Why were old transformers coated with tar?

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I remember a lot of old transformers, particularly the power
transformers on the old TV sets, were coated with tar. It was not a real
problem, but was kind of ugly, particularly when the wires had tar all
over them.  

What was the purpose for the tar? My guess was to eliminate chattering
of the steel laminations. Yet, it seems the practice of using tar
vanished for the most part in the 70s and later. Yet I have not seen any
untarred modern transformers chatter......  

I will add to this, that the most chattering I have heard from
transformers are the small ones which do NOT have bolts going thru the
core, such as filament transformers and doorbell transformers. Several
times over the years I have had people ask me why there is a noisy sound
in their basement and it never fails, it's the doorbell transformer.  

This could often be fixed by squeezing the outer metal piece against the
laminations, and applying something like laquer or silicone caulk.
However, for the small price they cost, I usually just replaced them
with a new one.  


Re: Why were old transformers coated with tar?
On 2019-01-15 03:24:49 +0000, snipped-for-privacy@myshop.com said:

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You are most likley correct. As for tar, it was cheap and epoxy hadn't  
been invented yet.
CP


Re: Why were old transformers coated with tar?
On Tuesday, 15 January 2019 03:24:57 UTC, snipped-for-privacy@myshop.com  wrote:

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Modern transformers use clear stuff for the same purpose. It stops noises & holds the windings etc in place.


NT

Re: Why were old transformers coated with tar?
I've been working for transformers.
Telecoms ones, but it's the same.

Once, winded and assembled, the pieces were put in a resin we call  
"jaja" in France.
And then put in an under-pressurized "bubble room" for completing the  
process.
When there was no more bubbles, the process was over.

Resin (or tar) is there for noise suppression, mechanical fixing of the  
windings and insulation improvement.


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Re: Why were old transformers coated with tar?
People put tar on airgun springs to remove noise and smooth out firing.

That's not the ideal way, but that requires precisely fitted spring guides adjusted for the individual airgun, and that doesn't happen in manufacturing.  

Re: Why were old transformers coated with tar?
 Look165 wrote:


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 ** The process is commonly known as " vacuum impregnation "  -  completed  
transformers are placed in  tank of resin or varnish and the air pressure r
educed until all air pockets are filled.  

Not always done with bobbin wound transformers but standard practice with l
ayer wound and old style cloth insulated toroidal mains transformers plus a
ny tranny that handles substantial voltages.

....   Phil  




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al
ny
nd
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es & holds the windings etc in place.
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Re: Why were old transformers coated with tar?
Excuse my English, I am French.


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Re: Why were old transformers coated with tar?
On 1/15/19 11:13 AM, Phil Allison wrote:
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Actually, the vacuum allows the bubbles to expand enough to
float to the surface and pop.

I did this with hand wound transformers at TRW using Poly
Urethane, and in my shop mostly with casting resins.

The vacuum chambers at TRW were nice, but I found that using
a modified pressure cooker works fine for the size jobs I have.


--  
"I am a river to my people."
Jeff-1.0
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Re: Why were old transformers coated with tar?
For me, it was telecom transformers for SNCF.


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Re: Why were old transformers coated with tar?
On Monday, January 14, 2019 at 7:24:57 PM UTC-8, snipped-for-privacy@myshop.com wrote:
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There are mechanical forces between wires carrying current, and in addition
to making 'hum' that puts some metal fatigue into the mix.    For
reliability, especially for switchmode, vacuum impregnation or some other
secure-the-wires provision is common in power inductors.   At low
frequencies, you can get away with oil and paper.  Or thick oil (asphalt, tar...).

Re: Why were old transformers coated with tar?
That's true, I forgot the magnetostriction.

But it's only true for LF ; with HF (some 10 kHz), the inertial forces  
are too strong and too much low in movement.


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Re: Why were old transformers coated with tar?
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It would be partly for that reason, partly because voltages tended to
be higher and insulation materials could be hygroscopic.

From the 70s, voltages in typical equipment dropped due to use of
semiconductors (although of course in a TV set the B+ still was there)
and the materials available for insulation changed from impregnated paper
to plastics and resins.

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