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Old 10th Dec 2015, 7:19 am   #21
joebog1
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Default Re: Transformer parts

I agree David!!
A "cheap" chinese vane pump in australia will set you back about $3000
thats why a smaller vacuum and a $100 piston compressor works fine for me.

With respect
Joe
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Old 10th Dec 2015, 12:18 pm   #22
MotorBikeLes
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Default Re: Transformer parts

I have had my compressor for almost 50 years. I bought it off a fitter who had scrapped a pottery making machine which incorporated a vacuum pump. So my compressor IS in fact a vacuum pump by design. A piston type which I use an old 1/2HP 'fridge motor to drive. It is OK for 100+psi (my blow off is set at that) and 40+ years ago I used it with a small Devilbis gun to spray my car. Slow but effective.
Not intending to impregnate any transformers right now, but you never know.
Les.
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Old 10th Dec 2015, 12:44 pm   #23
dominicbeesley
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Default Re: Transformer parts

So the next question is what lacquer to use. I know there are special one-part expoxy types available but as this is a one-off and I don't want to spend a fortune would something more readily available work. Originally I was just going to try dipping them in shellac and allowing them to cure at room temperature. Mainly because I have lots of left-over shellac!

Is it best to dip the whole assembled transformer or just the bobbin on its own and then the whole lot again to finish off once assembled. I can't tell if the noise is from the coil, the laminations or both.

D
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Old 10th Dec 2015, 2:28 pm   #24
kalee20
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Default Re: Transformer parts

Hi Dom, (Happy Birthday by the way!)

For minimum noise I have impregnated windings twice - one as a winding, once when assembled onto cores.

Transformer windings obviously have current flowing through - the more so when heavily loaded - and magnetic forces then exist between each length of wire which makes them move. So locking them with varnish virtually eliminates movement and acoustic noise. But the varnish has to go right through the winding.

Transformer laminations also move, due to forces between, but with a constant voltage applied the conditions in the core don't change with load, so core-created hum is constant. Varnishing will again cement the individual laminations together, eliminating buzz.

Finally, there's magnetostriction of the core material - changing in dimensions according to being magnetised - varnishing won't help here. But luckily it's small.

What varnish? Obviously, there are specialist varnishes available, try Wire Electric Supplies (Google them). Go for a solventless modified polyester alkyd varnish. Whether they will supply a litre or so remains to be seen. if not, I'd use a polyurethane exterior varnish.

Shellac I don't have much experience of, it does tend to crack but no doubt that's fixable with additives (linseed oil?).

One thing - a prolonged bake before impregnation (4 hours, 105 deg C) to remove moisture. Allow to cool but not get cold, a warm unit will help impregnant to flow. But a hot unit may start the impregnant gelling before it's fully penetrated.
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Old 10th Dec 2015, 4:07 pm   #25
dominicbeesley
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Default Re: Transformer parts

Thanks Kalee, and thanks for the birthday greetings!

As ever a full explanation for all things inductive...

I suspect that on these transformers it is mainy the lams that are noisy as I've just tried applying pressure to them and the noise changes. It will, like many of my other projects, have to go on the round tuit pile. So far my day off in the garage has ended up turning into buying a Christmas tree for my daughter!

The other reason I was looking at shellac is that it is relatively harmless, alcohol fumes, when drying. I was thinking of vaccum dipping then air-drying , however the more I think about it would the vacuum cause the alcohol to boil off the shellac?

I'll spend some time googling the available varnishes and the temperatures - I've no idea how hot I can get these Danbury transformer bobbins before they melt! I was thinking I could get my friend to stick them in his powder coating oven but I suspect they may get too hot...
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Old 10th Dec 2015, 6:52 pm   #26
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Default Re: Transformer parts

If they are anything like the noisy Danbury mains transformers I have here, they get unbearably hot to the touch in normal use.
Do you have a bobbin you can afford to "lose"? Just thought you could conduct an experiment to see what happens when subjected to a rising range of temperatures.
I am reading this bit of thread with great interest as I would like to try and immobilise the windings on my Danbury transformers to see if that quietens them. I have nothing to lose here and all to gain.
But i wont have access to any special tools or vacuum pumps, I was wondering if there was a method of just "steeping" the whole transformer in some sort of immobilising agent or varnish.
Ed Dinning has suggested immersion in Molten Paraffin wax but I am worried that it would just melt straight out with the heat from the windings.

Andy.
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Old 11th Dec 2015, 12:49 am   #27
joebog1
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Default Re: Transformer parts

What Kalee20 said !!! I tried to say that but my australian is not good enough

For biker, if the tranny gets so hot you cant touch it, its ( for me) a borderline transformer. Its probably well within spec, but will be running ( in my olde math) 500 circular mils for wire, and well over 64500 lines flux or in excess of one Tesla. This is standard modern proceedure and nothing is wrong with the traffo, its just made more cheaply and utilises the wire insulation rating, and the maxiumum flux density of the iron "to its maximum" .

Wax is OK but if the transformer runs hot anyway its gonna "shed" most of that wax onto the inside of the amp case, or onto your stereo cabinet, or both. ( Dont tell me the amp is on the french polished dining table or Ill be sending flowers )
PLUS when its new and gets hot it stinks and burns MY eyes, but some people seem unaffected.


Joe
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Old 2nd Apr 2016, 1:48 pm   #28
neonlamp
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Default Re: Transformer parts

I used to buy lams from Linton and Hirst, Swindon. They would sell M6, 0.35 grade (GOSS) suitable for audio and mains. About 10 years ago they were taken over and now seem only to sell toroids. Jemelec, Chesterfield, were able to supply both lams and bobbins but for the sizes I wanted next only in standard 0.5mm grade. Not so good for audio. Also, it may be better to phone them to discuss options.

For audio the split bobbin is useful, but don't wind the primary on one and the secondary on the other. You still need to layer both the primary and secondary on both halves to give the best coupling (lowest leakage inductance and balance for push-pull). For mains, the split bobbin is very useful in reducing primary to secondary capacitance.

If your transformer buzzes, it may be you have too few turns? I tend to limit Bmax to 1.2T for GOSS iron and 1.0T for standard iron cores. None of my transformers have ever overheated.

Cheers
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