David
Have been watching this thread, looking for an answer and am throwing this
theory out for consideration.
Sometimes with motors they can actually draw more current running un-loaded
than under full load all due to the power factor of the design.
I recommend one of those Kill-A-Watt testers that are readily available for
about $20 on line everywhere as a very handy to have around the shack or
home to see just what is going on with an appliance. Displays power and
power factor on 120vac loads. Realize you have 240 volt transformer but
would be interesting to see what readings are running on 120volt just to
test load and no load conditions.
Also, if you could look at sample of primary current on a scope that could
be a real eye opener.
A true rms clamp on meter measuring primary current ---another possibility.
Just some ideas for consideration
Paul K0UYA
----- Original Message -----
From: "David Craig" <davidhcraig@verizon.net>
To: <amps@contesting.com>
Sent: Wednesday, February 11, 2009 10:37 PM
Subject: [Amps] Transformer heating (problem?)
> 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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