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Transformer steel ageing?
Transformer steel ageing?
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Old 16th April 2018, 02:42 PM   #1
Johan Potgieter is offline Johan Potgieter  South Africa
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Default Transformer steel ageing?

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.

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Old 16th April 2018, 02:53 PM   #2
JMFahey is offline JMFahey  Argentina
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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.
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
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Old 16th April 2018, 03:14 PM   #3
Johan Potgieter is offline Johan Potgieter  South Africa
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Thank you kind Sir.

The originating remarks was then (as is so often the case ) over-reactive response to having read something somewhere.

I am relieved; couldn't believe that such was a real danger and my not having come across it in >6 decades of tube design.

Obliged, Jeff!
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Old 16th April 2018, 03:19 PM   #4
Andrew Eckhardt is offline Andrew Eckhardt  United States
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It's a real problem but not at consumer electronics temperatures.
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Old 16th April 2018, 06:12 PM   #5
pcan is offline pcan  Italy
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Transformer steel ageing?
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.
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Old 16th April 2018, 10:33 PM   #6
PRR is offline PRR  United States
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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
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.
Attached Images
File Type: gif FineMet.gif (67.7 KB, 374 views)
File Type: gif FineMet-blobs.gif (27.1 KB, 371 views)
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Old 17th April 2018, 12:20 AM   #7
multi is offline multi  Australia
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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.
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Old 17th April 2018, 02:35 PM   #8
lcsaszar is online now lcsaszar  Hungary
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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.
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Old 18th April 2018, 04:31 AM   #9
popilin is offline popilin  Argentina
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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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Last edited by popilin; 18th April 2018 at 04:35 AM.
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Old 19th April 2018, 09:48 AM   #10
Kay Pirinha is offline Kay Pirinha  Germany
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Originally Posted by popilin View Post
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.
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