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Transformers

timelessbeing

Jun 7, 2011
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What is the difference between these transformers?
Np=1, Ns=10
Np=1,000,Ns=10,000
Both have turns ratio = 1:10
 

(*steve*)

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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?
 

timelessbeing

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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.
 

daddles

Jun 10, 2011
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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...
 

timelessbeing

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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.
 

davenn

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Allow me to rephrase...
but you never see transformers with one winding. Why?

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
 

davenn

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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.

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.


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

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



Dave
 

timelessbeing

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you never see transformers with one winding.

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

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.

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.
 
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(*steve*)

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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.

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.
 

timelessbeing

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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?
 

timelessbeing

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

Ampacity means current carrying capacity, does it not?
 

davenn

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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
 

poor mystic

Apr 8, 2011
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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.
 

daddles

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Ampacity means current carrying capacity, does it not?
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.
 

(*steve*)

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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.

Just what I thought. It comes from a non-English speaking country. :p
 
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