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