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No. of poles of the Rotor.

  1. May 12, 2012 #1
    Hello,

    Generally and specially in AC motors, what is the effect of the no of poles of the rotor on the motor performance? if it's same or less or more than the no of poles of the stator?

    Thanks
     
  2. jcsd
  3. May 12, 2012 #2

    FOIWATER

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    The thing is, for an AC motor, you won't see a rotor w/ poles, generally the rotor is made of amortisseur winding/squirrel cage winding, or a wound rotor.

    When discussing rotor poles, you are more often than not discussing an alternator/generator with a rotating field and stationary armature.

    With a revolving field type alternator/generator - your armature is no longer wrapped around poles, as it is stationary the voltage is induced into it as the field revolves inside of it. The number of poles of the rotating armature tends to increase the amount of flux that is cut by time, increasing the overall value of alternator output voltage.

    The rotating field poles can be wound in one of two ways, salient pole rotor, or cylindrical pole (turbine) rotor - which type depends on the application..
     
  4. May 12, 2012 #3
    The number of poles on the rotor is analogous to the number of teeth on a gear, i.e. inversely proportional to the rate of rotation and proportional to the output torque

    I would think that the number of poles on the stator would be influenced by the size of the coils and all the constraints that come with it, such as the number of turns and the thickness of the wire, and these would in turn affect the performance of the motor... Torque can be increased by increasing the number of poles on the stator just like on the rotor, and also by increasing the number of turns in each coil to increase the magnetic flux... But increasing the number of coils also decreases the rate of rotation, while increasing the number of turns in each coil would increase their size and reduce the number of coils you can use...
    These are just the simpler examples of how the number of poles can affect the performance of the motor, and they are seldomly linearly related
     
  5. May 12, 2012 #4

    jim hardy

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    AC induction and synchronous motors want to run at speed of (one revolution per line cycle) divided by (number of pole pairs).

    Synchronous motors run at that speed, induction motors run a very few percent slower.


    So a two pole AC motor in a 60 hz counrty runs at or near 3600 RPM, or 3000 RPM in a 50 hz country. A four pole runs at half that.

    There exist so called "Universal" motors that run on either AC or DC. They have brushes and their pole count is not really involved in their speed . They are closer kin to DC motors than they are to normal AC motors. Your electric drill (which is probably variable speed) is an example.
     
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