1. HANSABA COLLEGE OF ENGINEERING & TECHNOLOGY, SIDHPUR
GOKUL TECHNICAL CAMPUS
HANSABA COLLEGE OF ENGINEERING & TECHNOLOGY, SIDHPUR
Subject: DC Machine & Transformer ( )
Name:-Sajidali ,
Hardik ,
Manish: ,
Nishant
Branch: Electrical Engineering
Division:- C1
Semester: 3rd
Year: 2014

2. Topic autotransformer

3. Introduction Auto Transformer
Auto transformer is kind of electrical transformer where primary and secondary shares same common single winding.
The Normal Transformer has separate Primary and secondary windings.
But the autotransformer is a special number in which a part of winding is common for the primary and secondary winding.
The construction of an Autotransformer is shown in the fig.
It consist of only one winding wound one a laminated magnetic core with a rotary movable contact.

4. So Voltage Per Turn Winding is V1
Hence, the voltage across the portion BC of the winding, will be,
V1*N2 And from the Figure above This voltage is V2
N1 
Hence,V1*N2 =V2
V2=N2=constant=K
V1 N1

5. Thus autotransformer can operate as a step down a step up transformer.

6. There are Two type of Autotransformer
(1)Step up Transformer
(2)Step Down Transformer

7. Step down Autotransformer

8. The Connection of autotransformer as a step Down Transformer is shown fig.
It shows That the two fixed Terminals A and B are connected to single phase AC supply V1, The winding AB acts as a Primary Winding.
A Part of the complete winding i.e. CB acts as secondary Winding Across Which the load is connected.
The Operating Principle of an autotransformer is same that of the normal transformer

9. V2=N2*V1 N1
There for the load voltage for this configuration is given by.
Where N2=number of turns coresponding to secondary i.e CB
N1=number of turns coresponding to primary i.e AB
As the number of turns coresponding to winding CB i.e. n2 is less then that of winding AB i.e.n1 this configuration operates as step down transformer

10. If we neglate an the losses the megnetistisng circuit an the leckage reactaence then the transformer ratio defined as
K=V2=N2=I2
V1 N1 I1
I1= primary current
I2=load current

11. Step up Autotransformer

12. 11/24/2017
The connection of an autotransformer for operating it as the step up transformer.
Note that the part CB of the complete winding acts as the primary winding. The ac input voltage V1 is applied between these terminal.
As the number of turns of winding AB is higher than the number of turns of winding CB, the autotransformer now acts as a step up transformer.

13. Neglecting the loss the magnetizing current and leakage reactances, the load voltage V2 is given by,
V2=N2*V1 N1
11/24/2017

14. Advantage of Auto Transformer
As only one winding is uses the copper required for the transformer is very less
The size and hence the cost is reduced as ccompared to the conventional transformer
The losses taking place in the winding are reduced the efficiency is higher then the conventional transformer
Due to reduced resistance the voltage regulation is better then the conventional transformer

15. Disadvantage of Autotransformer
11/24/2017
Disadvantage of Autotransformer
There is no electrical isolation between the primary and secondary windings. This can prove to be dangerous for high voltage applications.
It posses a low impedance, hence if the secondary circuit is short circuited, then a large current will flow on the secondary side.
If The common Part of CB The winding (windingCB) breaks (open Circuit) Then The transformer action is lost and full primary Voltage Apperas across the secondry.

16. Applications of an Autotransformer
It can be used as a variable, i.e. Variable ac supply to vary the ac voltage applied to the load smoothly from 0V to about 270 V.
In order to start the ac machine such as induction motors or synchronous motors.
To vary the supply voltage(as per requirements) of a furnace.
As a dimmerstat : when the variac autotransformer is used to control the intensity of lamps in the cinema halls etc., it is called as the dimmerstat.

17. Copper Savings in Auto Transformer
Now we will discuss the savings of copper in auto transformer compared toconventional two winding transformer.
We know that weight of copper of any winding depends upon its length and cross - sectional area. Again length of conductor in winding is proportional to its number of turns and cross - sectional area varies with rated current.
So weight of copper in winding is directly proportional to product of number of turns and rated currentof the winding.
Therefore, weight of copper in the section AC proportional to,

18. (N1-N2)I1
and similarly, weight of copper in the section BC proportional to,
N2(I2-I1)
Hence, total weight of copper in the winding of auto transformer proportional to,
(N1-N2)I1+N2(I2-I1)
N1I1-N2I1+N2I2-N2I1
N1I1+N2I2-2N2N1
2N1I1-2N2I1
2(N1I1-N2I1)

19. and similarly, weight of copper in the section BC proportional to,
Hence, total weight of copper in the winding of auto transformer proportional to,

20. In similar way it can be proved, the weight of copper in two winding transformer is proportional to, N1I1 + N2I2⇒ 2N1I1   
(Since, in a transformer N1I1 = N2I2) Let's assume, Wa and Wtw are weight of copper in auto transformer and two winding transformer respectively,

21. Auto transformer employs only single winding per phase as against two distinctly separate windings in a conventional transformer. Advantages of using auto transformer. For transformation ratio = 2, the size of the auto transformer would be approximately 50% of the corresponding size of two winding transformer. For transformation ratio say 20 however the size would be 95%. The saving in cost is of course not in the same proportion. The saving of cost is appreciable when the ratio of transformer is low, that is lower than 2.

22. THANK YOU 