Auto Transformer Definition, Theory & Diagram

What is Auto Transformer

An Electrical Transformer with only one winding is known as an Autotransformer (or auto Transformer). The “auto” prefix does not refer to any Automatic Mechanism, but rather to a single coil acting on its own (self in Greek). While an Autotransformer and a Two-winding Transformer are similar, they differ in how the transformer’s primary and Secondary Windings are connected.

Auto Transformer Theory

In an Autotransformer, one single winding is used as primary winding as well as Secondary winding. But in two Windings transformer two different Windings are used for primary and Secondary purpose. A circuit diagram of auto Transformer is shown below.

The primary winding is the winding AB of the total number of turns N1. The portion BC of this winding, which is tapped at point C, is regarded as secondary. Assume that N2 turns are necessary to get from point B to point C.

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,

It is clear that the value of the constant ′k′ is the turns ratio or voltage ratio of that auto transformer since the BC portion of the winding is regarded as secondary. Load current I2 begins to flow when the load is connected between the secondary terminals, or between ′B′ and ′C′. The difference between I2 and I1 is what determines the current in the secondary winding or common winding.

Copper Savings in Auto Transformer

We will now talk about the copper savings in the auto transformer as compared to the traditional two winding transformer.
We are aware that the weight of any copper winding is influenced by its cross-sectional area and length. Once more, the length of a conductor in a winding varies with the rated current and its cross-sectional area is proportional to the number of turns.
Therefore, the product of the number of turns and the rated current of the winding gives the weight of copper in the winding.

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

(N1 – N2)I1

and similarly, weight of copper in the section BC proportional to,
N2(I2 – I1)

Hence, total weight of copper in the winding of Autotransformer 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 autotransformer 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. A two winding transformer is less efficient than an autotransformer. This is brought on by lower core and ohmic losses brought on by a reduction in transformer material.
3. Due to the lower voltage drop in the single winding’s resistance and reactance, the auto transformer has better voltage regulation.

Disadvantages of Using Auto Transformer

The disadvantages of an auto transformer include:

1. The primary and secondary windings’ electrical conductivity makes the lower voltage circuit susceptible to being impressed by higher voltage. It becomes necessary to design the low voltage circuit to withstand higher voltage in order to prevent breakdown in the lower voltage circuit.
2. The impedance is low because there is little leakage flux between the primary and secondary windings. As a result, under fault conditions, short circuit currents become more severe.
3. Except when using interconnected starring connections, the connections on the primary and secondary sides must always match. This complicates things because the primary and secondary phase angles are changing, especially when there is a 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. When voltage adjustment tappings are present, it is harder to keep the winding’s electromagnetic balance. It should be noted that an auto transformer’s frame size is significantly increased by the availability of tapping. The benefits gained in initial cost are lost to a great event if the range of tapping is very broad.

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.