Transformer Hum and Load
Date: Fall 2012
I work as an electrician, and none of the instructors I learned from at the trade school I went to could answer the question of why electrical transformers hum. Also it seems that as more load is added to the secondary side of the transformer, the less it hums. So, my personal theory is that because these are inductive in nature, the hum maybe comes from the amount of amperage that is more readily available to try and permeate through and between the winding's to ground and as more and more amperes are demanded on the secondary side, there are less chances of eddy currents to develop and the electrons actually end up inside or "around" the wire rather than escaping.
Jose, the short answer is that when you magnetize metal, like that
which occurs in a transformer, the metal experiences bending and
stretching. Those mechanical forces cause the vibration that results
in the 'humming' noise. The phenomenon is called 'magnetorestriction'.
For more details, several transformer manufacturers have posted
similar information on their sites that go into more detail -- this is
the best one I found:
You can also find numerous articles about the physics of
magnetorestriction (just do an Internet search for that term).
Hope this helps,
You are on the right track with your thinking. The transformer windings create a magnetic field that cycles with the varying primary current. Each winding, whether in the primary or secondary, carries a current that creates a force on the winding according to the Lorentz force equation. It is the same principle as a motor in that a wire carrying current will experience a force depending on how magnetic field is oriented with respect to the wire. The transformer windings in this case experience an oscillating force that creates the hum. The windings are not the only parts creating hum, however, and your observation is correct that eddy currents in the laminations play a part. These currents are also subject to Lorentz forces and will hum as well. Typically, both windings and laminations are shellacked in order to reduce this hum. You further observation that the hum lessens with increasing secondary load is also correct. The secondary load creates a “counter EMF” that reduces the net overall transformer field. Lower overall field means less Lorentz force and thus a lighter humming noise. The actual physics of hum has been studied using electromagnetic simulations, and the “signature” of the hum can indicate the health of the transformer. If insulation degrades, or windings short, this hum can change in ways that point to the eventual failure of the transformer.
Kyle Bunch, Ph.D
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Update: November 2011