(ET) Compound motors

["Rhett T. George" <rtg ee duke edu>]

 - Greetings -

Kirk asked about compound motors.  Others may be interested so here is a
short description of the two types.

The cumulative compound has two field windings which add to produce the 
total field.  The connection and typical magnetic pole contributions
follow in this ASCII drawing.

        __________________________________________
      +                    |                     |
      |         - North -  (          - North -  (
      |     shunt winding   )    series winding   )
      |     many turns of  (       few turns of  (
      |       small wire,   )       large wire,   )
      |   high resistance  (     low resistance  (
      |                     )         - South -   )
      |                    (                     |
      |                     )                    |
      |                    (                    ***
      |                     )                 *     *
      D C                  (                 *       *
     supply                 )                *       *
      |                    (        armature  *     *
      |         - South -   )                   *** 
      _                     |                    |
        ------------------------------------------


The differential compound motor will be built the same way but the series
field will have the connections reversed so that the South pole is on top,
decreasing the effect of the North pole of the shunt field.  Then the 
North pole of the series field is at the bottom.

As torque demand on the cumulatively-compound motor increases, the series 
field adds to the total field strength.  Torque increases a bit more than
linearly with armature current.

As torque demand on the differentially-compound motor increases, armature
current goes up, net field strength drops, back EMF drops, and armature
current goes up more, further decreasing net field strength.  To avoid 
having the armature current increase to the point of no net magnetic 
field, 
the series winding for a differentially-compound motor should have no more 
than a few turns.

If you are still awake and have further questions, please ask.

Keep on ET'ing                                  Rhett George