Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page Visit Our Archives Ask A Question How To Ask A Question Question of the Week NEWTON Teachers Our Expert Scientists Volunteer at NEWTON! Referencing NEWTON Frequently Asked Questions About Ask A Scientist About NEWTON Education At Argonne Transformer Hum and Load

Name: Jose
Status: other
Grade: other
Location: NV
Country: USA
Date: Fall 2012


Question:
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.

Replies:
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:

(HTML) http://www.federalpacific.com/university/transnoise/transnoise.html

(PDF) http://www.federalpacific.com/literature/drytrans/10transformernoise.pdf

You can also find numerous articles about the physics of magnetorestriction (just do an Internet search for that term).

Hope this helps, Burr Zimmerman


Jose,

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


Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 223
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: November 2011
Weclome To Newton

Argonne National Laboratory