The transformer is made of electromagnetic induction. It consists of an iron core composed of silicon steel sheets (or silicon steel sheets) and two groups of coils wound on the iron core. The iron cores and coils are insulated from each other without any electrical connection, as shown in the figure. We call the coil connected between transformer and power supply side as primary coil (or primary side), and coil connecting transformer and electrical equipment as secondary coil (or secondary side). When the primary coil of the transformer is connected to the AC power supply, the magnetic line of force will change in the iron core. Because the secondary coil is wound on the same iron core, the secondary coil is cut by the magnetic line of force, and the induced electromotive force is generated on the secondary coil, which makes the voltage appear at both ends of the coil. Because the voltage of the secondary is alternating. And the frequency is exactly the same as the power frequency.

经理论证实，变压器初级线圈与次级线圈电压比和初级线圈与次级线圈的匝数比值有关，可用下式表示：初级线圈电压/次级线圈电压=初级线圈匝数/次级线圈匝数 说明匝数越多，电压就越高。因此可以看出，次级线圈比初级线圈少，就是降压变压器。相反则为升压变压器。

The manager has demonstrated that the voltage ratio between primary coil and secondary coil and the ratio of turns between primary coil and secondary coil are related, which can be expressed as follows: voltage of primary coil / voltage of secondary coil = number of turns of primary coil / number of turns of secondary coil, indicating that the more turns, the higher voltage. Therefore, it can be seen that the secondary coil is less than the primary coil, that is, the step-down transformer. On the contrary, it is a step-up transformer.