Among the many types of transformers, there is a special transformer in which the primary and secondary coils are directly connected in series with the same winding. This self-coupling structure gives these transformers the name autotransformers.

From a structural point of view, autotransformers share one winding, so their size is relatively small. The required transformer materials will be reduced accordingly, which will also reduce copper loss and iron loss, which can effectively improve power transmission efficiency and reduce costs.

According to structural subdivision, autotransformers can be subdivided into two types: adjustable voltage type and fixed type. The fixed transformer ratio is fixed and can be used in scenarios such as step-down starters. The voltage-adjustable autotransformer adjusts the voltage through a split device inside the winding or an additional transformer. Its main principle is to change the position of the line end.

Autotransformers are particularly economical when the transformation ratio is close to 1, so they are often used in high-power transmission transformers with similar voltages. In addition, in step-down starters for asynchronous motors above 10 kilowatts, autotransformers are Has been widely used.

However, due to its structural characteristics, we must also be aware that the short-circuit impedance per unit value of the autotransformer is smaller than that of the two-winding transformer. It has the characteristics of smaller voltage change rate but larger short-circuit current. In addition, there is a direct electrical connection between its primary and secondary sides. Overvoltage on the high-voltage side will cause severe overvoltage on the low-voltage side. Lightning arresters need to be installed on both primary and secondary sides, which is not suitable for all electrical environments.

In general, autotransformer is an excellent form of transformer design. It has the advantages of saving materials and large capacity in the same volume. In some cases where the transformation ratio is not large and no special protection device is required, it can be selected. Consider using an autotransformer.