# Auto-Transformer Formula help Please..

#### stinky111

Joined Apr 7, 2011
39
Hi All,

Im studying for a final & am having problems finding the formulas in my notes to solve the question below..

An auto-transformer is connected to a 230V a.c supply. the secondary voltage is 50V a.c and the load current is 8A. How much current flows to the common part of the winding??

Appriciate the formula to find the current flowing to common part of winding please guys?? (Final a week away & I have no more lectures beforehand)
Thx

#### jimkeith

Joined Oct 26, 2011
540
Iout = Iin + Icom

Icom = Iout - Iin

Iin = Iout /Eratio

Icom = Iout - (Iout /Eratio)

Try this and see if it works

#### EL7819

Joined Apr 15, 2011
20
Np/Ns=Vp/Vs=Is/Ip

#### t_n_k

Joined Mar 6, 2009
5,455
Np/Ns=Vp/Vs=Is/Ip
This is partly true for the auto-transformer - to the degree that it relates the input primary terminal current to the secondary terminal current but it doesn't tell you what the common current is in the 'shared' part of the winding. Jimkeith's method is correct.

#### stinky111

Joined Apr 7, 2011
39
Iout = Iin + Icom

Icom = Iout - Iin

Iin = Iout /Eratio

Icom = Iout - (Iout /Eratio)

Try this and see if it works
Thanks JimKeith,
so am i correct in saying:

Ratio = vp/vs= 230/50 = 4.6
Icom = Iout-(Iout/Eratio)
= 8a-(8a/4.6)
= 6.26A??
Thx..

#### t_n_k

Joined Mar 6, 2009
5,455
Correct.