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Isolated induction generator

  1. Apr 12, 2015 #1
    can anyone give me a detailed description of working of isolated induction generator or self excited induction generator

    it has been mentioned about it in the following link in the last paragraph but no details are given
    http://electricalstudy.net/lesson/poly-phase-induction-generator-or-asynchronous-generator-2/ [Broken]

    can anyone refer me a link or book from where i can study about it in detail.
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Apr 12, 2015 #2

    jim hardy

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    a demonstration here

    Typically it's explained as "The capacitor supplies excitation current."
    That's counterintuitive - a capacitor doesn't "supply" current, it allows current.

    So - you already know the answer, from your studies of parallel resonance.

    Excitation current is magnetizing current and that is inductive, 90 degrees behind terminal voltage.
    Capacitive current is 90 degrees ahead of terminal voltage.
    So if they're of equal magnitude, their sum is zero.
    In a parallel resonant circuit, current circulates between inductor and capacitor.
    Apply KCL to junction of L-C and you see there's hardly any external current , just enough to replenish the I2R losses.
    In a lossless circuit there'd be zero external current.

    In your induction generator , those losses are made up by shaft power instead of by external current.

    So, with enough capacitance the machine will make its own excitation current.
    Motors work the same way , so one must be careful applying power factor correction capacitors to motors. Never correct one all the way to unity .

    One of many hits from a google search:

    it's difficut to predict which explanation will "click" with an individual, we're all different.

    I find it useful to think of VARS as instantaneous real power .
    Vars are instantaneous real power that flows between machine and load(or source), reversing direction every quarter cycle.
    So averaged over a whole cycle VARS move no real power, hence their old name "wattles current".
    But -- instant by instant, joules are moving back and forth between the magnetic field in the machine and the electric field in the capacitor.
    When the balance is just right you have a self excited induction machine.
    As you saw in the video.

    Like any other tank circuit with an energy input, it's self sustaining

    Hopefully this simple mental model will help you work through the formulas.
  4. Apr 13, 2015 #3


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    Fundamentally, an isolated induction generator, with a capacitor bank, generates an AC frequency equal to the LC resonant frequency of the winding inductance with the capacitor bank.
    It will not start to generate until the rotation rate exceeds the speed synchronous with the LC resonant frequency.
    As you draw more current from the generator the slip will increase.
    If you draw too much current you will kill the excitation and so the system will protect itself.
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