dc motor basics?

So they cant be designed top down, because there is no public source of tradeoffs. Kind of like David Copperfield's magic tricks?

I figured someone would have a 'design tradeoff' rule of thumb. In the given example, 3 pole vs 5 pole, everything else (turns, wire size) equal, what measures different? L is the same, R is the same, B is the same. Only thing I can see is that the poles are energized fewer usec at the same rpm, so it has less time to charge up, so it has less torque, less rpm? What do you think John?

OK, I read it. Thanks for the link. I notice it doesnt mention a thing about number of poles, and neither did you. Will the 3 pole or 5 pole motor have more torque, everything else equal? How about rpm?

The torque is proportional to the current, the number of poles, the number of turns per pole, the diameter of the armature, and the strength of the permanent magnets. There are a lot of variables.

OK, so a mabuchi motor with 2 magnets is '1 pole', and the 3 coil and 5 coil armatures, if wound the same, have same torque and rpm. I'll be darned.

The trade offs are mostly what I listed. If you mean that hobbyist sources of motors do not have data sheets that cover those variables (and several others that are needed to calculate efficiency and speed), you are right. But manufacturers do produce data sheets with exactly this data, or its equivalent. The equivalent often consists of a torque per amp, stalled, a speed versus voltage constant, a magnetic and mechanical time constant and an efficiency at a usually full load.

I broke the information down into things you might be able to measure if you have motors to dissect.

The laws of proportionality I gave you apply all at once. If everything is the same except one of them, say, magnet strength, then the one with the stronger magnets would produce more torque per amp.

I didn't give you the rules for speed. Stronger magnets (all other things being equal) produce lower speed per volt (because the stronger magnetic field makes the motor generate more voltage at a given speed that bucks the applied voltage). You asked only about torque, so I gave the simplest case, zero speed torque. Speed adds another dimension to the problem.

You might read this tutorial on DC motor characteristics:

Unfortunately, the number of poles is not normally an independent variable. But for motors of the same physical dimensions and quality, more poles (of permanent magnet field or number of brushes, not armature coils) means more torque and lower speed. More armature coils usually just mean smoother torque during a rotation.

Nope. 2 magnets is 2 poles. Motors always have an even number of poles (pairs of 1 north and one south).

They can certainly be wound to produce similar torque speed curves. The 5 armature coil version will just run smoother at low RPM (smaller chunks of armature magnetization switch with each commutator bar passage). Changing the wire gauge and so, the number of turns that fit in each slot is the main way to trade speed for torque or vice versa. Of course, upgrading from ferrite field magnets to neodymium iron boron moves all the choices toward more torque, less speed and higher efficiency.

the 7 pole motor runs smoother. torque and rpm depend on other design factors.

Bye. Jasen