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.
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.
The current in the winding section of an auto transformer, which is common to both the windings (CD) is minimum (I1 – I2). Therefore, the cross-sectional area of that winding wire is minimum.
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.
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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- Ideal Transformer on No Load
- Construction of Three Phase Transformer
- Types of Transformers
- Equivalent Resistance and Reactance of Transformer
- Equivalent Circuit of Single Phase Transformer
- Power Loss in a Transformer
- Open Circuit Test of Single Phase Transformer
- Short Circuit Test on Single Phase Transformer
- Transformer Efficiency
- Regulation of Transformer
- Instrument Transformers
- Polarity of Transformer Windings
- Significance of Vector Group of Transformer
- Buchholz Relay Construction | Working
- Why current transformer secondary should not be opened