Why were old transformers coated with tar?

Question

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.

MOP CAP · Accepted Answer

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

tabbypurr · Answer

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

Look165 · Answer

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.

Tim R · Answer

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.

Phil Allison · Answer

** 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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es & holds the windings etc in place.

Look165 · Answer

Excuse my English, I am French.

Fox's Mercantile · Answer

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.

Look165 · Answer

For me, it was telecom transformers for SNCF.

whit3rd · Answer

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...).

Look165 · Answer

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.

Rob · Answer

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.

Why were old transformers coated with tar?

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