Using FEMM and how much flux density for mid driver - diyAudio
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Old 11th February 2013, 05:17 PM   #1
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Default Using FEMM and how much flux density for mid driver

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

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Old 11th February 2013, 05:32 PM   #2
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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.
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Old 11th February 2013, 06:12 PM   #3
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Quote:
Originally Posted by Jef Nuyts View Post
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.
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Old 11th February 2013, 06:30 PM   #4
sreten is offline sreten  United Kingdom
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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.
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Old 11th February 2013, 06:45 PM   #5
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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
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Old 11th February 2013, 07:00 PM   #6
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Quote:
Originally Posted by sreten View Post
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.
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Old 11th February 2013, 07:04 PM   #7
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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.
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Old 11th February 2013, 07:08 PM   #8
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Old 11th February 2013, 07:15 PM   #9
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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:

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Old 11th February 2013, 07:55 PM   #10
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Oké thx guys Bl = 7.2Tm so i need increase windings with 150% to achieve the desired 11Tm.

Oké next step...

Thx
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