In message <[email protected]>, dated Sat, 17 Dec 2005, Robert Johnson <[email protected]> writes >A couple things make a transformer hum. One is magnetostriction. The >physical size of the core actually changes size due to magnetic flux. >The amount depends on the material, size, shape, flux density, >temperature, frequency. You can probably tell something from the sound >frequency whether it is the fundamental or harmonics (I doubt) of a >distorted waveform..
I didn't see the original post, because I have had a bad virus in my computer. The audible hum is normally very rich in harmonics. I've added some further comments below. > >Another factor is vibration between two parts, particularly >laminations, caused by magnetic attraction. For large power >transformers, a typical fix is a ball peen hammer at the right place. >Varnish helps. > >Thermal vibration is also possible. You can hear that in some light >bulbs. I doubt it is an issue with transformers. > >Bob Johnson >ITE Safety > >Scott Douglas wrote: > Dear Colleagues, > > What makes a transformer hum? In case you were wondering, the > question is indirectly related to product safety. > > We have a product with a large (21 kg) toroid mains transformer. 5 > voltage primary (100/120/220/230/240), 4 secondaries. When safety > testing the original design, Class A (105 deg.) insulation, the > transformer got to 104.9 deg. C. So we asked the manufacturer to > change it to Class B (130 deg.) insulation. He asked could he also > modernize the 10 year old design and we said sure. The new design > allows 2800 W at a max. 80 deg. C temp rise. It runs 20 deg. C > cooler and loses 30 watts less to heat. More efficient we thought > and said yea. > > Now we have 2 of the product installed in the UK and the customer > complains of transformer hum. We replaced the units with two more > and got same result. Customer says with unit in standby (130 W on > mains) it hums, with unit at idle (360 W on mains) it hums a bit > louder, with unit working (300 - 2000 W on mains) hum is about same > as at idle. > > Changes to transformer included decrease I.D. of core center opening > by 5 mm, decrease turns of primaries and secondaries by about 5%, I think that is significant. It depends on how much the core area has increased due to the decreased ID. This would perhaps compensate for the increase in maximum induction (Bmax) due to the reduced number of primary turns. Running the core with Bmax even just a little to high can increase the sound output greatly. But there may be other reasons. The new transformer presumably has a different mass. This may be altering the natural frequency of the panel on which it is mounted, and Murphy's Law says that any such change will increase the noise! > deleted insulation wrap between primaries, and core is annealed now. > There may have been some other changes I can't recall right now. > > Customer site sent us scope pix of AC Mains showing a somewhat > distorted signal. 250 VAC pk-pk, I hope that's not the peak-to-peak value! In any case, 250 V is very high, even for UK. Noise increases rapidly with applied voltage. >looking more triangular than > sinusoidal. 0-90 slope fairly straight to narrow but rounded top at > 90 deg., then fairly straight slope 90-270 deg with a dent in the > 90-180 deg part of the negative slope. From 270-360 deg, again > fairly straight but with another dent like the first one. The 270 > point is also narrow but rounded. I did not see anything looking > like a switching power supply sort of waveform, just a sharp sine > wave or very rounded triangle wave, depending on your point of view. > Don't know if this scope pix is from AC mains with or without our > product connected and powered up. The mains waveforms in Europe are often quite distorted; that's why we have EN 61000-3-2 and -12. But your description indicates a very bad waveform indeed and I wonder what is causing it. > So, what makes this transformer hum in the UK and not in any other > place? Oh yeah, we have some of this product with the older design > transformers in the UK as far as I know with no complaints. > > After we figure that out, and if it is caused by the AC mains being > poor quality, are there any external conditioner type things that > could make the AC mains look more sinusoidal? Normally the mains waveform doesn't have much effect. > Sorry for the long story, but its got me stumped. Any and all input > is welcome and appreciated. If I can be of any help to your client, please put us in touch. It may be a lot easier to solve this in UK rather that transatlantically. > > Merry Christmas to all. > And to you. -- Due to a virus attack, I've lost all my clever signature tags. Are you sorry? OOO - Own Opinions Only. See http://www.jmwa.demon.co.uk and http://www.isce.org.uk John Woodgate - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
