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Transformers

Discussion in 'General Electronics Discussion' started by timelessbeing, Jun 22, 2011.

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  1. timelessbeing

    timelessbeing

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    Jun 7, 2011
    What is the difference between these transformers?
    Np=1, Ns=10
    Np=1,000,Ns=10,000
    Both have turns ratio = 1:10
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Assuming all other things are equal, one has 1000 times as many turns as the other.

    What difference do you think this will make?

    And what is this question related to?
     
  3. timelessbeing

    timelessbeing

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    Jun 7, 2011
    Allow me to rephrase...
    Transformers are used, among other things, to convert voltage. Both the transformers in the example I gave will step up voltage 10x, but you never see transformers with one winding. Why?

    If I knew what difference it would make, why would I post the question here?

    The question isn't related to anything. I'm just curious.
     
  4. daddles

    daddles

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    Jun 10, 2011
    The copper losses in the transformer with lots of turns would be lots higher if the wire sizes were the same. It would also be more likely to saturate the core for the same current.

    You do see transformers with one turn -- the usual GFI outlets in the US have two of them. Ditto for the zillions of current transformers used in the power industries.

    Which brings up a question for the EEs: when a wire passes straight through a toroidal current transformer, is that called 0 or 1 "turn" by convention? Clearly, there's magnetic coupling between the "coils", implying a mutual inductance; I was just curious what the convention was, if there is one. Hope that doesn't hijack the OP's thread too much...
     
  5. timelessbeing

    timelessbeing

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    Jun 7, 2011
    If you look at transformers in ordinary household appliances, lets say a simple halogen Ikea lamp, the turns number in the hundreds. If it were simply a matter of achieving the desired ratio, surely the transformer would have fewer of them.

    daddles, my guess is that number of turns in the straight conductor don't play a significant role in the transformer calculations.
     
  6. davenn

    davenn Moderator

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    Sep 5, 2009
    actually you do, its called an auto-transformer or otherwise known as a variac
    it has a tapping point that can be adjusted for a variable output voltage.

    a "transformer with just 1 winding is otherwise called an inductor or choke.

    A choke when its in series with a DC rail to help provide smoothing

    inductors, often but sometimes fixed tapped and adjustable slug core are used in RF circuits to name one place.

    Dave
     
  7. davenn

    davenn Moderator

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    Sep 5, 2009
    you still have to provide the appropriate load across the 120 (240) VAC primary side to that it doesnt look like a short circuit and promptly burn out. Hence still lots and lots of turns are needed.


    turns in a straight conductor ?? thats a contradiction ;)



    Dave
     
  8. timelessbeing

    timelessbeing

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    Jun 7, 2011
    Sorry that was a brain fart. I meant to say one turn. Like in the example in the original post.

    OK that makes sense. How many turns do you have to use? This Ikea lamp uses 120VAC mains to power a 12V 20W halogen bulb.
     
    Last edited: Jun 22, 2011
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Given that the "load" that the transformer appears to be is not resistive, consider the effect of more turns. This will increase the inductance. The increase in inductance will limit the rate of rise of current with voltage. Too low an inductance will result in a huge current flowing. Whilst this current does not actually represent power (because of the phase relationship) it still causes resistive losses and thus heating.

    So the number of turns is affected by the frequency of the mains.

    If you look at high frequency transformers you'll find that they have far fewer turns.
     
  10. timelessbeing

    timelessbeing

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    Jun 7, 2011
    So the number of windings will depend on the transformer ampacity. You want to match the reactance to the wire thickness (and possibly the circuit load rating in your home) . The thicker wires you use, the less windings you need, and the more current it can deliver (higher KVA?). Hence smaller transformers will have more windings and bigger ones will have less.

    Did I get that right?
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Not really.

    Remember that there is another winding there that is taking energy out.
     
  12. poor mystic

    poor mystic

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    Apr 8, 2011
    "ampacity"? Did you try to get that right?
     
  13. poor mystic

    poor mystic

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    Apr 8, 2011
  14. timelessbeing

    timelessbeing

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    Jun 7, 2011
    Thanks, I'll take a look at those links.

    Ampacity means current carrying capacity, does it not?
     
  15. poor mystic

    poor mystic

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    Apr 8, 2011
    It does not, or rather not in New Zealand, where I did my training; the word "ampacity" has never been heard.in that noble country.
     
  16. davenn

    davenn Moderator

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    not in Australia either, but I like it haha Ampacity, has a nice ring about it ;)
    send it off to the Websters Dictionaly for inclusion
    you could have more formally used Amperage, or current capacity

    Dave
     
  17. poor mystic

    poor mystic

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    Apr 8, 2011
    Dave I looked it up on Wikipedia
    c'est un fait accompli!
    They have completely %#@* our language
    I shall go into the garden and eat worms.
     
  18. daddles

    daddles

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    Jun 10, 2011
    Yes it does, in the US at least. I have engineering documents that use that term from 30-40 years ago and, I believe, it has been part of the NEC for quite a while.
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Just what I thought. It comes from a non-English speaking country. :p
     
  20. poor mystic

    poor mystic

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    Apr 8, 2011
    It's got a kind of a Walt Disney feel about it.
     
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