That's a good point. The filthy leakge inductance will be
spraying its flux all about. But there are things we can
do a to contain it.

A belly band around the winding (but not though the core) helps.
End bells (covers) bolted down over the windows work too.

There may also be fringing flux spewing out of the unavoidable
gaps if it's a butt stack lamination core. Interleaving the
laminations will help.

I've done all of that on a design with the transformer mounted
directly beneath the neck of the CRT in a video terminal.
A requirment of the project manager because he feared
SMPS noise and demanded a linear power supply. Of course
everything had to be jammed into the smallest possible
enclosure and a mu metal shield was deemed too expensive :P





Jim Higgins wrote in
:

It's not just the laminations that make noise. The windings can
vibrate if they're not locked in place with transformer varnish or
epoxy. No amount of tightening the laminations will help if the
windings are free to vibrate. This is probably more likely in larger
higher power transformers (multiple kW) than in a filament or screen
supply transformer, but it can happen.

Also, depending on how much magnetic leakage your core has you can
couple to the steel walls of the cabinetry the transformer is housed
in if you don't maintain adequate spacing from the walls, in which
case the steel wall and the transformer can vibrate and make noise
even though no parts on the transformer are loose.

On Thu, 30 Oct 2008, Grumpy The Mule wrote:

Date: Thu, 30 Oct 2008 01:37:32 +0100 (CET)
From: Grumpy The Mule
Newsgroups: rec.radio.amateur.homebrew
Subject: I built a 7.5VAC 21Amp transformer now it has lots of buzz


Howdy,

I think I would measure the line current with no load on the
transformer. Then let it idle a while, maybe three hours, then
give it the finger test. If it's not warm and the magnetizing
current is small, I'd then run it with a heavy load and check
it again. If it's not stinky hot (the winding is below about
110C more or less) I'd say it's OK at 50Hz and the breaker
tripping is from the inrush current.

There's some inrush current charging the core.

Don't forget that the inrush current is also going into at least one and
maybe two high capacity electrolytic capacitors that, instantaneously
before initial charging are going to look like zero ohms. Peak current, at
say peak AC volatage when the on switch is turned on, is going to be
higher than you think.

The manufaturers are just too cheap to put any kind of Soft-start
circuitry in to limit this inrush.

That's not a
defect though, it's just how things work. The first half
cycle peak might be five or more times the peak magnetizing
current. The inrush current charging the capacitors is more
prolonged. Depending on the curve of the circuit breaker one
or both might be to blame.

So if the soft-start circuit works for you just monitor the
temperature of the transformer. If it's reasonable don't
worry. Then look elsewhere for the cause of the buzzing.
The cabinet sheet metal you should be able to quiet with
the palm of you hand...

Ok, here's one crazy idea. The inrush current is mostly
on the first half cycle so it's magnetizing something, maybe
the cabinet who knows what, which is then a more efficient
transducer. If you soft start it either doesn't magnetizing
the part or perhaps degausses it.


73


Highland Ham wrote in news:KO-
:

==============
Followed this thread with great interest .
Reason : I have an Astron RS-50A , 13.8V -50 Amperes unit for 220V-AC
input which is buzzing heavily. Upon switch-on the group circuit breaker
in the house sometimes trips due to the high switch-on current surge .
This can be overcome by adding a soft start circuit which initially puts
a say 25 Ohms-50Watts resistor in series with the primary transformer
winding ,shorting this resistor after a second or so.

However I fear that the main reason for the buzzing is that the
transformer (of US origin) was designed for 60 Hz ,whereas here in the
UK (like in all European countries ) the supply frequency is 50 Hz.
Can someone on this NG please confirm/reject my suspicion.

Another possible reason could be that the cabinet sheet metal which is
very close to the transformer is vibrating (transducer effect) .


TIA for any constructive advice/opinion

Frank GM0CSZ / KN6WH

On Oct 29, 1:21*pm, Highland Ham wrote:


However I fear that the main reason for the buzzing is that the
transformer (of US origin) was designed for 60 Hz ,whereas here in the
UK (like in all European countries ) the supply frequency is 50 Hz.
Can someone on this NG please confirm/reject my suspicion.

Another possible reason could be that the cabinet sheet metal which is
very close to the transformer is vibrating (transducer effect) .

TIA for any constructive advice/opinion

Frank * * GM0CSZ / KN6WH

I can confirm the 50/60Hz problem. Have been moving line powered
equipment (including microwave ovens, Astron P/S, and more - even real
Wahl hair clipper) between US/Canada and Europe for over 10 years and
found the increased level of buzz when running 60Hz eqpt on 50Hz in
practically all cases. My Astron P/S is just a 4A unit and the buzz is
bearable. Other stuff ranges from OK to loud like hell.

73,
Rich OK8RF/VE3FSF

On Oct 25, 12:25 pm, raypsi wrote:
Hey all:

I tore down a transformer I got from ebay it was a Zenith TV power
transformer from days gone by. Re wound it with 17AWG wire on the
secondary and primary. It powers up my 4-1000A filament nicely with
7.25VAC from 120VAC mains. I used a penta-filar winding on the
secondary. Shimmed the bobbin in the core window with some 1/8 thick
virgin teflon. Epoxied the windings to the bobbin. This was a 240VA
transformer and now is a 83watt buzzer / 157watt filament transformer.

So what is the cheapest I can gits away with sealing these laminations
from buzzing, I used up all my epoxy sealing the windings.

73

n8zu

I finally got around to clamping the transformer down with grade 8,
1/4inch bolts and nuts That didn't help, luckily I found my 3 hour
epoxy, ran the transformer for 3 hours straight to heat it up, took
the bolts out and with a putty knife slapped on some microwave heated
epoxy on the lams. That took the buzz down to below the level of the
fan noise. In dead silence you can still hear it. Nowhere can you feel
the buzz.

73

n8zu

congrats on the fix..
now you have th epower and the time to contest..
Contesting is much like transformer winding..
Once you learn how to do one turn, you repeat and count until you reach
a goal...
In contesting you do one turn, then another till you reach the time
limit and count your turns.
hi hi
Hank
"raypsi" wrote in message
...
On Oct 25, 12:25 pm, raypsi wrote:
Hey all:

I tore down a transformer I got from ebay it was a Zenith TV power
transformer from days gone by. Re wound it with 17AWG wire on the
secondary and primary. It powers up my 4-1000A filament nicely with
7.25VAC from 120VAC mains. I used a penta-filar winding on the
secondary. Shimmed the bobbin in the core window with some 1/8
thick
virgin teflon. Epoxied the windings to the bobbin. This was a 240VA
transformer and now is a 83watt buzzer / 157watt filament
transformer.

So what is the cheapest I can gits away with sealing these
laminations
from buzzing, I used up all my epoxy sealing the windings.

73

n8zu

I finally got around to clamping the transformer down with grade 8,
1/4inch bolts and nuts That didn't help, luckily I found my 3 hour
epoxy, ran the transformer for 3 hours straight to heat it up, took
the bolts out and with a putty knife slapped on some microwave heated
epoxy on the lams. That took the buzz down to below the level of the
fan noise. In dead silence you can still hear it. Nowhere can you feel
the buzz.

73

n8zu