Working Principle of Autotransformer

The working principle of autotransformer and construction is similar to that of conventional two winding transformers. However, it differs in the way in which the primary and the secondary are inter-related.
In a two-winding transformer, primary and secondary are only magnetically linked by a common core but are completely insulated from each other. But in the case of an auto transformer windings are connected electrically as well as magnetically.
It consists of only one winding wound on a laminated magnetic core, with a rotary movable contact. The same auto transformer can be used as a step-down or a step-up transformer.
working principle of autotransformer

Working Principle of Autotransformer

The circuit diagram of an auto transformer is shown in Figure. When the single phase AC supply is connected between A and D terminals and output is taken from C and E terminals, this auto transformer will operate as a step-down transformer.
Because the number of turns in winding between A and D terminal (i.e. primary winding) is more than the number of turns in winding between C and E terminal (i.e. secondary winding).
On the other hand, when the single phase AC supply is connected between B and D terminals and output is taken from C and E terminals, the same auto transformer will operate as a step-up transformer.
Because the number of turns in winding between B and D terminal (i.e. primary winding) is less than the number of turns in winding between C and E terminal (i.e. secondary winding). We can make small variations in output voltage by taking the output from different tapings of the auto transformer.
auto transformer working principle
The current in the winding section of an auto transformer, which is common to both the windings (CD) is minimum (I1I2). Therefore, the cross-sectional area of that winding wire is minimum.

Advantages | Disadvantages | Applications of Auto transformer

Advantages of an Auto transformer

  • Continuously varying voltage can be obtained.
  • It needs less copper and is more efficient than a two-winding transformer of same ratings.

Disadvantages of an Auto transformer
If the winding (CE) breaks (open circuited) then the transformer action is lost and full primary voltage appears across the output. It can be harmful to the load when we are using an auto transformer as a step-down transformer. That is why an auto transformer is used for only making small variations in output voltage while using as a step-down transformer.
Applications of an Auto transformer
The auto transformers are used

  • as starters for induction motors and synchronous motors which are known as auto transformer starters.
  • in labs for obtaining a continuously varying voltage.
  • in voltage stabilizers as regulating transformers.
  • as booster transformer to raise the voltage in AC feeders.

Thanks for reading about working principle of autotransformer.

Transformer | Objective Type Questions


1. A transformer transforms

2. Which of the following is not a basic element of a transformer ?

3. In an ideal transformer,

4. The main purpose of using core in a transformer is to

5. Transformer cores are laminated in order to

6. A transformer having 1000 primary turns is connected to a 250-V a.c. supply. For a secondary voltage of 400 V, the number of secondary turns should be

7. The primary and secondary induced e.m.fs. E1 and E2 in a two-winding transformer are always

8. A step-up transformer increases

9. The primary and secondary windings of an ordinary 2-winding transformer always have

10. In a transformer, the leakage flux of each winding is proportional to the current in that winding because

11. In a two-winding transformer, the e.m.f. per turn in secondary winding is always.......the induced e.m.f. per turn in primary.

12. In relation to a transformer, the ratio 20 : 1 indicates that

13. In performing the short circuit test of a transformer

14. The equivalent resistance of the primary of a transformer having K = 5 and R1 = 0.1 ohm when referred to secondary becomes.......ohm.

15. A transformer has negative voltage regulation when its load power factor is

16. The primary reason why open-circuit test is performed on the low-voltage winding of the transformer is that it

17. No-load test on a transformer is carried out to determine

18. The main purpose of performing open-circuit test on a transformer is to measure its

19. During short-circuit test, the iron loss of a transformer is negligible because

20. The all-day efficiency of a transformer depends primarily on

21. In operating a 400 Hz transformer at 50 Hz

22. The marked increase in kVA capacity produced by connecting a 2 winding transformer as an auto-transformer is due to

23. Transformers are rated in kVA instead of kW because

24. When a 400-Hz transformer is operated at 50 Hz its kVA rating is

25. At relatively light loads, transformer efficiency is low because

26. A 200 kVA transformer has an iron loss of 1 kW and full-load Cu loss of 2kW. Its load kVA corresponding to maximum efficiency is ....... kVA.

27. If Cu loss of a transformer at 7/8th full load is 4900 W, then its full-load Cu loss would be .......watt.

28. The ordinary efficiency of a given transformer is maximum when

29. The output current corresponding to maximum efficiency for a transformer having core loss of 100 W and equivalent resistance referred to secondary of 0.25 Ω is ....... ampere.

30. The maximum efficiency of a 100-kVA transformer having iron loss of 900 kW and F.L. Cu loss of 1600 W occurs at ....... kVA.

31. Which of the following connections is best suited for 3-phase, 4-wire service ?

33. In a three-phase Y-Y transformer connection, neutral is fundamental to the

34. Of the following statements concerning parallel operation of transformers, the one which is not correct is

35. Instrument transformers are used on a.c. circuits for extending the range of

36. Before removing the ammeter from a current transformer, its secondary must be short-circuited in order to avoid

37. The kVA rating of an ordinary 2-winding transformer is increased when connected as an autotransformer because

38. The saving in Cu achieved by converting a 2-winding transformer into an autotransformer is determined by

39. An autotransformer having a transformation ratio of 0.8 supplies a load of 3 kW. The power transferred conductively from primary to secondary is.......kW.

40. The essential condition for parallel operation of two 1-φ transformers is that they should have the same

41. If the impedance triangles of two transformers operating in parallel are not identical in shape and size, the two transformers will

42. Two transformers A and B having equal outputs and voltage ratios but unequal percentage impedances of 4 and 2 are operating in parallel. Transformer A will be running over-load by ....... percent.


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  • September 9, 2018 at 5:47 am

    Yes I have build this website. I have jut migrated it from bloger to wordpress. A free theme is modified by css.


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