# Auto Transformer Principle of Operation,Working & Applications

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

## Why do we need to go for Auto Transformer?

We have some advantages of auto-transformer over normal two winding transformers.

1. Autotransformers 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.

## 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.

## Copper Savings in Auto Transformer:

If we compare conventional two winding transformers and autotransformer the amount of copper needed for autotransformer 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.

copper in the section AC proportional to,
(N1-N2)I1
The weight of copper in the section BC proportional to
(I1-I2)N2
Hence, total weight of copper in the winding of autotransformer proportional to,
In a similar way it can be proved, the weight of copper in two winding transformers is proportional to,
N1I1+N2I2
=2N1I1(Since, in a transformer N1I1=N2I2)

Let’s assume, Wa and Wtw are weight of copper in autotransformer and two winding transformers respectively,

∴ Saving of copper in autotransformer compared to two winding transformers,
∴ Wtw -Wa=kWtw