magnet design 101 - saturate iron with a ferrite magnet?

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We've had various discussions here on the advantages of AlNiCo, Neodymium and field coils. One thing that crystallized here was that it is advantageous to saturate the iron return system because that makes a modulation of the iron and magnet magnetization impossible, and it also keeps the pole piece from acting like a core that increases the inductance of the voice coil.

One point that I found in the literature was that ferrite magnets had too low a flux density to saturate iron (1.0-1.2 vs. 1.3-1.5 T). Nobody has challenged this view.

While this statement is certainly true if the magnet and the iron share the same cross section, I wonder if it is possible to saturate a part of the return system by letting it have a smaller cross section than the rest of the return system and the magnet. Where the iron becomes thinner, the field lines move together und the flux density increases. If the iron becomes thin enough, it will sature. Any "extra" field lines will have to go through the air until the system get back to its original thickness.

The gap should be placed in the thin area so that all iron that has contact with the field of the VC is saturated. Also, this will keep the stray field spread out, so that we won't have a strong assymetric stray field around the gap. The thin area should, however, not extend too far because this will keep the iron barely in saturation, making it easier for the VC's field to drive the iron out of saturation.

Any flaw in my reasoning? How come it's not done?
 
I think it is already done more often than not !
If you look at the cross section of the magnet and the cross section of the air gap the former is usually significantly larger than the latter one for the average driver with ferrite magnet.

I think the WHOLE iron part of the magnetic loop must be saturated to take any advantage of saturation.

Regards

Charles
 
Hi Charles,

theoretically, it should be sufficient to saturate anything that interacts with field of the VC. So the upper pole plate (both sides of the gap) should be saturated, and also the upper few cm of the pole piece.

In a standard system, if the inner side of the gap is just saturated, the outer side is probably not. Maybe a pole plate that becomes thinner as the radius increases is the solution?

Greetings,

Eric
 
Hi Guys,

Yes, it certainly is possible to achieve very high gap flux density with ceramic mags. Just to see what was possible, I recently created a finite element magnetic simulation of a ceramic-magnet motor with a steel and permandure return circuit. I achieved 2 tesla across the gap! But it took a *big* grade-8 magnet. I'll try to post a pic of the simulation after lunch.

theoretically, it should be sufficient to saturate anything that interacts with field of the VC. So the upper pole plate (both sides of the gap) should be saturated, and also the upper few cm of the pole piece.

Yes, I believe this is true.

In a standard system, if the inner side of the gap is just saturated, the outer side is probably not.

Unless you're thinking of a tapered T-shaped pole piece, the reverse is often true. Yes, the diameter of the top plate is larger than that of the pole piece, but the top plate is thin and projects most of the system flux from its inner perimeter. Therefore, while its inner perimeter may be saturated, the flux field expands vertically as it crosses the gap, often creating a trapezoidial field and not necessarily saturating the surface of the pole piece.

Here's a sim I did of my best guess at Adire Audio's (still secret!) XBL2 topology. Neither the pole nor top plate are saturated in this example, but you can see what I mean about the way flux spreads out between the top plate and pole. Incidentally, here's here's the thread where I expose this secret motor, if you're interested!

I think the WHOLE iron part of the magnetic loop must be saturated to take any advantage of saturation.

This would be ideal if it was practically achievable, but the trouble is that as iron approaches saturation, flux lines begin to project out from the surfaces and escape. To achieve full-circuit saturation, I think you'd need to spec a ridiculously large NdFeB magnet that would waste so much flux that it would distort a TV from across a room. I believe a more practical ideal is saturation of both sides of the gap.

Bill
 
Bill F. said:
Unless you're thinking of a tapered T-shaped pole piece, the reverse is often true

Hi Bill,

at last somebody who has played with these things! Yes, I was thinking about T-shaped pole piece. What do you mean by "tapered", i.e. what would be growing successively thinner, the vertical column of the T or the top?

What software do you use for FE modelling? I am a little surprised about the calibration. Your ferrite magnet seems to have a pretty uniform flux of below 0.36 T. I have not studied magnet catalogues, but I am told that regular ferrite can be magnetized at about 1 T, maybe even a little higher if magentization is done with a return system installed. What is the grade 8 vs. grade 5 stuff you are talking about in your linked thread?

Similarly, the iron seems to reach almost 2.0 T where I thought the limit was somewhere around 1.5 T. Is this a special alloy? If so, does it get used in common drivers?

Regards,

Eric
 
Hi Eric,

By tapered, I mean that the ears of the T taper down with increasing diameter toward the gap. If they are not tapered, the flux-conducting area is greater at the gap than where the T ears join the cylinder of the pole piece, creating a flux bottleneck that prevents saturation at the gap.

The program I use is FEMM. I haven't taken the time to check the values FEMM specs for ceramic magnets, I just plugged 'em in. My understanding is that there are two broad categories of industrial ceramic magnetic material--grade 5 and 8. The material properties in FEMM are easily modified if I need to.

The steel alloy I specd for the XBL2 motor sim is 1006, a very low-carbon steel with good magnetic properties. I believe P-Audio uses it in some models, and probably other manufacturers as well.

The simulation wasn't supposed to accurately reflect the flux numbers in an actual XBL2 driver, just the shape of the BL curve that results from the topology.

Driver design is my favorite hobby right now, in case you couldn't tell!

Bill
 
Bill F. said:
Hi Eric,

By tapered, I mean that the ears of the T taper down with increasing diameter toward the gap. If they are not tapered, the flux-conducting area is greater at the gap than where the T ears join the cylinder of the pole piece, creating a flux bottleneck that prevents saturation at the gap.

Ok, I am getting the point. Perimeter increases with r, so thickness has to decrease with r to kepp the cross section constant. So my idea isn't new :(

Of course, one could continue the tapering with the top plate, or at least the inner half inch of the top plate. Has it been done, with either the pole piece or pole piece + top plate?


So if acutal magnets are 3x stronger than you assumed in your calculation, you could easily saturate large parts of your geometry...

I gather FEMM is not exactly freeware?

Greetings,

Eric
 
Of course, one could continue the tapering with the top plate, or at least the inner half inch of the top plate. Has it been done, with either the pole piece or pole piece + top plate?

Of course, the effective cross-sectional area of a flat top plate already increases with r, but this is still a good way to ensure that your return circuit nears saturation only at the gap, thereby allowing it to efficiently conduct the flux.

I gather FEMM is not exactly freeware?

I see Rob gave you the good news while I was writing. Yes, FEMM is free and the best thing since sliced bread! Check out the FEMM forum on Yahoo to come up to speed. If you check the names on many of the posts, you'll notice that many audio industry who's-whos use this jewel of a program. Babb, Adire, Aura, etc.
 
Here's that model I cooked up of the ceramic-mag motor with over 2T in the gap--the outer limit of what would be achievable with ceramic. The majority of the return circuit is 1006 steel, but the piggy-back top plate and the pole cap are permandure--an unavoidable requirement to achieve 2T+.

Notice how much of the flux projects from the surface and is wasted. That is one of the downfalls of trying to achieve high flux densities with ceramic magnets. Rare-earth would be much better suited to this application.

Bill
 

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Hi Bill,

I guess that software will keep me happy at some point. Just need to find somebody to build the magnet systems for me...

In your new example, I am once more surprised at the low flux in the magnet, around 0.2 T this time where I would expect more than 1 T. I guess Permadure is the right choice if maximum flux is your aim. If you're shooting for saturation, sticking to steel might be a better idea.

Cheers,

Eric
 
sense in stacked magnets?

Double and even triple magnets are becoming increasingly popular. However, the field in the gap should be primarily determined by the cross-section of the magnet, not it's hight. So in first approximation, the added hight has no effect except to allow a longer VC stroke.

Secondary benefits might be:
- the energy stored in the magnets is bigger compared to the changing energy of the VC, so there is less modulation
- it is easier to concentrate the field onto the gap with a longer magnet assembly??
- ?????
 
I believe you're correct that maximizing magnet cross-section is the most efficient way to harness its flux. I've heard an an approximation that it takes close to 10x the stack height to double the flux density of a single magnet.

I don't know about field stiffness benefits of stacked magnets, but I believe stiffness is more directly linked to how much surplus flux is available at the gap to maintain saturation under power, independant of the geometry of the magnet that supplies the flux. I suppose you could think of this surplus flux as "transient headroom."

Another interesting thing is that the level of field modulation is inversely proportional to the efficiency of the voice coil's coupling to that field. For example, think of a ribbon driver in which there is a wide air gap flanked by poles saturated by very large magnets (necessary to achieve sufficient flux across the gap). The ribbon couples so inefficiently to the B field that it will cause negligible field modulation.

In a similar sence, driver efficiency is your friend. To achieve a given SPL, an efficient driver will generally have a stronger B field and a lower current in the VC--a recipe for reduced field modulation.

it is easier to concentrate the field onto the gap with a longer magnet assembly??

Yes, to a point. However, the thinner the magnet the more flux short-circuits around the edges. I think there's an optimum balance. You'll always lose some. Notice in the HE ceramic model I posted above, a large portion of the magnet flux flies off into space or short-circuits and is wasted, even though the return circuit doesn't approach saturation anywere but near the gap.
 
Bill F. said:
Another interesting thing is that the level of field modulation is inversely proportional to the efficiency of the voice coil's coupling to that field. For example, think of a ribbon driver in which there is a wide air gap flanked by poles saturated by very large magnets (necessary to achieve sufficient flux across the gap). The ribbon couples so inefficiently to the B field that it will cause negligible field modulation.

In a similar sence, driver efficiency is your friend. To achieve a given SPL, an efficient driver will generally have a stronger B field and a lower current in the VC--a recipe for reduced field modulation.

Aren't these two examples contradicting? The ribbon has inefficient coupling and the the efficient driver has efficient coupling.
The efficient driver is less effected because there is less AC current and because the magnet is bigger compared to a more inefficient driver.

Greetings,

Eric
 
how do they magnetize magnets?

I have read that modern ferrite magnets are magnetized with the iron system installed. Apparently, the assemblies are inserted into the machines with their back plate first, i.e. there is no adaptor that attaches do the pole piece and top plate.

So it would appear that they use a coil that is essentially wrapped around the circumference of the ferrite magnet. It's field will magnetize the ferrite ring accordingly. However, it also extend into the interior of the magnet ring. This space is filled by the pole piece. The field inside the magnet ring will be conducted through the plates into the pole piece, but inside the pole piece, the orientation is exactly opposite to the coil's field. So it will act to oppose the external field. No harm is done because the iron is not permanently magetizable, but it seems a waste of energy compared to coupling the field into top plate and pole piece.

How are field coil speakers built? It would seem the only sensible way is to build them similar to old AlNiCos with an outside return, with the magnet being the pole piece. If they were built like conventional ferrite designs with a center return, the flux there would be opposing the external field.
 
Aren't these two examples contradicting? The ribbon has inefficient coupling and the the efficient driver has efficient coupling.

Not contradicting, but perhaps not necessarily related. I guess what I'm trying to say is (assuming good design and saturated poles) the greater the ratio of B flux to coil current, the smaller the field modulation--obvioiusly.

In the case of the ribbon driver, there is a great deal of gap flux that the ribbon doesn't harness efficiently. It doesn't need to because it is so light.

In the case of the high-efficiency cone, the voice coil does not necessarily have to make efficient use of the B field. Consider an underhung topology.
 
Will this iron core saturate?

A friend, from Hungary, will build one of my amplifiers...but a special underpowered one..output may be 12 volts...so...1.5 amperes will cross the output coil.

The coil is very small, for instance, if he intend to use it with a 22 turns coil (air core is around 2uH.... what gonna be the resultant inductor value and i would like to know if you had that experience..if this saturates and produces an audible "tac" sound when a peak of audio crosses and saturates the core.

So.... three questions in the reality:

Will this saturate with only 12 volts and 1.5 amperes crossing?
What gonna be the resultant inductance of that coil, now over a ferrite core?
Had you experienced the audible "tac" noise of saturation?


If you have a chart, coil turns/coil wire diameter/Inductance.... please... send me or post it here:

carlos.eugenio1951@yahoo.com

regards,

Carlos
 

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What driver companies use are for profit not just performance. Feroba 2 is only used by Lowther, once it was common. The choice of voice coil formers are the same Royalties are paid on liscensed materials. Nomex is more neutral then aluminum and kapton, rarely used now . Pro musicians that play guitar like Celestion woofers due to the neutral sound the nomex formers give. Double stacked off centered magnets are the best. The cost is not profitable when off centering the magnets. Less leakage occurs in the magnetic field.. The same can be said of cone materials There have been some clasic designs that have not been recreated. I know of one company that had USA made stamped frame drivers that were superior to the European cast frame counter parts at a fraction of the cost that were superior in t-lines.
 
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