Date: Fri, 11 Jun 2010 11:08:10 -0700
From: Patrick Barthelow <apolloeme@live.com>
Subject: Re: [Amps] Transformer Calcs
For a KW level HV transformer, is there much difference between the temp that
you can touch and measure, say at the outside laminations, and perhaps the more
important temperatures in the core center, and the copper wire, bobbins,
windings, and paper/plastic insulation materials that we cannot see, inside?
## yes... typ inner windings are a good 10 deg C hotter than outside
windings.. and sometimes 15 degc C. They have stuffed sensors in there.. and
measured all this in the past.
And, a related question, from a cold room temperature start, if you could
measure the surface temperature of the laminations of a, say, Heathkit SB 220
amp that was being used with a lot of transmit time, on CW at 1KW out, How
long would it be before the outside lamination temperature could give a
representative temperature of the more critical inside core temperatures?
Could there be, say, 50 degree F temperature difference between inside and
outside when the Amp is producing 1KW RF output? If you can cool the outside
of the transformer with continuous airflow from a muffin fan, or better, would
that reduce significantly the inside core temperature?
## U can read all abt this on various commercial pole pig / vault xfmr make's
tech notes. Dry xfmr's can have their ratings increased a whole bunch with
forced air..... a lot more vs trying to air cool a xfmr in oil.
[which is also done].
## dahl rated his plate xfmr's for a 55 deg C temp rise over a 50 deg C
ambient. That's 105 deg C on the outside.. when maxed out.. which is HOT.
That also allowed for 'spot heating' of another 10 deg C fro the inner
windings.
## dahl's insulation was rated at a lot more than 105 deg C though.. he did not
go cheap on the insulation.
## some commercial vault xfmr's /pole pigs will rate em at as much as 165
deg C temp rise over a 40 deg C ambient... now that's really hot. The
idea here is.... if u want to cut costs.... use smaller ga wire on the pri/ and
/or sec.. and use higher temp rated insulation. .. = lower cost. We had a
pair of 3 phase step down vault xfmr's in one telco.. that dropped 600 vac
down to 208 vac... and talk abt hot. They were well over 290 deg F all the
time.. and convection cooled. You then lose it in air conditioning costs in
the summer time.. since the rooms these things are in are a blast furnace.
## bottom line is... if the outside casing of a plate xfmr is hot.... the
inner windings are even hotter. If you have a blazing hot xfmr inside a
poorly ventilated cabinet.... some airflow from a fan helps a lot. No point
in having the inside cab temp surrounding the xfmr sitting at say 50 deg C....
when the external cab temp is only 25 deg C. The fan will reduce the
internal cab's ambient air temp down to 25-30 deg C... and the xfmr's
outer/inner windings will also drop 20-25 deg C.... and prolong the life of the
xfmr. Or conversely... you can run more power from the xfmr.. by air
cooling it. Size the xfmr right the 1st time around.... and it will run
barely warm, if that... and last forever. This all depends on mode rtty vs
ssb........ and room temp etc.
later... Jim VE7RF
Best Regards,
73, de Pat Barthelow AA6EG
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