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16864Re: Poor performance (again ;-)

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  • Jeremy Brandon
    Oct 1, 2003
      --- "bill.foote" <bill.foote@v...> wrote:
      > There is no question that the current 5-pole motors have visibly
      > superior characteristics compared with the earlier 3-pole motors

      The problem with the 3-pole motors (I don't think this has been
      mentioned) is called "cogging". The three poles of the armature are
      strongly attracted to the two poles of the magnet, giving six stable
      positions per revolution. At slow speeds a relatively large torque is
      needed to turn the armature between these positions. (You can see and
      feel the cogging if you turn the motor with a toothpick!) Most
      controllers are not able to give just the right amount of "oomph" to
      kick the motor smoothly between the "cogs".

      5-pole motors have 10 "cogs" and need less torque to turn from one to
      another (try it!), so controllers have an easier job driving the
      motor slowly and smoothly.

      Cogging is avoided in can (Faulhaber/coreless) motors and by using
      skew windings. Cogging can also be overcome by a suitable controller.

      I have made a controller that copes with cogging, even in 3-pole
      motors, and drives nearly all locos smoothly at very slow speeds, and
      gives smooth acceleration and braking. Unfortunately I do not yet
      have a design ready for production. I am still working on it!

      Jeremy.
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