Auto Transformer
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An Auto Transformer is a transformer with only one winding wound on a laminated core. An auto transformer is similar to a two winding transformer but differ in the way the primary and secondary winding are interrelated. A part of the winding is common to both primary and secondary sides.
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Auto Transformer
- 2. AUTO-TRANSFORMERS An autotransformer is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting on itself and not to any kind of automatic mechanism.
- 3. Features of Auto-Transformer Primary and secondary are not electrically isolated Uses less copper and hence is cheaper Transformation ratio differs from unity Higher efficiency but smaller in size Voltage regulation is superior Lower leakage Lower losses Lower magnetizing current Increase kVA rating
- 4. Theory of Autotransformer From the above fig. We get
- 5. (That is I/P=O/P) Output Power The primary and secondary windings of an auto-transformer are connected magnetically as well as electrically. So the power transferred from primary to secondary is inductively as well as conductively.
- 6. Saving of Cu in Auto transformer For the same output and voltage transformation ratio an autotransformer requires less copper than the 2-winding transformer
- 8. Types of Auto-Transformer Step UP Transformer A transformer in which Ns>Np is called a step up transformer. A step up transformer is a transformer which converts low alternate voltage to high alternate voltage.
- 9. Step Down-Transformer A transformer in which Np>Ns is called a step down transformer. A step down transformer is a transformer which converts high alternating voltage to low alternating voltage.
- 10. Conversion of 2-winding Transformer into Auto transformer • ADDITIVE POLARITY (STEP-UP) • SUBSTRACTIVE POLARITY (STEP-DOWN)
- 11. ADDITIVE POLARITY (STEP-UP) In this case common current flow towards the common terminal
- 12. SUBSTRACTIVE POLARITY (STEP-DOWN) In this case common current flow away from common terminal
- 13. Advantages • An autotransformer requires less Cu than a two-winding transformer of similar rating. • An autotransformer operates at a higher efficiency than a two-winding transformer of similar rating. • An autotransformer has better voltage regulation than a two-winding transformer of the same rating. • An autotransformer has smaller size than a two-winding transformer of the same rating. • An autotransformer requires smaller exciting current than a two-winding transformer of the same rating. • In conventional transformer the voltage step up or step down value is fixed while in autotransformer, we can vary the output voltage as per our requirements and can smoothly increase or decrease its value as per our requirement.
- 14. Disadvantages • There is a direct connection between the primary and secondary. Therefore, the output is no longer d.c. isolated from the input. • An autotransformer is not safe for stepping down a high voltage to a low voltage.
- 15. Application Autotransformers are used to compensate for voltage drops in transmission and distribution lines. When used for this purpose, they are known as booster transformers. Autotransformers are used for reducing the voltage supplied to a.c. motors during the starting period. Autotransformers are used for continuously variable supply. Used in both Synchronous motors and induction motors. Used in electrical apparatus testing labs since the voltage can be smoothly and continuously varied. • On long rural power distribution lines, special autotransformers with automatic tap-changing equipment are inserted as voltage regulators, so that customers at the far end of the line receive the same average voltage as those closer to the source. The variable ratio of the autotransformer compensates for the voltage drop along the line. • In control equipment for 1-phase and 3-phase electrical locomotives.
- 16. Limitation • Because it requires both fewer windings and a smaller core, an autotransformer for power applications is typically lighter and less costly than a two-winding transformer, up to a voltage ratio of about 3:1; beyond that range, a two-winding transformer is usually more economical. • Like multiple-winding transformers, autotransformers operate on time-varying magnetic fields and so will not function with DC. • A failure of the insulation of the windings of an autotransformer can result in full input voltage applied to the output. Also, a break in the part of the winding that is used as both primary and secondary will result in the transformer acting as an inductor in series with the load .
- 17. Problem. Determine the saving in the volume of copper used in an auto transformer compared with a double-wound transformer for a 200 V:150V transformer. Solution: For a 200 V:150V transformer
- 18. Conclusion
- 19. In this case common current flow towards the common terminal ADDITIVE POLARITY