The sound of the transformer comes from inside the transformer. The transformer is based on the principle of electromagnetic induction. It has a primary side winding coil and a secondary side winding coil installed inside, with a silicon steel sheet of high magnetic permeability material in the middle. Under normal circumstances, the coil winding of the transformer calculates a coil called "ampere-turns ratio" based on the core cross-sectional area.
Since there is a coil winding, when it is connected to an AC 50Hz power supply, there will be an excitation current. There are two parts of loss in the AC core coil. The variable loss is the short-circuit loss, which is also the copper loss. It is also divided into two parts, namely the active power part and the reactive power part.
There are copper losses RI² on the coil resistance R and iron losses (hysteresis losses and eddy current losses) in the iron core. Iron losses are approximately proportional to Bm². When the power frequency is a constant value, the iron loss of the coil is related to the operating voltage. According to the concept of constant magnetic flux U=4.44fNBmS, it can be seen that Bm in the iron core is proportional to the applied voltage. In other words, iron losses are approximately proportional to the square of the applied voltage.
Because the transformer can judge the operating condition based on the operating sound. The method is to put one end of the listening stick on the oil tank of the transformer, and put the other end close to your ear to listen carefully to the sound. If there is a continuous "yu" sound, it means the transformer is operating normally. If the "yuyu" sound is heavier than usual, check the voltage, current and oil temperature of the transformer to see if it is caused by excessive voltage or overload. If there is no abnormality, it is mostly caused by a loose iron core. When you hear a "squeak" sound, check whether there is flashover on the surface of the casing. When you hear a "crackling" sound, it means there is a breakdown of the internal insulation.
