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'[EE] Electromagnet query'
2012\12\18@193327 by IVP

face picon face
Hi all,

just a reality check before embarking on tedious winding ....

I need to make a couple of electromagnets for kind of a magnetic
suspension (not levitating, motion damping)

What I'm repelling against are fairly strong permanent ferrite
magnets. These are about 25mm diameter and 6mm thick, with
the poles on the faces

So I first Googled for ideas on construction. After seeing several
people suggesting that strong electromagnets should be U-shaped,
I thought that was the way to go. However, on reflection, that's
probably not what I want. The U-shape has N and S poles at the
ends of the U. Which might be best for attracting metal but seems
to me would be ineffective for repelling another magnet

OK so far ?

What I should be looking at then is more like a solenoid or relay
coil, which will have a pole at each end of the core rod. If so, I've
seen a suggestion that the addition of a permanent magnet to the
opposite end of the core will pre-arrange the domains so that the
energising of the coil will be more effective

Is that something anyone has tried and found to be true ?

TIA

Jo

2012\12\18@205250 by Brendan Gillatt

flavicon
face
In this case it is useful to remember that magnetic fields are linear --
i.e. they can be superimposed upon one another. The field from your
electromagnet will, with some approximation, "add" to the field from
your permanent magnet. So in your case the magnet to be damped will see the
force from the electromagnet /plus/ the force from the permanent one. A
permanent magnet will not automatically make an electromagnet stronger. The
work done by a magnetic field has to come from somewhere
and permanent magnets don't work for free!

Whether or not the permanent magnet will be useful depends on your
arrangement. More details would really help: how will the ferrite magnet be
moving, where will the electromagnet be positioned, etc.

NB: the shape of an electromagnet is no indicator of its strength! The
U-shape simply confines the magnetic flux to a different path to a straight
electromagnet, giving rise to a different magnetic field outside of the
confines of the ferrite/steel it is made of.

Regards,

Brendan

On 19 December 2012 00:32, IVP <spam_OUTjoecolquittTakeThisOuTspamclear.net.nz> wrote:

{Quote hidden}

>

2012\12\18@221203 by Marcel Duchamp

picon face
Do a google image search on "electromagnetic field pattern".  You will see what the fields look like for various core shapes.

It sounds like you are interested in maximizing the field strength. Basically this can be done in two ways: maximum amp-turns and shaping the field density.

Maximum amp-turns works like this: you get the same field from one amp and 100 turns of wire as you do from 100 amps and one turn of wire.  Or 1 mA and 100,000 turns of wire.

Practical aspects quickly push you into a particular solution.  You probably have some maximum volume you can occupy and some power source with a maximum current available.  Wire has volume and resistance. Smaller wire gives more turns in the available volume but higher resistance means you need a higher voltage to get the current.  And so on.

As for field density, you mentioned the solenoid shape versus the horse-shoe or 'U' shape.  You can go farther and make a 'C' shape. Magnetic flux is concentrated in the iron core versus air.  In the solenoid core, the flux departs immediately at the ends of the core and bends back around to the other end.  The effect is that the flux density quickly reduces.  In the 'U' core, the ends are close and the field goes from one to the other with higher density thus higher field strength. In the 'C' core, the air gap determines the density and as the points get closer, the field strength increases.  The problem you are solving determines the shape that can be used so it's up to you.

As for permanent magnets, Brendan mentioned the fact that the fields add together.  This is up until the core is saturated in which case no further increase of field strength is had even if the current is increased.  For a quick and horrible demonstration of this, bring a high power rare earth magnet up to the inductor in a switching power supply under maximum current load - you can easily saturate the inductor and then destroy the switching circuit due to high current.

Wikipedia has a lot on this stuff as do certain various techie websites.

2012\12\18@230223 by IVP

face picon face
> Whether or not the permanent magnet will be useful depends on
> your arrangement. More details would really help: how will the
> ferrite magnet be moving, where will the electromagnet be
> positioned, etc

The closest analogy I could give would be a door and a rubber
doorstop, although my application is vertical. The permanent
magnet is on the door and the EM is the doorstop. My hope was
to detect the approach of the 'door', probably with either a Hall
Effect chip, or maybe even an induced voltage in the EM, and
energise the EM to prevent a hard collision. After which the field
could be reduced with low frequency PWM (as the core will be
slow-to-react iron) to allow soft contact

> NB: the shape of an electromagnet is no indicator of its strength! The
> U-shape simply confines the magnetic flux to a different path to a
> straight electromagnet, giving rise to a different magnetic field outside
> of the confines of the ferrite/steel it is made of

By that I take it to mean that, for a given electrical input, the total
energy in the flux is the same but the chosen core shape distributes
or focuses it appropriately for the application

Jo

2012\12\18@231418 by IVP

face picon face
> maximum amp-turns and shaping the field density

I plan, unless advised otherwise, to make the core long and slender,
probably 6mm soft iron rod. That way I can get the turns up, 26 or
28 gauge, so the distance from the core should be minimised, and
will help keep the current down

> Practical aspects quickly push you into a particular solution.  You
> probably have some maximum volume you can occupy and some
> power source with a maximum current available

The initial energising will be a DC dump via a FET and reservoir cap.
After that I can let the PSU take over. There's no real restriction on
parameters but obviously the coil and PSU don't need to be unnecessarily
big

..  Wire has volume and resistance.
> Smaller wire gives more turns in the available volume but higher
> resistance means you need a higher voltage to get the current.  And so on..

> In the solenoid core, the flux departs immediately at the ends of the core
> and bends back around to the other end.  The effect is that the flux
> density
> quickly reduces

But it would be strongest and most repulsive at the point it leaves the core ?

> The problem you are solving determines the shape that can be used so
> it's up to you

If what I just asked is correct, then a straight bar would be best to repel
a magnet as it's pole against pole, with the attracting pole at the other end
of the EM core

> As for permanent magnets, Brendan mentioned the fact that the fields
> add together.  This is up until the core is saturated in which case no
> further increase of field strength is had even if the current is increased

I think I picked up on that on my travels. My question was whether a
permanent magnet can improve the efficiency or response of the EM
by "wetting" the domains before current is applied

> For a quick and horrible demonstration of this, bring a high power
> rare earth magnet up to the inductor in a switching power supply
> under maximum current load - you can easily saturate the inductor and
> then destroy the switching circuit due to high current.

Much as I'd like to see that (on someone else's of course), I'll take
your word for it ;-))

Joe

2012\12\19@050547 by Nily

flavicon
face
The closest analogy I could give would be a door and a rubber
doorstop, although my application is vertical. The permanent
magnet is on the door and the EM is the doorstop. My hope was
to detect the approach of the 'door', probably with either a Hall
Effect chip, or maybe even an induced voltage in the EM, and
energise the EM to prevent a hard collision. After which the field
could be reduced with low frequency PWM (as the core will be
slow-to-react iron) to allow soft contact

Will you invert poles once the 'door' is near your EM ? Is it imperative to
use an EM ?

Cheers

Rodolfo

-----Mensaje original-----
De: .....piclist-bouncesKILLspamspam@spam@mit.edu [piclist-bouncesspamKILLspammit.edu]En nombre de
IVP
Enviado el: Martes, 18 de Diciembre de 2012 08:14 p.m.
Para: Microcontroller discussion list - Public.
Asunto: Re: [EE] Electromagnet query


> maximum amp-turns and shaping the field density

I plan, unless advised otherwise, to make the core long and slender,
probably 6mm soft iron rod. That way I can get the turns up, 26 or
28 gauge, so the distance from the core should be minimised, and
will help keep the current down

> Practical aspects quickly push you into a particular solution.  You
> probably have some maximum volume you can occupy and some
> power source with a maximum current available

The initial energising will be a DC dump via a FET and reservoir cap.
After that I can let the PSU take over. There's no real restriction on
parameters but obviously the coil and PSU don't need to be unnecessarily
big

..  Wire has volume and resistance.
> Smaller wire gives more turns in the available volume but higher
> resistance means you need a higher voltage to get the current.  And so on..

> In the solenoid core, the flux departs immediately at the ends of the core
> and bends back around to the other end.  The effect is that the flux
> density
> quickly reduces

But it would be strongest and most repulsive at the point it leaves the core
?

> The problem you are solving determines the shape that can be used so
> it's up to you

If what I just asked is correct, then a straight bar would be best to repel
a magnet as it's pole against pole, with the attracting pole at the other
end
of the EM core

> As for permanent magnets, Brendan mentioned the fact that the fields
> add together.  This is up until the core is saturated in which case no
> further increase of field strength is had even if the current is increased

I think I picked up on that on my travels. My question was whether a
permanent magnet can improve the efficiency or response of the EM
by "wetting" the domains before current is applied

> For a quick and horrible demonstration of this, bring a high power
> rare earth magnet up to the inductor in a switching power supply
> under maximum current load - you can easily saturate the inductor and
> then destroy the switching circuit due to high current.

Much as I'd like to see that (on someone else's of course), I'll take
your word for it ;-))

Joe

2012\12\19@053340 by IVP

face picon face
> Will you invert poles once the 'door' is near your EM ? Is it
> imperative to use an EM ?

Hi Rodolfo,

Simply turning off the current will make the 'door' magnet stick to
the end of the iron core. Which I think makes it and the EM a good
pair. If it was necessary to release the magnet with little or no force
(ie not manually) the current can be turned on to repel it. Still having
a think about that part of the mechanism

Jo

2012\12\19@054107 by alan.b.pearce

face picon face
> The closest analogy I could give would be a door and a rubber doorstop, although my
> application is vertical. The permanent magnet is on the door and the EM is the
> doorstop. My hope was to detect the approach of the 'door', probably with either a
> Hall Effect chip, or maybe even an induced voltage in the EM, and energise the EM to
> prevent a hard collision. After which the field could be reduced with low frequency
> PWM (as the core will be slow-to-react iron) to allow soft contact

Would it be enough to have an un-energised electromagnet, and short the ends of the winding together, so the induced current causes a back electromagnetic field? Probably work better with pole pieces on both items designed to form a more complete magnetic circuit than a disc magnet.
-- Scanned by iCritical.

2012\12\19@064400 by Nily

flavicon
face
Hello Joe

I thought something like this.

Glue a PM to the 'door' and as a rubber stopper use another Permanent magnet
(Neo, the best) inside a non magnetic tube with a plastic or wooden piston,
glue that PM and use a spring with a screw and nut  on the opposite side of
the tube, to regulate it's force.
Not very techy but will work... may be for your application.

Cheers

Rodolfo

-----Mensaje original-----
De: .....piclist-bouncesKILLspamspam.....mit.edu [EraseMEpiclist-bouncesspam_OUTspamTakeThisOuTmit.edu]En nombre de
IVP
Enviado el: Miercoles, 19 de Diciembre de 2012 02:34 a.m.
Para: Microcontroller discussion list - Public.
Asunto: Re: [EE] Electromagnet query


> Will you invert poles once the 'door' is near your EM ? Is it
> imperative to use an EM ?

Hi Rodolfo,

Simply turning off the current will make the 'door' magnet stick to
the end of the iron core. Which I think makes it and the EM a good
pair. If it was necessary to release the magnet with little or no force
(ie not manually) the current can be turned on to repel it. Still having
a think about that part of the mechanism

Jo

2012\12\19@064400 by Nily

flavicon
face
Hello  Joe

I thou

-----Mensaje original-----
De: piclist-bouncesspamspam_OUTmit.edu [@spam@piclist-bouncesKILLspamspammit.edu]En nombre de
IVP
Enviado el: Miercoles, 19 de Diciembre de 2012 02:34 a.m.
Para: Microcontroller discussion list - Public.
Asunto: Re: [EE] Electromagnet query


> Will you invert poles once the 'door' is near your EM ? Is it
> imperative to use an EM ?

Hi Rodolfo,

Simply turning off the current will make the 'door' magnet stick to
the end of the iron core. Which I think makes it and the EM a good
pair. If it was necessary to release the magnet with little or no force
(ie not manually) the current can be turned on to repel it. Still having
a think about that part of the mechanism

Jo

2012\12\19@071133 by IVP

face picon face
> Would it be enough to have an un-energised electromagnet, and
> short the ends of the winding together, so the induced current causes
> a back electromagnetic field?

Hmmm, possibly. I've wound a coil, I'll try it tomorrow when I knock up
a test jig

Jo

2012\12\19@071133 by IVP

face picon face
> spring with a screw and nut  on the opposite side of the tube, to
> regulate it's force. Not very techy but will work... may be for your
> application

Yes, it might just. I've got a pretty good range of compression springs,
have to see if I've got one that has the right stiffness

Jo

2012\12\19@105301 by John Gardner

picon face
Re springs - Small OHC IC engines often use double,
sometimes triple cylinder head valve springs. Inner springs
can be quite small.

Another approach might use a piston which fits snugly enough
in a cylinder to provide pneumatic damping, with a very light spring
to return the piston

2012\12\19@144757 by Roger Furer

picon face
>The initial energising will be a DC dump via a FET and reservoir cap.After that I can let the PSU take over.

I'd suggest that by sizing the cap correctly and perhaps adding a resistor to achieve finer timing control, you could let the cap's own discharge rate reduce the field strength. If you needed a quicker 'turn-off' then a second FET to directly discharge the cap could be used.

Cheers,

Roge

2012\12\19@161247 by IVP

face picon face
John, Roger

thanks for the suggestions. I think what will eventuate depends
largely on the physical set-up, ie weights, speed etc

An EM looks good in my mind's eye and I know there's a practical
solution, hopefully one that's flexible enough to have a reasonable
tuning range

Jo

2012\12\19@165151 by John Gardner

picon face
Hi Joe -

I was envisaging the magnet as the piston in the pneumatic damper.

I tend to Heath Robinson solutions - Sounds like a fun project...

Jac

2012\12\19@171910 by IVP

face picon face
> I was envisaging the magnet as the piston in the pneumatic damper

Me too, like a typical anti-slam door closer

The end effect I hope for is similar to catching a ball properly, eg no
bounce or undue mechanical stress and a smooth, gradual decelaration
and capture

Jo

2012\12\19@195445 by John Gardner

picon face
.... like a typical anti-slam door closer...

It can't be terribly abstruse if I thought of it...   :


'[EE] Electromagnet query'
2013\01\02@163756 by Barry Gershenfeld
picon face
On Wed, Dec 19, 2012 at 2:41 AM, <KILLspamalan.b.pearceKILLspamspamstfc.ac.uk> wrote:

> Would it be enough to have an un-energised electromagnet, and short the
> ends of the winding together, so the induced current causes a back
> electromagnetic field? Probably work better with pole pieces on both items
> designed to form a more complete magnetic circuit than a disc magnet.


What you are about to invent here is a damper that works on eddy currents.
A strong magnet moving in close proximity across a metal plate induces a
field in the plate that opposes the motion.  No power supply required, and
fully automatic--the faster the magnet moves, the more opposing force.  You
can demonstrate this yourself with a supermagnet and an aluminum plate.
For a thoroughly outrageous example, locate "The Strongest Magnet In The
World" on YouTube, skip to 6:30, and watch the show.  Or use this link:
bit.ly/XjCPl

2013\01\02@171251 by IVP

face picon face
> What you are about to invent here is a damper that works on
> eddy currents

Hi Barry,

I'm sure I've seen this on TV used in railway sidings. And also sure
it was somewhere in Holland/Germany. That was quite some time
ago, maybe it's common practice nowadays

Still tinkering, as holidays/family stuff permit

Jo

2013\01\02@175327 by alan.b.pearce

face picon face
> > Would it be enough to have an un-energised electromagnet, and short
> > the ends of the winding together, so the induced current causes a back
> > electromagnetic field? Probably work better with pole pieces on both
> > items designed to form a more complete magnetic circuit than a disc
> magnet.
>
>
> What you are about to invent here is a damper that works on eddy currents..
....
Exactly, I figured it would provide exactly the sort of damping that Joe seemed to be after.


-- Scanned by iCritical.

2013\01\02@175332 by alan.b.pearce

face picon face
> > What you are about to invent here is a damper that works on eddy
> > currents
> I'm sure I've seen this on TV used in railway sidings. And also sure it was
> somewhere in Holland/Germany. That was quite some time ago, maybe it's
> common practice nowadays
>
> Still tinkering, as holidays/family stuff permit

I believe that some (probably all) European high speed trains use eddy braking for initial braking from high speed, then use a combination of eddy and friction braking as the speed drops, finally using friction braking only at low speed.


-- Scanned by iCritical.

2013\01\02@191444 by Jonathan Hallameyer

picon face
On Wed, Jan 2, 2013 at 5:12 PM, IVP <RemoveMEjoecolquittTakeThisOuTspamclear.net.nz> wrote:

> I've seen this on TV used in railway sidings. And also sure
> it was somewhere in Holland/Germany. That was quite some time
> ago, maybe it's com
>

IIRC the "drop tower" sort of rides at amusement parks use eddy current
brakes also,  beefy permanent magnets mounted on the car, copper/aluminum
fins mounted on the tower. Nothing moving to fail, nothing that wears out.

-- Jonathan Hallameye

2013\01\02@195646 by IVP

face picon face
> IIRC the "drop tower" sort of rides at amusement parks use eddy
> current brakes also,  beefy permanent magnets mounted on the car,
> copper/aluminum fins mounted on the tower. Nothing moving to fail,
> nothing that wears out

I do believe I've seen that on a Discovery show, thanks for reminding
me. I think though that eddy currents are, AFAIK, made by disks
moving past each other in the same plane, eg similar to the tuning gang
in a radio, without ferromagnetism

I don't think the same plates would be an effective brake if they move
perpendicularly (agh, that's twice I've had to type that !!) to each other

Which is why I'm looking at permanent and electromagnets. Maybe it'll
lead somewhere, maybe it won't

Jo

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