Electromagnet & Spring: Attraction & Compression?

[sid_galt]

If we take an electromagnet and then place it at one end of a metal spring, the spring will get attracted to the magnet. Now if at the other end of the spring, a metal bar is placed, the metal bar will get attracted to the spring.

My question is that will the spring get compressed substantially in the process provided it is a spring with a low spring constant or will there be no compression?

 

[berkeman]

If you want to optimize the power of your bar-pulling arrangement, then no, that's not the best way to do it. To get the most force out of electromagnet-based devices, you want to minimize the air gap that the magnetic field has to cross. The farther away the bar is from your coil, the lower the force will be.

The more usual way to make an electromagnetic device to pull a bar against a spring force is to use a hollow solenoid of wire, and have the bar partially poking into the solenoid. The spring holds the bar mostly outside of the solenoid until a current flows through the coil. When the current flows, the bar is pulled into the center of the solenoid by the magnetic field -- the bar is steel or other ferrous metal so that it is strongly affected by the magnetic field, obviously.

I googled electromagnet solenoid tutorial force, and got lots of good hits. Hope that helps.


EDIT -- BTW, you also want to look at how the field returns from the back end of the solenoid back around to the other end of the bar. By providing a good magnetic return path, you can further increase the power of the pulling force. Just look at how good solenoid actuators are designed for more ideas.

 

[sid_galt]

well, the arrangement actually is an electromagnetic valve - a solenoid with a ferromagnetic hollow core. In the hollow core is a bar above which there is a plate. The max distance between the electromagnet and the plate is 4 mm. The purpose is to pull the plate with very high force (500-700 N).
I was thinking of a way to reduce the exponential decrease in force as the plate moves away from the electromagnet. As an example, at .9 mm, the same current produces 200 N of force which at 4 mm produces less than 60 N. So I was just pondering over ways to reduce it. Is there really no way? (Apart from using some other fluid instead of air)

 

[berkeman]

Oh, so it is the traditional solenoid structure after all. You raise a good question -- is there a better way to keep the force high for more of the stroke of the solenoid rod...

The only thing that comes to mind is for you to look at why the force is higher at one end of the stroke (where more of the bar is inside the solenoid)... Can you think of any way to extend that so that you can keep more like the maximum force throughout the stroke? Since the force is higher when more of the bar is in the solenoid coils, what would happen if you lengthened the bar and the solenoid? If the length of the solenoid is long compared to the 4mm stroke, does that improve things? What if you did a 2-solenoid combination with a lever arrangement, so that you got a push-pull combo force on the plate...? If the stroke of the rod is always a short time, you could store up some energy at a higher voltage for a higher initial pull current...make the current exponentially come down to what you have now to compensate for the exponential pull force relationship...Time to get creative...

 

[sid_galt]

The only thing that comes to mind is for you to look at why the force is higher at one end of the stroke (where more of the bar is inside the solenoid)... Can you think of any way to extend that so that you can keep more like the maximum force throughout the stroke?

Actually the structure is like this
http://img376.imageshack.us/img376/9660/16vc.jpg"
Taken from "Rendering the Electromechanical valve actuator globally asymptotically stable"

So although most of the work of pushing and pulling is done by springs, solenoids are required to moderate the motion. Which is why I was thinking of ways to increase the force to reduce the current reqd.

Since the force is higher when more of the bar is in the solenoid coils, what would happen if you lengthened the bar and the solenoid?

The bar is always in the solenoid. It is the plate which has to be pulled towards the solenoid.

If the stroke of the rod is always a short time, you could store up some energy at a higher voltage for a higher initial pull current...make the current exponentially come down to what you have now to compensate for the exponential pull force relationship

True, but the lower the current reqd. the less the power wasted. And this matters a lot in engine valves where they are opening sometimes at 3000 times a minute which is 50 times per second.

 

[berkeman]

Actually the structure is like this
http://img376.imageshack.us/img376/9660/16vc.jpg"
Taken from "Rendering the Electromechanical valve actuator globally asymptotically stable"

So although most of the work of pushing and pulling is done by springs, solenoids are required to moderate the motion. Which is why I was thinking of ways to increase the force to reduce the current reqd.

Wow, that is seriously cool. Is it from an internal combustion engine? Neat stuff.

 

[sid_galt]

Solenoid valves are not in production right now because they are encountering many problems. However lots of research is going on. The above diagram taken from the quoted reference shows a setup of the arrangement.

BTW, In the first example, how can the distance between the solenoid and the bar be a true airgap as there is a metal spring in between?

 

[berkeman]

I'm not sure that I understand the drawing with respect to the solenoids and the bars. It's starting to look like the coils attract the armature plate up and down. Not like a regular solenoid where there is a bar that gets pulled into the solenoid. Are the applicable air gaps between the coil faces and the armature faces? It looks like the rod going through the center of the coils is just for mechanical transfer of energy, not for attraction into a solenoid coil. When the rod goes all the way through the solenoid, there is no net attractive force.