- It consists of only single winding which is common to both primary and secondary winding.
- It has two end terminals ( PR) acts as primary and one end terminal ( either P or R ) and inter mediate terminal Q acts as a secondary winding ( Figure A ).
- The position of the intermediate terminal Q determines whether auto – transformer works as step down transformer or step up transformer.
- The theory and operation of the auto - transformer is similar to that of conventional two winding transformer.
- The auto transformer is cheaper than the ordinary transformer because of single winding uses less copper than the two winding.
- Since the voltage / turn is equal in both winding, the voltage develops across each winding depends on number of turns.

- Figure B shows a step down transformer in
which PQ winding acts as a primary having N
_{1}turns and QR winding acts as a secondary winding consists of N_{2}turns. - If the supply
voltage V
_{1}is given to primary winding

Induced emf per turn ( E

_{t}) = V_{1}/ N_{1}
As we know that the value of E

_{t}is same for both winding
Secondary voltage V

_{2}= E_{t }× N_{2}_{}

**Saving of Copper in Auto transformer as step down transformer**

- The weight of winding material ( Copper ) is proportional to the length and area of cross section of the conductors.

W a Volume of material

W a ( a × L )

- The cross section of the conductor depends on current ( I ) and length of conductors is proportional to the number of turns ( N ).
- Therefore the weight of copper is directly proportional to ampere - turns ( NI ).

Weight of the copper in the ordinary
transformer ( W

_{O}) = N_{1}I_{1}+ N_{2}I_{2}
Weight of copper in the auto transformer ( W

_{a}) =
Weight of copper in part PQ a I

_{1}( N_{1}– N_{2})
Weight of copper in part QR a ( I

_{2}– I_{1}) N_{2}
Total weight of copper in the auto
transformer a I

_{1}( N_{1}– N_{2}) + ( I_{2}– I_{1})
N

_{2}
a ( I

_{1}N_{1}+ I_{2}N_{2}– 2N_{2}I_{1})
Weight of copper in auto transformer ( W

_{a}) / Weight of copper in ordinary transformer ( W_{O}) = ( I_{1}N_{1}+ I_{2}N_{2}– 2N_{2}I_{1}) / ( I_{1}N_{1}+ I_{2}N_{2})
= ( I

_{1}N_{1}+ I_{1}N_{1}– 2N_{2}I_{1}) / 2N_{1}I_{1}( As N_{1}I_{1}= N_{2}I_{2})
= ( 2I

_{1}N_{1}/ 2I_{1}N_{1}) – ( 2N_{2}I_{1}/ 2I_{1}N_{1})
= ( 1 ) – ( K ) ( As K = N

_{2}/ N_{1}= I_{1}/ I_{2})
= 1 – K

Therefore W

_{a }= ( 1 – K ) W_{O}
Saving in Copper = W

_{O}– W_{a}
= W

_{O }– ( 1 – K ) W_{O}
= KW

_{O}

- Therefore the saving in copper material depends on the value of K ( Voltage transformation ratio ).
- Higher value of K, more saving in copper material

**Saving of Copper in Auto transformer as Step up transformer**

Weight of the copper in the ordinary transformer ( W

_{O}) = N_{1}I_{1}+ N_{2}I_{2}
Weight of copper in the auto transformer ( W

_{a}) =
Weight of copper in part PR a I

_{2}( N_{2}– N_{1})
Weight of copper in part QR a ( I

_{1 }– I_{2 }) N_{1}
Total weight of copper in the auto transformer a I

_{2}( N_{2}– N_{1}) + ( I_{1 }– I_{2 })
N

_{1}
a ( I

_{1}N_{1}+ I_{2}N_{2}– 2N_{1}I_{2})
Weight of copper in auto transformer ( W

_{a}) / Weight of copper in ordinary transformer ( W_{O}) = ( I_{1}N_{1}+ I_{2}N_{2}– 2N_{1}I_{2}) / ( I_{1}N_{1}+ I_{2}N_{2})
= ( I

_{1}N_{1}+ I_{1}N_{1}– 2N_{1}I_{2}) / 2N_{1}I_{1}( As N_{1}I_{1}= N_{2}I_{2})
= ( 2I

_{1}N_{1}/ 2I_{1}N_{1}) – ( 2N_{1}I_{2}/ 2 N_{1}I_{1})
= ( 1 ) – ( 1 / K ) ( As K = N

_{2}/ N_{1}= I_{1}/ I_{2})
= ( 1 – 1 / K )

Therefore W

_{a }= [ ( 1 – 1 / K ) ] W_{O}
Saving in copper = W

_{O}– [ ( 1 – 1 / K ) ] W_{O}
= [ 1 – 1 + 1 / K ] W

_{O}
= ( 1 / K ) W

_{O}- Therefore the saving in copper material depends on the value of 1 / K.
- Lower value of K, more saving in copper material

__Application__- The auto – transformer is used for starting of synchronous motor and induction motor. Its winding is used only during starting period of the motor for a few seconds.
- It is used as a furnace transformer for getting suitable supply voltage for furnace winding from 230 V supply.
- It is used as laboratory equipment to get suitable supply voltage as per requirement.
- To compensate the small voltage drop in a distribution cable by giving small voltage boost.
- It may be used as interconnecting transformer in 132 kV / 66 kV supply system.

__Connection of two winding transformer into Auto – transformer__- The winding connection diagram of an auto transformer from two winding transformer may be as ( I ) Step down transformer ( II ) Step up transformer

__Conversion of Two winding Transformer into Step down Auto Transformer__

__Conversion of Two winding Transformer into Step up Auto Transformer__
You may also like :

Closed focus transformers could genuinely show up positively in relationship with shell outline transformer for high voltage control transformer applications.

ReplyDeleteTransformer winding is one more standard bit of a transformer upgrade, since this is the ensured current bearing conductors contorted inside ring.Winding Machine

Really I enjoy your site with effective and useful information. It is included very nice post with a lot of our resources.thanks for share. i enjoy this post. text auto reply

ReplyDeleteBrisani bay in southern Albania will surprise you with the beauty of its nature. You will find the peace that you are looking for. Albanian Coastline

ReplyDeletethank you

ReplyDeleteLeading load cos(phi) pasor plz

ReplyDeleteThis is same as transformer vector diagram

ReplyDeletehttps://myelectrical2015.blogspot.com/2017/10/transformer-on-load.html

This is such a great resource that you are providing and you give it away for free. I love seeing blog that understand the value of providing a quality resource for free. purple amethyst ring

ReplyDeleteI havent any word to appreciate this post.....Really i am impressed from this post....the person who create this post it was a great human..thanks for shared this with us. garnet engagement ring

ReplyDeleteThis is a well crafted article, if perhaps just about all bloggers provided the same worth of content while you, the web will be a far better place. Please keep it up! Amethyst Rings

ReplyDelete