# Which of the Following is True for Auto Transformers

By Varadi Hema|Updated : July 22nd, 2022

Which of the following is true for an autotransformer?

1. Auto transformer requires less copper as compared to the conventional two-winding transformer of the same capacity.
2. Autotransformer provides isolation between the primary and secondary windings.
3. Auto transformer has less leakage reactance as compared to the conventional two-winding transformer of the same capacity.
1. Only 1 and 2
2. Only 1
3. Only 1 and 3
4. Only 2 and 3

Answer: C. Only 1 and 3.

## Auto transformer requires less copper as compared to the conventional two-winding transformer of the same capacity and have less leakage reactance as compared to the conventional two-winding transformer of the same capacity.

Solution:

If k=(number of turns in the primary winding of auto transformer)/ (number of turns in the secondary winding of auto transformer)

We know that,

Conductor volume ∝  (Conductor cross section area × conductor length)

Conductor cross-section area ∝  current ∝ MMF (NI)

⇒copper required for auto transformer/ copper required for two-winding transformer=1/(1-k)

So, if k>1, the autotransformer requires low copper compared to the two-winding transformer of the same rating. Hence statement 1 is correct.

The disadvantage of the autotransformer is that there is no isolation between the primary and secondary winding. Hence statement 2 is wrong.

Autotransformer's leakage is less than that of the two-winding transformer of the same rating. Hence statement 3 is correct.

Hence option C is the correct answer.

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