Hello,
I want to learn how to design transformer 220V AC to ~24V AC iron core what is calculation?

 

We cannot say until you tell us how many watts.

 

like Power 12V*3Amps=36VA

 

V*A product is the power rating of the transformer. The power on the input side must equal the power on the output side.
Example:
220V *1.0 amp= 24V*9.1 amp
The power rating of this transformer is then 220VA.

Your transformer should be marked in some manner to indicate its power rating.

What are you planning to do with it?

 

i am planing to learn how it work and design some in 11kv like that etc etc

Example:
220V *1.0 amp= 24V*9.1 amp
The power rating of this transformer is then 220VA.

what should i do now?

 

Go by the Old Thumb Rule -
1 Sq Inch core = (1*1) x 30 = 30VA
2 Sq Inch = (2*2) x 30 = 120 VA
3 = 270 VA

and so on.

 

11kVA ?
You should learn to design 1VA first.

 

You should learn to design 1VA first

How sir?

 

Do the math.

 

when i buy transformer core he ask for number...

 

that's what . learn to do the math in transformer calculation

 

I endorse the idea of starting out small. There are many things that can come out wrong in spite of careful calculation or use of graphs, and in the details of the construction of a transformer. It is much better if you get a feel for these things and experience problems with a low power and low voltage transformer. It will also make your life easier in terms of scrap material, burned fingers, and other undesirable side effects of learning the necessary techniques (at low voltage an accidental short to the core is unlikely to be fatal).

The article at the URL below is an excellent starting place for learning the formulas to which @R!ff referred.
http://electronicdesign.com/power/build-your-own-transformer
http://electronicdesign.com/power/build-your-own-transformer
I would save it for reference.

Similar information can be obtained from the manufacturers of materials for cores:
Ferroxocube
Ferronics
Magnetic Inc
TDK
Texas Instruments

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Magnetics Design 4 - Power Transformer Design​