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20W 50Hz isolation transformer

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ali_asadzadeh:
Hi,

I want to design and build an Isolated 380Vac to 220vac single phase(50Hz) 20W transformer, the main thing about it is that it should withstand 6Kv isolation for burst tests, I want to short the input and outputs and test the 6Kv isolation. The windings also should withstand 6Kv isolation to the core.

The problem with this transformer is that I have lack of information, first of all I know these formulas from the high school.

V1/V2 = n1/n2

But How should I calculate the core size, which is an Iron EI core, what’s the wire sizes? What’s the primary winding turns?

I have another 220 Vac to 220 Vac 200W transformer Which I have bought many years ago. I measured it’s primary winding @ 100Hz and it’s in the order of 275mH.

What should be the size of insulation and bobbin thickness to pass the 6kV isolation? Do we have a tool for calculations? Any Ideas are very welcome.


Thanks

soldar:
Some very, very, and I mean *very* rough rule of thumb calculations:

I assume 20 VA rather than 20W. This is a very small transformer. The core section depends of the characteristics of the core but I would start our with, say, 5 cm2.

At 3A / mm2 the 380V primary can use 0.15 mm diameter wire and the 220 V secondary 0.2 mm.

Say 3800 turns on the primary and 2300 on the secondary. That is my first rough iteration. Calculate the bobbin section and if not enough go up one size.

If you want more precise calculations we would need to know magnetic characteristics of the core, etc.

Regarding 6KV isolation, I suggest this is for someone who has more knowledge and experience. If I had to build it myself I would go for separate bobbins on separate branches of a C core as that would make things much simpler. If you wind both on the same bobbin you really need to be careful and know what you are doing.

In air you would want to keep at least 6 mm separation and I would go for more if possible. With other materials it can be less but you would need to make absolutely certain there are no gaps or cracks. As I said, separate bobbins on a C core make it much easier. Still you need to study the bobbin material and insulating characteristics.

ali_asadzadeh:
Thanks for the feedback :)



--- Quote --- The core section depends of the characteristics of the core but I would start our with, say, 5 cm2.
--- End quote ---
So what is the length and width and height of the core? is there any standard core available?

How did you calculate the turns, the core is Iron,does a 4mm ABS  bobbin withstand 6kV? how should I calculate it?

Also is there a certain way to turn the wires, I mean for the isolation?

soldar:
Why do you want to design and build this yourself? Is it only to save some money?  Because, to be honest, I get a feeling you would be much better off just ordering it from a professional manufacturer. They can design and build in no time. They have all the supplies they need and plenty of expertise.

You will have much difficulty in buying only what you need and will have to buy much which will be wasted. But, most important of all, while the design is not hugely complicated for someone experienced, it is really outside of your level right now. If you really want to do this I suggest you get started learning how to design and build transformers. It is not extremely complicated but there are many small things. What laminations do you have access to? What are their magnetic and mechanical characteristics? Where are you going to buy all your supplies? If it were for me I would overbuild it just to be extra safe.  You want extra voltage isolation and you need to get the characteristics of the materials you will use. Where would you get your supplies?

What is the purpose of this transformer? Should it have some protection?

Frankly, I do not feel comfortable designing a 380 / 230 V transformer with 6KV isolation for myself, and much less for someone else who is a beginner. If you just want the transformer I suggest you buy it from a professional. If you want to learn I suggest you start by reading -- a lot. And then start with something more modest.

Note that professionally built transformers are going to be much better built. For instance, they will vacuum and fill with varnish which seals and holds everything in place. You are not going to be able to do this as well. I mean you can just soak it in varnish but the result will not be the same. They have winding machines which you don't. It is easy to damage the insulation on the wire if you are not extremely careful. You could end up wasting a lot of time and supplies ... and maybe causing some damage.

Circlotron:
A 380V 3-phase supply measures 220V from any phase to neutral. If you have the neutral wire available, do you really need a transformer? What is the function you want to achieve?

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