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How Transformers work

  1. Nov 4, 2013 #1
    Hi,

    I'm trying to comprehend the idea of why the induced current of a transformer decreases as voltage increases. Is this in terms of an increased applied voltage? I know that if you have a high applied voltage through the primary coil, if it has a high number of loops, this will cause for a decreased induced voltage and thus, a decreased induced current? I know that the secondary coil will have a lower number of loops and that ε1 = ε2, which is equivalent for power and that say that p1 = p2 = I1ε1 = I2ε2. Is my understanding on this correct or am I missing something?
     
  2. jcsd
  3. Nov 4, 2013 #2

    berkeman

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    This recent thread in the EE forum may be of help:

    https://www.physicsforums.com/showthread.php?t=719852

    :smile:
     
  4. Nov 4, 2013 #3

    mfb

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    I think this is easier to see on the primary coil: for a given coil (and a given frequency), you want to double the transmitted power. Therefore, you have to double the magnetic flux in the coil. There are two ways to do this:
    - double the current
    - double the number of windings. But as the voltage per winding stays the same, you need twice the voltage

    As you can see, just the product I*U matters.

    The same analysis works for the secondary coil as well.

    Right. With more windings, the same current leads to a larger voltage, so you can get less current out of it (as the input magnetic field is fixed by the primary coil).
     
  5. Nov 4, 2013 #4
    not necessarily.
    First of all the primary/secondary can have any turns ratio it depends only on the application and some physical limits nothing more.
    There are 1:1 isolation transformers , step up transformers and step down ,
    Now I can have a say primary 230v ac wall socket winding and I can make the secondary say some 3000 volts , that would be a step up transformer the secondary would have mopre turns than the primary.
    Now the current in tat secondary doesn't have to be smaller or limited it is usually smaller only because the input , or the primary source has a limited current capability and if you have a say 230v 10amps and you want 3000 volts you can't have 10 amps anymore as energy doesn't just show up from nowhere it has to be conserved so you cant have more than the primary and if the primary has x volts and y amps then the secondary having x2 volts cant have y amps anymore , for something to increase something has to decrease in a situation where the source is limited.

    But if you could supply enough current you could increase the voltage on the secondary and keep the current the same.

    Oh I came too late some good answers have already been given, well anyways :)
     
  6. Nov 4, 2013 #5
    Thank you all for your response! On the thread link that was posted, someone wrote this:


    "Get your 'causes and effects' in the right order and it may make better sense.
    1. You apply your supply volts to the primary. 2. This causes a secondary voltage (transformer ratio). 3. That secondary voltage will cause a certain current to flow through the load (I = V/R).
    4. That secondary current will result in a primary current (transformer ratio again).

    So the turns ratio doesn't 'cause' a lower or higher current in the load."


    I tried asking my professor if resistance would have anything to do with an overall decrease in induced current and he mentioned transformers don't correlate to resistance for a decreased induced current with an increased voltage (although applied vs induced voltages were not distinguished). From the perspective of Ohm's Law, does not resistance not apply to how a transformer works? It's more based upon the number of loops?
     
  7. Nov 4, 2013 #6

    mfb

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    Resistances do not matter for the transformer itself. Resistances somewhere can influence the whole network, of course (like the power transferred in the transformer).
     
  8. Nov 4, 2013 #7
    resistance does come into account ofcourse when you have a circuit or a path for that secondary current to go , but impedance would be the right word for a transformer winding as it has to do with more than just typical resistance like in a dc resistor.

    The total current of a secondary winding is dependent of the circuit it is attached to ofcourse.

    In simple terms the secondary winding current is defined by the turns and wire diameter as the wire serves as the current carrying medium.

    http://en.wikipedia.org/wiki/Electrical_impedance
     
  9. Nov 4, 2013 #8
    Its all about the law of conservation of energy - energy cannot be created nor destroyed- thus if voltage increases current MUST decrease - if this did not happen it would certainly solve our energy issues because we could just snap our fingers and CREATE energy which according to the laws of physics CANNOT happen.
     
  10. Nov 4, 2013 #9
    Thank you all again for your responses! I feel like I better understand how transformers and the concept of voltage increase, current decrease. I realize too that there are two different types of voltage present: applied voltage and voltage induced. So it could also be thought of as if there is a greater amount of voltage applied, then the amount of voltage induced will decrease with the number of loops present in the coil and thus will be proportional to a decrease in induced current.
     
  11. Nov 5, 2013 #10
    not exactly , you said:
    " then the amount of voltage induced will decrease with the number of loops present in the coil and thus will be proportional to a decrease in induced current. " "

    if the turns in the secondary are lower than that of the primary then you have a step down transformer , in such case the current in secondary will be higher than that of the primary because you have fewer turns and less voltage so current goes up.The only thing limiting this would be the wire thickness you would have for the secondary.
     
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