Using FEMM and how much flux density for mid driver

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.

Jef Nuyts

Hi everybody,

I have a question but I can't find the answer on the net.
This is a simulation of a driver "motor" with FEMM (see picture) for a mid-woofer.
As you can see the neo magnets are in the center in the structure (yellow). I achieve a density of 1.7T (is this correct? i use planar in FEMM but i don't know if this is correct) and the coil winding is 0.24inch in height the wire is 0.006inch diameter. coil diameter is 1.47inch. I want a low Qes factor so Bl need to be high. I think simulating Bl is possible in Femm but i don't know how.

Thx

An externally hosted image should be here but it was not working when we last tested it.

speaker dave

If by 1.7 Tesla, you mean the average flux density in the gap, then you are half way to your answer. BL is in Tesla Meters, so you need to figure out a voice coil and find the length of winding inside the gap (this gives the effective L. Winding outside the gap doesn't count). Your B number times your winding length (as if unwond and measured for length) is the answer.

Look up wire guages and be aware that there needs to be a generous coil clearance inside the coil and outside the coil to prevent rubs. The windings tend to nest into a hex pattern. Allow for a voice coil former.

Also, you have to use coated wire dimensions (use heavy build, or 2 layer coating dimensions). This also reduces the copper in the gap. With all the corrections, you are lucky to get 10% of the ideal from a practical motor.

David S.

speaker dave

Hi everybody,

I achieve a density of 1.7T (is this correct? i use planar in FEMM but i don't know if this is correct) and the coil winding is 0.24inch in height the wire is 0.006inch diameter. coil diameter is 1.47inch. I want a low Qes factor so Bl need to be high. I think simulating Bl is possible in Femm but i don't know how.

1.7T or 17,000 Gauss is very high for a structure. The usual limitation is saturation in the plates (where you show 23,000 Gauss internal). I assume the FEMM takes steel characteristics into account?

You might try the same design with the plate overhangs removed (where their diameter is greater than the magnet). They encourage outer fringing flux which is wasteful. Traditionally the top and bottom plates area little less in diameter than the magnet (by a plate thickness for each radius) to reduce this factor. Try it in your simulation and see if it changes gap flux.

David S.

sreten

Hi,

I can't fathom the point of that magnetic structure at all.

Two magnets ? two gaps ? Why ? Why those bits stick out ?

rgds, reten.

Jef Nuyts

Oké in that case i can easely calculate BL. Does someone know if the settings are correct of FEMM? I use the following settings:
planar
depth 1
min angle 30

speaker dave

Hi,

Two magnets ? two gaps ? Why ? Why those bits stick out ?

rgds, reten.

Two magnets but only one gap, as far as I can see.

Oh, gaps below. Is this a push pull, similar to the JBL units?

David S.

Jef Nuyts

I use 1010 steel in FEMM. I have made a test motor (it was for testing purpose) with normal steel (construction steel) the speaker worked but the sensitivity is not high enough. So first I need higher Bl i guess. The reason for the overhang is to bolt the structure together instead of glueing it. So i can adjust it for centering. In femm it make almost no difference with or without. It is hard to explain why two gaps. But i try: The gap on the bottom is for the voicewinding (sound) + suspension winding (EM suspension). the second on top is for center the voicecoil with ferrofluid + I thought if i use two magnets on top and under the big gap i have a near lineair field (the simulation in femm is for 90% lineair) 15mm. I use a 6mm coil winding so i have Xmax of 4.5mm in one direction.

Jef Nuyts

The reason for the strange design is that the motor has to be a small diameter. So the motor is "in" the cone to minimalize the voicecoil length.
The driver is a dome speaker not a cone.
a simple picture:

An externally hosted image should be here but it was not working when we last tested it.

Jef Nuyts

Oké thx guys Bl = 7.2Tm so i need increase windings with 150% to achieve the desired 11Tm.

Oké next step...

Thx

speaker dave

Increase 50% (to a length of 150%).

Your short coil long gap design will give great linearity but unfortunatly wastes a lot of "B".

Note that Bl alone isn't usually enough information for a full comparison. If you add turns to the coil Bl goes up but DCR does too. In the end, the best comparison between drivers is via: B squared * l squared / Re. That is the measure of motor strength.

Efficiency is then related to the product of B squared * volume of copper (impedance normalized out). This generally leads to a conclusion that efficiency grows with more copper in the coil until the coil mass and diaphragm mass are roughly equal.

Regards,
David S.

sreten

Hi,

You can't drive a dome like that, from near the centre.

rgds, sreten.

Jef Nuyts

Hi,

You can't drive a dome like that, from near the centre.

rgds, sreten.

Why you can't? I don't see the difference? It is a cone with the motor on the other side. Are do I miss something?
It is a 7" driver out of carbon fibre, that will maybe change something ;-)

Last edited:

Jef Nuyts

Increase 50% (to a length of 150%).

Your short coil long gap design will give great linearity but unfortunatly wastes a lot of "B".

Note that Bl alone isn't usually enough information for a full comparison. If you add turns to the coil Bl goes up but DCR does too. In the end, the best comparison between drivers is via: B squared * l squared / Re. That is the measure of motor strength.

Efficiency is then related to the product of B squared * volume of copper (impedance normalized out). This generally leads to a conclusion that efficiency grows with more copper in the coil until the coil mass and diaphragm mass are roughly equal.

Regards,
David S.

Offcourse I mean the total lengt 150% dutch englisch, englisch dutch brrr confusing..
Oké but L has more weight in the formula than Re.
DCR on this moment for (6mm height) => 6ohm so i can raise a bit to 7.2 for +/- a 8ohm speaker. But that isn't enough damn. I have ordered alu wire to lower the mass. So maybe 9-10Tm is enough for a decent Q (I have to calculate that) So much variables to deal with...

Everyday is a good day to learn

Last edited:

Jef Nuyts

I was thinking this night for a solution to lower the resistance of the coil.
If you put 2 wires together for winding the voicecoil (winding with 2 wires at the same time) so you will have the two wires in parallel. DCR wil drop by half. I think the efficienty will raise. Not? I mean with the same resistance as therefore the lenght wil double so Bl wil raise..
B will drop a bit because the gap has to be higher. Thx,

Last edited:

Dave Zan

New idea!

To improve efficiency don't waste the flux in the front gap.
You could have an extended voice-coil there and a short coil in the back!
I think over AND under voice coils would be unique.
You could still put ferro-fluid there to help centre the cone or dome.

Serious now, have you looked at the JBL Dual Coil Drive (DCD)?
A clever way to totally eliminate flux modulation and they have other benefits too.
The JBL tech. note writer does not seem to understand clearly how they work but the note is still worth a read.

Best wishes
David

Last edited:

Jef Nuyts

Oke good thought Dave! Over and under coil would be a good solution.
The extra force would compensate the extra mass easely, especially with alu windings.

I looked at the dcd system. almost the same but I need underhung for the electromagnetic suspension (2 layer system).
The reason why the extended voicecoil is in the middle is for the higher flux density because of the two magnets are polarized in opposite direction. The gap is bigger but the density is the same as the small gap.
I'm gonna calculate and see if the parameters are good with these modification.
Keep u posted

speaker dave

I was thinking this night for a solution to lower the resistance of the coil.
If you put 2 wires together for winding the voicecoil (winding with 2 wires at the same time) so you will have the two wires in parallel. DCR wil drop by half. I think the efficienty will raise. Not? I mean with the same resistance as therefore the lenght wil double so Bl wil raise..
B will drop a bit because the gap has to be higher. Thx,

Read what I said about B square l squared over Re. (also B squared over V gap or V copper) If you go from 100 turns of a gauge to 2 x 50 turns of the same gauge you can parallel connect or series connect. If you parallel connect you have half the length and 1/4 the resistance. Same (Bl)2/Re. If you series connect you have the same coil again. If you shrink the gauge to allow two windings of the same number of turns your packing factor will reduce and efficiency will drop.

More copper in the gap gives higher efficiency (not voltage sensitivity, but true efficiency). Impedance, or how you slice the copper, is irrelevant.

Regards,
David S.

Baseballbat

i use planar in FEMM but i don't know if this is correct)

I guess your magnetics circuit is axisymmetric (like most drivers), so the correct setting would be exactly that: "Axisymmetric". "Planar" is more for planar drivers, like ribbons or magnetostatics.

I don't know if this gives a difference here.

@Dave Zan: a far more clever method to reduce modulation distortion is to saturate the magnetic circuit. For best results you need a magnetic material with a nearly rectangular B-H curve and with the "knee" that low that you can never drive the material out of saturation. It is not very easy, but is done today already.

Jef Nuyts

Read what I said about B square l squared over Re. (also B squared over V gap or V copper) If you go from 100 turns of a gauge to 2 x 50 turns of the same gauge you can parallel connect or series connect. If you parallel connect you have half the length and 1/4 the resistance. Same (Bl)2/Re. If you series connect you have the same coil again. If you shrink the gauge to allow two windings of the same number of turns your packing factor will reduce and efficiency will drop.

More copper in the gap gives higher efficiency (not voltage sensitivity, but true efficiency). Impedance, or how you slice the copper, is irrelevant.

Regards,
David S.

Yes i understand what you mean. The thrue efficiency is not affected. But most amplifiers are 4ohm stable so higher current gives more lorenz force and the length of the wire is the same (in the gap). Like you said voltage sensitivity wil raise and that is also oke for me. In other words the amplifier wil give more power on the same output level (so I use the amplifier to compensate). Not a problem for normal amplifiers but for tubes is another story... But the solution of Dave is better

Last edited:
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.