# autotransformer design

Discussion in 'Homework Help' started by Shahedul Ahsan, Oct 15, 2012.

1. ### Shahedul Ahsan Thread Starter New Member

Oct 15, 2012
1
0
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.

2. ### vk6zgo Active Member

Jul 21, 2012
677
85
Unless you have a very important reason to go with an auto-transformer,I would advise you to use a double winding transformer instead,for a number of reasons.

(1)---& most important!!
Auto-transformers offer no isolation from the mains supply,& rely entirely upon the winding connections being such that one side of
the tapped section of the winding is at mains Neutral.

If you get the connections right,the Electrician may have inadvertently transposed Active & Neutral at the socket.

If they are transposed,your "secondary" is 220V above Neutral (& hence,Earth) at one end,& 110V above Neutral at the other end.

(2)
Although simple in concept,Auto-transformers become messy in practice,as it is difficult to safely & securely attach the tap,& bring it out to the outside of the windings.

(3)
You have to wind all the 220V winding from scratch,whereas with a double winding,you may be able to recycle an existing transformer.

If there is enough room left on the bobbin,you can re-use the primary,remove the secondary,& wind a new secondary for your required voltage.

Take note of what the voltage of the existing secondary is,then count the turns as you unwind them.---This will give you the turns per volt,which you need to determine the number of turns needed for your new secondary.