Power transformer fails

[martin manning]

I was suspicious before, but now I can see..everyone who commented something about voltages, power and currents was wrong :P

How do you reach that conclusion? Looks like the HV winding ran hot long enough to melt out the varnish and destroy the insulation.

[bepone]

I was suspicious before, but now I can see..everyone who commented something about voltages, power and currents was wrong

How do you reach that conclusion? Looks like the HV winding ran hot long enough to melt out the varnish and destroy the insulation.

you can see on the picture carbonisation on the wire exit, nude wires, where it started IMO,

[katopan]

Cool photos.

[Reeltarded]

Reminds me of the In 60 Seconds disaster show.

YiKeS!

[bepone]

Finally found some time to rewind this transformer.. primary finished.

How the time is not limited, will be several varnishing as i have full can, today after primary layer, and tommorow after secondaries

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[R.G.]

We always varnished by dipping the finished transformer into a bucket of varnish and pulling a vacuum on the bucket for an hour or two. This was done specifically because the varnish did seep into the wiring layers, even if covered by tape, and so the build up of multiple layers of varnish on the coil former did not cause problems with stacking the laminations into the coil after varnishing. It took a long time to remove the built-up varnish.

I see you're using a plastic bobbin. That would make removing the varnish easier, I suppose.

We tested for full penetration of varnish into the wiring layers by cutting several of them open with a bandsaw to see how deep the varnish got. Vacuuming worked very well. You have to be careful not to pull such a high vacuum that the varnish can collapses. We used an old cooking pressure cooker after the first few collapsed can disasters and cleanups. We left the excess varnish in the cooker, and valved in a little CO2 from the lab supply so it didn't skin over between times.

[bepone]

This one is air drying varnish..and for me is enough to cover exposed external parts.. internals i wound very tight and is not important at all for varnish to penetrate there..

Also insulation i choose double thickness, impregnated paper which every layer added is multiplying breakdown value, wire is double enameled (4kV) so transformer will work probably lifetime in this way..

Soak primary and cure construction first, and after is easy for secondary

[R.G.]

I guess it depends on what you're trying to achieve with the transformer. If you're OK with not getting better heat transfer and lower internal temperatures, that's fine.

From your post, it seems like you're using varnish just to hold the windings in place ("Soak primary and cure construction first, and after is easy for secondary") and not for the other uses of varnish. I'm guessing that you rely on the wire insulation and paper ("double thickness, impregnated paper [...] wire is double enameled") for voltage resistance. Again, that is OK if your uses for the transformer don't require the other things that varnish filling gives you.

We were taught that complete filling with varnish (or today - non-solvent resins, like some epoxies) did four things: mechanical stability against vibration, to prevent wearing through the wire insulation; heat removal improvement by displacing the air between the wires; voltage insulation; and long life by excluding moisture. The moisture thing was tricky - it gets in by condensation as well as water splashing on the transformer. Transformers with long times of not being powered up and heated can grow molds in the air pockets inside and degrade the insulation on the wire, especially in tropical places. In the first half of the 1900s, transformers were impregnated with beeswax, and this could melt and run back out if the transformer overheated, as well as supporting mold growth.

But we were designing for manufacturing products to go to any climate. If your transformers are for intermittent use in a non-tropical climate, it probably is fine.

[bepone]

this will all be ok..

second varnishing after secondary wound.. TR is soaking the varnish as seen by bubbles..

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[bepone]

transformer is finished, but why not one more pot again?

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[bepone]

trafo is getting new life, and for sure will be used in some Dumble circuits (maybe Bluesmaster!)

[Oddvar R]

Or just use a regular transformer and add external series resistance to taste.
Copper losses should be pretty straight line linear, so a resistor should provide a very close result.

How would I do that, resistors in series?

[pdf64]

Or just use a regular transformer and add external series resistance to taste.
Copper losses should be pretty straight line linear, so a resistor should provide a very close result.

How would I do that, resistors in series?

Yes.
Try measuring the resistance of Weber's primary and HT windings, and compare to those of your replacement transformer.
Then add suitably beefy resistors to the HT side.
If there's any appreciable difference in the primary winding resistance, that can be ratioed up (by the primary to secondary voltage ratio) and added to the HT resistance.
Ideally the Weber should probably be measured when it's hot

[R.G.]

A power transformer can be modeled as an ideal transformer plus "imperfection" parts added. An ideal transformer has zero winding resistances, infinite primary inductance, no core losses or leakage and no leakage capacitances.
A real transformer has primary and secondary wire resistances, which can be measured by a suitable ohmmeter. This is only modestly complicated by the low ohms of low voltage windings, which may need special low-ohms meters to get the right number, but it's pretty good for 120V and higher windings.
A real transformer has primary inductance and core losses, which can be estimated very closely by measuring the no-load/open-secondary primary current. This is the magnetizing current, which can be thought of as the price you pay to keep the magnetic field going and have the transformer work. The core loss is generally so small that it's ignored if the transformer designer has done a good job of not driving the iron too hard. The primary inductance is estimated by L = Vprimary / 2 *pi * F where F is the line frequency. For most purposes, the primary inductance is ignored, as a good design will make this only a few percent of the transformer's power rating.
The turns ratio can of course be measured directly: Ns/Np is equal to the secondary voltage divided by the primary voltage under no load conditions.

With the turns ratio, the primary wire resistance, and the secondary wire resistance, you can estimate very closely what the transformer will do under load. A load on the secondary will cause some secondary voltage sag because current through the primary wire resistance drops the primary voltage available to the "ideal transformer" hidden inside the real transformer. The ideal transformer secondary voltage is then a bit lower, and the voltage to the load drops again when the ideal transformer secondary voltage travels through the secondary wire resistance. This is in fact the way that power transformers were designed with before computer models - say, before the late 1970s.

One final benefit from the measured wire resistances - they are an effective thermometer to detect the internal temperature in the transformer. Copper's resistance increases by 0.393% per degree C. If you know the resistance at room temperature, you can run the thing under full load for a long time (it can take hours for a heavy lump of iron and copper to reach temperature stability), then disconnect the primary and re-measure the primary resistance. If it went up by 3.93%, the wire got 10C hotter.