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From: "Peter Gottlieb"
References: <0001HW.B9DF7C5F00256614162B2870@news.covad.net> <firstname.lastname@example.org> <0001HW.B9DFECA2002E26AD162B2870@news.covad.net>
Subject: Re: Boosting output from 120V inverter
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Mon, 28 Oct 2002 00:26:57 GMT
NNTP-Posting-Date: Sun, 27 Oct 2002 19:26:57 EST
A lot of those transformers are varnish or epoxy impregnated and the oil
will not circulate freely.
Big UPSes are available cheaply at hamfests. Why go through all this to try
and boost a tiny one?
"N. Thornton" wrote in message
> OK. Lets go...
> First, loading up the TF will tend to take it out of saturation, not
> into it.
> Re the difference heatsinking makes, lets look at TF heat paths. Youve
> got copper winding, which is an around 80% solid copper block, with
> the remaining 20 or so % being mostly air. So thats a good heat
> conductor. Then youve got the metal / air interface, which we know is
> a poor conductor of heat. Youve also got the iron core which is
> intermediate in conduction.
> So the big heat loss block is the metal / air interface, and fan
> cooling increases that heat flow point greatly. Thats why fan cooling
> makes such a big difference.
> Lets throw a number up. Say the breakdown point is 200C, and the TF
> gets upto 100C on its outer winding surface - I'm picking easy numbers
> here. Now if we reduce the outer winding surface to 50C by fanning,
> the temp drop across the copper has gone from 100C to 150C. So the
> hotspot temp wil be the same with 1.5x the power diss in the TF.
> And with heatsinks and aggressive fanning one could hopefully do
> better than that.
> Actually no, I assumed there all the hewat was dissed at the hot spot,
> whcih of course it isnt. So we should get better figs than that IRL.
> Bottom line, I think its do-able, but of course not risk free.
> >I couldn't have said it better myself. It's foolish to risk damage.
> I'd say it depends what youre driving. If youre powering the fridge or
> a light I wouldnt worry unduly. The control tronics is not going to
> suffer from heat, and a brief soak test will check that runs properly
> - eg driving some light bulbs. From then on I doubt the thing is
> suddenly going to start putting out excess V, so you could run almot
> anything off it without worry I think. Power failure would be the real
> Obviously this is not a risk free project, and you live with the
> results - which will most likely be positive I tihnk.
> >>If you're real determined, theres always the outer edge option of
> >>putting the TF in a well heatsunk tin full of oil - no fan power
> >as long as you cool the oil somehow. otherwise it will just heat up
> >slowly and then stay just very hot for a long time. as you say,
> >passive cooling may work, just so long as you watch the temperature
> >the oil to make sure it stabalizes at a reasonable temperature.
> Yup. If the TF is put in oil in a tin, I'm thinking one automatically
> has larger contact surface with the air, plus considerebale thermal
> capacity of oil too. I would choose thin oil - well, I wouldn't choose
> oil at all really.
> >You have to consider that each winding in the transformer is an
> >(easily thought of)... Now each of these seperate inductors will be
> >designed such that they have a maximum saturation current equal to
> >maximum current the device is rated at. Any current higher than the
> >saturation current will begin to cause a much faster rate of heating,
> >will begin to lose serious wattage through the iron and copper losses
> of the
> >transformer. Couple this with the normally very tight windings of
> small (as
> >these kind of transformers are designed) transformers and you have a
> >heating problem. It's just not worth the extra effort...
> >Baloney! The transformers used in the typical low cost inverter (and
> I can
> >speak from experience here) are designed to run close to their
> >limits. Push them harder (even if you cool them) and they start going
> I've always understood that more load will reduce saturation effect.
> Certainly it will send copper losses way up, but I'll address that. Am
> I wrong?
> >You may not be able to get much more out of the transformer because
> >resistance and leakage inductance may be limiting the output power.
> Lets see, invertors are normally specced at around 90% efficient: now
> even if we attributed every last drop of that loss to TF copper
> losses, we could still get far more power output. I dont think R is a
> limiting factor myself.
> Could you explain how leakage inductance would affect it? Thats
> something I'm not so hot on myself.
> >Unless you understand the principles of high frequency transformer
> >and can redesign for higher power throuput, you should forget about
> >entire thing. It ain't gonna fly.
> Well, I bet it will.
> >Transformers generally aren't the best items to heatsink...
> They heatsink reasonably well. I've done it before, every microwave
> oven I've ever seen does it too. Try running an old zapper with no
> fan, it wouldnt survive long. The fan makes a sgnificant difference.
> I'd be interested to hear what happens if you try it, I am optimistic
> Regards, NT
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