Is the Transformer in My Adapter Damaged? A Question for Physics Assignment

[Sora no tori]

Hi guys, I am rather new here and would like to pose a question about transformers.
As physics assignment I am supposed to make a model of an AC transformer, for which I grabbed an AC/AC step down adapter (something like these here: http://www.digikey.com/product-sear...internal-off-board/ac-ac-wall-adapters/590573) , my model (with coils on two sides around a soft iron core) and a multimeter. So I cut open the wire in the AC to AC adapter (230V to 12V 0.7A) and plugged in my adapter which would be connected to my model's primary coils, and the secondary coils would then be connected to my multimeter. My multimeter showed that it stepped about 12~V down to 6.0 V for coil ratio of 50:100, so I thought it was fine, until the adapter kind of stopped functioning.
My question is: Is anything wrong? Did the transformer in the adapter spoil due to a short circuit or something? If so how do I fix it/ prevent it if I get a new adapter? Please do help me out, thanks!

 

[sophiecentaur]

It's anybody's guess as to what's going on.
Am I right in thinking that you have made your own adapter? What did it consist of? That could be highly relevant.

 

[Sora no tori]

Uhh so basically the adapter is bought to step the voltage down from 240V (from wall plug) to a safe experimental 12V 0.7A AC current, and then I use my model like the one in the link below to step it down further to 6.0v~. Basically I am required to demonstrate how a AC transformer works through my model, and my adapter to give me lowered voltage AC current kind of stopped working after some time and I am unsure why. My guess is that there is no resistors and the heat from short circuit messed up something, but I ain't sure so I am asking: Why did the adapter kind of stopped working? (The adapter which was bought, not my transformer model)
Then again, I used about 0.6mm thick copper wire coated with enamel for my model, and coiled 100 primary coils and 50 secondary coils around a soft iron core.
Hopefully this clarifies my problem. :)

(http://upload.wikimedia.org/wikiped...3d_col3.svg/1280px-Transformer3d_col3.svg.png)

 

[sophiecentaur]

It seems to me that your transformer, despite having the right turns ratio, has too few primary turns and its primary inductance is too low - allowing far too much current to flow, even with the transformer unloaded. Try ten times the number of turns and see how it works then.

 

[Sora no tori]

Hmm... I'll take that into account. Thanks a lot for your help. On another note, what causes the vibrations through the iron core when the current flows through??

 

[anorlunda]

Hmm... I'll take that into account. Thanks a lot for your help. On another note, what causes the vibrations through the iron core when the current flows through??

Are the vibrations of the same frequency as the AC voltage?

 

[sophiecentaur]

Are the vibrations of the same frequency as the AC voltage?

Twice the frequency, actually (most likely). There is a max in the current amplitude, twice every AC cycle and those peaks of striction within the core are at twice the frequency of the AC.

 

[anorlunda]

Twice the frequency, actually (most likely). There is a max in the current amplitude, twice every AC cycle and those peaks of striction within the core are at twice the frequency of the AC.

I don't think so. I believe the vibrations he asked about is actually the power line hum. See below from Wikipedia. It is the fundamental, not the first harmonic that we hear.

 

[Sora no tori]

Hm... Thanks a lot guys. :) How do I close this thread though? My queries have pretty much been answered.

Edit: Ok wait the vibration thing is kind of confusing though. Which is it? :O
I did check the hum sound out, it kind of sounds like the hum along the power lines.

 

[sophiecentaur]

I don't think so. I believe the vibrations he asked about is actually the power line hum. See below from Wikipedia. It is the fundamental, not the first harmonic that we hear.

I find that very surprising, bearing in mind that a transformer is not like a loudspeaker voice coil (i.e. there is no permanent magnet to set up a 'reference' H Field). Where is the direction to a force that will change with the polarity of the AC? I would seriously have expected mostly second harmonic.
It may be that the transformer sound spectrum picture is from a three phase power transformer, which I might have expected to produce sound at the fundamental frequency. The fact that there is a strong peak at the third harmonic seems to confirm my suspicion.

 

[davenn]

Hm... Thanks a lot guys. :) How do I close this thread though? My queries have pretty much been answered.

you don't need to just leave it as is

thankyou for saying thanks to those that helped

Dave