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L-27(TB)(ET) ((EE)NPTEL)

# L-27(TB)(ET) ((EE)NPTEL) - Module 7 Transformer Version 2...

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Module 7 Transformer Version 2 EE IIT, Kharagpur

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Lesson 27 Auto-Transformer Version 2 EE IIT, Kharagpur
Contents 27 Auto-Transformer 4 27.1 Goals of the lesson ………………………………………………………………. 4 27.2 Introduction ……………………………………………………………………… 4 27.3 2-winding transformer as Autotransformer ……………………………………... 5 27.4 Autotransformer as a single unit ………………………………………………… 6 27.5 Tick the correct answers ………………………………………………………… 9 27.6 Problems …………………………………………………………………………. 9 Version 2 EE IIT, Kharagpur

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27.1 Goals of the lesson In this lesson we shall learn about the working principle of another type of transformer called autotransformer and its uses. The differences between a 2-winding and an autotransformer will be brought out with their relative advantages and disadvantages. At the end of the lesson some objective type questions and problems for solving are given. Key Words : tapping’s, conducted VA, transformed VA. After going through this section students will be able to understand the following. 1. Constructional differences between a 2-winding transformer and an autotransformer. 2. Economic advantages/disadvantages between the two types. 3. Relative advantages/disadvantages of the two, based on technical considerations. 4. Points to be considered in order to decide whether to select a 2-winding transformer or an autotransformer. 5. The difference between an autotransformer and variac (or dimmerstat). 6. The use of a 2-winding transformer as an autotransformer . 7. The connection of three identical single phase transformers to be used in 3-phase system. 27.2 Introduction So far we have considered a 2-winding transformer as a means for changing the level of a given voltage to a desired voltage level. It may be recalled that a 2-winding transformer has two separate magnetically coupled coils with no electrical connection between them. In this lesson we shall show that change of level of voltage can also be done quite effectively by using a single coil only. The idea is rather simple to understand. Suppose you have a single coil of 200 turns (= N BC ) wound over a iron core as shown in figure 27.1. If we apply an a.c voltage of 400 V, 50 Hz to the coil (between points B and C), voltage per turn will be 400/200 = 2 V. If we take out a wire from one end of the coil say C and take out another wire tapped from any arbitrary point E, we would expect some voltage available between points E and C. The magnitude of the voltage will obviously be 2 × N EC where N EC is the number of turns between points E and C. If tapping has been taken in such a way that N EC = 100, voltage between E and C would be 200 V. Thus we have been able to change 400 V input voltage to a 200 V output voltage by using a single coil only. Such transformers having a single coil with suitable tapings are called autotransformers .
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L-27(TB)(ET) ((EE)NPTEL) - Module 7 Transformer Version 2...

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