Thanks I redraw the magnetic circuit in FEMM axisymmetric and now i got a more realistic B value. 0.9T for the long gap 1T for the smal gap. But this looks oke for a good motor.
regards
This is an older method to reduce flux mod. but is a somewhat wasteful of (expensive) BH.
The JBL technique eliminates flux mod. completely, not just reduce, and wastes no BH at all.
An improvement in every aspect is what I call clever. Have I missed some point?
There is no reason that a DCD could not be implemented as short coil/extended field
It would be more expensive but looks like the best possible structure for performance.
I have wanted to try it for years but too many other projects in line. So I really hope you do this!
Best wishes
David
The flux turn can still only reduce the modulation. The DCD still seems better.
Have you analysed the idea?
The Tech Note looks incorrect and I would love to have some expert confirm that or correct me.
Best wishes
David
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Dave,
I wouldn't argue whether the JBL magnetic circuit is an improvement over a standard magnetic circuit but the math doesn't appear to add up. If you have the same magnetic mass in the circuit and now two instead of one gap the total energy in the gap should actually be lower, this is equivalent to increasing the gap width which would lower the flux density. Something is wrong with the written explanation and I will have to read a better paper on the subject to comment further.
I wouldn't argue whether the JBL magnetic circuit is an improvement over a standard magnetic circuit but the math doesn't appear to add up. If you have the same magnetic mass in the circuit and now two instead of one gap the total energy in the gap should actually be lower, this is equivalent to increasing the gap width which would lower the flux density. Something is wrong with the written explanation and I will have to read a better paper on the subject to comment further.
Gedlee,
I have been looking into electrical steel from Carpenter , Advanced specialty, titanium and powder metallurgy materials from Carpenter Technology, which has a better saturation than typical 1008 or 1010 steels. As far as the shorting ring there is much to be desired in many implementations of that to reduce eddy currents. I agree it doesn't make sense many of the claims for the JBL magnetic circuit. The same amount of wire distributed over two gaps instead of one would add nothing to the BL factor and two gaps of equivalent clearance of a single gap is going to reduce the flux density total. Isn't marketing great!
I have been looking into electrical steel from Carpenter , Advanced specialty, titanium and powder metallurgy materials from Carpenter Technology, which has a better saturation than typical 1008 or 1010 steels. As far as the shorting ring there is much to be desired in many implementations of that to reduce eddy currents. I agree it doesn't make sense many of the claims for the JBL magnetic circuit. The same amount of wire distributed over two gaps instead of one would add nothing to the BL factor and two gaps of equivalent clearance of a single gap is going to reduce the flux density total. Isn't marketing great!
The improved symmetry means that a shorted turn is totally unneeded. Even the writer doesn't mention that so I assume s/he is not the inventor.
JBL do not claim any improvement to the BL/Re factor, in fact they emphasise that it is identical.
JBL also emphasise that the coil widths is halved. So ideally two in series will have the same reluctance as one wider one.
What JBL brush over is that a narrower voice coil still needs some clearance and this does not halve. Need some data on typical wire ribbon width and clearance to evaluate impact of this.
The fact that the new drivers used in their own systems use DCD imply that is more than spin. My analysis bears this out to some extent, despite a few omissions.
Best wishes
David
Eddy currents are different issue, as you say, so I don't quite understand your point. Flux mod. manifests as 2nd harmonic and is usually solved with a shorted turn. Eddy currents in the pole piece manifest as 3rd. The DCD solves flux mod. but they make no claims about eddy currents in the pole piece and I never mentioned them.
Best wishes
David
Just saying this is far from the perfect design as you seen to be stating. I highly doubt that they have truly controlled flux modulation. How do you propose that that was actually accomplished. These are mostly a nice marketing feature. I would like to see any magnetic motor that eliminates flux modulation without a shorting ring. The voicecoil is going to be what modulates the flux in this design. That is just not a true statement in the least but I don't feel like arguing the point. It isn't real.
This is the correct simulation. (I changed an example of Femm itself, now it must be correct)
Maybe using a shorten ring. Would it be possible to use a very thin insulation between the structure to prevent some distortion?
In this case I have a BL of 11Tm. DCR is 10ohm (1cm high coil) so i have to use parallel winding (DCR drops too low) or thicker wire. Is there a better solution, or what is the best solution to maintain the strength and get 6ohm DCR?
Maybe using a shorten ring. Would it be possible to use a very thin insulation between the structure to prevent some distortion?
In this case I have a BL of 11Tm. DCR is 10ohm (1cm high coil) so i have to use parallel winding (DCR drops too low) or thicker wire. Is there a better solution, or what is the best solution to maintain the strength and get 6ohm DCR?
At first it will be a mid. But afterwards I'm going to try to extend it to a mid-sub for two way speaker systems (with high lineair excursion)
Why I think it's possible? This design has no spider, no surround. The suspension is a current driven (no voltage driven to reduce EMK) electromagnetic coil. With the same lineairity as the voicecoil (is pretty good because of the underhung system) So Xmax 90% lineair. One problem that is solved with this system: At higher load the suspension will be more stiff to control the cone on higher volumes. So low volume Cms wil be high (low stiffness) in conjunction to volume the stiffness wil change to control the cone and achieve higher powerhandling.
The voicecoil go through the upper gap but this is a good thing for centering the system with ferrofluid (this also will increase magnetic permeability) also there wil be ferrofluid in the under gap for centering. This results in a fully centered system without spider or suspension.
Why I think it's possible? This design has no spider, no surround. The suspension is a current driven (no voltage driven to reduce EMK) electromagnetic coil. With the same lineairity as the voicecoil (is pretty good because of the underhung system) So Xmax 90% lineair. One problem that is solved with this system: At higher load the suspension will be more stiff to control the cone on higher volumes. So low volume Cms wil be high (low stiffness) in conjunction to volume the stiffness wil change to control the cone and achieve higher powerhandling.
The voicecoil go through the upper gap but this is a good thing for centering the system with ferrofluid (this also will increase magnetic permeability) also there wil be ferrofluid in the under gap for centering. This results in a fully centered system without spider or suspension.
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Dave,
Yes, of course. You limit the total flux. With DCD, you reduce flux in the gap (by factor 2), too, and you cannot compensate by making the gap just half that wide (because of the clearance).
Yes, in theory. In practice? Never!
Yes: a more complex magnet structure.
Earl,
a well designed saturated circuit (pole piece saturates) has already very low flux modulation, I don't think a short circuit ring improves it that much that it's worth using it.
Yes, of course. You limit the total flux. With DCD, you reduce flux in the gap (by factor 2), too, and you cannot compensate by making the gap just half that wide (because of the clearance).
Yes, in theory. In practice? Never!
Yes: a more complex magnet structure.
Earl,
a well designed saturated circuit (pole piece saturates) has already very low flux modulation, I don't think a short circuit ring improves it that much that it's worth using it.
If I make the outerdiameter smaller this would saturate the structure. Would it be a good solution? Saturation of "normal" steel is 2T?
It's not that hard to simulate...
rgds
No this is not the case because I'm going to try to use only the lower gap for the voicoil (lower THD). If it is not possible to gain enough strength (I am trying to get Q under 0.5) from the motor then i will use both (under + overhung). This would be better as only overhung but not as good as only underhung.
What you mean with "rocking modes"? Because of no damping on the outside (surround)?
Thx
This is a surroundless woofer. High mass but the concept works probably.
Twin Spider, speaker surround-less bass driver
Twin Spider, speaker surround-less bass driver
Jef,
Not seeing the complete device it is hard for me to see what is going to be your restoring force to bring the voicecoil back to the resting centered position? Rocking will be caused by any amount of out of balance condition on a moving mass. The fact that you are using two voicecoils should help to keep things inline but I am not sure that alone will be enough to keep one or both ends of the moving former from wanting to deform in any way. From my practical experience I will tell you that voicecoil formers are often not perfectly round, there is a slight amount of run-out in the circumference, check one sometimes and I am talking about high quality manufacturers still having this situation. Another thing to remember is that in a long gap construction the actual gap volume is a significant volume and ferro fluids are expensive. If this was a commercial product you would soon see how this would affect the cost of manufacturing but since this is a one off you can just pay the price. You are going to have to think about the venting of the areas around the gap as the ferro fluids do have a habit of moving if put into a hydraulic situation where you can physically push them out of the gap. It could migrate to either the top or bottom of the plates if you are not careful and then your centering function will be lost and the speaker will fail as the voicecoil rubs in the gap.
Not seeing the complete device it is hard for me to see what is going to be your restoring force to bring the voicecoil back to the resting centered position? Rocking will be caused by any amount of out of balance condition on a moving mass. The fact that you are using two voicecoils should help to keep things inline but I am not sure that alone will be enough to keep one or both ends of the moving former from wanting to deform in any way. From my practical experience I will tell you that voicecoil formers are often not perfectly round, there is a slight amount of run-out in the circumference, check one sometimes and I am talking about high quality manufacturers still having this situation. Another thing to remember is that in a long gap construction the actual gap volume is a significant volume and ferro fluids are expensive. If this was a commercial product you would soon see how this would affect the cost of manufacturing but since this is a one off you can just pay the price. You are going to have to think about the venting of the areas around the gap as the ferro fluids do have a habit of moving if put into a hydraulic situation where you can physically push them out of the gap. It could migrate to either the top or bottom of the plates if you are not careful and then your centering function will be lost and the speaker will fail as the voicecoil rubs in the gap.
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