8.What is humming in transformer

:    Transformer noise is caused by a phenomenon called magnetostriction. In very simple terms this       means that if a piece of magnetic sheet steel is magnetized it will extend itself. When the                          magnetization is taken away, it goes back to its original condition. A transformer is magnetically         excited by an alternating voltage and current so that it becomes extended and contracted twice           during a full cycle of magnetization.   
                                                                                                                     

     We have established that the transformer hum is caused by the extension and contraction of the         core laminations when magnetized. Under alternating fluxes, we can expect this extension and 

     contraction to take place twice during a normal voltage or current cycle. This means that the               transformer is vibrating at twice the frequency of the supply, i.e. for 60 cycles per second supply         frequency, the noise or vibration is moving at 120 cycles per second. This is called the                             fundamental noise frequency