# Building an electromagnet to work over an air gap

I am currently working on a project where I am building an electromagnet that works over an air gap. Specifically a gap of about 0.3 inches. The electromagnet type was selected because I need to be able to turn it on and off based on input sensors to my micro-controller. Also all of my design work has been done in solid works using their Emag plugin.

How for the real grit. The essential problem I am having is that to make the electromagnet strong enough, I have to increase the size to a point where it is becoming too heavy for my project (too heavy is roughly over 3 lbs). Also the magnets should have a pulling force of about 50 lbs over the air gap from before. Furthermore I have dabbled into designing electro-permanent magnets but they, one also become to heavy, and two do not entirely turn off.

So here is my questions for you all:

1. Has anyone had experience making electromagnets that work over air gaps that could give me pointers?
2. Or has anyone had experience making electro-permanent-magnets that work over air gaps that could give me pointers?

EDITS:

The magnetomotive force required is calculated below. It takes into account the reluctance of the core, the air gap, and the material it is attracting. The material is 1008 12 guage steel. The core is either Vacoflux 27, which is 27% cobolt and 73% iron, or we may use high purity Iron, around 99.98% purity. Currently we have been using SolidWorks EMS plug in to do FEA analysis of the magnetic fields instead of using equations. N=number of turns; I=current; F=desired physical force; Lm=length of magnet; Lp=length of plate; Z=length of airgap; Uo=permiability of air; Up=permiability of plate material; Um=permiability of core material; Sm=cross sectional area of magnet core; Sp=cross sectional area of plate;

• Perhaps you could also describe the problem you're trying to solve with the electromagnet. It's quite possible that the trouble you're having could be solved with an entirely different approach. – Samuel Aug 11 '15 at 17:32
• There are commercial electromagnetic door locks available. Maybe it would be easier to buy one of those. There is one on amazon advertised as having 130 lbs of holding force. The shipping weight is only 1.1 lbs. Otherwise, maybe you can take us through the calculations you have done, and let us know what type of magnetic core material you are using for your magnet. – mkeith Aug 12 '15 at 0:50
• Is it DC or AC electromagnet? Looking at the gap and holding force requirements, an iron-clad (jacketed) electromagnet will be perfect. Look at here as well electronics.stackexchange.com/questions/139862/… – GR Tech Aug 12 '15 at 2:49
• @Samuel , I am working on a walker that has two electromagnets in the front legs for emergency situations. In my test environment there would be a steel sheet of metal under some carpet. When someone was falling backwards, they would instinctively grab tighter on the walker which would have force resistors hooked up to a micro-controller circuit. The electromagnets would activate and anchor the walker to the ground. Then, when the grip is released, so would he magnets. – Will Aug 12 '15 at 18:26
• @mkeith , I have tried commercial magnets, however the problem I am running into is that they have to be light enough so an elderly or handicap person is still able to utilize their walker. My solution was to design a custom magnet to more effectively use space and weight. I will edit my post to include calculation soon if that helps. – Will Aug 12 '15 at 18:32