Autotransformer

Autotransformer is to wind two or more coils on a closed iron core. When one coil is connected to AC power supply (the primary coil), alternating current flows through the coil, and this alternating current is generated in the iron core Alternating magnetic field, the alternating main magnetic flux generates its own induced electromotive force in the primary coil, and at the same time the other coil (the secondary coil) induces a mutual induced electromotive force. By changing the relationship between the primary and secondary coil turns ratio to change the primary and secondary coil terminal voltage to achieve voltage conversion, the general turns ratio is 1.5:1~2:1. Because the primary and secondary coils are directly connected, there is a risk of inter-level leakage. Therefore, it cannot be used as a running light transformer.

1. The autotransformer is a special transformer that shares a set of coils for output and input. The step-up and step-down are realized by different taps. The voltage of the taps that are less than the common coil will decrease, and the voltage of the taps that are more than the common coil will increase. high.

⒉In fact, the principle is the same as that of an ordinary transformer, except that its primary coil is its secondary coil. In general transformers, the primary coil on the left uses electromagnetic induction to make the secondary coil on the right generate voltage, and the autotransformer influences itself.

⒊The autotransformer is a transformer with only one winding. When used as a step-down transformer, a part of the turns are drawn from the winding as the secondary winding; when used as a step-up transformer, the applied voltage is only applied to part of the winding Turn up. Usually the part of the winding that belongs to both the primary and the secondary is called the common winding, and the rest of the autotransformer is called the series winding. Compared with the ordinary transformer, the autotransformer of the same capacity is not only small in size, but also more efficient, and the transformer The larger the capacity, the higher the voltage. This advantage becomes more prominent. Therefore, with the development of power systems, the improvement of voltage levels and the increase of transmission capacity, autotransformers have been widely used due to their large capacity, low loss, and low cost.

Ⅰ. The advantages of autotransformer:

The transformation ratio of the autotransformer in the step-down starter is fixed, while the transformation ratio of the contact voltage regulator is variable. Compared with general transformers of the same capacity, autotransformers have the advantages of simple structure, less materials, and small size. Especially when the transformation ratio is close to 1, it is particularly economical, so it is used more in high-power transmission transformers with similar voltages. In addition, it is widely used in step-down starters for asynchronous motors above 10 kilowatts. However, because the primary and secondary windings share a single winding and are electrically connected, they are not suitable for use in some occasions, especially not used as a running light transformer. Compared with ordinary dual-winding transformers, autotransformers have the following advantages:

1. It consumes less materials and has low cost. Because the amount of silicon steel sheet and copper wire used in the transformer is related to the rated induced potential and rated current of the winding, that is, it is related to the capacity of the winding, the autotransformer winding capacity is reduced, the material consumed is also reduced, and the cost is also low.

2. Low loss and high benefit. As the amount of copper wire and silicon steel sheet is reduced, at the same current density and magnetic flux density, the copper loss and iron loss of the autotransformer are both less than that of the dual-winding transformer, so the benefit is higher.

3. Easy to transport and install. Because it is lighter than the double-winding transformer of the same capacity, the size is small, and the floor space is small.

4. Improve the ultimate manufacturing capacity of the transformer. The ultimate manufacturing capacity of transformers is generally restricted by transportation conditions. Under the same transportation conditions, the capacity of autotransformers can be larger than that of dual-winding transformers.

Ⅱ. Disadvantages of autotransformer:

The use of autotransformers in power systems will also have adverse effects. The disadvantages are as follows:

1. Increase the short-circuit current of the power system.

Since there is an electrical connection between the high and medium voltage windings of the autotransformer, its short-circuit impedance is only (1-1/K) square times that of the ordinary double-winding transformer of the same capacity. Therefore, after the autotransformer is used in the power system, the Make the three-phase short-circuit current increase significantly. In addition, since the neutral point of the autotransformer must be directly grounded, the single-phase short-circuit current of the system will greatly increase, and sometimes even exceed the three-phase short-circuit current.

2. Cause some difficulties in voltage regulation.

There are three possible voltage regulation methods for the autotransformer mainly due to the electrical connection of the high and medium voltage windings. The first is to install a voltage regulation device with load to change the position of the splitter in the autotransformer winding; One is to install additional transformers on high-voltage and medium-voltage lines. These three methods are not only difficult to manufacture, uneconomical, but also have shortcomings in operation (such as affecting the voltage of the third winding), and the solutions are not ideal.

3. Make the overvoltage protection of the winding complicated.

Due to the auto-coupling of the high and medium voltage windings, when either side falls into a lightning shock wave with an amplitude that matches the insulation level of the winding, the amplitude of the overvoltage shock that appears on the other side may exceed the insulation level. In order to avoid this phenomenon, a set of valve-type arresters must be installed at the outlet ends of both high and medium voltages.

4. Make the relay protection complicated.

Although autotransformers have certain shortcomings, countries still attach great importance to the application of autotransformers, which are mainly inseparable from the development of power systems to large-capacity and high-voltage. As the capacity increases, the voltage rises, and the The advantages of the coupling transformer are even more significant.