What is an Auto Transformer : Construction and Its Working

As we know that a transformer includes two windings and the main function of these windings is to change the voltage level to the desired level. The two winding transformer includes two separately coupled magnetic coils without electrical connection among them. In this article, we will discuss the transformer which changes the voltage level through a single coil. Since the voltage level can also be converted through a single coil quite effectively using an autotransformer. So we can step down the voltage level from 400 V to 200 through a single coil transformer with appropriate tapings. This article discusses an overview of what is an Auto Transformer, construction with working and its applications.

What is an Auto Transformer?

Definition: A transformer that has a single winding is known as an Auto Transformer. The term ‘auto’ is taken from a Greek word and the meaning of this is single coil works alone. The working principle of the autotransformer is similar to a 2-winding transformer but the only difference is, the portions of the single winding in this transformer will work at both sides of the windings like primary & secondary. In a normal transformer, it includes two separate windings that are not allied with each other. The autotransformer diagram is shown below.


Autotransformers are lighter, smaller, cheaper comparing with other transformers, but they will not provide electrical isolation between two windings.

Auto Transformer Construction

We know that the transformer includes two windings namely primary and secondary which are connected magnetically but insulated electrically. But in autotransformer, a single winding is used like both the windings

There are two types of autotransformer based on construction. In one type of transformer, there is continuous winding with the taps brought out at convenient points determined by the desired secondary voltage. However, in another type of autotransformer, there are two or more distinct coils that are electrically connected to form a continuous winding. The construction of Autotransformer is shown in the figure below.


The primary winding AB from which a tapping at ‘C’ is taken, such that CB acts as a secondary winding. The supply voltage is applied across AB, and the load is connected across CB. Here, the tapping may be fixed or variable. When an AC voltage V1 is applied across AB, an alternating flux is set up in the core, as a result, an emf E1 is induced in the winding AB. A part of this induced emf is taken in the secondary circuit.


In the above diagram, the winding is represented as ‘AB’ whereas the total turns ‘N1’ is considered as the primary winding. In the above winding, from the ‘C’ point it is tapped as well as the ‘BC’ section can be considered like secondary winding. Assume the number of turns among the points B&C is ‘N2’. If the voltage ‘V1’ is applied across the winding AC, then the voltage for each turn within the winding will be V1/N1.

Therefore, the voltage across the BC section of the winding will be (V1/N1)*N2

From the above construction, voltage for this BC winding is ‘V2’

Therefore (V1/N1)*N2 =V2

V2/V1 = N2/N1 = K

When the BC section in the AB winding can be considered secondary. So ‘K’ is the constant value, it is nothing but the ratio of voltage or turns in the transformer.

Whenever the load is connected in between the BC terminals, then the load current like ‘I2’ will starts flowing. The flow of current within the secondary winding will be the main difference of currents ‘I1&I2’.

Copper Savings

In autotransformer, the copper savings compared to conventional two winding transformers can be discussed. In the above winding, the weight of copper mainly depends on its length as well as the cross-sectional area.

Again conductor’s length within the winding can be proportional to the no. of turns as well as cross-sectional area changes with the rated current. So copper weight within the winding can be directly proportional to the product of no. of turns & rated current of the winding.

Thus, the copper weight within the AC section is proportional to I1 (N1-N2). Similarly, the copper weight within the BC section is proportional to N2 (I2-I1).

Therefore, the whole copper weight within the winding of this transformer is proportional to,

= I1 (N1-N2) + N2 (I2-I1)

= I1N1-I1N2 + I2N2-N2I1

= I1N1+I2N2-2I1N2

We know that N1I1=N2I2

= I1N1+I1N1-2I1N2

= 2I1N1-2I1N2 = 2(I1N1-I1N2)

In this way, it is proved, then the copper weight within two winding transformers can be proportional to N1I1-N2I2

Since in a transformer, N1I1=N2I2

2N1I1 (Since in a transformer N1I1 = N2I2)

In autotransformer, let’s assume the weights of copper like Wa & Wtw as well as two windings respectively,

Thus, Wa/Wtw = 2 (N1I1-N2I1)/2N1I1

= N1I1-N2I1/2N1I1 = 1-N2I1/N1I1

= 1-N2/N1 = 1-K

Therefore, Wa = Wtw (1-K) = Wtw- k Wtw

So, saving of copper within the transformer when we evaluated with two winding transformers is

Wtw- Wa = k Wtw

This transformer uses simply single winding for each phase as against two particularly separate windings within a conventional transformer.

Advantages of Auto Transformer

The advantages are

  • It uses single winding, so these are smaller & cost-effective.
  • These transformers are more efficient
  • It needs lesser excitation currents to compare with the conventional type transformers.
  • In these transformers, the voltage can be changed easily and smoothly
  • Enhanced regulation
  • Fewer losses
  • It needs less copper
  • Efficiency is high due to low losses in ohmic and core. These losses will be occurred because of the reduction in transformer material.

Disadvantages of Auto Transformer

The disadvantages are

  • In this transformer, the secondary winding cannot be insulated from the primary.
  • It is applicable in restricted areas where a small difference in the o/p voltage from i/p voltage is necessary.
  • This transformer is not used for interconnecting systems like high voltage & low voltage.
  • The leakage flux is small among the two windings so the impedance will below.
  • If the winding in the transformer breaks, the transformer will not work then the full primary voltage comes into view across the o/p.
  • It can be dangerous to the load while we are utilizing an autotransformer like a step-down transformer. So this transformer is used only to make small changes within the o/p voltage.

Applications of Auto Transformer

The applications are

  • It increases the voltage drop for the distribution cable
  • It is used as a voltage regulator
  • It is used in audio, distribution, power transmission and railways
  • Autotransformer with several tappings is used to start the motors like induction as well as synchronous.
  • It is used in laboratories to obtain a varying voltage continuously.
  • It is used like regulating transformers in voltage stabilizers.
  • It increases the voltage in AC feeders
  • It is applicable in electronics testing centers wherever frequently changing voltages are required.
  • It is used where high voltages are necessary like boosters or amplifiers
  • It is used in audio devices like speakers to match the impedance as well as to adjust the device for nonstop voltage supply.
  • It is used in power stations where the voltage needs to step down and step up to equal the voltage at the receiving end which is necessary for the device.


1). What is the function of autotransformer?

This transformer is used to control the voltage in the transmission line and also changes the voltages once the ration of primary to secondary is close to unity.

2). Why autotransformer is not used as a distribution transformer?

Because it doesn’t give electrical isolation among its windings as a normal transformer does.

3). What is the role of an autotransformer in substation?

Autotransformer is frequently used in substations for step-up or step-down the voltage wherever the ratio of high voltage to low voltage is small.

Thus, this is all about an overview of an autotransformer, construction, working, advantages, disadvantages, and applications. Here is a question for you, what is the main difference between autotransformer and power transformer?