The transformer is made based on electromagnetic induction. It consists of an iron core laminated with silicon steel sheets and two sets of coils wound on the iron core. The iron core and the coils are insulated from each other without any electrical connection. The coil connecting the transformer and the power supply side is called the primary coil, and the coil connecting the transformer and the electrical equipment is called the secondary coil. When the primary coil of the transformer is connected to an AC power source, changing magnetic lines of force will be generated in the iron core.
Since the secondary coil is wound on the same iron core, the magnetic field line cuts the secondary coil, and the induced electromotive force is inevitably generated on the secondary coil, causing voltage to appear at both ends of the coil. Because the lines of magnetic force are alternating, the voltage of the secondary coil is also alternating. And the frequency is exactly the same as the power frequency.
It has been confirmed by theory that the voltage ratio between the primary coil and the secondary coil of the transformer is related to the ratio of the number of turns between the primary coil and the secondary coil, which can be expressed by the following formula: primary coil voltage/secondary coil voltage=primary coil turns/secondary coil turns It shows that the more turns, the higher the voltage. Therefore, it can be seen that the secondary coil is less than the primary coil, which is a step-down transformer. The opposite is a step-up transformer.