Scott, The joke answer is that it hums because "it doesn't know the words".
The short answer is that it hums because of movement. I am guessing that your toroidal core is laminated from a strip of steel. If not properly restrained it will resonate at the operating frequency. This is why line-frequency magnetics hum and higher frequency devices "sing". If the laminations and windings are not properly secured with an impregnation of varnish like material they will vibrate. If these vibrations couple to a sheetmetal chassis the sound will be amplified (metal movement causes air movement = sound). Toroidal power transfomers are usually mounted using elastomer isolators. This helps to decouple vibrations. If the problem exists mainly in the U.K. due to the 50 Hz you might be able to dampen vibrations or isolate components to reduce noise. If the noise is coming directly from the transformer itself you will have to work with your magnetics vendor to solve the problem. As far as the distorted AC waveform is concerned it is hard to be sure without seeing the actual waveform but it sounds very much like the effect of capacitor charging currents. In a typical transformer-rectifier-capacitor supply the capacitors only draw current near the peak of the waveform. At the transformer primary it looks like a normal sinewave with a tall narrow "blip" at the peak. On the other side of a mains filter it may appear somewhat more triangular. This is the reason why power factor correction is becoming so important. Non-linear loads cause problems for the electric utilities. The problem is getting worse as more electronic devices are in use. Carefully chosen series inductors may improve the power factor. I have seen them help in other situations. However, since I assume your product had to pass some kind of standards before you could sell it in the U.K. I would guess that your power factor was within acceptable limits. This is the kind of "perceived quality" issue that I have always warned about. The customer notices something that seems odd such as noise, smells or whatever and starts poking around the unit. They notice something else such as the distorted waveform and assume they are related. Now your job gets tougher. You fix the noise but still have to explain the distorted waveform which may be normal. Whew! You have my sympathy. I've been through it. I wish you luck and hope that you can resolve this easily. Scott Lacey On 16 Dec 2005 at 21:17, 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%, > 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, 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. > > 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? > > Sorry for the long story, but its got me stumped. Any and all input is > welcome and appreciated. > > Merry Christmas to all. > > Scott Douglas > [email protected] > > - > ---------------------------------------------------------------- > 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 > - 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
