dc motor basics?

If I have 2 12v motors, each 1 inch in diam, one with 3 poles, one with
5 (or 7) poles, which has more torque? more rpm? Whtas the rule of
thumb?

 

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.

 

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

 

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.

 

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?

 

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:
http://www.globe-motors.com/dc_motor.pdf

 

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?

 

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.

 

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.

 

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