Shorted turn?
- Robert -
On Fri, 16 Dec 2005 21:17:56 -0500
Scott Douglas <[email protected]> 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]
>
> -
>
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-
This message is from the IEEE Product Safety Engineering Society
emc-pstc discussion list. Website: http://www.ieee-pses.org/
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Instructions: http://listserv.ieee.org/request/user-guide.html
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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]
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All emc-pstc postings are archived and searchable on the web at:
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