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Author Topic: Magnetic Core  (Read 7805 times)

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It's not as complicated as it may seem...
Does a magnet perform as a good core for an inductor?

Obviously, a magnet is ferromagnetic, but how does it perform as an inductor core?

If not, what is the limitation? Careful, it's a bit tricky.

.99
   

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Yes, magnetized inductors offer some very
interesting possibilities.

When such an inductor is used with a blocking
oscillator circuit to drive an LED the effect is
striking.  Particularly when it is possible to change
the polarity and strength of the permanent magnet
to make comparisons.

Fertile ground for experimentation.


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"Truth: the most deadly weapon ever discovered by humanity. Capable of destroying entire perceptual sets, cultures, and realities. Outlawed by all governments everywhere. Possession is normally punishable by death." - John Gilmore (1935- ) Author
   

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Does a magnet perform as a good core for an inductor?

Obviously, a magnet is ferromagnetic, but how does it perform as an inductor core?

If not, what is the limitation? Careful, it's a bit tricky.

.99

It depends upon what you want from the inductor.

I await answers from the experts. My posts on this subject are likely to be met with 'you are wrong', 'here is the formula', etc., etc., etc.


---------------------------
"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   

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It's not as complicated as it may seem...
Thanks for the replies.

For starters, I want the core to increase the inductance.

If you go to induce a current in this inductor through mutual inductance, how if at all does the magnetic core influence the induction?

.99
   
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Particularly when it is possible to change
the polarity and strength of the permanent magnet
Can this be done with the expenditure of a modest amount of energy?

If so, and if the process is not too slow, all sorts of OU magnet motors
should suddenly become possible.
   
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It's turtles all the way down
Indeed a tricky question.

Permanent magnets have been used to bias inductors closer to saturation in ferroresonant power supplies, particularly in the TV industry.  Also they find use in TV inductors to create a degree of non linearity.

There are many qualifiers here e.g. what is the intended frequency of operation? this will determine the type of magnet that may be used without suffering significant eddy current losses at high frequencies, ruling out sintered metal alloys such as Alnico or Samarium Cobalt over ceramic based magnets, which have an insulating binder by nature of manufacturing and design.

There also is the demagnetizing that will occur if certain types are operated at very high power levels.

Temperature stability is also an issue since magnets flux varies with temperature in certain types, and this factor is used to temperature compensate ferroresonant power supplies.

The simple answer is yes it will perform as an inductor core, but in the design, consideration must be given to the intended use of the inductor.


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Does a magnet perform as a good core for an inductor?


A magnet does not make a good core material for an inductor for at least three reasons:


1)   it has very low permeability at the operating point assuming no DC bias, because the B-H curve is nearly flat at H=0, since the material is close to saturation.

2)   the core gets demagnetized as the inductor operates, so the coil inductance is unpredictable and changes while in operation, thus changing the circuit performance.

3)  I should also mention that depending on the magnet material, it may not be a good performer at high frequency and too lossy.


however, a demagnetized magnet, or magnetic material that is not magnetized, will make a good core material, from the point of view that it has high permeability properties.  But it may not be so good a material for high frequency operation, depending on the material.  All materials have limitations which should be understood beforehand.



EM
   

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A magnet does not make a good core material for an inductor for at least three reasons:


1)   it has very low permeability at the operating point assuming no DC bias, because the B-H curve is nearly flat at H=0, since the material is close to saturation.

2)   the core gets demagnetized as the inductor operates, so the coil inductance is unpredictable and changes while in operation, thus changing the circuit performance.

3)  I should also mention that depending on the magnet material, it may not be a good performer at high frequency and too lossy.



Depending upon the design requirements, these can also be good attributes.

Relating to .99's words:

In general, adding a magnet to a core or using a magnetized core will decrease values related to inductance. I any case I can think of measuring inductance of such a coil will show a decrease.
The fun part is the above is only true for AC usage and some DC usage. So, there may be cases where you measure one value and the coil performs with a different value.

Non-alternating radial polarization of a magnetized core is interesting. In most cases just think of the coil as being part of a magnetic amplifier with a fixed or variable control bias.

<Now wearing my Arc 100 cal/cm² Flash Coverall Suit>


---------------------------
"As far as the laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality." - Einstein

"What we observe is not nature itself, but nature exposed to our method of questioning." - Werner Heisenberg
   

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An easy way to evaluate the effect of
strong magnetism on inductor/transformer
performance is to construct a simple
blocking oscillator circuit (such as the
Joule Thief) with conventional ferrite
cored inductor.

I use a radial leaded 470 uH inductor (not
a toroidal inductor) with the feedback winding
(about 20 turns) added around the outside.
The feedback winding is then properly phased
to support oscillation.

With an LED load, add small Neo or Samarium
magnets to the inductor while observing the
effect both visually and with instrumentation.

You can easily increase/decrease the magnetic
bias as well as change its polarity.

You will see very interesting differences.


---------------------------
"Truth: the most deadly weapon ever discovered by humanity. Capable of destroying entire perceptual sets, cultures, and realities. Outlawed by all governments everywhere. Possession is normally punishable by death." - John Gilmore (1935- ) Author
   

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It's not as complicated as it may seem...
All good points gentlemen, thanks.

I am interested not only in general, but in terms of guitar pickups.

Guitar pickups work in a slightly odd manner; in order for an electric field to be produced and picked up by the winding, the magnetic field around the pickup is "disturbed" (caused to fluctuate) by the vibrating metallic strings above it. The magnetic field is produced either by magnetized poles, one for each string, or by using low remanence poles biased with a magnet placed beneath them, or perhaps both in some cases.

I agree with EM in that a highly saturated, high remanence magnet (such as a neo or samariam cobalt for eg.), will perform poorly as an inductor core (if the goal is to increase inductance), in particular with such puny signals found in a guitar pickup.

.99
   
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I tired back in 2009 to wind two coils over a speaker magnet but there was nothing special to write about. At the time I called it a Magoid (for magnet toroid). If I remember correctly, the wire was to thin with 144 turns at 29AWG. Too really get a magnet to start moving its field or to be useful in any inductive manner, you would need thicker wire and alot more juice. I remember pulsing it a high frequencies gave me a headache.

wattsup


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