# How does a transformer work?

Integral
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The field inside of a solenoid is not zero. The field outside is small but not exactly zero unless you have a toroid or an infinitely long solenoid.

It does not matter where you wrap the secondary, inside or outside as long as the secondary encloses the interior of the solenoid.

It is the CHANGE IN FLUX inside of a loop which induces current. When the current changes in the solenoid the magnetic flux inside the solenoid changes this change in flux is what induces the current in the secondary. This works as long as the change in magnetic flux is inside the loop. It is not the current in the wires you are enclosing it is the magnetic field inside the solenoid.

This trouble you are having hits pretty close to home. We were doing the Roland Ring experiment, that is a small secondary warped around the OUTSIDE of a toroid coil. Class room work had taught me that there was ZERO field OUTSIDE of the ring, how could this possibly work. But it did! I took my questions to my Prof, he stood me in front of the black board and had me write out Maxwell's equations. I had to explain the meaning of each term until if finally sunk in. Thank you Dr. David Burch, When I finally understood, the earth shook beneath my feet as some long held mental models came crashing to the ground.

Learning is the ability to overcome preconceptions.

Thanks for the input,
Ok I see that the equation is based of current enclosed, so lets say I took a 1" solenoid and wrap a 10 foot loop around it, the solenoid is enclosed, even though B will be proportional to 1/r, I should still see a slight EMF right? Lets say I want to run some experiments with a large solenoid, maybe around 2 ft large, would you have any suggestions on what to use for a core? I would like to be able to calculate the impedance, that way I don't blow another circuit breaker Now all my experiments have a 2 amp fuse with a variac along with current shunt I've built.
Philip

Integral
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Originally posted by philipc
Thanks for the input,
Ok I see that the equation is based of current enclosed, so lets say I took a 1" solenoid and wrap a 10 foot loop around it, the solenoid is enclosed, even though B will be proportional to 1/r, I should still see a slight EMF right?
Not necessarily, you have left out an important factor, you must CHANGE the current to see and induced emf in the outer loop.

Lets say I want to run some experiments with a large solenoid, maybe around 2 ft large, would you have any suggestions on what to use for a core? I would like to be able to calculate the impedance, that way I don't blow another circuit breaker Now all my experiments have a 2 amp fuse with a variac along with current shunt I've built.
Philip
2' is not a very helpful dimension for this. We need length, diameter and number of windings and the frequency of the current to compute the impedance.

Does the core also play role on impedance? Sorry I should of mentioned the dimensions in more detail, I was thinking 2' for the length, and the diameter will be based of the core material I can find. Number of turns, well I'm not sure yet, but maybe something like 200? Guess I just want to copy some experiments I've seen done, and play with secondary loops to see it for myself. I just haven't had much luck building transformers, need more theory :)
Philip

Integral
Staff Emeritus
Gold Member
If you really want to learn something useful you need to find a copy of a Physics text such as Fundamentals of Physics by Halliday and Resnick. To get the most out of it you will need to understand basic calculus. Study the chapters on Electro Magnetism.

My apologies but I cannot reproduce the necessary equations from memory, and do not have time to do the reserach for you.

Originally posted by philipc
Does the core also play role on impedance? Sorry I should of mentioned the dimensions in more detail, I was thinking 2' for the length, and the diameter will be based of the core material I can find. Number of turns, well I'm not sure yet, but maybe something like 200? Guess I just want to copy some experiments I've seen done, and play with secondary loops to see it for myself. I just haven't had much luck building transformers, need more theory :)
Philip
Yes the core plays a role , that's why in tuned circuits ( if your familiar at all with them ) use inductors with slugs that go thru the center of the coil. The slugs are made of ferrite or powered iron, as you insert more of the slug into the core the inductance, hence the Fq response of the circuit changes.That's why power transformers are so heavy more core means easier saturation and more efficient transfer of energy from s1 to s2 .If you understand field collapse and the subsequent induction of current, I wouldn't worry too much about the physics rather look up transformer construction on the net. The concepts are fairly simple. Windings are usually stated in ratios I.E 1:1 equals the same amount of windings on both sides, 2:1 equals 2 windings on s1 for every 1 winding on s2, ect. The only equally important factor is that as you half voltage you double current ( the inverse is true also) so if you have a 2:1 transformer with a supply of 100v 1@ s2 output will be 50V 2@.. If you use a low V wall transformer you can play all day and not worry about frying anything

edit to add: Inductance constraints are important in tuned circuit and in output stage transformers not usually a BIG worry in power x-frmers ( just remember Ohms law, wall current usually doesn't like 2 ohm loads =) )

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The only thing is I don't know where to find a 2 foot ferrite rod, I have steel pipe, would this be ok to use? Or do I need something solid? I'm only looking to run some experiments on Ampere’s law so efficiency is not of matter. Right now my main concern is building a solenoid that won't blow a fuse
Philip

I'm not quite sure what your attempting but in power X-formers and solonoids solid is best Steel is great ( in a solonoid the core is hollow and when energiezed "pulls in" the solid slug.. I only mentioned ferrite in example of tuned circuits. ( those components are small and very low current. ) My first x-former was bell wire and a 10 penny nail . If your building a solonoid it is one winding around a plastic or some other non-ferrous meterial The two are different , one (x-former ) uses saturation and field collapse to induce a current on phyically paralell winding, the other ( solonoid ) uses a coil to generate a magnetic field in order to pull a solid slug into it's core. Is this a school project or are you experimenting? Radio shack used to have books on basic electronics when I was a kid ...cheap and not too drenched in theory.

A 2Ft. Solonoid..?? You trying to build a rail gun? If you are the electronics are advanced, expensive and LEATHAL if you don't know what your doing.

No rail guns here, I just finished up Physics II and my wanting to do this experiment comes from the question I ask a few post earlier,

"My question is, does it matter if the primary is on the outside or inside? And if not, why not, because in my Physics class when we found the magnetic field inside a solenoid using Ampere’s law, we took the path outside the solenoid to be approximately zero, but if I wrap a coil around the same solenoid I would see an induced EMF? So if there is approximately zero magnetic field outside a solenoid, how does wrapping a wire around it create an induced emf?"

So the truth is I'm trying to prove Ampere's law to myself, unfortunately we did not go very deep into transformer theory, that will be in the future classes, but on my Christmas break I wanted to play around with a few of the things I've learned in Physics II. I'm having a hard time building a transformer that doesn't blow my 1 amp fuse, much less even think about building a rail gun. So my idea was to build a 2 ft solenoid for I could run various experiments, but have no clue on how to build one the proper way.
Philip

Integral
Staff Emeritus
Gold Member
Phillip,
You do not need a 2ft long solenoide to proof this, A nail should do nicly.

Ah I think I see your disconnect. The issue is one of permeability .
Air= Mu of 1 ,iron = mu of 1000.

When a coil , like a solenoid generates a mag field , there is a small mag field on the outside, while the shape and diameter of the hole ( usually <1/2') tends to focus the field towards the interior, wrapping an additional coil around the first mearly absorbs the existent mag field and even though the Mu of wire ( copper) is ~ to air it can actually couple and channel the emf to be used as an inductive current. The result however will be an inefficient transfer of the mag field, now if we add an iron core, the subsequent field is orders of magnitude higher and the field collapse transfers more emf to the additional coil.

This is a good site for understanding transformers,solenoids,generators, ect.
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/transf.html#c1

Originally posted by david90
since dc won't work with a transformer, what if u charge a capacitor with dc circuit then discharge it to a transformer, would the voltage be amplified? why?

When u first connect a dc power to a transformer, doesn't the voltage gets amplified then drops to zero? change in magnetic field right?

also, i'm hearing people say that amp kills not voltage. So if u cut yourself and stick a 9v bat to the cut, it would kill u? Would a 1.5v bat kill if the resistance of ur body is low enough to get high amp?

The van graf machine makes 10k volt so why is safe to touch it? Would u feel a shock if ur barefooted and touched the dome?
it should be metioned that it is neither voltage nor amps that kill it's power (the combination there of) if you could crank low voltage at many amps or high voltage at low amps either will kill.

Transformer operation

I am a journeyman electrician and I had a helper ask a question I really didn't know how to respond to. If you apply 480V to the primary of a transformer and step down to 120/208, how exactly is the voltage "created." I understand the principal of induction but the way he was asking it was "In an electrical circuit you utilize the energy through a light bulb or motor, etc. How is it able to alternate when you're not using it?" The best way I could explain it is that the induction is using the field that the primary creates and the induction is mutual between the coils. I'm not sure if that was the best response..... any ideas?