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dc motor basics?

Discussion in 'Electronic Basics' started by BobG, Aug 29, 2005.

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  1. BobG

    BobG Guest

    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?
     
  2. 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.
     
  3. BobG

    BobG Guest

    So they cant be designed top down, because there is no public source of
    tradeoffs. Kind of like David Copperfield's magic tricks?
     
  4. 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.
     
  5. BobG

    BobG Guest

    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?
     
  6. 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
     
  7. BobG

    BobG Guest

    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?
     
  8. 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.
     
  9. BobG

    BobG Guest

    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.
     
  10. 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.
     
  11. Jasen Betts

    Jasen Betts Guest

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

    Bye.
    Jasen
     
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