Difference between Conventional Transformer and Autotransformer

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.



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 Weight of copper in normal two winding transformer Weight of Copper in Autotransformer
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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