[Amps] Transformer heating (problem?)

[Angel Vilaseca]

I observed the same phenomenon with microwave oven transformers. The 
core gets very hot when power is applied to the primary even for a few 
minutes, even with no load on the secondary. This is because the primary 
hasn't enough turns. This saves copper (and money), but the core gets 
hot because of saturation. The microwave oven relies on fan cooling. 
Having a saturated transformer also helps perventing overcurrent to the 
magnetron.

The solution I found for my power supply is to use two identical 
transformers with both primaries and secondaries in series. The cores 
remain cool now, with only half of mains voltage on every primary.

73 de HB9SLV


David Craig a écrit :
>     Corollary to a previous issue with a B+ transformer I passed thru the group a while back, which before I repaired it had a current loop due to bad/no insulation on the core bolts.  Transformer has a 240V primary, 2800-3000V secondary rated at 2 amp CCS.  Is installed & in use on a HB GS35b 6m amp.  When "module" testing the amp the transformer taps met specs, measured with a Simpson 260.  Calculations based on measurements then indicated a 12.5 to 1 ratio.  
>     Fast-forward to completed amp...After some occassional use (~30 minutes of total operating time) am having the following "issues."  The 3000VAC secondary tap is now only putting out 2700VAC with line voltage measured at 243VAC  (240V mains line is 10 guage wire, a 10 ft run from the breaker box & then 6 feet of 12 guage line to the amp...  original measurement taken with same lines).   The even more disconcerting problem is that after a couple hours of sitting at "idle" (amp on, power to B+ but no RF) the transformer iron is getting hot- enough that you wouldn't want to keep your hand on it for long, and particularly so near the left & right sides of the core. (Yes, I checked the plate meter & no current thru tube, bias relay fine). 
>      Lamination bolts remain cold so no more current loop around the mounting brackets....   I am baffled as to why this thing is getting hot just idling.  There is no "bleeder" per se on the PS, just 6 x 220k ohm Rs across each of twelve 450V caps in series.  The only other constant current draw is a 5 MEG dropping R to the B+ meter.  I am baffled as to why the transformer laminations would get hot with almost no current draw, and am concerned that once I really work this thing out during a contest or Es opening the heat is going to short the secondaries.
>     Any ideas as to what could be going on?  Is this normal, or as I am beginning to think, an 86 lb misdesigned piece of junk??? Any input appreciated.
>
>     73 Dave N3DB  
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