Auto-transformer design

I want to design an step-down auto-transformer of 220/110 V. I have read a book where it says that there is no difference between the two winding transformer and a single winding auto-transformer. So i calculated required turns for the input and the output. I used a core limb with 11.52 sq. cm cross sectional area. I got 660 turns in input according to normal rules. But i got another calculation from internet according to which input is of 1100 turns. I don't know where this calculation came from. I just got the equations, inserted values and found 1100 turns. Can anyone help me with the design of this transformer? i am a newbie.

Hello Shahedul Ahsan,

I can help you, but I need an information about the power you want to transfer and the core you have.

The calculation of an auto - transformer is different to a normal transformer. You can use a smaller core for the same power.

Regards

Rainer

The calculation of an auto - transformer is different to a normal transformer.

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Not necessarily, but you need to understand the operation of an auto transformer. A 220/110V 1000VA auto transformer is essentially a normal 110/110V 500VA transformer.

Hallo FvM,

A 220/110V 1000VA auto transformer is essentially a normal 110/110V 500VA transformer.

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I agree with you, but I think you have a typing error.

In my opinion it must be : "A 220/110V 1000VA auto transformer is essentially a normal 220/110V 500VA transformer," otherwise you have only a isolation transformer without voltage transformation. In this case the correction factor is zero.

Regards

Rainer

I meaned 110/110 as written. That's excatly the way to use a regular transformer as auto transformer: Connect primary and secondary winding of a 110/110 transformer in series.

Hello FvM,

Connect primary and secondary winding of a 110/110 transformer in series.

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If you write it like this, it's OK. I've understand it in other way, like I write. Sorry.

Regards

Rainer

Hello rfredel ,
I am using a core cross sectional area of 11.52 sqr.cm. And i want a 60 VA output

11.5 cm² sounds like M85b, would be good for 115 VA as regular transfomer, respectively 230 VA as auto transformer.

Hello Shahedul Ahsan,

**broken link removed**

I hope, I have make no mistake.

Regards

Rainer

Dear Rfredel Sir,

I am from India and i am new in transformer calculation as on 27.04.2013 i join myself to edaboard.com. i have read a lot of books also see many articles over the internet during this time rewind many transformer but at last while read your article which was for step down autotransformer i am satisfied. i always honor to Germans technology also. sir i want the same article for a step up autotransformer i will be highly obliged for same. here is attached of your step down autotransformer page.

**broken link removed**

In this article i want to understand some item. as follows.
1. Power of a normal transformer P=(Fq / B)² why B value is 1.3 ( i understand core is CRGO ) ?
2. P auto transformer = P normal transformer / {1-(U2/U1)} here is U2 = output volt & U1 is input volt what will be calculation for step up autotransformer ?
3. N/V = 10*10*10*10 / (4.44 * 1.3 * 11.52 * 0.9 * 50 ) what is meaning for 0.9 filling fector ?