[Amps] HV transformer buzzing!! Help Please

[Manfred Mornhinweg]

Hi Pat, and all!

> Which raises another question...I have heard that over the long term, the 
> failure mechanism of high power, high voltage transformers usually finds 
> heat as the antagonist.  What is the mechanism?

Usually the insulation breaks down from heat degradation. Many of the 
transformers I have rewound had the paper charred, and the wire 
insulation flaking off.

 >  I have  felt the core
> frames,  of Kilowatt Amp power supply transformers at the time of heavy 
> sustained service, and they are REALLY hot.  That is the outside surface.  
> Would the internal core laminations be much hotter?  How hot? 

The laminations will not be very much hotter inside than on the surface, 
because silicon steel is a reasonably good thermal conductor. But the 
wire deep in the middle of the winding assembly will be much hotter! The 
electrical insulation is unfortunately a poor heat conductor, and on top 
of that the wire typically generates much more heat than the steel. So 
the hot spot is in the wiring, and not the steel.

 > Does that
> kind of heat eventually char inter winding paper insulation wraps, causing 
> conductive paths, and failure,

Yes, that's quite common.

 > or does it maybe melt plastic Bobbins that
> hold and insulate  the widings, also causing failure?

This also happens, but is a rather extreme case. When the bobbin melts, 
usually something else has failed before.

It's good to know that transformer insulation material exists in several 
temperature classes. The humblest of them is class A, which can work up 
to 105 degrees Celsius. The typical pressed paper (known by the German 
name "Pressspan" much over the world) falls in this class. Significantly 
better is class B, good for 130 degrees. Mylar falls into this class. 
There are several higher classes, some reaching about 180 degrees 
Celsius, using rather expensive materials such as Nomex.

The wire insulation also comes in several classes. Thin wire usually 
comes with an enamel that turns into solder flux at normal soldering 
temperature. This insulation of course cannot take very much heat in 
normal use. I'm not sure now, but I suppose it's something like 120 
degrees. The other kind of enamel, which cannot be soldered through, is 
able to resist any temperature the materials above can take.

So, depending on mostly on the insulation material, a transformer might 
work at maximum internal winding temperatures varying from 105 degrees 
to about 180 degrees, but the lower grades are the more common ones. 
Depending on the size and impregnation level of the transformer, and on 
air motion around it, this might equate to surface temperatures between 
perhaps 50 and 80 degrees Celsius.

Another failure mechanism is loose wires in the winding vibrating until 
they chafe through their insulation and form shorted turns. Then they 
will get very hot and burn any insulation, even if the transformer is 
not running hot!

> When building amps, or enhancing reilability of commercial (ham)  amps, is 
> it a worthy cause to add extra airflow, into/around the power transformer?  
> with a  wide selection of cheap, tiny "muffin fans" perhaps you can place 
> them strategically near/on/in your power transformer and forstall a 
> replacement during your lifetime.
> Same... with HV electrolytic caps, any reason to keep cooler?

It would probably help, but it would be a noisy kludge. I prefer to 
trust that the builder of the amp did the math correctly, and designed 
the amplifier to last. Additional beefing up should come in only when 
you want to use the amplifier outside the specified duty cycle rating.

> Another question.  Has anyone quantified the temperatures involved when a 
> technician does the "thumb" test by touching a component of an electrical 
> device? 

I prefer not quantifying them, because this thumb test is very silly! If 
you don't know what temperature the part can safely take, and how much 
hotter it's inside than outside, then it's meaningless to measure the 
surface temperature. Be it by thumb or with a real surface thermometer 
(which, by the way, are available very inexpensively).

> I know from experience, say, touching a normal 2 watt resistor in a 
> circuit dissipating a couple of watts.  In my opinion, it should be 
> uncomfortably warm, say 2 seconds max with my thumb.. that might be normal.  
> If I instantly recoil, with an ouch!  I know a clue is there that something 
> is wrong, that there is a lot more power/heat there.

Unfortunately this method is not correct at all!  In olden times, 2 watt 
resistors measured about 5cm long and 1cm diameter. These were carbon 
composition ones, which changed characetristics if they got too hot, so 
they had to work quite cool, and that forced manufacturers to make them 
big to get that 2 watt dissipation rating. Such a resistor should be 
quite warm at most, but never uncomfortably hot. If instead you take a 
modern glassed metal resistor rated at 2 watts, this thing measured only 
about 1cm long and 4mm diameter, and it will work burning hot at 2 
watts. Your method would take this temperature as abnormal, but it is 
not! These resistors can work at that high temperature for many tens of 
thousands of hours.

 > I remember in the
> olden days, as a kid, putting my stocking feet on the power transformer of 
> my Drake TR-3, under the operating table.  If I was transmitting a lot, it 
> was really hot.

The method is useful as a RELATIVE test: If you usually heat your feet 
on the transformer, and one day you notice it's getting much hotter than 
usual, you know something has to be checked. But as an absolute test, 
it's useless.

> So, using all due caution, doing a thumb, or palm test, what might be the 
> temps of the following:
> 
> Transformer frame, Palm on frame,
> Hot, 3 seconds max comfort zone:
> Hot  6 seconds comfort zone:
> 
> Two watt Resistor (CAREFUL, NOT in HV Circuits)
> Mildly warm, 8 seconds comfort zone
> Very Warm  3 seconds comfort zone
> Hot, 1-2 seconds
> VERY HOT, 1/2 second, painful recoil

This cannot be calibrated, because different people react so very 
differently to heat! My mother is able to take a cooking pan from the 
stove with her bare fingers. I absolutely cannot do that! For her, it's 
hot but endurable, while for me it's way into the red zone!

Manfred.

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