John,
You have helped me to think about this differently. To tell if it is the
transformer secondary load that is the culprit one can disconnect the
load. If it still hums then the load is not the problem.
What can the problem be then? It could be the transformer design or it
can be the distorted AC waveform being fed to the transformer. Or a
combination in which a transformer deficiency is revealed by the
distorted AC waveform.
The previously mentioned experiment to apply a swept sinewave to the
transformer can reveal a transformer defect. This signal can be applied
to the transformer primary or the secondary.
A manufacturing change was made where the core area was reduced along
with a reduction in the number of windings of 5%. This would tend to
make the transformer saturate at a lower voltage - less volt-seconds
will do it. This should have made the hum worse, all other things being
equal, if transformer saturation is the issue.
It can be of great academic interest to get to the root cause of a
problem but sometimes in the interest of time I prefer to solve a long
distance problem such as this by fixing more than one thing at once. We
never now for sure which fix solved the problem but the problem is gone.
That is why I like the idea that was suggested of potting the noisy
critter. There was also the suggestion of mounting it on some type of
vibration isolation.
I can't stand humming transformers myself. If I hear the slightest AC
hum I have to unplug the thing. I suppose it is a case of audible
asthetics or noise pollution.
Dave Cuthbert
Micron Technology
From: [email protected] [mailto:[email protected]] On Behalf Of John
Woodgate
Sent: Tuesday, December 20, 2005 6:33 AM
To: James, Chris
Cc: Scott Douglas; EMC-PSTC
Subject: Re: What Makes a Transformer Hum in the UK?
In message <[email protected]>,
dated Mon, 19 Dec 2005, "James, Chris" <[email protected]> writes
>So look at the current wave forms not the voltages - I expect the UK
>ones will be more peaky which will cause any mech "weaknesses" to sing
>more noticeably.
Yes, if the transformer is feeding a rectifier with capacitive filter,
the peak current can be very high indeed. If the noise is due to winding
vibration, the high peak current makes it worse. But if the hum is
related to core saturation, the high peak current tends to pull down the
peak supply voltage and thus the peak induction.
--
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
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