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What I think is that maybe insulation is gradually removed between plates, and eddy currents start passing from one plate to another and this produces sound. Am I right?

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  • $\begingroup$ What makes you say new transformers don't produce noise? I recently visited a place with a brand new (a few months old) dry-type 20 kV transformer and it buzzed surprisingly loudly. $\endgroup$ – ntoskrnl Nov 15 '14 at 15:02
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Transformers generate oscillating magnetic fields at the mains frequency and the fields produce an oscillating force on:

  1. anything nearby that's ferromagnetic (like the core)

  2. anything nearby that is carrying a current (like the windings)

The sound you hear is because various bits of the transformers are moving in response to the oscillating fields and this movement creates the sound. It is a purely mechanical process.

When transformers are new they are very tightly constructed so nothing within them can move in response to these oscillating forces, and they generate no or minial sound. However over time the transformer loosens up and this allows some movement and the corresponding vibration. I would guess the major cause of this is temperature cycling and the associated expansion and contraction.

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  • $\begingroup$ Also, the cyclic stress from the oscillations themselves should play a role, shouldn't it? If some sort of glue is used to freeze the coils in place, that will eventually become brittle and crack. $\endgroup$ – zwol Nov 15 '14 at 17:43
  • $\begingroup$ @Zack: But isn't fatigue caused by cyclic strain rather than cyclic stress? The two are correlated, of course, due to Hooke's Law; but if the glue is initially rigid, as this explanation assumes, then it will (to a large extent) resist the strains that would fatigue it. $\endgroup$ – ruakh Nov 15 '14 at 20:13
  • $\begingroup$ @ruakh I spoke informally. Anyway, I don't actually know much about the materials science of glue; but I do know that quite a few glue compounds become brittle and lose adhesion over time even if they're not under any kind of load. $\endgroup$ – zwol Nov 15 '14 at 22:15
  • $\begingroup$ @Zack: Oh, certainly. But in that case it's not due to "the cyclic stress from the oscillations themselves". $\endgroup$ – ruakh Nov 15 '14 at 22:59
  • $\begingroup$ @ruakh I see now it looks like I'm implying a causal link between those two sentences, but I meant them as two independent thoughts on the same topic. Sorry. $\endgroup$ – zwol Nov 15 '14 at 23:01
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Cores of the transformers a made of ferromagnetic material, which can change the shape due to magnetostriction phenomena. When transformer works with typical grid frequency of 50Hz..60Hz, it can be heard. As it was answered above, generated noise depends on mechanical construction. New power supplies use frequency converters, which drive the transformers with frequency high above audible 20kHz, so they are silent (when are stable).

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