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Author Topic: HV transformer thermal shut down switch temp??  (Read 2575 times)
2E0ILY
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Posts: 131




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« on: December 01, 2012, 02:10:25 PM »

My HV transformer has a broken bi-metallic thermo switch bolted to the laminations. I am curious as to what might be considered a "too high" temperature for a 240 volt laminated core HV transformer, and whether it's worth finding a new one and using it as an additional safety feature? The HV supply will be remote from my shack room, so I might not smell smoke or see flames Smiley Thanks
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Best regards, Chris Wilson.
KM3F
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« Reply #1 on: December 01, 2012, 10:26:16 PM »

If your going to do that, consider the primary fuse size so it will blow if to much current is drawn or a fault occurs.
You should have the parts list to determine the cut out temp.
There are thermo switches of many different specs so you need to know before ordering.
The specs have cutout temp, how far down cooling temp has to be before it resets, how much current it's rated for etc.
You need to know this .
Good luck.
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K8AXW
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« Reply #2 on: December 02, 2012, 05:46:24 PM »

If your transformer is running hot enough to open the cutoff switch in the laminations, you're overloaded!  You can tell this by simply using the power supply for a period and feeling the transformer laminations.  If they're hot to the touch, then it's overloaded.  In order for most cutoff switches to open, the transformer has to be quite hot.
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W8JX
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« Reply #3 on: December 02, 2012, 05:59:41 PM »

My HV transformer has a broken bi-metallic thermo switch bolted to the laminations. I am curious as to what might be considered a "too high" temperature for a 240 volt laminated core HV transformer, and whether it's worth finding a new one and using it as an additional safety feature? The HV supply will be remote from my shack room, so I might not smell smoke or see flames Smiley Thanks

I would say no higher than 60C because o insulation breaking down at higher temps. Actually 55c would be better as windings will be warmer than core.
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KD0REQ
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« Reply #4 on: December 03, 2012, 09:56:56 AM »

so, what is the usual standard insulation temp rating for the magnet wire?  let's check a wire maker

http://www.temcoindustrialpower.com/product_selection.html?p=magnet_wire_faq#temperature

I seem to remember 55 C from long ago, and if you run the conversion, 155 degree class wire converts to 54.11 C.  you might expect the HV windings to have been done with 180 F wire for a margin, as a core will develop warmer spots, but figure low and live longer, right?

now, you need a thermal cutout at 55 C or lower.

http://www.nteinc.com/specs/8000to8999/pdf/TCOs.pdf  if axial lead "bullet" type

looks like NTE8076 gets you there.

http://www.nteinc.com/circuit_protection/disc_thermostats/disc_thermostats.pdf  bolted type

leading to the NTE-DTO140.

why would the HV transformer get that freaking hot in the first place?  overload.  start troubleshooting.  while you're at it, if you use a glass tube in your linear, check the bottom of the tube pins for solder gaps.  if you have them, you need to do two things... beef up the cooling,a nd replace the tube sockets.  wear on the socket results in crummy contact, which generates thermal resistance, which generates extra heat.
« Last Edit: December 03, 2012, 10:00:31 AM by KD0REQ » Logged
KE3WD
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« Reply #5 on: December 03, 2012, 11:05:24 AM »

I've seen quite a few instances of failed bimetallic breakers of this type with absolutely no problems with the circuit or device the breaker was protecting. 

Bimetallic button breakers are electromechanical devices and over time due suffer from failure. 

That failure can occur simply due to the daily flexing of the bimetal element during normal heatup and cooldown cycles.  In some failures, I have also suspected moisture intrusion may hasten failure due to corrosion or rusting as well. 

In all cases, when presented with such on the testbench, my standard routine has been to first bypass the bimetal breaker device and then bring the whole thing up on the Variac and Bulb to check for proper operation, proper Current draw, any signs of overheating of the protected device, be it a heatsink, a transformer or whatever and if the thing runs fine with no overheating for a period of at least a half hour I will go ahead and order and replace the bimetal breaker.  In certain instances, might set the thing aside on one of the burn-in benches for a longer session of running, maybe even all day, and recheck the temperature before replacing the thermal breaker. 

The breaker itself can and does fail, actually this happens fairly often, especially in app;liances that are pushing the operation life envelope into years or decades. 


73
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2E0ILY
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« Reply #6 on: December 03, 2012, 12:00:02 PM »

I have managed to decode a number on the original bi-metallic switch and it was a 75 degrees Celcius one. As the transformer is rated for continuous industrial usage and I won't be pushng it that hard, i have opted for a 60 degree Celcius one. Hopefully it'll never be needed, but it's a bit of additional insurance. The original switch was on the frame and used convection as much as conduction. I'll mount the new one directly on the core laminations. Thanks for the help. Oh, and yes, I have replaced the cracked Paxolin with some decent material, and cleaned it up and had it on heat soak for a few weeks Smiley http://www.gatesgarth.com/transformer.jpg
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Best regards, Chris Wilson.
WX7G
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« Reply #7 on: December 03, 2012, 01:02:16 PM »

The minimum temperature rating for materials in magnetics is 105 deg C with many specified to 150 deg C.

I would be comfortable running the core of any HV transformer at 80 deg C.
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W8JX
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« Reply #8 on: December 03, 2012, 02:10:25 PM »

The minimum temperature rating for materials in magnetics is 105 deg C with many specified to 150 deg C.

I would be comfortable running the core of any HV transformer at 80 deg C.


Maybe a new one but not a older one.
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