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
