Auto Transformer: What is it? (Definition, Theory & Diagram)

What is an Autotransformer?

An autotransformer (or auto transformer) is a type of electrical transformer with only one winding. The “auto” prefix refers to the single coil acting alone (Greek for “self”) – not to any automatic mechanism. An auto transformer is similar to a two winding transformer but varies in the way the primary and secondary winding of the transformer are interrelated.

Autotransformer Theory

An auto transformer uses a single winding for both the primary and secondary purposes, unlike two-winding transformers which use separate windings. The diagram below illustrates this concept.

The winding AB of total turns N₁ is considered as primary winding. This winding is tapped from point ′C′ and the portion BC is considered as secondary. Let’s assume the number of turns in between points ′B′ and ′C′ is N₂.

If V₁ voltage is applied across the winding i.e. in between ′A′ and ′C′.

Hence, the voltage across the portion BC of the winding, will be,

As BC portion of the winding is considered as secondary, it can easily be understood that value of constant ′k′ is nothing but turns ratio or voltage ratio of that auto transformer. When load is connected between secondary terminals i.e.between ′B′ and ′C′, load current I₂ starts flowing. The current in the secondary winding or common winding is the difference of I₂ and I₁.

Copper Savings in Auto Transformer

Now we will discuss the savings of copper in auto transformer compared to conventional two winding transformer.
We know that weight of copper of any winding depends upon its length and cross-sectional area. Again length of conductor in winding is proportional to its number of turns and cross-sectional area varies with rated current.
So weight of copper in winding is directly proportional to product of number of turns and rated current of the winding.

Therefore, weight of copper in the section AC proportional to,

and similarly, weight of copper in the section BC proportional to,

Hence, total weight of copper in the winding of auto transformer proportional to,

In similar way it can be proved, the weight of copper in two winding transformer is proportional to,

N₁I₁ + N₂I₂ ⇒ 2N₁I₁ (Since, in a transformer N₁I₁ = N₂I₂)

Let’s assume, W_a and W_tw are weight of copper in auto transformer and two winding transformer respectively,

∴ Saving of copper in auto transformer compared to two winding transformer,


Auto transformer employs only single winding per phase as against two distinctly separate windings in a conventional transformer.

Advantages of using Auto Transformers

The advantages of an auto transformer include:

1. For transformation ratio = 2, the size of the auto transformer would be approximately 50% of the corresponding size of two winding transformer. For transformation ratio say 20 however the size would be 95 %. The saving in cost of the material is of course not in the same proportion. The saving of cost is appreciable when the ratio of transformer is low, that is lower than 2. Thus auto transformer is smaller in size and cheaper.
2. An auto transformer has higher efficiency than two winding transformer. This is because of less ohmic loss and core loss due to reduction of transformer material.
3. Auto transformer has better voltage regulation as voltage drop in resistance and reactance of the single winding is less.

Disadvantages of Using Auto Transformer

The disadvantages of an auto transformer include:

1. Due to the electrical conductivity between the primary and secondary windings, the lower voltage circuit may be affected by the higher voltage. To prevent breakdown, the low voltage circuit must be designed to withstand the higher voltage.
2. The leakage flux between the primary and secondary windings is small and hence the impedance is low. This results into severer short circuit currents under fault conditions.
3. The connections on primary and secondary sides have necessarily needs to be same, except when using interconnected starring connections. This introduces complications due to changing primary and secondary phase angle particularly in the case of delta/delta connection.
4. Because of common neutral in a star/star connected auto transformer it is not possible to earth neutral of one side only. Both their sides should have their neutrality either earth or isolated.
5. Maintaining the electromagnetic balance of the winding is harder when voltage adjustment tappings are used. Adding tappings increases the transformer frame size, and if the tapping range is large, the initial cost savings are significantly reduced.

Applications of Auto Transformers

The applications of an auto transformer include:

1. Compensating voltage drops by boosting supply voltage in distribution systems.
2. Auto transformers with a number of tapping are used for starting induction and synchronous motors.
3. Auto transformer is used as variac in laboratory or where continuous variable over broad ranges are required.