[Amps] Transformer Calcs

[Jim Thomson]

Date: Fri, 11 Jun 2010 11:08:10 -0700
From: Patrick Barthelow <apolloeme at 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