Electrical Engineering ⇒ Topic : Autotransformer

An auto-transformer is a transformer having a part of its winding common to both the primary and secondary circuits as shown in Figure (a). Its principle of operation is the same as that of a two-winding transformer discussed earlier, but it is cheapter because it has only one winding and hence uses less copper for the winding. It is normally used under conditions where the voltage transformation ratios are not much different from unity. In the auto-transformer shown in Figure (a), the primary winding AB has N₁ turns while the secondary winding CB has N₂ turns. C is a tapping on winding AB and the load is connected across CB. If V₁ and V₂ are the primary

                              FIGURE   (A)

and secondary terminal voltages whose currents are /₁ and /₂, then neglecting the losses, the leakage reactance and the magnetising current, we have

                                            .........1

However, the current in section CB of the winding will be the resultant of the two currents /1 and /2. Since these two currents are practically in phase opposition, and since /₂ is greater than /₁, the resultant current will be (/₂- /₁).As already shown in Figure, the characteristics of all transformers is that the primary ampere turns should be equal to the secondary ampere turns. In the case of an auto-transformer shown in Figure (a), we have

ampere turns due to the section CB of the winding = N₂(I₂ - I₁)

    = ampere turns due to section AC of the winding

AUTOTRANSFORMER

It is a transformer with one winding only in which a part of the winding is common to both the high voltage and low voltage sides. A single-phase autotransformer is shown in Figure (a).From Figure (a), it is clear that the primary and secondary are not electrically isolated from each other. Here XV is the primary winding having number of turns N₁ and Z_Y is the secondary winding having number of turns N₂. V₁ is the supply voltage to primary and V₂ is the terminal voltage of the secondary. I₂ is the secondary output current. The portion ZY of the full winding XZY is common to both the primary and secondary sides. Hence it is called the common winding and the smaller winding XZ is called the series winding as it is connected in series with the common winding ZY.

The transformer action takes place between winding XZ and ZY. Therefore, the volt amperes across winding XZ are transferred by transformer action to the load connected across ZY. The rest of the input VA conducted directly.

                                             ............ (1)

                                            .............. (1)

If we neglect the transformer losses, the following relation holds good:

                      Figure (a) Single-phase autotransfornner

                                        .................. (1)

If 1/a = 0.2, saving of conductor material is 20%. If 1/a = 0.8, saving of conductor material is 80%. Therefore, the use of autotransformer will be more economical when 1/a approaches unity