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Potted transformers

semitone

Member
2008-01-07 9:34 pm
Hi all. My first question (I am a new member) arises from the demise of the PT of my first amp that I finished last week. I basically rebuilt a Heathkit W3-AM using new parts, other than the original transformers. Other than aesthetics is there any other purpose for potting transformers? It seems to me that there is a fairly significant decrease in cooling properties over unpotted. I'm pretty sure the reason my Heath PT failed was due to too much idle current being drawn by the amp. I'm pretty sure it will be difficult finding another (Heath part # 54-13) in the near future. Hammond has a close replacement that would fit in the old "pot" but Hammond recommends against potting their transformers. Any thoughts on this? Thanks. -Paul
 
Semitone

As you noted the problem with potting a transformer is heat. A 1/4" of epoxy raises the hot spot temperature by 5 C, a layer of metal, end bell or can, raises it another 5 C. Hammond makes "commercial" transformers and so they are already running their devices at about a 65 C rise, in an ambient of 20 C.

A total of 85 C is not going to hurt the insulation system in a modern transformer. Modern class B materials will tolerate 130 C total temperature for 300k hours, mtbf. However, the 90 to 95 C temperature in your amp is going to degrade a lot of other components. Also, external box covers, over the entire device, add another 5 C.

Back in the day, the transformers were not able to operate at these temperatures, for the extended mtbf and so were designed more conservatively.

If you want to pot the Hammond in a new box, then a thermally conductive filled epoxy must be used. I do not know of any that are self curing so you will need to have a transformer company, with dedicated ovens bake it out for you. You cannot do this in your kitchen oven, period, don't even think about it.

Probably a better path is to look through the Plitron catalog for seperate B+ and filament toroids. They will almost certainly both fit into the box, have lower emitted fields and run cooler than one single E/I transformer. For power, it really is hard to beat a toroid. For audio signal, the two formats are much more evenly matched.

Anyway, welcome to the forum. Scout around a bit, there is an amazing amount of information available and a ton of polite folks willing to help, or argue with you, if you so choose.

Bud
 
I have successfully repotted a heathkit power transformer recently. I baked the old transformer out with a blow torch (goo!) and repotted a stancor PC8411 using medium casting resin from ebay. The transformer was from the infamous (for bad power transformers) W4 series. Works like a charm so far, and looks stock...well almost..I have to repaint it, but I'm too busy listening to it. I have recently devised a new plan for new metal potting cans, and will report back when I get the parts and try it out.
 
1. Magnetic and electrical field shield.
2. Protection against humidity and micro-organisms.
3. Protection against mechanical damages.
4. Aesthetics (questionable: depends on a taste; for my personal taste what is more functional that looke better. I don't like a metal painted and looked like a wood, neither a stove in a gear rack; when a material reflect it's properties I like it better).

However, thermal resistance of a compound matters increasing an overheating of a wire and it's insulation that may be damaged by high temperature, but an air gap between a transformer and a pot is usually less thermal conductive.

I would rather put grills on tubes to protect them mechanically and kids from their heat.
 
semitone said:
Hi all. My first question (I am a new member) arises from the demise of the PT of my first amp that I finished last week. I basically rebuilt a Heathkit W3-AM using new parts, other than the original transformers. Other than aesthetics is there any other purpose for potting transformers? It seems to me that there is a fairly significant decrease in cooling properties over unpotted. I'm pretty sure the reason my Heath PT failed was due to too much idle current being drawn by the amp. I'm pretty sure it will be difficult finding another (Heath part # 54-13) in the near future. Hammond has a close replacement that would fit in the old "pot" but Hammond recommends against potting their transformers. Any thoughts on this? Thanks. -Paul


You could call edcor or heyboer and tell them your situation, they could recommend an overrated transformer that would still fit in the can (you might have to specify dimensions to them) and then do what I did to pot a new one in there.
 

semitone

Member
2008-01-07 9:34 pm
Thanks!

Such great feedback. Thank you. Based on your responses, I'm torn between trying to keep the look of my original design with the big potted OT in between the potted OT and choke, or just going for the simplicity of the naked PT. SO, Boris-the-Blade, you say the PT on the W4 was notoriously bad. Was it the same as the 54-13 on my W3? 'Cause that would make me feel a little better about having blown mine up!

Hmmm, so what do you guys think about me removing my dead Heath PT from the pot, mounting a new PT in the chassis (I am going to look into the sources you suggested), adding vent holes around the PT on the chassis, adding vent holes to the top of the old pot and then mounting the pot over the new PT. So, it's kind of an unpotted-potted transformer? I'd think with adequate ventilation this would work. Is this rookie thinking?
 
BudP said:
Semitone


If you want to pot the Hammond in a new box, then a thermally conductive filled epoxy must be used. I do not know of any that are self curing so you will need to have a transformer company, with dedicated ovens bake it out for you. You cannot do this in your kitchen oven, period, don't even think about it.


Bud


Guess who found out this the hard way while a teenager, and stunk out the whole house for a month!


:bawling:
 
semitone,

If you can remove the old transformer, without deforming the can or having to use a blow torch to remove old potting material. then there is no no other reason why you should not perform this swap.

I would not bother with cooling holes anywhere. A single metal cover is only going to raise the hot spot temp by 5 C. This should not be enough to cause failure in Hammond's product. If it does, they need to be notified quickly, because a serious mistake has been made in design.

Again, modern materials allow a much higher operating temperature than did materials as recently as 1980. If you were planning on using another NOS transformer for this, I would be much more uncomfortable. Hammond is a reputable manufacturer, but they are also a competitive manufacturer and the truth is, their customers are more than willing to trade heat, for money taken from the BOM.

My suggestions about toroids was to get you to seperate the high current low voltage, from the low current high voltage windings, by putting them on seperate core. This always sounds better in an audio amplifier, especially one based on minimal money power supply design. This being where large amounts of capacitance is traded for a power transformer with less than 3% no load to full load regulation and just enough capacitance to remove ripple.

Other than all of this verbiage I see no other problem with your scheme. Just make sure you are not exceeding the current draw limitations of the transformer. Psud2, with the no load sec voltage and DCR of primary and secondary entered in the edit box, will provide a very good estimation of true RMS AC current draw, for the transformer. The other acceptable method, is to calculate the primary AC current draw, plus DCR losses, on both sides of the transformer windings, calculate the resultant heat rise. Then measure the primary AC draw, under max duty, with an iron vane meter to make certain there is not excess current being drawn by the load. Psud2 is easier and faster and can be found here, if you don't already have it.http://www.duncanamps.com/psud2/download.html


Bud.
 
Just a general note on transformer temperature rise and user safety.

Eight hours of rated load are required for thermal equilibrium, all materials are the same temp. The first two hours will see 80% of that rise with the first hour seeing 80% of that second hours temp rise.

When UL, CSA or CE/VDE test a transformer, for suitability to use, in a specific application, they load all secondaries to full load for eight hours and then one by one short circuit each secondary until either the thermal limiter trips, all the short circuit tests are completed, or the transformer emits enough smoke or liquids to stain a piece of cheese cloth under and over the unit under test, at which point it has failed.

When one of these conditions is met, the unit is immediately subjected to dielectric strength tests, that are applicable for that category of usage. Then the unit is dissected to see if any of the safety shields, bobbin walls or other required materials have suffered deformation, incineration or physical movement. Then creepage and clearance requirements are judged.

The dielectric strength tests are, for the vast majority or categories, comprised of applying 3750 vac between primary and ground and primary and secondary, for one minute each, but ramped up to that voltage, in that order. Then a secondary to ground test is applied, but it rarely exceeds 900 vac,.

If the transformer passes these tests and has a recognized and previously tested insulation system, it passes.

The BOM driven folks in the world want something that will pass this sort of rigor, for cheap.

Bud
 
BudP said:
Boris,

Please contact GE's tech support folks and ask them about thermal generation during cure and what effect different material thicknesses have upon their temp rise. This looks like a conformal epoxy coating, intended for circuit boards and perhaps not aimed at a filled container with 1/2 inch thick filled volumes. Just a a precaution.


I realize that, but I'm not going into any commercial business with this. 3M does make an aluminum oxide mix for greater thermal conductivity, but it is mucho expensive, at least at most places I can find it...perhaps I could find some aluminum oxide power and blend it when I try this....this is diy audio after all...
 
Boris,

Sorry, I was only concerned about heat generated by the compound itself, as it self catalyzes. Not whether you should or should not use it, because of lack of thermal conduction. Just how hot will it get when it is in a situation where volume to surface area causes the internal heating of the resin to exceed about 250 F. Unless I am misunderstanding the intended usage, you may have a greater amount of material catalyzing as a block than they recommend. Potted transformers, using a self catalyzing resin have been known to burst into flames, from the catalytic heat, of too thick a coating of resin around the transformer.

I am also just adding this cautionary note for individuals who, unlike you, might not have done their homework.

Bud
 
BudP said:
Just how hot will it get when it is in a situation where volume to surface area causes the internal heating of the resin to exceed about 250 F. Unless I am misunderstanding the intended usage, you may have a greater amount of material catalyzing as a block than they recommend. Potted transformers, using a self catalyzing resin have been known to burst into flames, from the catalytic heat, of too thick a coating of resin around the transformer.

Right you are, judging by how hot the heathkit W4 transformer became while curing, I would suspect that a fast casting (under 1 hour) resin would become extremely hot. The medium casting resin I was forced to use before (they stopped stocking slow casting resin) became moderately hot, but within reason. No flames:D It was a trial after all, and it worked out good enough to use permanently. Of course, onthe old stancor PC8411, all leads are cloth and no melty innards (no PVC). At my old job, i've used better potting materials (casts over maybe 8 hours, and I could outgas there), but I can't seem to get them in low qty as a hobbyist. bummer, eh?
 
BudP said:
Just a general note on transformer temperature rise and user safety.

Eight hours of rated load are required for thermal equilibrium, all materials are the same temp. The first two hours will see 80% of that rise with the first hour seeing 80% of that second hours temp rise.

(big snip)

If the transformer passes these tests and has a recognized and previously tested insulation system, it passes.

The BOM driven folks in the world want something that will pass this sort of rigor, for cheap.

Bud
Bud,
When did this transformer testing start and what seal or mark does a transformer that passes the test get?
 
Kevin Graf

The typical UL stamp seen on a transformer, can only relate to the insulation system used. At one time, a construction listing was available, that allowed you to "group" transformers within a VA rating for a stand alone approval, based upon construction details and materials, from testing two examples from the extremes within the range. This was cheated on pretty severely by the bottom feeder transformer mfg's and UL eventually removed this as a category.

Now, with all safety agency approvals, the transformer can receive an approval only in context of the end product. Except for Class 1 and Class 2 electrical transformers, for specific uses in furnaces and door bells, etc.

The insulation system test is pretty rigorous. They take a glass tube with a twisted pair of appropriate insulation system class coil winding wires, of #18 gauge and the amount of impregnated coil dielectric materials that would be found in a coil, on one layer, on both sides and out to the next wire tangent to the insulation for these two turns of this wire size, in a VA size that would use this wire size.

So, a good few square inches of material. They thermally age this evacuated tube for 7 days and heat it to a temperature 10% above the maximum rating of the desired temperature class for that time. The wires are then pulled hot and hit with a dielectric withstand voltage, in rising steps, to see where it fails. If it fails at the same point that a cold control pair of twisted wires, from the same spool of wire, on either side of the lengths that went into the tube, fail, the test is successful.

Insulation and impregnation mfg's have sponsored tests using their materials and any mfg can buy into those tests for their own insulation system. This does not in any way imply that the transformer in question will pass the dielectric strength, overheating or construction requirements for any UL approval in any kind of usage. Just the insulation system is approved, if the mfg actually follows it, not the design, or your eventual usage. This is not to imply that mfg's are cheating, when they put a UL approved sticker on their transformers, this insulation system approval is the only one they can get. All other approvals are only in finished goods.

Bud
 
Well, in the usual transformer, the copper loses are about double those of the core. Adding a 1/4" of anything to the coil will bring the hot spot temp up 5 C. With endbells that are not form fit, you will have about 3/8" of whatever you use to dampen the noise.If you add a goo plus aluminum sheet to the endbells it will still add about 15C to the hot spot. The end bells will ring but you can stop most of that with a crdboard tube made of corrugated packing cardboard wedged end on between coil and endbell or a suitable rubber grommet glued to the coil face and compressed slightly between the two surfaces. OR just get rid of the endbells.

Depending upon the VA and the type od hum another trip through a potting tank might be the best bet. Any B+ transformer over 250 VA is going to have a growl. This comes from the shrinking and swelling of the core sheaves and there is nothing to be done about it. A buzz is usually a loose I piece and if you run a bead of super glue over the gap joints and across the long side of the core stack, where the I's are, this may kill that noise.

The coil, which you are talking about coating in something, does not make an audible noise.

Something to do is to energize the transformer and hold it up in the air, on finger tips if possible. If the unit does not hum then, the real problem is one of shrinking and swelling mass and bending drum head chassis. Try mounting the transformer on a couple or three small as possible steel washers, or mount it on rubber hole grommets.

Bud