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Homework Help: 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]

    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


    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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