power in transformer's windings.

Discussion in 'General Electronics Chat' started by electr, Jun 9, 2009.

  1. electr

    Thread Starter Active Member

    May 23, 2009
    I learn these days about transformers.
    I read that the power going into the primary coil is Pin = Vp*Ip*cos(θ), and that in an ideal transformer, the power going into the secondary coil equals Pin.

    A coil (an indcutor) doesnt consume real power, so in that case doesnt Pin equal 0?
    In addition, isnt the power transferred from the primary coil to the secondary coil a reactive power and not real (active) power?

  2. The Flavored Coffee Guy


    Nov 7, 2006
    Any given current will produce a specific number of magnetic lines in a loop of wire. In an inductor, you have a time constant associated with it. So, from the time a switch is turned on, there is a delay before the coil actually reaches it's maximum current. If I double the current, I double the number of magnetic lines. If I double the number turns I would double the number of magnetic lines.

    For the most part, Lorentz forces in the secondary consume the energy from the primary. Bascially, what happens is that the primary generates magnetic lines. When confronted with a shorted coil/a shorted secondary the secondary produces magnetic lines in the same polarity of the primary. That magnetic field from the secondary is nearly equal to primary. The magnetic lines produced by the primary are cancelled out by the magnetic lines produced/induced by the secondary with north facing north and south facing south. In the sense of for every action there is an equal but opposite reaction, the trasnformer is the same. The primary's magnetic field changes as the current increases or decreases. The secondary then produces an equal but opposing magnetic field and an equal and opposite direction of current flow. If you wound a toroid as a transformer, and kept track of the direction of the windings etc. you would find out that this true to some extent. The magnetic field wouldnt' be apparent because it will trapped in the magnetic core.

    Self inductance is the point at which any transformer with two isolated windings is working. The V=L(di/dt) formula is then expressed on the secondary. But, the current through the primary coil is converted to magnetic lines, and di is transferred through core as field lines.

    Simplified, the primary converts the current into magnetic lines. The secondary converts the change in the number of magnetic lines into electrical current. In the core the magnetic lines are cancelled as magnetic lines of the same polarity produced by the secondary.

    When you take the North pole of a permanent magnet and stick it to unmagnetized steel, the south pole/the opposite pole of the magnet is induced in the steel. This is basic static charge law, and some part of that charge law operates the same way with magnetic fields as it does with electric fields. My socks, when charged with static electricity will induce the opposite pole on any conductor. So, if my socks are negatively charged, a grounded object will become positive in respect to the space charge/static voltage. That voltage is on the grounded object is induced.

    MIT Opencourseware

    Lectures by Walter Lewin.

    If what I said doesn't make sense, you can view all of the lectures will make it make sense.

    The Primary in a transformer is actually doing the same job as a magnet in a generator. It produces a changing magnetic field. Your equation included cosine, and frequency plays a key role in a transformer's ability to operate. If there is not enough change in the magnetic field, it will not work based upon turn ratios. There is a two part recipe for a working transformer, one is the loading frequency, and the other is the loading current. One or the other must be high enough to place a voltage on the primary when the secondary is open. So, if you apply 100 volts, it will measure 100 volts across the primary. If the loading frequency or current are too low, then you will measure a lower voltage than applied. Change, has to happen at a rate high enough for the conditions to exist in a useful fashion within the transformer.

    Like with a generator, there is kick EMF involved, and that is where you get the 740 watts per horsepower equation from. Those coils always present a changing magnetic field with the same polarity as it sees changing. So, north faces north.
    Last edited: Jun 9, 2009
  3. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    that's why transformers are sized in VA, not watts.