Autotransformer schematic diagram

An autotransformer is a special transformer in which the output and input share a set of coils. The voltage step-up and step-down are implemented with different taps. The tap voltage of the part with less than the common coil will decrease. The voltage of the tap of the part with more common coils will increase. In fact, the principle is the same as that of an ordinary transformer, except that its primary coil is its secondary coil. In a general transformer, the primary coil on the left uses electromagnetic induction to generate voltage in the secondary coil on the right. The autotransformer is influenced by itself. Own

An autotransformer is a transformer with only one winding. When used as a step-down transformer, a portion of the turns are extracted from the winding as the secondary winding; when used as a step-up transformer, the external voltage is only applied to a portion of the turns of the winding. superior. Usually, the part of the winding that belongs to both the primary and secondary windings is called the common winding, and the remaining part is called the series winding. Compared with ordinary transformers of the same capacity, self-coupling transformers are not only smaller in size, but also more efficient, and the transformer capacity is larger. The higher the voltage, the more prominent this advantage becomes. Therefore, with the development of power systems, the improvement of voltage levels and the increase of transmission capacity, auto-coupling transformers have been widely used due to their large capacity, low loss and low cost.

According to the principle of electromagnetic induction, a transformer does not necessarily have to have separate primary windings and secondary windings. Only one coil can achieve the purpose of voltage conversion. In Figure 1, when the primary winding W1 of the transformer is connected to the AC power supply U1, the transformer The voltage drop of each turn of the original winding is the same, and the voltage is evenly distributed in the original windings 1 and 2 of the transformer. The voltage of the secondary winding W2 of the transformer is equal to the voltage of each turn of the original winding multiplied by the number of turns 3, 4. When U1 remains unchanged, change Different U2 values ​​can be obtained by the ratio of W1 and W2. This kind of transformer in which the primary and secondary windings are directly connected in series and self-coupled is called a self-coupling transformer, also called a single-turn transformer.

The primary and secondary windings of an ordinary transformer are insulated from each other. They are only connected magnetically but not electrically. Depending on the number of coil groups, this kind of transformer can be divided into a double-turn transformer or a multi-turn transformer. By electrical The principle of magnetic induction shows that it is not necessary to have separate primary windings and secondary windings. Only one coil can achieve the purpose of voltage conversion. In Figure 1, when the primary winding W1 is connected to the AC power supply U1, the voltage of each turn of the primary winding The voltage drop is the same, and the voltage is evenly distributed between the original windings 1 and 2. The voltage of the secondary winding W2 is equal to the voltage of each turn of the original winding multiplied by the number of turns 3 and 4. While U1 remains unchanged, change the ratio of W1 and W2, Different U2 values ​​can be obtained. This kind of transformer in which the primary and secondary windings are directly connected in series and self-coupled is called an autotransformer, also called a single-turn transformer.

Figure 1

Figure II

The relationship between voltage, current and number of turns in an autotransformer is the same as that of a general transformer, that is: U1/U2=W1/W2=I2/I1=K

The biggest feature of the autotransformer is that the secondary winding is part of the primary winding (auto-step-down transformer as shown in Figure 1), or the primary winding is part of the secondary winding (auto-step-up transformer as shown in Figure 2).

The current directions of the primary and secondary windings of the auto-coupling transformer are opposite to those of ordinary transformers.

Neglecting the excitation current and losses of the transformer, the following relationship can be obtained

Buck:I2=I1+I,I=I2-I1

Boost:I2=I1-I,I=I1-I2

P1=U1I1,P2=U2I2

In the formula:

I1 is the primary winding current, I2 is the secondary winding current

U1 is the primary winding voltage, U2 is the secondary winding voltage

P1 is the power of the primary winding, P2 is the power of the secondary winding