PSN-L Email List Message

Subject: Re: Sensor and Damping magnets
From: ChrisAtUpw@.......
Date: Tue, 28 Mar 2006 20:59:10 EST


 
In a message dated 28/03/2006, ivey@.......... writes:

 
 
Chris, 
 
>The 5 mm plate that  I used at first was only just thick enough to take the 
flux of the 1/2" x 1/4"  thick magnets. 
Did you calculate this or measure it  somehow? 
Thanks, 
Jack




 
Hi Jack, 
 
    I did both. You can look up the flux densities of  the magnets and the Bm 
saturation for mild steel. If you want to get very low  leakage flux, all the 
flux has to go sideways through the mild steel plate. In  my design, I allow 
a 1/2" border around the periphery of the magnets for  the internal field to 
spread out and also to catch stray flux. You get  a flux concentration near the 
magnet join and field from the edges of the  magnets themselves.
    I also mounted various arrangements of magnets  on one side of a larger 
mild steel plate and then used a transformer  lamination strip suspended with 
cellotape on the other side to check if  there was any field penetration. With 
the 5 mm plate and 20x10x5 mm magnet  pairs, there was no effect. With 20x20x5 
mm magnets there was a small joint area  which attracted the strip. Going to 
1"x1/2"x1/4" magnet pairs, I could just  detect some attraction. Doubling it 
up to 1"x1"x1/4" magnet pairs produced a  fairly strong attraction area. In 
contrast, the 1"squarex1/8" sensor magnet  pairs seemed OK. 
    I chose 1/4" mild steel (6.35 mm) with the  1"x1/2"x1/4" magnet pairs. I 
may be being 'a bit fussy', but I don't want  any stray field about and I do 
want the maximum field in the magnet gap. Using  mild steel bolts to secure the 
plates was of considerable help - no two magnets  have an identical flux and 
I am using 4 off ! I was satisfied the this was 'good  enough'.
    The maximum damping effect is near the  central join. To get maximum 
damping the copper plate needs to overlap both  ends of the 1" magnet square by 
about 1/2". 
 
    Regards,
 
    Chris Chapman






In a message dated 28/03/2006, ivey@.......... writes:
<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>

Chris, 

 <= /FONT>>The 5 mm plate= that=20 I used at first was only just thick enough to take the flux of the 1/2" x=20= 1/4"=20 thick magnets.

= Did you calculate this or measure i= t=20 somehow?

 Thanks,

Jack

Hi Jack,
 
    I did both. You can look up the flux densities=20= of=20 the magnets and the Bm saturation for mild steel. If you want to get very lo= w=20 leakage flux, all the flux has to go sideways through the mild steel plate.=20= In=20 my design, I allow a 1/2" border around the periphery of the magnets fo= r=20 the internal field to spread out and also to catch stray flux. You= get=20 a flux concentration near the magnet join and field from the edges of the=20 magnets themselves.
    I also mounted various arrangements of mag= nets=20 on one side of a larger mild steel plate and then used a transform= er=20 lamination strip suspended with cellotape on the other side to check if= =20 there was any field penetration. With the 5 mm plate and 20x10x5 mm magnet=20 pairs, there was no effect. With 20x20x5 mm magnets there was a small joint=20= area=20 which attracted the strip. Going to 1"x1/2"x1/4" magnet pairs, I could just=20 detect some attraction. Doubling it up to 1"x1"x1/4" magnet pairs produced a= =20 fairly strong attraction area. In contrast, the 1"squarex1/8" sensor ma= gnet=20 pairs seemed OK. 
    I chose 1/4" mild steel (6.35 mm) with the=20 1"x1/2"x1/4" magnet pairs. I may be being 'a bit fussy', but I don't wa= nt=20 any stray field about and I do want the maximum field in the magnet gap. Usi= ng=20 mild steel bolts to secure the plates was of considerable help - no two magn= ets=20 have an identical flux and I am using 4 off ! I was satisfied the this was '= good=20 enough'.
    The maximum damping effect is near the=20 central join. To get maximum damping the copper plate needs to overlap=20= both=20 ends of the 1" magnet square by about 1/2".
 
    Regards,
 
    Chris Chapman

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