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Re: [Electronics_101] Re: Hand mixer speed control

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  • John Popelish
    On 10/08/2011 07:35 PM, laser100 wrote: (snip) ... You are welcome. Universal motors are not hard to understand, in a sort of hand waving way, but the details
    Message 1 of 8 , Oct 9, 2011
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      On 10/08/2011 07:35 PM, laser100 wrote:
      > Thank you so much for your help. I'm not
      > sure I understand the different taps on the coil. Does it
      > adjust voltage to the motor?

      You are welcome.

      Universal motors are not hard to understand, in a sort of
      hand waving way, but the details get a little tricky.

      The most important fact of their operation is that the field
      coils and armature are in series, so both sets of poles (the
      stationary ones and the rotating ones are energized by the
      same current. The interaction between these two sets of
      poles is proportional to the strength of each, multiplied
      together, which makes it proportional to that single current
      to the second power.

      The second important fact about universal (or series wound)
      motors is that their speed is very poorly regulated and
      thus, highly dependent on their load torque. Here is a
      little hand waving to explain why.

      The speed settles at whatever is required for the armature
      to generate almost the full line voltage (since the field
      voltage drop is usually a small part of the total line
      voltage, except near stall conditions, when the high current
      produces large resistive and inductive drops). But that
      speed depends inversely on the field magnetic strength. The
      load determines the torque required at any given speed (and
      most loads require higher torque for higher speed) so the
      speed is the intersection of that load speed torque curve
      and speed torque of the motor with a given number of field
      windings. Lower load torque runs the speed up the motor
      curve, where the armature generates lots of voltage, driving
      the current down, which weakens the generated voltage which
      runs the speed up higher, etc., until load and motor torque

      Switching in extra field winding turns increases the field
      strength at all currents, which causes the armature to
      generate more voltage at every speed, current combination,
      so that slower speeds buck the current down than with fewer
      field turns. This shifts the motor speed torque to more
      torque at all speeds so that it will intersect the given
      load curve at a lower speed.

      That is a bunch of words trying to substitute for
      simultaneous second order equations.


      John Popelish
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