The normal transformer has two windings; primary winding and secondary winding. The autotransformer is a special kind of which has a common winding for primary and secondary winding.

The operating principle of autotransformer is same as the normal transformer. But, the construction of autotransformer is different compared to the Conventional transformer.

Sr No	Parameter	Two Winding Transformer	Autotransformer
1	Separate Winding	Primary and secondary windings are separate.	A part of winding is common between primary and secondary.
2	Movable contact	No movable contact	There is a movable contact.
3	Type	It can operate as step up or step down.	It can work as step up as well as step down.
4	Copper saving	None	Copper saving takes place.
5	Possibility of getting a variable secondary voltage	–	Yes
6	Electrical isolation	Yes, there is electrical isolation between primary and secondary.	No electrical isolation between primary and secondary.
7	Size	Large	Small
8	Cost	High	Low
9	Losses in the winding	High	Low
10	Efficiency	Low	High
11	Regulation	Poor	Better
12	Applications	Mains transformer, power supply, welding, isolation transformer.	Variac, starting of ac motors, dimmer stat.
13	Symbol
14	Induction	Mutual inductance	Self-induction
15	Winding Material	More winding material required compared to autotransformer	Less material required
16	Leakage flux and resistance	High	Low
17	Impedance	High	Less
18	Excitation Current	Large current is needed	Small current is needed
19	Output Voltage	Constant (if tapping is not provided)	Variable

Advantages of Autotransformer over Conventional Transformer

Autotransformer is better in some terms compared to the normal transformer. Here, listed some advantages of autotransformer over conventional transformer.

• The copper required for the autotransformer is very less compared to the copper required for normal two winding transformer.
• The size of Autotransformer is small. Therefore, the cost of the entire device is less compared to the normal transformer.
• It requires less copper. So, the loss in winding (copper loss) is less. So, transformer losses are less and it increase the efficiency of transformer.
• Voltage regulation of autotransformer is better than the conventional transformer.

Disadvantages of Autotransformer

Here below lists shows the disadvantages of autotransformer over conventional transformer.

• There is no electrical isolation between the primary and secondary winding. This is very dangerous for high voltage transformers.
• Due to any reason if the secondary winding is short circuited, a large amount of current will flow on the secondary side because it possesses low impedance.
• Here, on single winding divided in two parts, if the common part of winding (CB) breaks, the transformer action is lost and full primary voltage appears across the secondary.

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