Auto Transformer: Definition, Theory, and Diagram

In this article, we will study the theory and diagram of an autotransformer(or auto transformer). An auto transformer is a type of electrical transformer used to transform or change the value of voltage or current.

The unique point about an auto transformer is that it has only one winding for transforming electric parameters, i.e., voltage and current. This detailed article on autotransformers will explain all the important concepts related to it and are essential for all electrical students and professionals. So, knowing everything about an auto transformer in electrical engineering, we suggest you read this complete article.

What is an Auto transformer?

An autotransformer is an electrical machine that contains only one winding and is used to transform or change the level of voltage or current in an electrical system or circuit.

It is named an autotransformer because it consists of only one winding acting as the primary and secondary windings.

The function of an autotransformer is similar to a two-winding transformer. The only difference is that it has only one winding serving as primary and secondary.

Therefore, an auto transformer transforms electrical energy both electrically and magnetically.

Let us now discuss the theory of autotransformers to understand how it works.

Theory of Auto Transformer

A circuit diagram of a typical autotransformer is depicted in the following figure.


It can be seen that it consists of only one winding acting as the primary and secondary. The winding is taped at different points to get the secondary winding terminals.

In this diagram, the winding AB has N1 turns and acts as the primary winding. The winding BC has N2 turns acting as the secondary winding.

The primary winding AB has a tapping at C, and part of the winding BC acts as a secondary winding. The input supply voltage is connected across the winding AB, and the load is connected across the winding BC. The tapping may be fixed or variable. When an AC voltage V1 is applied across AB, an alternating flux is generated in the core, inducing an emf E1 in the winding AB. A part of this induced EMF is transferred to the secondary circuit.

Let us consider V1 as the primary winding voltage and V2 as the secondary winding voltage.


Since the number of turns in the secondary winding is N2. Hence, the voltage available across the secondary winding will be,


Therefore, the transformation ratio of the auto transformer will be,


Since the secondary ampere-turns are opposite to primary ampere-turns, the current I2 is in phase opposition to I1. This means that the secondary voltage is lower than the primary one, and as a result, the current I2 is greater than I1. Therefore, the total current flowing through section BC is equal to the current (I2 – I1).

Ampere turns in section BC are equal to the current flowing in the BC section and the number of turns.

ampere-turns in -bc-section-of-auto-transformer

Ampere turns in section AC are equal to the current flowing in the BC section and the number of turns.

ampere-turns in -ac-section-of-auto-transformer

From equations (1) and (2), it is clear that ampere-turns of section BC and AC of auto transformer are equal, which is characteristic of the transformer action.

As we know, the autotransformer has only one winding. Therefore, there is a certain amount of copper saving in the autotransformer compared to a two-winding transformer for the same kVA rating.

Let us derive the expression for the weight of copper saved in an autotransformer compared to a two-winding transformer.

Copper Saving in Autotransformer

In an electrical transformer, the weight of copper required for constructing windings depends on the length and cross-sectional area of the winding.

As we know, the length of the winding is directly proportional to its number of turns. The cross-sectional area varies with the rated current of the winding.

Therefore, the weight of copper required in the winding is directly proportional to the product of number of turns in the winding and the rated current.

Hence, the weight of copper required for the part AC of the winding is directly proportional to (N1 – N2)I1.

The weight of copper required for the part BC is proportional to (I2 – I1)N2.

Thus, the total weight of copper required for making the winding of an autotransformer is directly proportional to (N1 – N2)I1 + (I2 – I1)N2.

It can be written as,

 total-copper-weight-required-for- winding

As we know, for a transformer,


Similarly, the weight of copper required for two winding transformers is directly proportional to



On comparing the two weights, we get,


Thus, the saving of copper in an autotransformer as compared to a two-winding transformer is,


Advantages of Auto Transformer

The following are some key benefits of an autotransformer over a normal two-winding transformer:

  • Smaller Size – For the same rating, an autotransformer has approximately half the size of a typical two-winding transformer.
  • Less Manufacturing Cost – An autotransformer requires less manufacturing material, resulting in appreciable cost savings.
  • Reduced Core and Copper Losses – An autotransformer has fewer core and copper losses due to reduced iron and copper material.
  • Higher Efficiency – An autotransformer can transform electrical energy through electrical and magnetic conductions. Hence, it is more efficient than a two-winding transformer that relies only on magnetic conduction.
  • Better Voltage Regulation – An autotransformer has better voltage regulation. It is because it has less voltage drop as there is only a single winding causing resistance and reactance.
  • Variable Voltage – The output voltage of an autotransformer can be adjusted by changing the taping on the winding.

Disadvantages of Autotransformer

An autotransformer has several advantages. However, it also has various disadvantages listed below:

  • High Insulation Required – Due to common primary and secondary windings, the low-voltage winding is also subjected to high voltage. Hence, to avoid breakdown in the low voltage winding, it has to provide insulation to withstand the high voltage.
  • No Isolation between Primary and Secondary – It is the major disadvantage of the autotransformer. Due to single winding, there is no electrical isolation between the primary and secondary winding, which can cause severe problems in the short circuits.
  • High-Fault Current – During short circuit fault conditions, the load is directly connected to the supply, which is limited by the internal impedance of the transformer. Hence, there is a very high fault current under fault conditions.
  • Full Supply Voltage at Output under Open Circuit Fault Condition – In an autotransformer, if there is an open circuit fault in the common part of the winding, the full supply voltage will appear across the load.

Applications of Auto Transformer

The main applications of an autotransformer are given below:

  • Autotransformers are used in testing labs to test repaired electrical devices.
  • Autotransformers are used in transmission and distribution lines to compensate for the line voltage drop.
  • It is used for temperature adjustment in resistance heating.
  • An autotransformer is also used to start and control the speed induction motors.
  • Autotransformer is used as the main component in a voltage stabilizer.


Hence, in this article, we have studied everything about an autotransformer. In conclusion, an autotransformer is a type of electrical transformer that contains only a single winding and is used to provide a regulated output voltage.

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