Transformer current

Discussion in 'General Electronics Chat' started by mentaaal, Apr 7, 2008.

  1. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    0
    Hey guys, just beeing reading the transformers section on aac for like the 5th time and am trying to understand, the difference between a CT and a PT.
    One question that will help solve this is, when a PT's secondary winding is open circuited, is the magnetising or exciting current small and when the secondary is loaded, is the magnetising current larger?

    When a CT's secondary is open circuited, does this inductor impede the flow of the flux through its core so as to drop more voltage across the primary winding and thus stepping this higher voltage to dangerous levels?

    It is from this line of thinking that i am guessing that the magnetising current changes with the load on the secondary but i am not sure.
     
  2. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    Sorry, i have been thinking about this and please allow me to rephrase my initial question, when the secondary coil is open circuited, is the core flux very small as the open circuited secondary coil acts like a large reluctance?
     
  3. subtech

    Senior Member

    Nov 21, 2006
    123
    4
    Hello mentaaal

    I'll try to take this one step at at time.
    In a properly designed potential transformer with secondary terminals that are not connected to any load:
    The magnetising current will be relatively small. The magnetising current stays relatively constant as load is progressively added to the secondary side of the transformer. The amount of current necessary to magnetise (or "excite") the transformer core will be a function of the core type and material from which it is constructed as well as the primary winding details. The magnetising current will essentially lag the applied voltage by nearly 90 degrees as the transformer primary winding is largely inductive.

    The flux that is present in the core at any given moment is relative. As you probably already know, in a potential transformer (or power transformer) the amount of energy taken from the primary(or source side of the tx) and delivered to the secondary side is done so by means of magnetic coupling.(a properly designed core will concentrate and focus the magnetic flux created by the current flow in the primary winding into the secondary winding)
    As the amount of energy being transferred increases, so does the flux present in the core. As the amount of transferred energy approaches the limit of the core/winding capability, "saturation" of the core is reached and the amount of flux (for that particular core) is maximum.
    As I'm sure you have deduced, transferring ever larger amounts of energy requires ever larger transformer cores that can accomdate the tremendous concentrations of flux.

    I hope this helps.
     
  4. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    Hi subtech, thanks for that reply. I have one question though, say for example, we have a transformer with just the primary coil on the core, that is, the secondary coil is completely removed. This is basically an inductor and a magnetic flux will flow through the core which is fine but my question is this, if we suddenly put on a secondary coil which is open circuited onto the core, will the flux through the core decrease?
     
  5. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    One more question, if a secondary coil is physically removed from a transformer such that only the primary coil is inserted, will the flux in a transformer be at a maximum in this condition (assuming the primary is energised of course)
     
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