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[Induction Motor] help to understand this concept

  1. Nov 15, 2013 #1
    Hi all

    I am trying to understand the concept of 3 phase wound rotor induction motor.
    I understand why when you feed in 3 phase current to the stators that are separate spatially 120 degree will get you a rotating magnetic field.


    However, I am having much trouble in understanding the picture below; the representation of the rotor as the "3 phase transformer secondary".

    I know the rotor is only like squirel cage, so graphically we could represent it as a bunch of current loop wrapping on the inner circle. However, this figure presents it as the "3 phase transformer secondary winding" confuse me.

    How am I suppose to interpret this?
    http://forum.allaboutcircuits.com/attachment.php?attachmentid=61444&stc=1&d=1384497568
     
  2. jcsd
  3. Nov 16, 2013 #2
    I think this could be an equivalent diagram of any three-phase induction motor.
    The stator winding are replaced by coils of the same number of turns and the rotor winding –nevertheless conventional each bar is considered another phase.
    In order to explain a certain phenomenon one may replace a distributed winding with a lumped one. The criteria to do this may be preserving same magnetic field.
    If we take the stator phase A and we presume we "shoot" the current at maximum [IA=I.cos(wt+fi) t=0 fi=0 IA=I IB=cos(-2/3*pi)=-0.5 IC=cos(-4/3*pi)=-0.5
    That means the A coil is the maximum North and B and C coil is ½ South each.
    When t=1/3/f wt=2.pi. f*1/3/f=2/3.pi then
    IA=I.cos(2/3.pi)=-0.5 IB=I.cos(2/3pi-2/3.pi)=I IC=I.cos(-2/3.pi)=-0.5
    The North is deployed 120 degree without to move the winding. After another 1/3/f seconds the North reach C position and so on.
    The magnetic field is rotating. But since the coil will “inject” the magnetic field in one point only, the field will be full of harmonics which will brake the rotor and will produce a lot of losses. So we have to spread the windings in an even way around the stator periphery.
    The stator “rotating” field induces EMF in the stator itself and in rotor bars. This EMF induced in rotor produces a current and a “counter” magnetic field which will interact with the stator magnetic field providing the rotor rotating force[and torque].The rotating rotor magnetic field poles are organized in the same number as stator poles are, nevertheless the emf induced will differ from bar to bar. The emf frequency will follow only the difference between stator rotating field and the rotor velocity [the “slip”] but the rotor magnetic field will follow synchronously the stator field.
     
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