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Old 22nd Jun 2018, 1:44 am   #21
Argus25
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Default Re: Hot LOPT's!

Quote:
Originally Posted by 1100 man View Post
I put a shorted turn of wire round the winding which killed the ring completely. I had never tried this with a working transformer powered up however.
I put a turn of 1.5sq mm pvc insulated wire round the overwind and soldered the ends together and switched on. The transformer struggled manfully into life but there was barely a glow from the U26 heater and a large, dark picture. After 30 seconds, smoke started to come from the insulation on the shorted turn: I switched it off!!

So I would doubt that there are any shorted turns in the overwind
I still wonder because that wire you used for the shorted turn would have a much larger cross sectional area and lower resistance than the wire the overwind is made from. Try a few turns of similar gauge wire to the overwind shorted out and repeat the experiment. Probably it will melt off the enamel, but the test would be to see if it fused or not, and how much that dropped the EHT, compared to the current loss of EHT (4kV) you already have documented.
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Old 26th Jun 2018, 2:29 pm   #22
Philips210
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Default Re: Hot LOPT's!

Hi.

One thing that might be a possibility is a crack in the hot core which may not be evident until the LOPT is dismantled due to it being obscured by the overwind.

Regards,
Symon
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Old 30th Jun 2018, 11:26 pm   #23
1100 man
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Default Re: Hot LOPT's!

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Originally Posted by Argus25 View Post
I still wonder because that wire you used for the shorted turn would have a much larger cross sectional area and lower resistance than the wire the overwind is made from. Try a few turns of similar gauge wire to the overwind shorted out and repeat the experiment. Probably it will melt off the enamel, but the test would be to see if it fused or not, and how much that dropped the EHT, compared to the current loss of EHT (4kV) you already have documented.
Not having any enamelled copper wire (I don't do coils!) the finest I could find was a single strand from some mains flex. This was at least in the same ball park as the overwind but still a little thicker.
With either a single or double turn, the wire glowed a dull red. The EHT rectifier was not lit.
I'm not sure if one layer of winding shorted to another layer would cause more or less effect than this?

I still can't get the core apart even when hot. The coil former of the primary won't budge and I'm sure that's what's holding the two halves together.
I would still like to get it apart just to satisfy my curiosity!

Cheers
Nick
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Old 1st Jul 2018, 10:53 pm   #24
Philips210
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Default Re: Hot LOPT's!

Hi

I wonder if a tool could be devised similar to a puller. This could consist of a couple of semi-circular section plates to fit around the core halves. Each plate would have a recess to accept a rotating stud. A long hex nut, located between the two plates, would accept a stud each end. Tightening the studs would then force the plates apart and hopefully separate the cores. In order to turn the studs, a couple of nuts tightened to a stud would work. It would be risky to apply too much force, the last thing you want is a cracked core.
If a simple puller could be used then very minimal tension should be applied in conjunction with a little amount of heat. This should work if care is exercised.

Regards
Symon.
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Old 1st Jul 2018, 11:24 pm   #25
Argus25
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Default Re: Hot LOPT's!

I would avoid applying any tool to the core surfaces or local forces from a tool. Its probably only safe if you can pull it apart by hand. I would place it in a convection oven to about 80 to 90 degrees C for 20 minutes, less than 100 degC, then with gloves on or a cloth, try to manually separate the cores with a pulling and slight twisting action by hand.

I think with more shorted turns, say in series, since the turns per volt are the same, each one would get to a similar temperature, but the total power loss or dissipation would be higher forcing the induced voltage downwards. So you could conclude, probably, that if a whole layer was shorted out, each turn in that layer would not get quite as hot as a single shorted turn of the same wire. But since you got the single added shorted turn to red heat, it suggests if there were any overwind shorted turn/s they would have to be at least that hot too. However, they will be cooled by thermal coupling to other turns/layers of the winding, unlike the thin wire in free air you tested.

I cannot think of a mechanism how the core itself, or a crack in it, could be responsible for the local heat production which appears to be under one side of the overwind. That is if those temp measurements are basically correct. You mentioned the wax bubbling out of the overwind, is that happening on the side of the overwind where the core is hot ?
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Old 2nd Jul 2018, 6:41 pm   #26
PJL
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Default Re: Hot LOPT's!

Sounds like the usual problem of losses in the overwind (although in this case it is not 'over'). The primary is warming up due to the losses in the overwind. The liquid wax at the centre of the winding transferring heat to the core. As Ed says, the thermal coefficient of copper reducing the EHT.

Maybe we need an arrangement to ship them out to Arizona for a roast in the desert or a trip to space for desiccation. Maybe a visit to the chippy and drop them in boiling oil to remove the wax?
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