The Autotransformer

The Autotransformer

In the two-winding transformer we have considered thus far, the primary winding is electrically isolated from the secondary winding. The two windings are coupled together magnetically by a common core. Thus, the principle of magnetic induction is responsible for the energy transfer from the primary to the secondary.

When the two windings of a transformer are interconnected electrically, it is called an autotransformer. An autotransformer may have a single continuous winding that is common to both the primary and the secondary.

Alternatively we can connect two or more distinct coils wound on the same magnetic core to form an autotransformer. The principle of operation is the same in ether case

The direct electrical connection between the windings ensures that a part of the energy is transferred from the primary to the secondary by conduction. The magnetic coupling between the windings guarantees that some of the energy is also delivered by induction.

Autotransformers may be used for almost all applications in which we use a two-winding transformer. The only disadvantage in doing so is the loss of electrical isolation between the high- and low-voltage sides on the autotransformer. Listed below are some of the advantages of an autotransformer compared with a two-winding transformer.

It is cheaper in first cost than a conventional two-winding transformer of a similar rating.

It delivers more power than a two-winding transformer of similar physical dimensions.

For a similar power rating, an autotransformer is more efficient than a two-winding transformer.

An autotransformer requires lower excitation current than a two-winding transformer to establish the same flux in the core.

We begin our discussion of an autotransformer by connecting an ideal two-winding transformer as an autotransformer. In fact, there are four possible ways to connect a two-winding transformer as an autotransformer, as shown in Figure 1.31.

Let us consider the circuit shown in Figure 1.13a.

The two-winding transformer is connected as a step-down autotransformer. Note that the secondary winding of the two-winding transformer is now the common winding for the autotransformer. Under ideal conditions,

(1.39)

where a = N₁/N₂ is the a-ratio of a two-winding transformer, and a_T = 1 + a is the a-ratio of the autotransformer under consideration.

The a-ratio for the other connections should also be computed in the same way. Note that A_t is not the same for all connections.

In an ideal autotransformer, the primary mmf must be equal and opposite of the secondary mmf. That is,

Figure 1.31 Possible ways to connect a two-winding transformer as an autotransformer.

From this equation, we obtain

(1.40)

Thus, the apparent power supplied by an ideal transformer to the load, S_0a, is

(1.41)

where S_ina is the apparent input to the autotransformer.

The above equation simply highlights the fact that the power input is equal to the power output under ideal conditions.

Let us now express the apparent output power in terms of the parameters of a two-winding transformer. For the configuration under consideration,

and

However, for the rated load, I_1a = I₁. Thus,

where S₀ = V₂I₂ is the apparent power output of a two-winding transformer.

This power is associated with the common winding of the autotransformer. This, therefore, is the power transferred to the load by induction in an autotransformer. The rest of the power, S₀/a in this case, is conducted directly from the source to the load and is called the conduction power. Hence, a two-winding transformer delivers more power when connected as an autotransformer.