In the AC circuit, the equipment that will increase or decrease the voltage is called transformer. The transformer can change any value of voltage into the voltage value we need with the same frequency to meet the requirements of power transmission, distribution and use. For example, the power generated by power plants has a low voltage level, so the voltage must be raised before it can be transported to a far away power consumption area. The power consumption area must be changed into a suitable voltage level through voltage reduction to supply power equipment and daily electrical equipment.