3 pole dc motor with commutator?

If all you want is speed control, just use PWM. What you're suggesting could only have value as a learning experience; you can already get brushless DC motors off the shelf.

Cheers! Rich

Hi Rich. I only converted the commutator to slips rings in my head as a mental what if. I'm trying to figure out how to compute torque so I can get a good tight spreadsheet on electric vehicle kwhrs per mile and argue with the ice heads. Let me rephrase my main question.... there is a big honkin magnet in skycraft that is holding a big steel bolt straight up in the air on a chain. You can pull it away from the magnet sideways, but aint no way you're gonna pull it away from that bad boy on axis. Lets say you have to pull 50lbs just for talking purposes. I figure if I built a motor out of those things, and it had a 6in radius, There would be 25 lb-ft of torque just from the magnet attracting the iron pole piece. Now if I put enough NI (ampere turns) in the coil to equal the B from the permanent magnet, I assume I'll have 50 lb-ft of torque. If I double the NI in the coil again, if thermally possible, I dont get any more torque increase, right? I'm 'swamping out' the permanent magnet?. So I want to calc how much torque I need to accelerate my EV at .3Gs etc. Anyone want to agree or disagree with this shade tree mechanic analysis of dc motor torque?

(snip)

All coils are energized at all times, except when a brush shorts across the gap between two commutator bars. Then the coil connected between those two sees no voltage. There are

3 coils, one connected across each of the 3 commutator gaps. As long as the two brushes are in contact with only two bars, all three coils are energized in one of two directions, with either the whole supplied voltage across them, or half the supply (where there are two gaps between the brush contact points. When a brush shorts a gap, one coil gets no voltage, and the other two coils get full voltage (in opposite directions). A brush crosses a gap every 60 degrees, so you might make a timing diagram with waveforms representing the voltage across each coil, showing how every 60 degrees, either coil passes through the shorted (pick some number of degrees less than 60, say, 30 that it takes for the short to pass across the gap) situation and then has its voltage reversed.

So you have 30 degrees with constant voltage on the coils, one getting full voltage and two getting half voltage in the opposite direction.

30 degrees with a short across one of the half voltage coils (show that as zero volts) while the other coil that had half voltage now gets full voltage.

30 degrees with the just shorted coil getting half voltage, reversed from what it last had, and the one that first had full voltage drops to half, also. The coil that popped up to full voltage in the last step continues to get full voltage.

etc.

Thanks John P. I rewound slot car motors back in '68 in hi school, but I always thought in my simple mind that one needed 6 poles to get pairs energized simultaneously. Anyway, I'm glad to know more about how they really work, but can you help me with my torque question? That is, is the max torque possible when the pull from the coil matches the pull from the magnet?

I might not be much help, since I am not very fluent in the units of magnetism. I have to look them up every time I have a problem to solve. But I can tell you that the peak torque from any armature pole (around which a coil is wound) occurs when that pole is in the crack between a pair of permanent magnet poles. In that place, it is simultaneously repelled by one magnet and attracted to the next. This is also the place it should be in the middle of its maximum voltage phase (in the diagram I talked about, earlier), since this is where the peak rate of change of flux through the coil occurs (assuming the motor is turning), and so, generates the peak generated EMF the supply must exceed to keep forcing current through the coil.

One important limitation to torque any permanent magnets can produce is the field that it takes to demagnetize them. So if you have high coercivity magnet material, like neodymium iron boron, you may be able to push the armature flux above the magnet flux (actually pushing flux a little backwards or sideways from the permanent field), but if you have a motor made with low coercivity material (like some ALNICO that will be partially demagnetized if you simply pull the armature out of the field), the armature flux may have to stay well below the permanent flux to preserve the magnetization.