Envelope-to: dave@picks.force9.co.uk
Delivery-date: Sun, 23 Oct 2005 12:00:11 +0100
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From: "James Moritz" <james.moritz@btopenworld.com>
To: rsgb_lf_group@blacksheep.org
References: <20051021193636.64011.qmail@web86605.mail.ukl.yahoo.com><Pine.OSF.4.56.0510211830330.10074@plato.ucs.mun.ca> <1664918137.20051021232424@t-online.de><00de01c5d6bb$bab3c860$4711f4cc@p1i5f0> <Pine.OSF.4.56.0510221606260.23896@plato.ucs.mun.ca>
Date: Sun, 23 Oct 2005 11:56:37 +0100
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Subject: Re: LF: VO1NA - transformers
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Dear Joe, LF Group,

I may be wrong, but I believe the term "control transformer" just refers to
relatively small mains transformers in industrial applications intended for
powering ancilliary control circuits, as opposed to big MVA-rated
transformers for huge motors, furnaces, etc.

As far as the rating goes, the VA rating normally refers to a resistive
load - if you have a rectifier/capacitor connected to the secondary, the
transformer should be derated, because the high crest factor of the peaky
current waveform being drawn by the rectifier leads to greater heating
effect within the transformer. By how much depends on the type of
transformer, rectifier and filter component arrangement, but in the worst
case, derating by about 0.7 for a capacitor-input filter seems to be normal.
The rating is also "thermal", i.e. the main limiting factor is the
temperature rise inside the transformer, so an intermittent load can be
considerably more than the rating without overheating the transformer,
provided there are low-load cooling down periods. This would be the case
when transmitting CW and QRSS, so it might well be OK for  the full output,
provided you are not operating with 100% key-down modes like Jason or DFCW
(nearly). If it is in an enclosure, removing the enclosure ought to reduce
the temperature rise a bit (unless you really need the "waterproof" feature,
of course!). Another factor might be the regulation, i.e. reduction in
output voltage under load, which would distort the keying waveform -
probably more of an issue for manual CW than QRSS. A transformer with good
regulation also implies higher peak currents drawn from the mains, which
might potentially cause problems with popping fuses without some kind of
"soft start" -having said all this, there seem to be plenty of PSUs around
for linears, etc. that work quite happily on normal mains supplies at this
kind of power level. My suggestion would be to try it and see what happens -
good luck!

Cheers, Jim Moritz
73 de M0BMU


----- Original Message -----
From: Joe Craig <jcraig@mun.ca>
To: <rsgb_lf_group@blacksheep.org>
Sent: Saturday, October 22, 2005 7:49 PM
Subject: Re: LF: VO1NA


> Hi Steve (and Mike WE0H),
>
> You are quite preceptive... a chainsaw weekend indeed.  I cut a 100'
> line in the woods this afternoon. And as for warming a Decca (and
> eventually some worms under the QTH) I have acquired a Hammond
> No. 93895 1.1kVA type "Q" transformer 117 to 55 volts.  I have no
> idea what a type "Q" is.  From what I've read it's a "control" xfmr,
> but this tells me little.  Will it melt if I use it to heat the
> Decca?  It looks a bit small for 1.1 kVA.  Interestingly, it's potted
> and waterproof!  Its maximum temperature rise is 80 deg. C. I'm hoping
> it will survive QRSS120.
>
> 73
> Joe