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AFAIK transformers hum because alternating magnetic field causes pieces of laminated core to slightly deform and vibrate and that is simply a byprocess of how transformers work. However for the same model, the same power load and the same age some transformers hum more than others. The latter is sometimes claimed to coils being winded slightly less tightly so that over the transformer lifetime it first vibrates less and that vibration causes coils to stretch a little bit and allow for more vibration. Another scenario is when a transformer ages and laminated pieces slightly grind the thin insulation between themselves.

I'd guess that increased vibration should require some power to work and that would decrease transformer efficiency.

Does transformer humming cause any notable waste of energy?

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  • \$\begingroup\$ Define "notable". It is obvious that some energy gets wasted, but if it is notable depends entirely on your definition. \$\endgroup\$ – PlasmaHH Apr 24 '15 at 9:12
  • \$\begingroup\$ @PlasmaHH I guess it has to be comparable to other losses to be "notable". Otherwise if it's next to nothing compared to resistive losses why would we care? \$\endgroup\$ – sharptooth Apr 24 '15 at 10:30
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    \$\begingroup\$ browse around this site, people care about the weirdest things... \$\endgroup\$ – PlasmaHH Apr 24 '15 at 10:32
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The humming itself is probably not a significant loss factor, however frictional heating as laminations move (of which hum is a byproduct) due to magnetostriction can be. In fact, attempts to reduce the audible noise can increase losses.

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  • \$\begingroup\$ Right. Acoustic power is amazingly low. The defined threshold of hearing, 0dBSPL, is only 1pW/m^2. (picowatt per square meter) The defined threshold of pain, 130dBSPL, is 10W/m^2. I'd guess that your transformer is likely to be somewhere around 30dBSPL about a meter (3 feet) away, which is 1nW/m^2. (nanowatt per square meter) A sphere of radius 1m has an area of 4*pi ~= 12m^2, for a total power loss of 12nW. That may vary by an order of magnitude or two (10-20dB), but it's still tiny. \$\endgroup\$ – AaronD Apr 24 '15 at 15:34
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Of course it does cause a "notable" waste of energy. Evidently, there's enough acoustic energy radiated for you to hear it - so it's notable to you already, by definition.

Yet, the acoustic radiation is not the only way that transformers lose energy. There is also acoustic energy conducted to the building structure, the electromagnetic radiation, the resistive losses in the windings, and various kinds of core losses.

For the acoustic radiation to be significant, rather than simply notable, you'd have to measure its power, as well as the power lost to other losses. Finally, you'd be able to try and reduce the acoustic radiation while not increasing the other losses. This is a design optimization problem, and it may well turn out that it's either impossible to reduce acoustic radiation while not increasing the sum of other losses. Or it may simply be that the cost of the energy lost via acoustic radiation, amortized over the life of the transformer, is much lower than the amortized increase in transformer cost that would be required to mitigate the acoustic radiation in an environment where nobody cares about such radiation. Finally, you might be targeting an acoustically sensitive environment, where the mitigation makes sense in spite of increase in cost.

You can't really decide the design tradeoffs unless you have a particular market and application in mind. So, the rephrased question of whether the energy loss is significant or not, needs a context that we simply don't have here - and thus cannot be answered.

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