83Views8Replies

Author Options:

Transformer current restriction? Answered

I am aware mostly how transformers (AC - AC) work, but I am lest informed on a few key parts.  I know there is a direct relationship between windings and voltage as well as an inverse relationship between voltage and amperage. The thing I don't understand is what limits the current produced by the transformer.  I know you can keep droping the secondary windings to increase amperage, but eventually you get to a point you can no longer do that.  To overcome this one can use three sets of windings wired in parallel which will retain voltage but increase amperage.  The amount of amperage is still limited to the maximum throughput of the primary, but how do you actually find this out.  If one wanted to make a variable current AC welder, it would be relatively easy to predict the voltage, but establishing a potential amperage is more difficult.  What actually establishes the maximum output of a transformer because in some cases the max current is reached long before the actual failure, and is there a way to predict the end product. 
P.S. I am not looking for the answer they use voltage/current limiting circuts ect...  I am looking at something like a pole pig which maxes out output before internal failure and contains no additional circutry, not a wall wart

Discussions

0
None
steveastrouk

Best Answer 4 years ago

Ultimately the transformer's power handling is limited by the properties of its iron and its copper. This article has a bit of hand-waving in it, but seems a reasonable intro to me

http://en.wikibooks.org/wiki/Electronics/Transform...

If you want the full run-down, you'll find that in books on "Electric machine design" - which a transformer is considered.

Once you pull so much primary current that the iron saturates, your transformer's reached its limit.

0
None
jj.incsteveastrouk

Answer 4 years ago

Thank you, I never quite made that connection before and the term saturated makes a lot of sense.

0
None
steveastroukjj.inc

Answer 4 years ago

Here's a nine KVA transformer I designed.

20121115_155448.jpg
0
None
jj.incsteveastrouk

Answer 4 years ago

That is a really nice, and large transformer I must say. What is its use as the primary and secondary seem relatively matched. Also, How many windings are on each coil and what is the wire size, I am trying to figure out how much wire one would need to make a stick welder, I have seen many small welders and spot welders made with microwaves, but I was thinking of a transformer somewhere between yours and a microwave to use.

0
None
steveastroukjj.inc

Answer 4 years ago

It weighs a lot more than a man. The primary is around 2.5mm^2 and the secondary is twin, paralled 16mm^2: The output is 36V @ 250A continuous. You won't be able to source that kind of iron and core formers. Find out what you can get. Also, the transformer in a stick welder is DELIBERATELY "bad", with LOTS of leakage inductance, so that the current is limited by the intrinsic design

0
None
jj.incsteveastrouk

Answer 4 years ago

As welders are rather lossy transformers I imagine the need for lamination is fairly minimal. I plan to simply cast or purchase an iron cylinder between 5 and 10 cm in diameter and 10 to 20 cm in length. This core will be wrapped with a few mm gap and it will be lifted from the center of the coil similar to a control rod in order to varry the amperage. I plan to wrap the secondary outside the primary which is another very lossy thing. I only hope that I can reach my power goal. As of now I am aiming for 150 to 200 amps at somewhere between 20 and 30 volts. I am worried I wont be able to achieve this much power at this voltage though so it will be rather experimental

0
None
steveastroukjj.inc

Answer 4 years ago

Believe me, the iron losses will kill you. Take apart an old transformer from a buzzbox and see how its made.

0
None
jj.incsteveastrouk

Answer 4 years ago

The losses of using pure iron instead of steel or the losses of not laminating it. As of right now I don't have access to any old ones to open. I don't really have a good idea of any other way to smoothly vary the current. I know some have multiple transformers in series or parallel and can switch some on and off, but I would really prefer a smooth change.

I see the way current is varied here, but I don't actually know the processes behind it.http://www.mig-welding.co.uk/forum/threads/78-buzz-box.8879/

0
None
steveastroukjj.inc

Answer 4 years ago

The non-lamination losses mainly. If its soft iron you should be OK on that front.