Autotransformer – Introduction, Advantages and Applications

Autotransformer – Introduction, Advantages and Applications.

In my previous post you read why the transformer is rated in KVA but not in KW, now in this post I am going to describe a type of transformer used in electrical and electronics domain. This post is about autotransformer. Well autotransformer is also called as ‘Variac’, because it is basically used to continuously vary the voltage supplied to the circuit.

Some of you may know about the general type transformers used in electrical power systems. In those kinds of transformers, there are 2 types of winding one is primary winding while the other is secondary winding. Both of these windings are physically separated or insulated from one another but are magnetically linked to each other.

In the case of autotransformers, the both the windings that is primary and secondary windings are connected to each other both physically and magnetically. There is one common winding which forms both primary and secondary winding in which voltage is varied by changing the position of secondary tapping on the body of the coil.


Now let us discuss some facts related to the advantages of autotransformers over the conventional transformers of the same rating.

Advantages of Autotransformers:

  1. Its efficiency is more when compared with the conventional one.
  2. Its size is relatively very smaller.
  3. Voltage regulation of autotransformer is much better.
  4. Lower cost
  5. Low requirements of excitation current.
  6. Less copper is used in its design and construction
  7. In conventional transformer the voltage step up or step down value is fixed while in autotransformer, we can vary the output voltage as per out requirements and can smoothly increase or decrease its value as per our requirement.

Talking about some of the modern day use of autotransformers:

  1. Used in both Synchronous motors and induction motors.
  2. Used in electrical apparatus testing labs since the voltage can be smoothly and continuously varied.
  3. They find application as boosters in AC feeders to increase the voltage levels.

If you have any doubts or queries related to this post, you can post your comments below. I would be glad to answer them. Do share this if you liked the post.


  1. Anonymous6/19/2012

    thanks very helpful

  2. Anonymous8/23/2012

    Is conventional transformer used as a auto transformer?

    1. Anonymous11/08/2012

      No. Smaller conventional customers use off load tape changers and although bigger ones use auto tape changers their voltage regulation response is not as swift as for autotransformers.

  3. Auto Transformers are used in variety of purposes and we offer complete range of auto transformers world wide.

    Auto Transformers

  4. As autotransformer can stepup voltage...and low cost...can we replace conventional transformer wd autotransformer..if yes then how and if no then why??
    What is the use of autotransformer in EHV ac trnasmission

  5. Nice Article mate on Auto Transformer, Keep Posting.

  6. i want now the application of autotrasformer in general >> I mean where is use and some figure to support the description ..

    1. it is used as variac.. used as a variable power source in labs,,

    2. used as variable power source..
      where we have to give variable input voltage, manily used in lab

  7. i want to know what is the application of autotrasformer << I mean in general (( where are they use & with figure to support the description ))

    1. Anonymous9/30/2013

      * It transfom transfom a voltage level.
      * It transform a current level.
      * Transformers is used for impedance's matching purpose.
      * It filters DC from AC.
      * It issolates two system.
      Transformer are used in big and small industries,power stations,Daims,ect.The main purpose to use trnsformer is change tha voltage level,by using step up and step down transformer.

  8. how many windings are there does it depends on the manufacturer or what ?

  9. an auto transformer having 2000 turns has to step up 220 to 250V and deliver 40 amps (10kVA) Obviously the primary 220 has to be applied between 0th and 1760th turn. The 240 turns have to carry 40 amps. What about the 1760 turns. It does not have to carry 40 amps. So what will be design aim. Can one use a max current of 2 amps for these windings?

  10. Hi very nice blog regarding Auto Transformers. Found it very useful. Thanks for posting.

  11. Hi very nice blog regarding Auto Transformers. Found it very useful. Thanks for posting.

  12. can u use autotransformer for step up voltage purpose,..

    for stepping up purpose we need to have more turns in secondary, but here secondary winding is tapped to primary..

    pls xplain.. if it can be used for stepping up purpose

  13. I have just started manufacturing of autotransformers. Kindly help me as to where it is used so that I can approach them and increase my sales.

  14. For Electrical Engineering Information and Interview Questions


    Electrical Engineering Info

    Like my fb page

  15. internal circuit design of variac should be published for better understanding.

  16. Here is some helpful information I found about autotranformer applications

  17. Here is some good information about autotranformers

  18. i have a 500 kva transformer , can i use this for step down like 380v to 220 v ,if i use it as step down how much AMP.will be available .
    please reply me

  19. l purchased a auto transformer rated for 45kw and i want to run 37kw motor but during testing my main breaker is tripping due to high current ,i have 70% tapping please let me know the how to solve this issue,i tried with 40% but motor is not rotating .

  20. Advantages of Auto transformers are,

    It has efficiency is more when it was compared with the conventional one.
    Its size is relatively very smaller.
    It good because, its cost lower.
    It uses very low requirements of excitation current.
    Less copper is used in its design and construction.

    Power Transformers in India | Transformer Manufacturer in India

  21. 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.

    Advantages of Auto-Transformers:
    Less costly
    Better regulation
    Low losses as compared to ordinary two winding transformer of the same rating.

    Power Transformers in India | Transformer Manufacturer in India

  22. I have bought some machines from other country with specifications of 480V, 3 phase and now I want to install the same with power supply of 415V, 3 phase. Application is extruders with precise control by VFDs and load is around 600-700 kVA. Request to suggest should I go with Auto transformer or conventional transformer with advantage and disadvantage.

  23. Can we replace 220kva conventional transformer with 220kw auto transformer


If you liked this blog then Join me at:

You might like