transformer residual flux remanis forever or decays with time? 4

[Bronzeado]

Hi "magnetic people",

When we de-energize a transformer, some residual flux remains in its ferromagnetic core. I wonder if somebody knows if the residual flux remains forever or decays with time?

I have already

Best regards,

H. Bronzeado

 

[EdStainless]

The amount that stays will depend on the quality of the core iron. Roughly speaking, the better the material the lower the residual field. There will always be some residual field.

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Plymouth Tube

 

[Bronzeado]

EdStainless,

Thank you for your reply.

I am talking about ferromagnetic core of large power transformers which are normally made of "soft stell".

As I am working with controlled switching of these transformers, I need to know the value of the residual after the transformer be de-energized to adjust the instant at wich the circuit breaker pole will be closed.

As the transformer normally stays de-energized for a long time (1 hour, 1 day, 1 month, etc...) I have my doubts about if the of the residual flux stays with its initial value (forever) or if it dacays with time.

Discussing with some colleagues, I found that 50% of them says that the residual flux reamains, 30% says that it decays with time and 20% do not know.

So far I have not a good explanation on this matter. I would appreciate if you or somebody else could give one.

Regards,

Herivelto Bronzeado

 

[MJR2]

Steel has some residual and with time it decays. The decay may be really slow or non-linear. I think you will need to quantify/qualify your own steel to be able to answer this question. To achieve what you are trying for it will be necessary to very carefully specify and purchase your own steel.

Six months ago you asked it in another group and got the same answers and gave the same reply. You were given a link that was quite interesting. But it would seem to be not good enough for you.

Mike

 

[Bronzeado]

Thank you MJR2, for your reply.

Yes, I have asked this question to another group (electrical people) but I still having doubt as I have not got a good explanation. So I move for the "magnetic people" to see if they could give me somo "light".

You said that "steel has some residual and with time it decays. The decay may be really slow or non-linear."

1. Have you got any evidence on that or is it only a "guess"?
2. How can we explain this dacay (if exists)?
3. What do you mean for "decay may be realy slow"? 1% per day, for example?

I would apreciate you thought on that.

Best regards,

Herivelto Bronzeado

 

[electricpete]

One thing to mention. The phenomenon of residual magnetism in soft steels (such as cores) is analogous to the magnetism retained in magnetized hard steels (such as a ferrite permanent magnet that you use to hang on your refrigerator).

Loss of residual magnetism is similar to a diffusion process and depends on temperature. Above the curie temperature, the residual magnetism disappears immediately.

Residual magnetism on soft magnetic materials can be seen in a number of ways.

When testing transformers, dc testing creates resiudal magnetization that affects the ac results. So we prefer to do the dc test last.

When testing motors using PDMA "rotor influence test", rotor core residual magnetism affects the signal. You can find a paper about it on pdma website.

Under some circumstances, CT core can become magnetized.

I have read about these several places. When a procedure is recommended for demagnetization, it is invariably to apply a slowly decreasing-magnitude ac field. I have never heard anyone suggest to wait for the residual magnetism to decay. So my suspicion is that it would take months or years.

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[IRstuff]

If there is residual magnetism, then it will decay over time, for the simple reason that high temperatures scramble magnetic alignment, making the transition a thermal process. Therefore, at notmal temperature we would expect a slower process.

TTFN

FAQ731-376

 

[MJR2]

Normally the rate of decay is not an issue. When it is we use devices to force a decay. What I have seen leads me to believe in my answers as given. I think it is called an educated guess from a lot of years of observation.

Perhaps the other answers explain it much better. However it appears that no one is going to be able to give you data.

Mike

 

[EdStainless]

In large transformers you can't shut the current off instantaneously, there is a decay to it. And since this is an AC field that is decaying, in effect you are demagnetizing the core when you de-energize the transformer.

The effect of residual magnetization of the core is very real as Pete has said. There will always be some value of residual magnetization in the core.
However the absolute value of the field can be quite different from one transformer to another.
What you need to do is watch the rate of change of that field. If it is changing slowly then you should be safe.

When we built machines to de-magnetize permanent magnets we used a 'ringing' circuit to give us a slow decay of the field.

I don't know how fast the current and associated field take to decay in a transformer like this, but my gut feel is that you will see all of the measurable change within minutes. While the field may continue to decay for days, you are probably dealing with the last trace levels that are not measurable in the engineering sense.

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Plymouth Tube

 

[Bronzeado]

EdStainless,

I have measured indirectly the residual flux in large transformers (230/138kV, 100MVA) just integrating over time the decaying voltage on all three phases. The measured results seems to be very good as when I use those information values to making the controlled closing I have got a very small inrush current. However, normally the controlled closing is done just 1 or 2 hours after de-energizing the transformer.

What I do not know is: if I wait one day or one month to do the correct controlled closing, may I still using those values of residual flux I measured or I have to consider some decay?

Regards,

Herivelto

 

[EdStainless]

Bronzeado,
What happens if the actual field level in the transformer is lower than what you assume for your closing control? Does that hurt anything?
I don't believe that the filed levels will decay any further after that first couple of hours.

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Plymouth Tube

 

[Bronzeado]

EdStainless,

The methodology of transformer controlled switching relies on knowing the residual flux value which define the instant of the voltage point on wave at which the circuit break will be closed.

As I said, the residual flux is "measured" taking into account the decaying voltage during the transformer de-energizing. We really measure the voltage and calculate the integral of this voltage over time. When the voltage reaches zero after oscilating, we assume that the value obtained from the integral is the residual flux. If it changes to another value, the controlled switching will fail.

Best regads,

Herivelto Bronzeado

 

[BobM3]

I came across this while looking for something else in a brochure from Magnet Aplications, Inc. They make permanent magnets so I don't know how much is applicable to electrical steels:

5.1 Time

The effect of time on modern permanent magnets is minimal. Studies have shown that permanent magnets will see changes immediately after magnetization. These changes, known as "magnetic creep", occur as less stable domains are affected by fluctuations in thermal or magnetic energy, even in thermally stable environment. This variation is reduced as the number of unstable domains decreases. Rare Earth magnets are not as likely to experience this effect because of their extremely high coercivities. Long term time versus flux studies have shown that a newly magnetized magnet will lose a minor percent of its flux as a function of age. Over 100,000 hours, these losses are in the range of essentially zero for Samarian Cobalt materials to less than 3% for Alnico 5 materials at low permeance coefficients.

I see they have a website:

http://www.magnetapplications.com

They make magnetizing equipment so they may have experience with materials not used for permanent magnets.

 

[hacksaw]

You can start by looking them up or requesting them from the transformer supplier, or if you are eager do the testing your self...though often an expensive option. Not all transformers designs have low remanence, it depends on their design and the core material.

 

[drmagnet]

Let me start by saying I am NOT a transformer expert. But I suspect the issue is related to the material from which the transformer is made.

That a transformer becomes warm (or even hot) is an indication that there is conversion of electrical energy into heat from hysteresis loss. The hysteresis loss is a function of the coercivity of the material - - actually the area within the hysteresis loop as the material is polarized first in one orientationa nd then in the opposite direction. Since coercivity is a major descriptor of the hysteresis loss (though not the only one) let's stay with it.

Coercivity is also a measure of the material's "resistance to de-magnetization." This resistance to demag is greatest in permanent magnets, less so in (permament) magnetic steel, even less in materials such as screwdrivers, and minimal in Electrical Steel (both grain oriented and non-grain oriented). (Strictly speaking it is the materials Intrinsic coercivity that is the resistance to demag, but in soft magnetic materials Hcj ~ Hcb).

Transformers are made from many different grades of steel with the less expensive ones being low carbon steel, intermediate grades using silicon steel and the highest grades using the very expensive materials such permender.

I should note for us that a permanent magnet remains magnetized, essentially unchanged, until exposed to a demagnetizing stress: high temperatures, reverse field, a ringing AC field, etc.

The second issue is that since the transformer is essentially a closed magnetic circuit, even a very low coercivity will result in retained magnetic field.

As an interesting side note: nanocrystalline transformer core material is excellent, but is so sensitive to physical strain increasing the Hcb that it must be annealed after forming into the transformer.

There is a paper at

http://www.arnoldmagnetics.com/mtc/pdf/SMMA_Fall_Constantinides_081017.pdf

which discusses silicon iron and the loss factors. You might find it interesting.

 

[Bronzeado]

drmagnetic,

Thank you for your reply.

In your words "since the transformer is essentially a closed magnetic circuit, even a very low coercivity will result in retained magnetic field".

So, in short, we can say that "the residual flux will remains in the transformer core after de-energizing".

Best regards,

Herivelto Bronzeado

 

[drmagnet]

Herivelto,

That is correct.

It also helps to remember that magnetic flux does not "flow". It is an energy state. So once the field is established, it takes no additional energy to stay that way.

Steve (drmagnet)

 

[Bronzeado]

Steve,

Thank you!

Herivelto