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I have a number of mains-powered devices that have begun humming (I assume at 50Hz, though I haven't measured it). These include clock radios, a speaker system, a lamp (20W halogen with a transformer before the switch), and a temperature-controlled soldering iron. With some of these devices, it can be really annoying (like the radio and lamp and the speaker that vibrate the floor, and my bed, and make it hard to get to sleep). I assume it's also wasting a small amount of energy. I have tried taking one of the clock radios apart, and cleaned some dust and crap off the PCB (I couldn't see any other obvious problems), and it did reduce the hum a little, but not much, and it started getting worse again.

Is there a common cause for these hums? And is there anything that can be done about it?

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  • \$\begingroup\$ It's usually the AC transformer. I've also noticed a lot of old people whistling LOL. \$\endgroup\$
    – Andy aka
    Sep 24, 2014 at 7:30
  • \$\begingroup\$ @Andyaka: Ok, in that case, what is it about the transformer that causes the hum? \$\endgroup\$
    – naught101
    Sep 24, 2014 at 7:33
  • \$\begingroup\$ I had a bad case of this where a dimmable ceiling lamp below my room (2nd floor) caused my guitarr amplifier to hum when the lamp was not fully on. Solved by using a grounded outlet to the amplifier. \$\endgroup\$
    – Dejvid_no1
    Sep 24, 2014 at 7:38
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    \$\begingroup\$ delamination of the transformer core. \$\endgroup\$
    – Majenko
    Sep 24, 2014 at 7:38
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    \$\begingroup\$ As one grows older, one forgets the words. \$\endgroup\$ Sep 24, 2014 at 16:53

6 Answers 6

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A transformer, as you well know, is made up of two or more coils around a core of ferrous material. That ferrous material is not a solid lump of metal, but a series of plates laminated together with adhesive. This is done because:

Early transformer developers soon realized that cores constructed from solid iron resulted in prohibitive eddy current losses, and their designs mitigated this effect with cores consisting of bundles of insulated iron wires. Later designs constructed the core by stacking layers of thin steel laminations, a principle that has remained in use.

-- Wikipedia

So you have lots of steel plates stuck together, but not only that:

Each lamination is insulated from its neighbors by a thin non-conducting layer of insulation.

Lots of metal plates, each with an induced magnetic field. That magnetic field acts between the adjacent plates stretching and squeezing the adhesive and insulation between them. Over time that adhesive starts to break apart and the laminated layers separate from each other slightly. This is the humming noise you can hear. It's always present, but once the adhesive starts to break it gets louder. These micro-fractures in the adhesive may not be visible to the naked eye, but in extreme situations they may be so bad the layers of lamination become loose and the transformer literally rattles as you shake it.

Also, the more current you draw through a transformer the larger the induced magnetic fields, and thus the louder the transformer hums (and the shorter its life span).

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  • \$\begingroup\$ Ok, so with these cheapish appliances, there'd be basically nothing your could do about it, short of replacing the transformer (which might be hard to ID)? \$\endgroup\$
    – naught101
    Sep 24, 2014 at 9:36
  • \$\begingroup\$ Correct. Unless you fancy unwinding, re-laminating, and then re-winding the transformer. However, transformers are pretty common and finding a replacement may not be as hard as you'd at first think. \$\endgroup\$
    – Majenko
    Sep 24, 2014 at 9:38
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The laminations are moving wrt each other. They're generally stuck together at the factory and over years the varnish gets brittle and the forces can cause the laminations to no longer be stuck together. Magnetic forces from the field cause the hum.

If it's a valuable item, you can remove the transformer and take it to a motor rewinding shop and ask them to vacuum impregnate it for you.

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I used super glue. I applied it all over the transformer, used pliers to squeeze it in order to get the glue to penetrate deeply between the laminations, applied more glue where necessary, and then used a few C clamps to hold it together. I put my amp back together 12 hours later and the hum is gone.

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OK I know very little about laminated transformer cores. But it was my understanding that the hum was due to magnetostriction of the iron.

Size change depends on the strength of the B-field and so happens at twice the AC mains frequency.

This size change gets transmitted as sound to whatever holds the transformer. I don't think it has to do with a failing lamination's. (But I'm happy to learn something if I'm wrong.)

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A noisy transformer is often the sign of a failing transformer. I had the case in my Copal FP-220 flip clock radio. The transformer was buzzing, and worked for 30 seconds, then shut down. I replaced it with a generic 220v-9v transformer, and everything is working like new now!

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fix for noisy transformers Here is a wonderful tip to fix buzzing or loud transformers i used it to fix my copal RD150 this can be done with the transformer connected or for best results remove...

you will need . wood shellac....http://www.bunnings.com.au/feast-wat...lakes_p1567019

one eye dropper or a container to soak transformer in .

simply make up shellac to directions soak transformer in solution of shellac and Denatured alcohol, also called methylated spirits DO NOT PLUG in clock for at least 6 hours to be sure that the shellac has dried !!!!!!!!!!!!!!!!!! or leave transformer in the the clock and drop in and around the (transformer) wires with and eyedropper.......once dry (6 hours) to be sure.. plug in your flip clock and bingo no more buzz or hum....hope this helps Last edited by [email protected]; 1 day ago.

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  • \$\begingroup\$ Welcome to EE.SE! When copying content from other sites you should cite where it came from. Also the copy/paste has broken the Bunnings link. \$\endgroup\$
    – PeterJ
    Feb 20, 2016 at 7:14
  • \$\begingroup\$ @PeterJ: Evidently, it's just a link to a particular brand of shellac flakes, so not terribly important to the answer. \$\endgroup\$
    – Dave Tweed
    Feb 20, 2016 at 13:39
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    \$\begingroup\$ Would be interesting to know if this actually works though. And how long it works for.. \$\endgroup\$
    – naught101
    Feb 22, 2016 at 23:16
  • \$\begingroup\$ unattributed copy/paste from flipclockfans.com/forum/forum/clock-show-and-tell/… \$\endgroup\$ May 21, 2017 at 15:43

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