Question about crossed magnetic fields

[chadj2]

I am not very familiar with magnetic design so please bear with my laymans terms. I am preparing to do an experiment on crossed magnetic fields. I am attempting to see what kind of interaction perpendicular magnetic fields have on one another. For this experiment I am planning on having a 1 inch cube of steel wound with two seperate coils. One coil will be on the vertical axis and the other coil on the horizontal axis both of approx 100 turns. Now suppose I set up a H field of 1000 on the vertical axis by way of the first coil with the corresponding flux density and poles resulting. Once the flux is established on the vertical axis and I were to introduce a H field of 1000-1500 on the horizontal axis by means of the second coil. What would be the impact on the flux density from the first coil or vertical axis. Would the north and south poles still be on the vertical axis or would they shift or maybe cancel out? If someone could point me in the right direction as to where I can find information pertinent to this scenario I would be appreciative.

Thanks,

 

[Skogsgurra]

Since field lines cannot cross, the resulting field will be the vector sum of coil 1 and coil 2.

In your example, if both coils are the same strength, you will get a resulting field at 45 degrees. The geometry of the core will also have some influence, but the vector addition principle holds.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[chadj2]

Skogsgurra,

Thank you again for answering so quickly. Do you know a good source where I can read more on the vector addition rule or principle?

 

[Skogsgurra]

It is rather simple. Magnetic fields are representaed as collections of field lines. These are fictious, but good for visualising the field. Have a look at

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfie.html

If you now want to calculate the field strength from two coils, you can simply calculate the two separate field strengths and then superimpose them. Use ordinary geometric math (which is what vector math is, but a little more formalized) to add the vectors (field lines). If you have an H vector H1 pointing to the north and another one H2 pointing to the south, the resulting vector H3 is simply H1 - H2. If H2 points to the east (perpendicular to H1) then the resulting vector H3 = sqrt(H1^2 + H2^2). For other angles, you need to add squares of sine and cosine and then take sqrt(sum).

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[logbook]

There are a few practical considerations with skogs has neglected to mention.

In air there is no “interaction” between magnetic fields. The principle used to solve problems is called “linear superposition”. The two fields act as though the other is not present. You do find a resultant using a vector rule as skogs has said.

Now iron is not a linear material. Linear superposition may or may not work depending on the field strength involved. For example if one field takes the iron up near saturation, the second field will not increase the field greatly, although the direction will not change. Furthermore iron is not necessarily isotropic, meaning its magnetic permeability may not be the same in one axis compared to another. In this case the B field and the H field may not be in the same direction!

 

[Skogsgurra]

I was specifically using only the H fields. Just to avoid confusion. I think that a guy that is approaching this subject first time needs some spoon-feeding. Dumping everything on him at once is both error-prone (because you cannot get everything right and 100 percent complete, how hard you try) and confusing.

We have no idea if his iron is saturating or not. If it is - and that is not what he wants - then he has to re-design. If he is happy with a saturated core, fine. BTW, the resultant field direction does change when the second coil carries a current. It is not determined by the coil that first saturates the iron.

That is probably what you meant when saying "although the direction will not change". But I had to read that several times to realize that there is a "not" that shouldn't be there.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[chadj2]

Logbook,

Thank you for the information about there being no interaction in air. That was going to be a future experiment which you saved me the time of trying. Skogsgurra is correct that I do need a bit of spoon feeding, but I do appreciate the more indepth explanation in addition to the simplified stuff. The reason I am even trying this experiment is to see how much I can control the direction and intensity of the steady magnetic flux from the first coil by adjusting the field strength from the perpendicular coil. I have another question somewhat on the subject. If the magnetic field from a permanent magnet encounters a ferromagnetic material like iron which is already saturated by another field which is running perpendicular to the permanent magnet; will permanent magnet's field treat the iron like it is air?

 

[logbook]

Quite right about the surplus "not". It's a pity we can't edit postings :-(

 

[Skogsgurra]

That question (about another field entering an already saturated ferromagnetic material) is best answered by seeing what happens to the composite H field. If it is perpendicular to the original field, it cannot reduce that field, so the resultant H field will be greater than the original field. So the iron will still be saturated.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[logbook]

Another advanced piece of information which I failed to mention is that bulk iron is effectively not ferromagnetic above 100kHz. This takes some explanation. At 1kHz if you put a solid iron core inside a coil, the inductance increases. The iron allows the flux to flow more easily than through air.

At 100kHz a solid iron core actually reduces the coil’s inductance because of eddy currents. The iron no longer appears like an easy path for flux, and is actually worse than air. This is why iron cores in transformers are laminated and cores in inductors are made from powered iron suspended in an insulating binder (iron dust cores).

If your experiment is being done above 5kHz you may get some confusing results.