Magnetizing current for Primary Coil

Discussion in 'General Electronics Chat' started by JStitzlein, Mar 8, 2011.

  1. JStitzlein

    Thread Starter Member

    Dec 6, 2010
    I thought the primary coil current was supposed to lag the supply voltage by 90 degrees. Doing simulations with different coupling coefficient values changes this phase shift. Can someone please tell me why
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Post a specific example which will form the basis for discussion.

    As a general observation - if the question relates to transformers - the unloaded primary magnetizing current will lag the source voltage by ~90°. This would assume a primary winding with zero net losses. If the secondary is loaded and you change the coupling coefficient then it is likely the total primary current (magnetizing + load) phase lag will vary and be less than 90°.
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  3. subtech

    Senior Member

    Nov 21, 2006
    In an "ideal" transformer, we consider the excitation current to lag the voltage sine wave by 90 deg.
    The ideal transformer does not exist in reality, but we use it as an approximation in engineering when absolute precision isn't required or practical. It's also used when learning (while in school)
    because dragging in all of the precise parameters does little to help you learn the foundational principles involved. That is not to say that you shouldn't ever learn all of the technical details,(in time I hope you certainly will) but at first it would be more of a obstacle than anything.

    I'll not give you a direct answer to your question, but I'll point you towards what you should be considering carefully.
    Think of the primary coil in terms of XL and R.
    By introducing another coil (or an iron core for that matter) in close proximity to the primary coil, how is the effective X of the primary coil changed?
    How is the R of the primary coil changed?
    Thinking in these terms, how would you expect the angle of current to change with the introduction of the second coil?

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  4. JStitzlein

    Thread Starter Member

    Dec 6, 2010
    Given that the primary must generate a back emf opposing the secondary I'd have to quote the book and say that the currents are 180 degrees apart.

    Sad to say i've been thinking about your questions all day, but i can't answer them. In a series circuit with two inductors, the reactance would go up and cause less current draw per inductor. In a situation where they are mutually inductive, they oppose each other almost like they don't exist in the circuit, leaving the source with the secondary load.

    How can the primary coil balance the source and the secondary? If this were a series RL, I see how the current would shift to balance all the elements to sum to the voltage source.

    basically, I am so confused!