Features and Functions of Autotransformer

Autotransformer is a special transformer that does not need to be insulated between primary and secondary. Its primary and secondary windings are on the same adjustable winding, it is a transformer with only one winding, and its output and input share a set of coils. When the autotransformer is used as a step-down transformer, a part of the turns are drawn from the winding as the secondary winding; when the autotransformer is used as a step-up transformer, the applied voltage is only applied to part of the turns of the winding. Because the direct current cannot be directly boosted, it needs to pass through electronic components.

A.Autotransformer function:

Autotransformers are used in occasions where primary and secondary isolation are not required, and have the advantages of small size, less consumables, and high efficiency. Fundamentally speaking, it is to save a coil to reduce the cost, and has the function of an ordinary transformer, that is, to increase or decrease the circuit voltage, isolate and reduce impedance. In addition, it can be made into an AC voltage regulator to flexibly change the output voltage for use in the laboratory.

The autotransformer power supply mode is very suitable for the power supply of large-capacity loads, and has less interference to communication lines, so it is widely used by passenger dedicated lines and heavy-duty freight railways. In the current power grid, there is almost no autotransformer below 220KV, and autotransformers only start from the 220KV voltage level, which are mostly used as connection transformers between the power grids. 

B. Main features of autotransformer:

1. The primary side and the secondary side have not only a magnetic connection, but also an electrical connection, while an ordinary transformer is only a magnetic connection.

Due to the direct electrical connection between the primary and secondary autotransformers, when the high-voltage side is over-voltage, it will cause serious over-voltage on the low-voltage side. In order to avoid this danger, lightning arresters must be installed on both the primary and secondary windings. Don't think that the primary and secondary windings are connected in series. Once the primary and secondary windings are installed, the secondary can be omitted.

2. Since the autotransformer winding is composed of two parts: a primary winding and a common winding, the number of turns of the primary winding is relatively reduced compared with the number of turns and height of the primary winding of an ordinary transformer, the current of the common winding and the leakage reactance generated.

3. Since the short-circuit impedance per unit value of the autotransformer is smaller than that of the dual-winding transformer, the voltage change rate is smaller, but the short-circuit current is larger.

4. If the autotransformer is equipped with a third winding, the third winding will occupy the common winding capacity, which will affect the autotransformer operation mode and exchange capacity.

5. The autotransformer is small in size, light in weight, easy to transport, and low in cost.

Because the calculated capacity of the autotransformer is less than the rated capacity. Therefore, under the same rated capacity, the main size of the autotransformer is smaller, and the effective materials (silicon steel sheets and wires) and structural materials (steel) are reduced accordingly, thereby reducing the cost. The reduction of effective materials reduces copper loss and iron loss correspondingly, so the efficiency of the autotransformer is higher. At the same time, due to the reduction of the main size and quality, a single transformer with a larger capacity can be manufactured under allowable transportation conditions. But usually only when k ≤ 2 in the autotransformer, the above advantages are obvious.

6. In general transformers, the on-load voltage regulating device is often connected to the neutral point of the ground, so that the voltage level of the voltage regulating device can be lower than that of the line end voltage regulation. The neutral point voltage regulation side of the autotransformer will bring so-called related voltage regulation problems. Therefore, when the autotransformer is required to adjust the voltage under load, only the end-of-line voltage adjustment method can be used.