Mail Archives: geda-user/2011/09/21/12:12:47
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I posted a few weeks ago asking about how magnetic exercise bikes work.
Below I copied something Peter Clifton said and John Doty's comment in
response;
> If you're after significant resistance, I would go for a copper disk,
> about 5mm or thicker, with strong magnets - either an electromagnet on
> an iron core - placed quite close (within a few millimetres) of the
> spinning disk, OR - some neodymium hard-disk magnets (for example).
>
> You could use an aluminim disk (much cheaper, and easier to obtain I'd
> imagine) - but I would up the thickness.
The torque will fall off at low speed, when the magnetic diffusion depth
exceeds the thickness of the disk. The diffusion depth is given by:
sqrt(dm*t)
where dm is the magnetic diffusion coefficient, about 130 cm^2/s for Cu,
230 cm^2/s for Al. The time parameter, t, is essentially the time a point on
the disk remains in the vicinity of the magnet.
This is, of course, just dimensional analysis: a detailed model of the
field configuration is needed if you need to be more quantitative.
The same physics leads to the concept of "skin depth".
I'd like to order material to start prototyping. I would like to be able to
calculate approximate torques for given size disks of aluminum or copper and
approximate size, quantity and strength of magnets mounted to a disk a given
proximity away. Does anyone know the theorey and how I would try to model
this to calculate the variables and purchase materials to begin a
prototype? Or refer me to a source covering the theory?
Thanks,
Rob
On Fri, Sep 9, 2011 at 2:54 PM, Rob Butts <r DOT butts2 AT gmail DOT com> wrote:
>
> Wow, this is fantastic everyone. Thanks!
>
> I knew I came to bthe right place!
>
> Rob
>
>
> On Fri, Sep 9, 2011 at 12:41 PM, John Doty <jpd AT noqsi DOT com> wrote:
>
>>
>> On Sep 9, 2011, at 5:29 AM, Peter Clifton wrote:
>>
>> > If you're after significant resistance, I would go for a copper disk,
>> > about 5mm or thicker, with strong magnets - either an electromagnet on
>> > an iron core - placed quite close (within a few millimetres) of the
>> > spinning disk, OR - some neodymium hard-disk magnets (for example).
>> >
>> > You could use an aluminim disk (much cheaper, and easier to obtain I'd
>> > imagine) - but I would up the thickness.
>>
>>
>> The torque will fall off at low speed, when the magnetic diffusion depth
exceeds the thickness of the disk. The diffusion depth is given by:
>>
>> sqrt(dm*t)
>>
>> where dm is the magnetic diffusion coefficient, about 130 cm^2/s for Cu,
230 cm^2/s for Al. The time parameter, t, is essentially the time a point on
the disk remains in the vicinity of the magnet.
>>
>> This is, of course, just dimensional analysis: a detailed model of the
field configuration is needed if you need to be more quantitative.
>>
>> The same physics leads to the concept of "skin depth".
>>
>> John Doty Noqsi Aerospace, Ltd.
>> http://www.noqsi.com/
>> jpd AT noqsi DOT com
>>
>>
>>
>>
>> _______________________________________________
>>
>> geda-user mailing list
>> geda-user AT moria DOT seul DOT org
>> http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
>>
>
>
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<div>I posted a few weeks ago asking about how magnetic exercise bikes work=
.=A0 Below I copied something Peter Clifton said and John Doty's commen=
t in response;</div><div>=A0</div><blockquote style=3D"margin-right: 0px;" =
dir=3D"ltr">
<div>=A0> If you're after significant resistance, I would go for a c=
opper disk,</div><div>=A0> about 5mm or thicker, with strong magnets - e=
ither an electromagnet on</div><div>=A0> an iron core - placed quite clo=
se (within a few millimetres) of the</div>
<div>=A0> spinning disk, OR - some neodymium hard-disk magnets (for exam=
ple).</div><div>=A0>=A0</div><div>=A0> You could use an aluminim disk=
(much cheaper, and easier to obtain I'd</div><div>=A0> imagine) - b=
ut I would up the thickness.</div>
<div>=A0</div><div>=A0The torque will fall off at low speed, when the magne=
tic diffusion depth exceeds the thickness of the disk. The diffusion depth =
is given by:</div><div>=A0sqrt(dm*t)</div><div>=A0where dm is the magnetic =
diffusion coefficient, about 130 cm^2/s for Cu, 230 cm^2/s for Al. The time=
parameter, t, is essentially the time a point on the disk remains in the v=
icinity of the magnet.</div>
<div>=A0This is, of course, just dimensional analysis: a detailed model of =
the field configuration is needed if you need to be more quantitative.</div=
><div>=A0The same physics leads to the concept of "skin depth".</=
div>
</blockquote><div>=A0</div><div>I'd like to order material to start pro=
totyping.=A0 I would like to be able to calculate approximate torques for g=
iven size disks of aluminum or copper and approximate size, quantity and st=
rength of magnets mounted to a disk a given proximity away.=A0 Does anyone =
know the theorey and=A0how I would try to model this=A0to calculate the var=
iables=A0and purchase materials to begin a prototype?=A0 Or refer me to a s=
ource covering the theory?</div>
<div>=A0</div><div>Thanks,</div><div>Rob</div><div>=A0</div><div>=A0</div><=
div>=A0</div><div>On Fri, Sep 9, 2011 at 2:54 PM, Rob Butts <<a href=3D"=
mailto:r DOT butts2 AT gmail DOT com">r DOT butts2 AT gmail DOT com</a>> wrote:</div><div>=A0<=
/div><div>
></div><div>> Wow, this is fantastic everyone.=A0 Thanks!</div><div>&=
gt; =A0</div><div>> I knew I came to bthe right place!</div><div>> =
=A0</div><div>> Rob</div><div>> =A0</div><div>> =A0</div><div>>=
On Fri, Sep 9, 2011 at 12:41 PM, John Doty <<a href=3D"mailto:jpd AT noqsi=
.com">jpd AT noqsi DOT com</a>> wrote:</div>
<div>> =A0</div><div>>></div><div>>> On Sep 9, 2011, at 5:29=
AM, Peter Clifton wrote:</div><div>>> =A0</div><div>>> > If=
you're after significant resistance, I would go for a copper disk,</di=
v>
<div>>> > about 5mm or thicker, with strong magnets - either an el=
ectromagnet on</div><div>>> > an iron core - placed quite close (w=
ithin a few millimetres) of the</div><div>>> > spinning disk, OR -=
some neodymium hard-disk magnets (for example).</div>
<div>>> ></div><div>>> > You could use an aluminim disk (=
much cheaper, and easier to obtain I'd</div><div>>> > imagine)=
- but I would up the thickness.</div><div>>> =A0</div><div>>> =
=A0</div>
<div>>> The torque will fall off at low speed, when the magnetic diff=
usion depth exceeds the thickness of the disk. The diffusion depth is given=
by:</div><div>>> =A0</div><div>>> sqrt(dm*t)</div><div>>>=
; =A0</div>
<div>>> where dm is the magnetic diffusion coefficient, about 130 cm^=
2/s for Cu, 230 cm^2/s for Al. The time parameter, t, is essentially the ti=
me a point on the disk remains in the vicinity of the magnet.</div><div>
>> =A0</div><div>>> This is, of course, just dimensional analys=
is: a detailed model of the field configuration is needed if you need to be=
more quantitative.</div><div>>> =A0</div><div>>> The same phys=
ics leads to the concept of "skin depth".</div>
<div>>> =A0</div><div>>> John Doty =A0 =A0 =A0 =A0 =A0 =A0 =A0N=
oqsi Aerospace, Ltd.</div><div>>> <a href=3D"http://www.noqsi.com/">h=
ttp://www.noqsi.com/</a></div><div>>> <a href=3D"mailto:jpd AT noqsi DOT com=
">jpd AT noqsi DOT com</a></div>
<div>>> =A0</div><div>>> =A0</div><div>>> =A0</div><div>&=
gt;> =A0</div><div>>> ____________________________________________=
___</div><div>>> =A0</div><div>>> geda-user mailing list</div><=
div>>> <a href=3D"mailto:geda-user AT moria DOT seul DOT org">geda-user AT moria DOT se=
ul.org</a></div>
<div>>> <a href=3D"http://www.seul.org/cgi-bin/mailman/listinfo/geda-=
user">http://www.seul.org/cgi-bin/mailman/listinfo/geda-user</a></div><div>=
>> =A0</div><div>></div><div>> =A0</div><div>=A0</div><div>=A0<=
/div><div>
=A0</div>
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