To first order it is true that the magnetic flux in the core does not
change with load current. It would be exactly true if the permeability of
the core were infinite. For real materials with finite permeability, a load
current on the secondary does force some flux out of the core into the air.
One can confirm this empirically by holding a permanent magnet near a
transformer, and then loading and unloading the secondary. This flux going
into the air causes the coupling coefficient to come up just short of 1.
Perhaps the flux in the air is interacting with some other nearby metal
(magnetic or not).
Going back to the start of this discussion: The inner diameter of the
transformer was reduced. There is something in the back of my head telling
me that perhaps the part of the core at the inner diameter is going into
saturation. Maybe someone with a field solving engine could give this a go.
If this is so, could this cause more hum via magnetostriction? The
transformer overall would not exhibit saturation effects due to a
sufficient amount of unsaturated core material at the outer diameters.
Don Borowski
Schweitzer Engineering Labs
Pullman, WA
drcuthbert@micron
.com
Sent by: To
[email protected] <[email protected]>,
<[email protected]>
cc
12/22/2005 08:54
AM Subject
RE: What Makes a Transformer Hum in
the UK?
Peter,
When John Woodgate said that secondary current does not change the core
flux I went back to the books to see where I went wrong. Here is what I
think is going on:
The primary current that is caused by the secondary load creates a
magnetic flux in the core. The secondary current creates a magnetic flux
in the core. These magnetic fields are equal and opposite and therefore
they cancel. Loading a transformer secondary does absolutely nothing to
the magnetic core flux. The core flux is set by the magnetizing current
and not the load current. Does this sound correct to the rest of the
group? This has come as a revelation to me, I must say. But, it is
intuitively obvious. The formulas support this but none of the papers I
read pointed out why, or if they did, I missed it.
Dave Cuthbert
Micron Technology, Inc.
From: [email protected] [mailto:[email protected]] On Behalf Of Peter
Tarver
Sent: Tuesday, December 20, 2005 10:43 AM
To: [email protected]
Subject: RE: What Makes a Transformer Hum in the UK?
Dave -
Removing the load removes flux from the core. My experience has shown
that some transformers vibrate only for higher current load conditions
(not necessarily at saturation) and there's negligible or no vibration
with only magnetization current supporting flux in the core. Replacing
the in situ load with a resistive load would demonstrate if the
phenomenon John is describing is the root cause. This is more
seat-of-the pants than your method but could be more direct with regard
to effects load type.
Regards,
Peter L. Tarver, PE
[email protected]
> From: [email protected]
> Sent: Tuesday, December 20, 2005 9:01 AM
>
> 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.
>
________________________________________________________________________
_____
Scanned by Sanmina-SCI eShield
________________________________________________________________________
_____
-
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
This e-mail may contain SEL confidential or
legally privileged information. The opinions
expressed are not necessarily those of SEL.
Any unauthorized disclosure, distribution or
other use is prohibited. If you received this
e-mail in error, please notify the sender,
permanently delete it, and destroy any
printed copies.
-
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
