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 <joecolquitt@clear.net.nz> wrote:

> 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
>
> Joe
> --
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Brendan Gillatt
http://www.brendangillatt.co.uk
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