# Transformer ?

Discussion in 'Physics' started by Mathematics!, Jan 5, 2009.

1. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
I am learning about transformers. And I am wondering why the magnetic flux
thru the primary coil will be the same as the flux thru the secondary.

Assuming an ideal transformer.

The derivation is using faradays law. But in the derivation why do we have
v1 = -N1 * F' and v2 = -N2 * F'. My many confussion is why F' is the same for both is it because of the iron core?
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Main Question!!!!!!!!!!
How do we know that their will be any magnetic flux thru coil 2. Just because we have flux thru primary coil how does this imply flux thur secondary coil ?
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My many problem is the above flux question.

With the assumption of the flux we then have v1/v2 = N1/N2 => v2 = N2/N1*v1 , N2 > N1 it is step up transformer and if N1 > N2 then it is a step down transformer.

Thanks for any help in understanding transformers.

Apr 20, 2004
15,815
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3. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,043
292

"Assuming an ideal transformer"

As with anything else in electronics, we have to start with ideal components and work down from there. A well designed power transformer will have upwards of 95% flux coupling. The remainder of this will show up as "leakage reactance.'

Certain types of transformers, such as Transmission Line transformers, common in R.F. circuitry can achieve even greater coupling factors.

The key to obtaining close coupling has a lot to do with the permeability of the core....this tends to confine the flux lines into a very small region, so they don't wander off into space. There IS no perfect transformer...but one has to start somewhere.

eric

4. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
But my problem is how does the magnetic flux thru primary coil get transfered to the secondary coil in the first place. I would think this would only work if
the 2 coils are close together? So that the magnetic field lines of primary coil go thru both coils?

5. ### triggernum5 Active Member

May 4, 2008
216
0
Consider the shape of magnetic field lines, and the position of the coils.. The core compacts those field lines into itself so that coupling is efficient.. The core can be considered a magnetic circuit pathway..

6. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Why?

And could we still assume the flux is the same thru both coils even if we didn't have a iron core. What happens if their was no core (air core).

So in most circuits if 2 inductors are close together they will have the same flux going thru them?

I would think their would always be a difference in flux regardless of ideal or not. I know it would probably be negtable but a difference none the less.
So these equations are not fully correct for the ideal transformer either they maybe of be an epsilon or so. Which is Ok for me if that is what they are assuming in the simple derivation of v1/v2= N1/N2.

Thanks for any clarity

7. ### n_prithivi New Member

Jan 4, 2009
2
0
i think you are the right person............. i have a 3V solar panel that gives output 0-3V(o/p is cntinsly changing ) if i convert it into AC an then use a step up transformer, will it be possible for me to get an output of 6VDC or greater......... or suggest me what to do for getting 6+V as a constant output using a solar panel

8. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
Why do you think a transformer has a 'core' ?

The core is made of a material which 'concentrates' the magnetic flux within and close to itself.
The term 'reluctance' is used to describe how easy or difficult it is for magnetic flux to pass through a material. This is analogous to resistance for current.
Just as we find the most current in a path containing the lowest resistance, we find the most flux in a path containing material of lowest reluctance.

Soft iron has low reluctance. This is why transformer cores are made of alloys of this.

If the windings are close to the surface of the core they will receive the 'benefit' of the enhanced flux.

Yes all inductors can pick up magnetic flux from other inductors, even air cored solenoids.
there are two forms of inductance noted.

Self Inductance.
This is the value quoted for an inductor in a circuit and represents the generator effect of passing curent through the inductor on its own windings.

Mutual Inductance.
This is much harder to specify and represents the generator effect of one coil's magnetic field upon another.
Its value depends partly upon the orientatin of the axes windings, relative to each other and is zero if they are at right angles.

9. ### triggernum5 Active Member

May 4, 2008
216
0
Ideal is an imaginary situation that we can never actually reach in a real world situation..
An inductor creates a magnetic field when current passes exactly like a bar magnet does.. Two bar magnets beside each other will create fields that interfere either constructively, or destructively depending on orientation.. One bar magnet beside an iron bar is analogous to two inductors (one attached to a current source) beside each other with no core.. The iron will weakly magnetize because the magnetic field is widely spread out.. (It leaves the magnetic poles as if they were antennae)..
If you were to take 2 more iron bars though, and use them to connect the top and bottom of the initial bar to the ends of the magnet, the bar would magnetize much more strongly, because the magnetic energy would be leaving the poles into the iron as if it was electricity travelling across a solder joint, not as if the magnetic poles were antennae..
Nothing is perfect though, the core doesn't contain 100.0% of the magnetic energy, but for that matter, energy is lost to leakage fields along every wire and connection in every device.. Its noticable at HV in the form of corona, but it happens in a potato battery as well..
(Notice the similarity between magnetic and electric energy?)

Last edited: Jan 7, 2009
10. ### GirishC Active Member

Jan 23, 2009
58
0

I think I will give you analogy, if you have over head water tank, and if you have a pipe coming out of tank at the bottom. Then water flows from top to bottom. Now add one more pipe with higher diameter. Then more water flows through bigger pipe. And that's the key to your answer. You need to think little bit...many times your next question answers previous question.

But good thinking...trust yourself

11. ### GirishC Active Member

Jan 23, 2009
58
0

You are right, 3VAC will get stepped up to 6VAC, but transformer can not generate energy nor destroy (that's the universal law). So what it does is it maintains energy same from the input to output (ideal condition). So at the output of step up transformer, you may have 6VAC but current will be less by factor of 2.