Why do transformers hum?

The short and simple answer is that transformers hum because of an effect known as 'lamination rattle' caused by DC voltage on the line or poor construction or both. 'Lam' 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.

Remember when we mentioned that this humming problem was due to either power line conditions or the quality of the transformer? Here's why: when there's DC on the line, we have an asymmetrical field which causes greater vibrations. The laminations are 'pushed' together in one direction because of the DC.

Because transformers work when the coil magnets switch poles from North to South and back again, 60 times a second, DC forces the input coil to always sit in one direction and this makes the transformer a little crazy so it hums.

To reduce these noises, transformer manufacturer have several tricks up their sleeves: they can varnish, or use super glue to stick the laminations together so they rattle less, and they can make bigger transformers that don't have to work so hard, even in the presence of DC. The harder a transformer has to work, the more stress and strain is placed on the laminations.

By: J. Peter Maciel
For: DMF Lighting