Auto Transformer Principle of Operation,Working & Applications


An auto transformer is a electrical transformer having only single winding which acts as both primary and secondary,so in input and output connected to shared single winding.

Why we need to go for Auto Transformer ?

We have some advantages of auto-transformer over normal two winding transformer.
1.Auto transformers usually smaller in size,because one winding is eliminated.
2.as size is small cost also low(so cheap in cost)
3.as the winding is same so leakage reactance will be less.
4.increased kVA rating.

Auto Transformer

Construction, Principle of Operation Of Auto Transformer: 

In Auto Transformer, one single winding is shared as primary winding as well as secondary winding.as in transformer copper wire wound on silicon steel.as shown in the figure input connected at fixed positions.but on other side we employ some tapping to get variable output voltages.Variable turns ratio at secondary can be obtained by changing the tappings of the winding.

Auto Transformer

Copper Savings in Auto Transformer:

If we compare conventional two winding transformer and  auto transformer the amount of copper needed for auto transformer is less.weight of copper of any winding depends upon its length and cross - sectional area.and length depends on no.of turns,cross - sectional area varies with rated current.

Auto Transformer

copper in the section AC proportional to, 

(N1-N2)I1

weight of copper in the section BC proportional to

(I1-I2)N2

Hence, total weight of copper in the winding of auto transformer proportional to,



In similar way it can be proved, the weight of copper in two winding transformer is proportional to,


N1I1+N2I2
=2N1I1(Since, in a transformer N1I1=N2I2)

Let's assume, Wa and Wtw are weight of copper in auto transformer and two winding transformer respectively,



∴ Saving of copper in auto transformer compared to two winding transformer,
∴ Wtw -Wa=kWtw