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Single phase induction motor

  1. Jan 8, 2016 #1
    1. The problem statement, all variables and given/known data
    I studied that as the single phase motor speeds up the backward induced field currents are larger than the forward field currents. So , the corresponding opposing rotor mmf causes backward field to be greatly reduced whereas the smaller forward currents leads to enhancement of forward flux wave..
    I am not getting this fact as if the backward current is higher so even the backward flux should be greater . Then , on what basis the forward flux is greater???

    2. Relevant equations


    3. The attempt at a solution
    The backward field currents has a lower power factor whereas the forward currents have higher power factor .
     
  2. jcsd
  3. Jan 16, 2016 #2

    jim hardy

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    Ahhhh single phase motors

    i dont have right now a good word picture to paint for you.

    I've done this hands on experiment hundreds of times :
    energize a single phase motor with start winding disconnected
    hear and feel it hum as locked rotor amps vibrate the windings
    give it a spin either direction, watch it smoothly accelerate and grow quieter and feel vibration diminish.

    Clearly something cancels out nearly all of one of those counter-rotating fluxes.


    Here's a link with some mathematics trying to explain it
    but to me he does it in a roundabout manner
    http://ee.lamar.edu/gleb/tps/Lecture%2003%20-%20Special-Purpose%20Motors.pdf [Broken]
    upload_2016-1-16_6-32-0.png

    Since, in that equivalent circuit, resistance is where torque is produced , most of the torque will appear in the upper resistor

    His mathematical explanation, same link

    upload_2016-1-16_6-46-15.png



    Hmmm... see if these words help you accept what we empirically know to be true... and what the author cited above asserts:

    Remember an induction motor is essentially a transformer with shorted secondary.
    Since in a transformer secondary mmf cancels primary mmf, backward component of current in rotor cancels backward component of primary mmf.....

    actually i think it only cancels most of it. Single phase motors do have small pulsating torque at 2x line frequency.

    Any help ?

    Clearly my thinking on this subject needs some polishing. I look forward to your improved presentation.

    old jim
     
    Last edited by a moderator: May 7, 2017
  4. Jan 22, 2016 #3

    jim hardy

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    I'm still trying to reconcile what happens at the rotor.... a physical picture not an equation

    The key to an intuitive picture i think lies iwith envisioning the rotor bars and their relation to the stator poles.
    I might try to draw something when get back home...


    Envision a simple two pole machine with one turn stator windings, one per pole at 12 o'clock and 6 o'clock.
    Freeze frame your thinking to instant of maximum flux, stator voltage zero crossing
    Next , envision the rotor bars in plane 3 o'clock-9 o'clock; clearly they're perpendicular to flux so link it all
    and they're the ones that cancel stator flux

    next envision the rotor bars in plane 11:59 o'clock-5:59 o'clock not quite aligned with poles
    they link very little flux
    but what happens if the rotor is turning slowly ?
    Flux linked by 3-9 o'clock loop is not changing
    but flux linked by 11:59-5:59 o'clock loop is changing considerably.. Lenz will try to do something about that....

    old gray cells are not quite there yet

    how're your younger ones ?

    old jim
     
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