Mains transformer goodness

Something to do while watching some less engaging TV. Unwound another large toroidal mains transformer. Suspecting doubled up winding of the primary and then for UK use joining opposite ends and so relying on 2 thicknesses of lacquer to resist high voltage. Of course somewhere near the middle at some point it fails catastrophically. Yes, burnt spot weld buried in the middle of the primary. Anyone know what this duff winding technique is called ? (reduces the number of shuttle passes by 2 must be the reason). Is there a way of testing an unknown , but good , transformer for this winding pattern ? How come this wiring procedure is not outlawed ? More generally, someone in production told me that a "goodness" test for a mains transformer is an open secondaries, no load, monitoring of the primary current is useful, any truth/rationale in that. ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook
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Not that I know of.

Most of the transformers wound in this way don't burn out?

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It tells you the inductance of the transformer. It doesn't tell you how much heat is being dissipated in the windings and the core - for that you have load the seconaries and watch how fast the output voltage falls off with incresing load.

Since the permeability of the core declines with increasing temperature, the inductance falls away as the transformer heats up, so that magnetising current - and the power dissipation - goes up as the transoformer gets warmer.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

"N_Cook"

** " Standard practice " ??
** Only by testing to destruction.

** It works, is not a recognised safety hazard and saves cost.

BTW: I have a 300VA toroidal with EXACTLY the same kind of failure here at the moment, waiting for a replacement to be made. But it is lasted 8 or

9 years before shorting.

** Yes.

If you checked a run of transformers and found one with a significantly higher I mag than the others - then the core assembly or the primary winding has been bodged. For example - an E core tranny where the laminations are not fully interleaved or tightly enough packed to eliminate air gaps between the Es and Is will show high I mag on test.

Toroidals normally have very low I mag figures, 10 to even 100 times lower than similar sized e-cores. However, if the primary voltage exceeds the maker's design value, expect to see I mag go through the roof. It will also go through the roof if you try to use a toroidal made for 60 Hz mains power on 50Hz.

...... Phil

Reply to
Phil Allison

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I'm amazed they last as long as they do , 400 turns or so of paired primary wires layed together touching and thermal cycling / chaffing possibility along the lacquer . Certainly not produced in a clean-room so any amount of microscopic grit between the wires, what is the thickness of 2 layers of lacquer to resist pk-pk voltage of 350V , let alone mains surges?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

Way back in my dark past I worked in a plant where we wound our own transformers. Indeed, the doubling of the primaries is standard practice. Note that if your primary voltage is 240VAC the voltage between the adjacent wires will be approximately 180V maximum.

Of course, these were using 'I and E' laminates. We tested the wound bobbin for the correct number of turns and leakage between the wires of the paired windings. After the laminations were added the entire transformer was vacuum impregnated with varnish. Not much chance for movement between the windings after that!

PlainBill

Reply to
PlainBill

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Did you ever metal saw across any of them to actually check on the impregnation. ? Not transformers, but I've seen such a check on a similar process with much the same viscosity of fluid and most of the interior was not impregnated, vacuum gave out to viscosity down fine pathways.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

Sounds like fun, in front of the telly, something I would do.

But.................

I would really worry about inhaling/handling the slightest amount of PCBs. Especially if the unit was manufactured before the '70s. (@_@)

Reply to
Kalarama

Was this PCB stuff overblown about being dangerous? I worked for Westinghouse motor and transformer repair in the 40's. Was up to my elbows some times in PCB oil on transformers. I am still here. As for all coils and motor windings they were submerged in varnish over night. then drained and baked. No failures. WW

Reply to
WW

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In the ideal yes, but this Tx and another similar there is no interlayer insulation between each of the 3 or 4 primary "layers". Compact winding only on the inside diameter, not the outside , where windings could easily slip a "layer". So worst case voltage could be 240/0.707 , peak to peak, without any mains spikes.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

Then you'd be somewhat less than brilliant. No mention of transformer oil was involved, and nobody is likely to uncan an oil filled transformer in their living room. A dry, varnished/lacquered transformer has no oil, so no PCBs.

--
Cats, coffee, chocolate...vices to live by
Reply to
Ecnerwal

PCBs.

I did think elf 'n' safety a week ago, opening a pack of silicone rubber sleeving. All this fine glistening deposit everwhere , will I get silicosis ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

"N_Cook" PlainBill

** Nevertheless, vacuum impregnation works very well with transformers.

The basic idea is to remove all the air spaces between windings and fill them with insulation material - so the unit will never suffer from "corona" discharge.

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Corona discharge is the biggest killer over time of transformers that handle AC mains or higher voltages - particularly valve output transformers and EHT transformers.

Until fairly recent times, vacuum impregnation was standard practice for all mains voltage transformers - toroidal types were wound with porous cloth tape insulation to allow this step to be done.

Now makers use polyester tape insulation along with tougher, flexible enamels and impregnation is impossible.

IMO - a mains toroidal with dual primaries ought to have them wound separately and with at least one layer of poly tape in-between.

..... Phil

Reply to
Phil Allison

peak to peak is half of that between series-connected windings.

Reply to
Jasen Betts

Though if the deposit had been from something dowsed down from a fire at over 300degC, it's likely to be bloody painful amounts of Hydrofluoric acid. Be ready to lose fingers :-(

--
Adrian C
Reply to
Adrian C

Probably French chalk, AKA talc. Put there as a lubricant.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
 Click to see the full signature
Reply to
Fred Abse

"Ecnerwal" wrote

So, you're willing to bet there are never any toxic additives (as in polyvinylchloride) in these varnishes and/or lacquers?

Reply to
Kalarama

Bs.

s

There's three issues with PCB oils. First, they're fireproof (a safety plus). Second, the oil can overheat and decompose, into dioxins and other toxins/carcinogens. This, is a safety minus. I've heard of a contaminated animal feed episode that got LOTS of political attention. The third one, is kinda scary: like DDT, the compound persists and spreads in the natural environment, so it can cause problems in future decades or centuries if it isn't collected and neutralized. No one wants to take the risk of causing the next species extinction and/or lawsuit flood.

Reply to
whit3rd

PCBs are really dangerous stuff. They are carcinogenic, mutagenic bioaccumlative toxins. I must have waded through thousands of pages of technical and medical data on them. The parts i printed stacked up to a couple of inches. .

Reply to
JosephKK

It ain't necessarily so.

Start with this for current US regulation:

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Reply to
JosephKK

Anyone care to speculate on the timeline of failure? There was no local hotspot and no lacquer damage more than a mm from the "spot weld", and a bit of very localised smoke staining travelling a cm or so along the affected wires in each direction. Nothing to suggest that the initial bridge was between the 2 bifilar wires of the primary, so not running a 120 volt primary in effect on 240V ac for any time. For 240V use the 2 primaries seriesed to give about 2.8 ohm originally , after failure then about 0.4 ohm. The 2 primaries broke into 5 lengths

9.6,14.6,20.1,20.1 and 25.3m long , measured to about 0.2m accuracy. So originally probably 2 x 45m. Don't know for sure as did not think to check but the weld was probably 20.1m from one end, but I would suggest that bridge occured after an arc to another layer (higher pd) and then localised heating to bridge across to the bifilar fellow wire.

In summary , no evidence that bifilar wiring itself was the reason for failure but more due to the lack of any interlayer insulation. Because of the uneven wire spacing between inside and outside faces of the toroid it is too easy for the layers to be jumbled.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

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