Study notes on Auto‐Transformer For Electrical Engineering Students

By BYJU'S Exam Prep

Updated on: September 25th, 2023

In the field of electrical engineering, the study of transformers holds immense importance. Transformers are essential devices that facilitate the efficient transmission and distribution of electrical energy. One specific type of transformer that warrants attention is the auto-transformer. Auto-transformers are widely used in various electrical systems, including power transmission networks, industrial applications, and even household appliances. Understanding the principles, working mechanisms, and applications of auto-transformers is crucial for electrical engineering students to gain a comprehensive understanding of the subject.

In this article, you will find the study notes on Autotransformer which will cover the topics such as Autotransformer, Design of Autotransformer, Advantages, Disadvantages of Autotransformer. This collection of study notes aims to provide concise information on auto-transformers, specifically to the needs of electrical engineering students.

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What is Autotransformer?

This single winding is “tapped” at various points along its length to provide a percentage of the primary voltage supply across its secondary load. Then the autotransformer has the usual magnetic core but only has one winding, which is common to both the primary and secondary circuits.

Therefore in an autotransformer, the primary and secondary windings are linked together both electrically and magnetically. The main advantage of this type of transformer design is that it can be made a lot cheaper for the same VA rating, but the biggest disadvantage of an autotransformer is that it does not have the primary/secondary winding isolation of a conventional double-wound transformer.

The section of winding designated as the primary part of the winding is connected to the AC power source with the secondary being part of this primary winding. An autotransformer can also be used to step the supply voltage up or down by reversing the connections. If the primary is the total winding and is connected to a supply, and the secondary circuit is connected across only a portion of the winding, then the secondary voltage is “stepped-down” as shown


The voltages developed in the windings are dependent on the flux linkages. The windings are wound on the same magnetic core so they link the same flux.

  • V1 / N1= V2 / N2.

So whenever voltage V1 exists across the primary winding, then voltage V2 will be induced across the secondary winding irrespective of changes in connections.

Similarly, the magnetic circuit demands that mmf should be balanced. It implies the primary side ampere turn should equal the secondary side ampere turn.

  • I1 . N1= I2 . N2


  • Autotransformers are also used for voltage regulation in distribution networks, for starting induction motors, and as lighting dimmers. Autotransformers are also used in electric traction.
  • One main disadvantage of autotransformers is that the primary and secondary are electrically connected. So the electrical disturbance i.e. high voltage transients from one side can be easily transmitted to the other side.
  • The other disadvantage is that the impedance of the autotransformer is considerably low, so the short circuit current will be more.
  • More an open circuit in common winding results in full primary side voltage across the load which is harmful.


Disadvantages of an Autotransformer

Here are the few disadvantages of the auto transfer given below, that must be understood properly.

  • The main disadvantage of an autotransformer is that it does not have the primary to secondary winding isolation of a conventional double-wound transformer. Then an autotransformer can not safely be used for stepping down higher voltages to much lower voltages suitable for smaller loads.
  • If the secondary side winding becomes open-circuited, current stops flowing through the primary winding stopping the transformer action and resulting in the full primary voltage being applied to the secondary terminals.
  • If the secondary circuit suffers a short-circuit condition, the resulting primary current would be much larger than an equivalent double-wound transformer due to the increased flux linkage damaging the autotransformer.
  • Since the neutral connection is common to both the primary and secondary windings, earthing of the secondary winding automatically Earth’s primary as there is no isolation between the two windings. Double-wound transformers are sometimes used to isolate equipment from the earth.

The autotransformer has many uses and applications including the starting of induction motors, used to regulate the voltage of transmission lines, and can be used to transform voltages when the primary-to-secondary ratio is close to unity.

An autotransformer can also be made from conventional two-winding transformers by connecting the primary and secondary windings together in series and depending upon how the connection is made, the secondary voltage may add to, or subtract from the primary voltage.

All the Best

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