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What is an Auto Transformer? Learn About the Definition and Theory

What is an auto transformer? An auto transformer is a transformer with only one winding wound on a laminated core. Discover more about auto transformer along with it's definition and theory in this comprehensive article.
authorImagePratibha Bohra6 Dec, 2023
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What is an Auto Transformer?

What is an Auto Transformer?: An auto-transformer is a type of transformer in which primary and secondary windings are connected electrically in such a way that a portion of the winding is common to both sides.

Auto Transformer In the figure shown above, winding BC is common to primary as well as secondary terminals. So, BC is called common winding. From mmf balance, mmf balance In auto-transformer, power transfer is due to inductive as well as conductive action. Total power rating Total power rating

Copper Saving in Auto-transformer:

When an auto-transformer is made by connecting both the windings of a two-winding transformer in series, the power rating of auto-transformer is high as compared to the 2-winding transformer. It means, by using same amount of copper (winding), more power can be transferred in case of auto-transformer. Since, Weight = Density x Volume Volume = Cross sectional area x Length If cross sectional area is more, the current rating will be more and length of copper wire is proportional to number of turns. Copper Saving in Auto-transformer Hence, when an auto-transformer supplies rated load, Copper Saving in Auto-transformer Transformation ratio of auto transformer is always than 2 . Lesser the transformation ratio, copper saving will be more.

Connecting 2-winding Transformer as an Auto-transformer :

Let as take a conventional 2-winding transformer rated as 60 kVA, 3000/1000V as shown in figure below. Connecting 2-winding Transformer as an Auto-transformer This transformer can be reconnected as auto-transformer in 4 different possible ways.

Connection 1:

This is additive polarity and high voltage winding AB is Taken as common Connection

Connection 2:

This is also additive polarity but, here winding CD is taken as common winding. Connection

Connection 3:

This is subtractive polarity and winding AB is taken as common winding. Connection

Connection 4:

This is subtractive polarity and low voltage winding CD is taken as common Connection

Note:

  • In all the four possible connections, inductive power transfer component remains same and equal to rating of 2-winding transformer.

Conductive power transfer = kVA rating of Auto-transformer – Inductive power transfer

  • Connection 4 is worst connection and it is shown just to do the theoretical calculations.
  • When 2-winding transformer is reconnected as auto-transformer and the auto-transformer supplies the load as per the theoretical rating, the efficiency of auto-transfer is more as compared to 2-winding transformer because copper loss and iron loss remain same.

Applications of Auto-transformer :

  • In electric traction, it is used as booster transformer for the compensation of line drop.
  • In laboratories. It is used as a variable-voltage supply.
  • It is used to connect two transmission or distribution lines at different voltage levels (the voltage transformation ratio should be up to 2).
  • It is used to start three-phase induction motors.

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