out of this motor:
1) measure or infer current (you know voltage in from PWM and if you are driving with it, you know the RPM -- that gives you a good method to calculate current).
2) given current and voltage, you can calculate electrical power input and the efficiency of the motor at that speed/load/voltage point
3) given power input and effeciency you can determine how much heat you are generating at that moment in time
4) given the heat input at any moment in time, you know how much energy is put into the system and (assuming initial conditions and the heat out is proportial to the temp of the motor) you can do a basic energy balance calculation temp over time.
5) if you want to get fancy you can even incorporate a motor speed based cooling factor for the motor (faster speed -> more cooling).
6) after the temp rises above a certain number, THEN and only then, I would limit POWER INPUT to the amount that I can dissipate at the current speed

Note that #6 in effect is done by limiting voltage, but the difference is important. You are only limiting voltage when needed and then only by the amount needed to keep the temperature from further.

In theory, this is a pretty easy thing to do. In fact, because temperature is a slow process I think you can do this calculation 3 or 5 times per second and still get good data.

Has anyone implemented a current limit based on such ideas? To be honest we have talked about it for 2-3 years now. We never implemented it mostly because we haven't used the F-P for a drive system in a while.

Any volunteers to share their experience or to take on this subject in a whitepaper?

Joe J.

*where the windings cannot stand the centrifuge effect and they mechanically disconnect from the armature, forming a kind of bird cage.