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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