Why does electricity at power plants and transformers make a humming noise?
Transformers hum because their laminations and windings rattle mechanically. This is best known as mechanical hum. This can be caused by several things, including poor construction and poor AC power conditions or both. If your equipment has a noisy transformer, you’ve probably also noticed that it’s intensity varies depending on the time of day, sometimes even the time of month. The reason it varies is due, in large part, to the quality of the AC line voltage and how much DC is on it. Why do transformers hum? The short and simple answer is that many times, transformers hum because of an effect known as ‘lamination rattle’ or ‘winding rattle’ caused by DC voltage on the line. ‘Lam’ or ‘winding’ rattle occurs in all transformers to some degree, that degree being related to the quality of the transformer and the quality of the line voltage. One of the problems we find on the AC line is when there’s an unwanted DC component. DC (like battery voltage) leaks into most AC lines but its level
