|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. I 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.|
|This extension and contraction is not uniform, consequently the extension and contraction varies all over a sheet.
A transformer core is made from many sheets of special steel. It is made this way to reduce losses and to reduce the consequent heating effect. If the extensions and contractions described above are taking place erratically all over a sheet, and each sheet is behaving erratically with respect to its neighbor, then you can get a picture of a moving, writhing construction when it is excited. Of course, these extensions are only small dimensionally, and therefore cannot usually be seen by the naked eye. They are, however, sufficient to cause a vibration, and as a result noise.
The act of magnetization by applying a voltage to a transformer produces a flux, or magnetic lines of force in the core. The degree of flux will determine the amount of magnetostriction (extensions and contractions) and hence, the noise level.
|The obvious question is why not reduce the noise in the core by reducing the amount of flux. Why? Because it is not that simple.
Transformer voltages are fixed by system requirements, and the amount of magnetization, by the ratio of these voltages to the number of turns in the winding. The decision on what this ratio of voltage to turns will be, is made for reasons, mainly economic. It means that the amount of flux at the normal voltage is invariably fixed, thus setting the noise and vibration level. Also, increasing (or decreasing) magnetization does not increase or decrease the magnetostriction by the same amount. In technical terms the relationship is not linear. Therefore, when we are asked, as we invariably are, can you reduce the noise level at the source? the answer is that it can be done, at a cost and for not much improvement in noise level.