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When the primary of a transformer is connected to A.C. mains

  1. Aug 28, 2009 #1
    When the primary of a transformer is connected to A.C. mains, the current in it...
    a) is very small if the secondary circuit is open, but
    b) increases when the secondary circuit is closed.
  2. jcsd
  3. Aug 28, 2009 #2


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    Re: Transformers

    What are your thoughts on this?
  4. Aug 28, 2009 #3
    This http://www.animations.physics.unsw.edu.au/jw/AC.html" [Broken].
    Last edited by a moderator: May 4, 2017
  5. Aug 28, 2009 #4
    Re: Transformers

    My thoughts:
    When the circuit is closed, the load placed in the secondary circuit draws current...Is that right?
  6. Aug 29, 2009 #5


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    Re: Transformers

    So, where does this power come from?
  7. Aug 29, 2009 #6
    Re: Transformers

    The power comes from the A.C. generator. (P=IV)
  8. Aug 29, 2009 #7


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    Re: Transformers

    Right. And if the voltage in the primary is 3 times the voltage of the secondary, then the current through the secondary will be three times the current in the primary; and power in = power out (neglecting losses).
  9. Aug 29, 2009 #8
    Re: Transformers

    When the secondary is open there is no current flow thus it consumes no power, power = voltage*current (there is also an additional term 'power factor' that we can neglect for this example).
    When the secondary is attached to a load, the primary coil feeds the secondary power through induction. A voltage is induced onto the secondary coil in the amount equal to the ratio of secondary windings/primary windings. The power required by the secondary comes from this voltage.
  10. Aug 30, 2009 #9
    Re: Transformers

    A real transformer does consume power with no load.

    The output voltage is not the ratio of the turns x the input voltage for a real transformer... certainly not with with more than just a very light load on it.
  11. Sep 14, 2009 #10
    Re: Transformers

    As you said when the secondary is open the primary current is very small; when the secondary is closed current flows in it and the primary current increases.

    To understand what is going on you should have some knowledge of electromagnetic fields and Lenz's law.

    When curren is flowing through any conductor a magnetic field is produced. Now, when a curren is flowing through the primary winding of a transformer the produced magnetic field is flowing in the iron core of the transformer. In this case the magnetic field isrepresented by lines known as flux lines. This lines intersect the secondary winding and induce a voltage across it.

    http://www.clayeng.net/Training/myweb6/Module04/Output/images/t-f3.gif [Broken]

    A) Secondary is open:
    No current flows through the secondary and consequently there is no magnetic field produces by the secondary. The only field in the core is that due to the primary current (which is small).

    B) Secondary is closed:
    Current is flowing through the secondary and a magnetic field is produced (so, now there are two magnetic fields: from Primary and Secondary). The secondary magnetic field opposes the primary magnetic field according to Lenz's law. So, the total magnetic field in the core which is now the resultant of primary and secondary magnetic fields is reduced. Since, EMF = N dф/dt , by reduction of flux the induced emf in the primary winding is reduced causing more current to flow in the primary winding. This continues untill a balance is achieved.

    Firstly, it seems to be complicated... but it isn't.
    Last edited by a moderator: May 4, 2017
  12. Sep 14, 2009 #11
    Re: Transformers

    You said that when the secondary circuit is closed, a current flows through the circuit which produces its own magnetic field. From where does this current come from? I thought that it is the induced current that flows through the secondary circuit. (Induced when the primary current being AC fluctuates between positive and negative peak emfs and causes change of flux to occur through the secondary coil)
  13. Sep 14, 2009 #12
    Re: Transformers

    The current flows because there are an induced voltage across the secondary winding.

    There are voltage...by closing the circuit there will be current.

    What do you mean by Induced Current?
    If you mean the current flowing due to the induced voltage, you are right.
    Induced voltage causes the induced current to flow.
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