electromagnetic field measurement 1

[GMick]

Hi guys, I'm new to this forum and I have what I think is a basic question. I'm not much of an electrical engineer, so I hope it does not sound too silly. I have the basic equations that characterize the magnetic field on axis with a solenoid shell at a distance away from the solenoid. However since the field falls away so quickly I wanted to concentrate the flux with a solenoid with some sort of core (probably ferrite). I only need a magnetic field a few millimeters away from the electromagnet, but it will not make contact with the magnetic substrate. I have tried to find equations to help characterize the field away from a solenoid with a core but have not been able to. I also looked for commercial electromagnets that may have specific field equations for that particular electromagnet, but none I found have the formula, plus they are all contact electromagnets.

Any ideas on where I can get (or I guess make?) an electromagnet to attract a magnetic material from a distance of a few mm and its magnetic field equation so I can model it?

I'm currently working on specifics, as to how much of a field will be necessary to create the force I need.

Thanks already,
George

 

[gphatch]

The force acting on a mobile entity, that responds to a magnetic field, is proportional to the product of the field magnitude and the gradient of that field, at that point in space. So to maximize the force, you need to maximize this product - the easiest way is to use a "reasonable" value for each part of the equation, rather than a low value for one and a high value for the other.

Gareth P. Hatch, Ph.D.
Director of Technology
Dexter Magnetic Technologies

http://www.dextermag.com

 

[UKpete]

George, I must put my hands up and say I was wrong when I said that the force wasn't proportional to the gradient of flux density.

Gareth, I take your point although you will agree I think that to get a reasonable value for field magnitude you do want to minimize the length of the air path - even a small airgap drastically reduces the flux density. It has never been normal practice to deliberately introduce longer air paths in order to increase the attractive force (although there may be special circumstances when you might do so, such as a requirement for linearity of force).

So I stick to my original point, minimize the airgaps. Taking for example a large active magnetic bearing such as manufactured by my employer (but not designed by me!) the total flux path for each pole is approximately 0.5m but the two airgaps are just 1.3mm, the mechanical clearances to allow shaft rotation.

 

[gphatch]

Yes George - I agree that if the system can cope with a minimal air gap, that's a good approach to take.


Generating large forces within smaller volumes though, is where permanent magnets just can't be beat, particularly if you put two magnets together side by side - the gradients become the dominant term in the force equation.

Gareth P. Hatch, Ph.D.
Director of Technology
Dexter Magnetic Technologies

http://www.dextermag.com