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Excitation current & primary current in transformer

  1. Dec 24, 2012 #1
    Transformer equivalent circuit shows that excitation current is just a tiny friction of the primary current. I always thought the pricinple of transformer was like this: voltage on the primary winding generates the current there, then flux in the core, then voltage on the secondary winding and current.

    But obviously excitation current is not the same as the primary current and only the tiny part of primary current could generate the flux in the core. But I couldnt understand. Why cant the whole primary current generate the flux on the core since the whole winding is there?

    Anyone can help me with this like I just got two candies from Santa in my company :)
     
  2. jcsd
  3. Dec 24, 2012 #2

    NascentOxygen

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    Staff: Mentor

    Ideally, with no secondary load there would be zero primary current. But nothing is perfect. With the secondary winding open-circuited, you have basically a magnetic core with many turns of the primary winding. This is an inductor, and you know what happens when you connect an inductor across an ac voltage—it draws some current. The resistance in the equivalent circuit is there to account for losses associated with each reversal of magnetism in the core, together with a little due to the wire's series resistance. The usual equivalent circuit is only approximate, but so long as it gets us to within a few percent of the exact answer, it is considered good enough.
    http://img803.imageshack.us/img803/4666/holly1756.gif [Broken]
     
    Last edited by a moderator: May 6, 2017
  4. Dec 25, 2012 #3
    The secondary's current also creates a flux, or in fact, reduces the flux, because the flux creates this current.
    When a current flow in the secondary, the current in the primary has to increase to keep the flux, hence keep the primary voltage. Only the difference between N1*I1 and N2*I2 creates the flux, and in a well-designed and properly used transformer, this difference is tiny.
     
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