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