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Transformer steel ageing?

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Not sure where this belongs, but seeing as how valve amplifiers use output transformers:

While vaguely knowing the principles of transformer core steel ageing, I recently found the alarming news that transformers can become seriously lossy after some 20 years. Then googling, I found one study (for the degree of M.Electronics) in fact finding that an 80 hour test including heat treatment etc. slightly improved steel characteristics. Most studies/analyses concerned mainly large (industrial) power transformers.

As we have some metallurgical fundis on this forum, I hope to be informed as to what degree this can/might influence the life of 'small' transformers as mainly used for audio work.

Thanks!
 
No such a thing.
Large distribution transformers and electric motors (think Metro trains and similar) have been running smoothly for 100 years or more in some cases with no problem or degradation.

An "80 hour test" clearly shows some experiment on fresh made steel or manufacturing, fine with me, but any commercial product you might conceivably use will definitely be a "finished" and stable product, a completely different situation.

Only physical parameter which I guess might affect transformer steel can be overheating , but we are talking near red hot temperatures, way above what would burn insulation, melt plastic bobbins and maybe even melt copper , nothing you would even approach under any circumstance.

And in that case it would not be "a transformer" any more but a piece of burnt scrap.
 
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Vibration can abrade the core interlaminar insulation of high power transformers. This will increase core loss, resulting in higher temperature (for a given load) and therefore slight increase in conductor resistance (approx 0.4% per degree C) and resistive losses. The transformer is gradually more and more hot over time.

Smaller transformer aging mainly results in degraded transformer insulation. which is directly related to the temperature of the transformer windings. As consequence, older transformers are more susceptible to insulation faults due to voltage spikes. They work fine, until one day they suddently develop a short circuit. Adding a fuse in series to the power transformer winding on vintage equipment is a useful precaution.
 

PRR

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I would have said that transformer iron does not age. People who re-wind60+ year old iron just do not worry about it. The measurements after re-wind seem quite comparable to original specs.

BUT-- when you get to "super core stuff", apparently there is such a thing. Or rather, Hitachi boasts that their FineMet has far less aging than 'Co based amorphous'.
Nanocrystalline Soft Magnetic Material FINEMET<sup>(R)</sup> | Hitachi Metals
https://www.hitachi-metals.co.jp/products/elec/tel/pdf/hl-fm9-h.pdf
Note that this is at 100 deg C, some hotter than we normally run. Running 50C cooler *may* extend the scale 32X longer; that still includes the life of many vintage audio amps.

The blob-chart shows FineMet against Permalloy, Co based amorphous, and Si-Steel. Frankly this is interesting. Not for "power"; I am sure FineMet is SO much more costly then Si-Steel that we would "always"(?) choose a big lump of iron over a kilo-buck of FineMet. However in the small-core case where Permalloy is king we *may* get a smaller core (less C) with FineMet. (And negligible aging!) But "Permalloy" is widely sourced at good price, and Hitachi probably holds-up Finemet prices because nobody else has it.

(Yes, somebody is/will be winding OTs on FineMet, just because it costs a lot.)

If working with Co based amorphous, OTOH, you want to know your aging.
 

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Lots of Tube amps were stored in damp sheds and transformers, damp transformers do not work very well. I bought a transformer for 2A3's had to run it with no load to dry it out; with a 240 watt globe in series at first lit up very bright after running it for several days light did not light up now using in a 2A3 amplifier.
 
Crystal domains can grow over time. I remember this has been demonstrated at uni where a piece of metal bar was heated up and cooled down very slowly (over days). Nice large monocrystal domains grew up. Such crystal domains can be observed on silicium steel sheets of transformer cores. I am not sure if such phenomenon happens at room temperature, but we are talking about decades here. And magnetic properties are certainly change as crystal structure changes. Perhaps it has been investigated scientifically.
 
Mother Nature is wise enough, and magnetic materials minimize their magnetic energy by forming magnetic domains, i.e. regions with uniform magnetization. To grow up, a magnetic domain needs energy, e.g. heat, a magnetic field, mechanical stress, etc.

To modify permanently a magnetic domain it is needed a huge stress, of the order of MPa. As we are careful with our transformers let's aside mechanical stress.

Heat depends on temperature, temperature required to alter permanently magnetic domains is so high that transformer bobbins would melt, as pointed up Juan Manuel. Again, as we are careful with our transformers, let's ignore temperature effects.

How about a magnetic field? Again, as we are careful with our transformers, working well below saturation, at the end of a musical session, we end up with the transformer the same as before.

Sure, there are pathological cases, a huge magnetic field (AC and/or DC) probably saturates the core, and it reaches a permanent magnetization, but fortunately this is mostly reversible.

On the above reasoning was excluded exotic materials, topic already covered by PRR.
 
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Sure, there are pathological cases, a huge magnetic field (AC and/or DC) probably saturates the core, and it reaches a permanent magnetization, but fortunately this is mostly reversible.

This situation happens every time when a power transformer accidently is powered up at, or near the input voltage's zero crossing. In the next half wave a big current surge occurs, limited mainly by the primary's DC resistance. This may magnetize and saturate the core instantaneously. But within the next few full waves this magnetization vanishes.
That's the reason why we provide soft start circuitries to big power trannies.
Best regards!
 
This situation happens every time when a power transformer accidently is powered up at, or near the input voltage's zero crossing. In the next half wave a big current surge occurs, limited mainly by the primary's DC resistance. This may magnetize and saturate the core instantaneously. But within the next few full waves this magnetization vanishes.
That's the reason why we provide soft start circuitries to big power trannies.
Best regards!

Thanks for a very clear explanation. Fortunately it is a reversible process.

The same reversal mechanism is (was) used in CRT TVs at switch on, a "degaussing coil" (From Gauss I guess) uses the inrush current surge to remove the possible permanent magnetization of the CRT (its internal mask really)

Freundliche Grüße!
 
Yes, over time, steel loses its magnetic permeability. Although insignificant. There are several reasons. One of them is the aging of steel due to the heating of the magnetic circuit due to losses during magnetization reversal of the cores and the release of heat by magnetizing windings. Electrotechnical steel is a magnetically soft material and features a small area of ​​the hysteresis loop.
The main parameter determining the area of ​​the hysteresis loop is the coercive force Hc.
Hc is mainly affected by internal stresses and non-metallic inclusions. The main reason for the growth in losses due to magnetic hysteresis is related to the distortion of the domain structure, while the carbon and nitrogen impurities have the greatest negative impact here. Those. heating impurities leads to a change in the internal structure of the metal. (the process is very, very long). If your transformer is not heated very much, then you can, do not worry about its magnetic properties.
 
The same reversal mechanism is (was) used in CRT TVs at switch on, a "degaussing coil" (From Gauss I guess) uses the inrush current surge to remove the possible permanent magnetization of the CRT (its internal mask really)

Not exactly the same. No CRT loves magnetizing the internal mask during the power up sequence. Due to a PTC in series and a NTC in parallel, the AC current through the degaussing coil starts at a high value and decreases within a few seconds to almost zero.

Best regards!
 
Yes, over time, steel loses its magnetic permeability. Although insignificant. There are several reasons. One of them is the aging of steel due to the heating of the magnetic circuit due to losses during magnetization reversal of the cores and the release of heat by magnetizing windings. Electrotechnical steel is a magnetically soft material and features a small area of ​​the hysteresis loop.
The main parameter determining the area of ​​the hysteresis loop is the coercive force Hc.
Hc is mainly affected by internal stresses and non-metallic inclusions. The main reason for the growth in losses due to magnetic hysteresis is related to the distortion of the domain structure, while the carbon and nitrogen impurities have the greatest negative impact here. Those. heating impurities leads to a change in the internal structure of the metal. (the process is very, very long). If your transformer is not heated very much, then you can, do not worry about its magnetic properties.

i) Steel does not lose its magnetic permeability over time, always is ferromagnetic.
ii) The increase on loses and decrease on magnetic permeability over time is called magnetic ageing.
iii) The magnetic circuit does not heat up, the magnetic material does.
iv) The solubility of C in Fe-3% Si is about 0.005% at 700ºC (!)
v) There is experimental evidence that the magnetic ageing is not only dependent on precipitate fraction (impurities) but also on other factors such as grain size and texture.
vi) On reasonable quality magnetic materials, the total losses (hysteresis and eddy current loss) decrease after heat treatment (80 hours at 225ºC). Furthermore, grain size and precipitate fraction increase whereas the texture slightly improves.

Experimental results mentioned by lcsaszar on post#8 are consistent with results from other researchers.


Not exactly the same. No CRT loves magnetizing the internal mask during the power up sequence. Due to a PTC in series and a NTC in parallel, the AC current through the degaussing coil starts at a high value and decreases within a few seconds to almost zero.

Best regards!

The mechanism is the same, but degaussing coil is designed to… how to say… degaussing. :D

In all my years as a TV repairman, never saw an NTC in parallel, just the PTC in series.

The NTC is to prevent the inrush current surge due to the uncharged big electrolytic capacitor on the input of PSU, and it has nothing to do with the degaussing circuit.
 
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PRR

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Totally off-topic: iron losses and aging was a big deal over a century ago. The old plain iron had high idle losses. For utility companies powered-up 24 hours with significant load only a few hours a day, this hurts. Transformer makers experimented, but didn't always know what they were doing. There is one case of guaranteed low-loss transformers that had to be taken-back after 4 months because they aged into high-loss transformers that quickly.

By 1910-1912 the makers knew about impurities. Some Silicon is good. Too much Carbon is bad. Raw iron has *many* other impurities, some of which do alter the iron over time, and the metallurgists learned to measure and control these.

The time period is interesting. Recall that the 1912 sinking of the Titanic partly involved unsuspected influence of Phosphorous and Sulfur in steel, making it brittle when cold. It was maybe the best they had at the time; we can wish they knew more.
 
Metallurgists now know how to deal with Carbon content, Nitrogen content is even easier to deal with.

I must wind my own transformers, for high quality the most available here is Brazilian M4 GOSS from Aperam South America, this company also published some papers, and the Carbon content for a cheap lamination (non-oriented grain, 2% Silicium content) is about 40ppm.

I want to think that the highest quality material is even better. ;)
 

PRR

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Metallurgists now know how to deal with ...

1912 was a long time ago.

...Brazilian M4 GOSS from Aperam...
I want to think that the highest quality material is even better. ;)

I strongly suspect your M4 GOSS is about the same as M4 GOSS from anywhere else. Aperam is a global operation and everything they learn in europe or asia goes back to the Brazillian steel-makers. I do note that the Brazil plant cooks with eucalyptus charcoal instead of coal-coke; but as you say "none" of that carries over to the steel. The blast furnace turns iron-oxide ore plus carbon into carbon-dioxide and iron; ideally all the carbon burns out. Some lingers but a later step (perhaps the electric hearth) can adjust carbon up/down as needed for hard knives or mild laminations.
 
I strongly suspect your M4 GOSS is about the same as M4 GOSS from anywhere else. Aperam is a global operation and everything they learn in europe or asia goes back to the Brazillian steel-makers.

I do not know others, but Aperam M4 GOSS has high mechanical deviations in the same batch, from 0.23mm to 0.28mm, maybe is my provider, lamination quality is quite good.


I do note that the Brazil plant cooks with eucalyptus charcoal instead of coal-coke...

Are you sure? Energy difference is high, mineral has about double than vegetal.

It can be seen in Aperam catalogs an eucalyptus forest, and the bio-friendly chatter, but do not trust so much in people from this side of the world. :D

Ah, by the way, carbon content of Aperam GOSS seems to be (11.7 ± 0.9)ppm, but who knows...
 
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In all my years as a TV repairman, never saw an NTC in parallel, just the PTC in series.

The NTC is to prevent the inrush current surge due to the uncharged big electrolytic capacitor on the input of PSU, and it has nothing to do with the degaussing circuit.

German color TV's usually used a three pin component that combines both the PTC and the NTC which work as I've described. If present at all, the PSU's NTC was quite another story,

Best regards!
 
German color TV's usually used a three pin component that combines both the PTC and the NTC which work as I've described. If present at all, the PSU's NTC was quite another story,

Best regards!

German color TVs used a three pin component which has two PTCs inside, one series connected to degaussing coil and the other paralleled with mains, replace one of them by an NTC is a recipe for disaster. :rolleyes:

Dismantle an old Siemens or Telefunken 3 pin PTC and measure by yourself. ;)
 

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