What are the Main disadvantages of auto transformer ?

Auto transformer consist of single winding (no primary secondary winding) so the main disadvantage of auto transformer is that it provides no galvanic isolation. Used only in those application where a small voltage change is required from input voltages so this is another disadvantage of auto transformer.    

• An electrical connection between the primary and secondary is not always desirable, particularly when the voltages levels on both the sides of transformer are quite different. If the neutral point is not solidly earthed, the lower voltage side can be subjected to high potential in the event of earth fault on high voltage side. In general practice, auto transformers are only used on distribution systems where the neutral is connected to the earth
• Auto-transformers are particularly sensitive to the atmospheric over-voltages. Therefore the auto-transformer requires much highs standard of insulation compared to normal transformer. Surge Arrester protection is required
• As the short circuit impendace values of the auto-transformer are low, short circuit currents can reach higher critical values.

An auto transformer just like a current transformer used for amps reading, while in voltage/ instrument transformer use for volt reading purposes. There is no disadvantage about it. Just on what will be the function of a transformer.

we can’t say disadvantage ,because it is designed to meet certain requirement ,that is can’t  be accomplished by other type of transformer.

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I AGREE WITH MR. MUSHRIQ ABDUL MAJEED

I apologize this is not my specialist

Thanks for invitation -

Agree with both answers for E.Manish - E.Muhammad Qasim *

The Main Disadvantage of the Autotransformer is that it does not have the Primary to secondary winding Isolation of a conventional Double Wound Transformer.

1. Auto-transformers are particularly sensitive to the atmospheric over-voltages. Therefore the auto-transformer requires much highs standard of insulation compared to normal transformer. Surge Arrester protection is required 

2. An electrical connection between the primary and secondary is not always desirable, particularly when the voltages levels on both the sides of transformer are quite different. If the neutral point is not solidly earthed, the lower voltage side can be subjected to high potential in the event of earth fault on high voltage side. In general practice, auto transformers are only used on distribution systems where the neutral is connected to the earth,

3.  Because of Low short circuit impedence value ,short   circuit currents can reach higher critical value.

There is no separation between winding, So, during charging it 

always sences higher charging current a compared to normal transformer.

In Ferro-Alloy industries, Auto tranformer ack as a safeguard to Main Ferro Transformer.

"The main disadvantage of autotransformers is that, unlike ordinary transformers, there is a direct physical connection between the primary and secondary circuits, so the electrical isolation of the two circuits is lost