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Induction motor as transformer

  1. Jul 15, 2015 #1

    cnh1995

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    It is observed that the stator current of IM increases on loading. In transformers, the increase in primary current due to loading is significant . Is it same in induction motor? Does the air gap affect it?
     
  2. jcsd
  3. Jul 15, 2015 #2
    The principle is basically the same, yes. Air gap will have most of its effect with the efficiency which will have effects on loading currents. To get a better idea, you could do a bunch of analysis with different loads on the transformer model and the induction motor equ. circuit model.

    Here's the link to some info I found: http://myelectrical.com/notes/entryid/251/induction-motor-equivalent-circuit
     
  4. Jul 17, 2015 #3

    cnh1995

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    But in transformer, secondary flux is physically in direct opposition to the primary flux and in IM, rotor flux is circular around the rotor bars and stator flux is perpendicular to the bars. How does this weaken the stator flux? Could anyone explain with a diagram?
     
  5. Jul 17, 2015 #4

    Averagesupernova

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    Haven't you answered your own question here? The stator in an induction motor can be considered the primary winding of the transformer and the rotor can be considered a short circuited secondary. When the rotor comes up to synchronous speed it is almost as if the secondary has been taken out of circuit.
     
  6. Jul 19, 2015 #5

    cnh1995

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    Yes, I know the transformer action,but I don't understand how it takes place in IM. In transformer, secondary flux tries to cancel out primary flux because it is physically opposite in direction to that of primary flux. So, secondary emf tries to "oppose the cause" by means of secondary flux, as per Lenz's law. On the other hand, as per my limited knowledge, IM rotor tries to oppose the cause by rotating and reducing the relative speed, thereby inducing smaller emf than that at standstill. I don't understand how rotor flux opposes the stator flux ? They are not physically in opposition. Stator flux lines are perpendicular to the rotor bars and rotor flux lines are circular around the rotor bars.
    full196.jpeg
    How does this weaken the stator flux and make the stator draw more current from supply?
     
  7. Jul 20, 2015 #6

    jim hardy

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    Let us go to a simpler picture
    shaded.gif
    Remove the copper shading ring and it's still a single phase motor, just it can't self start.

    Now let us simplify our thinking a LOT .

    First consider it when rotor is locked.
    Stator flux is vertical through the entire height of rotor.
    The rotor bars in horizontal plane are a shorted secondary, so large current flows and resulting MMF opposes stator flux, just like in a transformer. Primary current goes up accordingly.
    Rotor bars in vertical plane link no flux so might as well not be there.
    Remember right hand rule -
    Rotor MMF is vertical and opposing stator MMF, so no torque is developed.
    That's why a single phase motor needs a start winding.

    Now unlock rotor and give it a spin.
    Rotor bars in horizontal plane now have velocity relative to stator flux
    and so do rotor bars in vertical plane
    so now both will have induced current.
    Rotor MMF is vector sum of MMF's from both the horizontal and vertical rotor bars.
    Remember right hand rule again?
    One of those MMF's is vertical(from the horizontal bars) and the other is horizontal(from the vertical bars).
    That sum is no longer aligned with stator MMF, so there's a net torque.. That's why you can give an induction motor a spin by hand either direction and it'll take off running that way.


    As rotor speed approaches synchronous there's less and less relative velocity between rotor bars and stator field, so both amplitude and frequency of rotor current decrease. As you approach synchronous speed, frequency of rotor current becomes lower and lower. In an unloaded motor slip may be just 1RPM , how far is that from DC?
    So as it approaches synchronous speed, the induction motor comes to resemble more and more closely a permanent magnet (or maybe reluctance) synchronous motor.. though it never quite arrives.

    That's the mental shortcut i use.
    It will be re-inforced if you watch an unloaded motor run under a strobe (or fluorescent lights) .
    In motors over a horsepower or two, slip is so slow one gets impatient waiting for it to accumulate a single shaft rotation.

    Was that any help?
    For the single phase motor presented, there's one piece of explanation missing. Care to point it out?
     
    Last edited: Jul 20, 2015
  8. Jul 23, 2015 #7

    cnh1995

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    What is the role of copper shading ring? Also, I studied the RMF generation for 2 pole IM. But I can't analyse a 4 pole motor. How are 4 poles formed? How are individual fluxes oriented? For 2 pole motor, they are at 120 degrees apart physically..What about 4 pole motor?
     
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