From: John Woodgate
Subject: Re: Doubling wallwart power rating
Date: Tue, 8 Oct 2002 17:11:59 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Tue, 8 Oct 2002 17:47:29 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Tony Williams
wrote (in <firstname.lastname@example.org.
uk>) about 'Doubling wallwart power rating', on Tue, 8 Oct 2002:
>In article ,
> Phil Allison wrote:
>> ** I can view the primary current wavefrom
>> - pure magnetising current.
> Interested question Phil. Was the no load current
> purely in quadrature, or was there a significant
> inphase component.... indicating large iron losses?
>> > If you design a transformer so that the core is deep into saturation at
>> > nominal mains voltage (which is what is implied by the input current
>> > being the same on no-load as on full load), it WILL overheat under
>> > safety testing at high mains voltage (+6% or +10% depending on the
>> > standard; one even requires +15%!).
>>** Well, plug paks tend to get 24 hour, 365 day
>> usage and often with low or no loads.
> Best efficiency is when iron losses = copper
> losses, but never possible at 50Hz. Or is it?
Yes, it is possible and supply system transformers achieve it. But small
laminated EI cores are 'scrapless' - two E's and two I's make up an
exact rectangle. This doesn't give enough window area (which determines
the copper loss) to allow the losses to be made equal unless the core is
run deep into saturation at nominal supply voltage, which is bad news if
the supply volts go up, as I pointed out in an earlier post. In any
case, without heroic efforts at ventilation, such a transformer runs too
> I do believe that this method of designing/using
> transformers is an attempt to get the no load
> iron losses near to the full load copper losses.
> After all who says that a transformer should only
> run hot at full load..... if it runs as hot at no
> load, then this must be a condition of minimum
> iron in the design.
Well, it's the condition for maximum efficiency. But two things make it
inadvisable as a design goal:
- the rapid increase of iron loss with supply voltage, that I already
- the variation in the properties of the iron from one batch to another.
Modern heat treatment methods have reduced the variation (we allowed
about 20% to 50% upward variation, according to core size, in the 1970s,
and that was at 1.2 T!), but it's still there. Maybe Phil Allison has
some magnetizing current test data on a batch of nominally identical
samples. I did have some, but I did that work about 14 years ago and I
can't find it now.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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