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View Full Version : Magnetic circuits and solenoids.....?????



The Artful Bodger
10-09-2011, 05:22 AM
I am messing around with a project that involves a solenoid coil, 500 Ohms, about 3 inches long and 2 inches diameter, 3/4 inch or so bore.

I was looking for a longer effective stroke so I turned up a stainless steel armature which is a nice easy slide in the bore and about 4 inches long. The stainless exhibites some permeability but it does not stay magnetised.

The new armature pulled into the solenoid but rather weakly, weak that is compared to what I was hoping for. So I added a neo magnet to one end which made it a bit more effective but still not remarkable, so I put another magnet on the other end which improved it a little, but still not what I was hoping for.

More out of 'lets see what happens' than anything else I flipped the magnet on one end so I then had a bar magnet with like poles on each end and the opposite pole in the middle. I later checked with a piece of paper and a sprinkling of dust from under my grinder and the three poles was confirmed.

Now, when the bar is at one end of the travel and I apply DC power it is pulled firmly into the coil but it does not stop and travels to the opposite end of the travel! If thats not clear, it starts with 1 inch sticking out the left (for example) and continues until there is 1 inch sticking out the right. 2 inches of travel and with quite significant force all the way. If I reverse the polarity it moves all the way back.

It might move further but my magnets are bigger diameter than the bore so that limits the travel.

Now this had me really puzzled until I thought about it.... my conclusion is that the induced centre pole is very strong and with only 2 inches of possible travel this pole is always right inside the coil and presumably an even longer armature might give 3 or even more inches of 'power stroke'.

That might be useful information to store away!:D

winchman
10-09-2011, 06:39 AM
I wish you'd make a longer version of your plunger to see what effect the added length has on the travel.

Then we'd have even more useful info to store away.

I'm thinking the travel will be determined by the length of the coil, not the length of the plunger. One nice feature of this arrangement should be the soft landing at the end of the stroke.

Now we need to think up a use for it. Hmmmm.

The Artful Bodger
10-09-2011, 07:46 AM
I wish you'd make a longer version of your plunger to see what effect the added length has on the travel.

Then we'd have even more useful info to store away.

I'm thinking the travel will be determined by the length of the coil, not the length of the plunger. One nice feature of this arrangement should be the soft landing at the end of the stroke.

Now we need to think up a use for it. Hmmmm.


You are probably right that the length of the coil limits the length of the power stroke but in my example the plunger is too short to allow travel even that far.

Yes, I have more of the same stainless steel for a longer plunger but I wonder what happens to the strength of the central pole(s) as the plunger is made longer. Maybe if I had material with a higher Mu......:confused:

Chris S.
10-09-2011, 08:18 AM
Now we need to think up a use for it. Hmmmm.

Hang a crankshaft and flywheel on it. Then power the coil from an "H" Bridge driven by a Flip-Flop and you've got a cylinder driven, reciprocating electric engine. :D

Weston Bye
10-09-2011, 08:27 AM
John, describe your coil construction. A wound bobbin with no steel end plates? I am presuming that that is the case.

I have worked with variations of your experiment with the exception of the mildly magnetic armature. An interesting variation worthy of some experimentation to see what value there might be. I suspect that it would serve to spread out the flux pattern somewhat along the length of the armature rather than bunching it up in the middle where it encountered the flux of the opposing magnet. What value this would have is fodder for experimentation. Force vs stroke position studies would reveal this.

If I am envisioning your arrangement correctly, your force curve would have maximum force at the extremes of the stroke and minimum force in the middle. Lengthening the armature would reduce the force in the middle even more, eventually to become ineffective. There are practical limits.

If your magnets were able to slide inside the bore of the coil, you would find that when the coil was energized, that the magnet would seek equilibrium in the middle of the magnetic field of the coil. The armature would offset the magnet somewhat as it extended the field of the magnet.

DICKEYBIRD
10-09-2011, 10:53 AM
...power the coil from an "H" Bridge driven by a Flip-Flop...Care to elaborate with a schematic with some component part numbers or a link to an existing circuit? Preferably 24v power to the solenoid please.:)

Unfortunately I know just enough ee-leck-trickal stuff to visualize what you've described but not even close to enough to design & build one. I'm collecting parts to build a solenoid engine and am interested in a way to switch the power without mechanical switching.

Sorry for the hijack John. By the way, why stainless? Wouldn't you want to use iron?

Chris S.
10-09-2011, 12:06 PM
Care to elaborate with a schematic with some component part numbers or a link to an existing circuit? Preferably 24v power to the solenoid please.:)

Unfortunately I know just enough ee-leck-trickal stuff to visualize what you've described but not even close to enough to design & build one. I'm collecting parts to build a solenoid engine and am interested in a way to switch the power without mechanical switching.


I half heartedly meant that as a joke but if you're interested read through this thread on AAC.
http://forum.allaboutcircuits.com/showthread.php?t=47688
It describes the technology of an "H" Bridge to control the direction of a DC motor by reversing its polarity. The obnoxious, intolerant and ill tempered guy named CDRIVE is me. I appear to be in one of my better moods when I wrote that stuff. :rolleyes:

Chris

DICKEYBIRD
10-09-2011, 01:05 PM
Thanks Chris. ZING! (The sound of the thread in your link going right over my head.):eek: Sigh, I have too many hobbies already to take up "real" electronics at my age.

I guess I'll switch the power with cams & switches....that'll be challenge enough for me.:)

Chris S.
10-09-2011, 02:22 PM
Milton, go here..
http://bbs.homeshopmachinist.net/showthread.php?p=705727#post705727

Scottike
10-09-2011, 02:29 PM
You might be able to do your switching with magnetic reed switches mounted at the ends of your shaft travel.
like these:
http://www.meder.com/standard-switches5.html?&tx_jppageteaser_pi1[backId]=2841

The Artful Bodger
10-09-2011, 04:30 PM
Thanks for the comments everyone...

Weston, yes, it is a wound bobbin with no end plates.


A simple iron filings experiment showed a very strong concentration of flux lines at the mid point, it looked like I imagine two like poles forced together would show although my scatterings did not show an obvious null I expect it must be there.

I dont have magnets that will go into the coil, not big ones that is. However, if I did, and the armature was able to travel to that point of equilibrium would there be full force available when the voltage reversed to return the armature?


To everyone else, yes, this did start as a solenoid engine project. I saw early on that this solenoid in its original form was very unsuitable for a solenoid engine. It was in a metal frame with a plunger going into a cone shaped space in one of the pole pieces. It was very strong but only over the last 3mm ( 1/8") so most of the effort was going into stretching the connecting rod as the end of stroke corresponded with TDC of the crank.

The first modification I did was to slot the connecting rod so that the plunger reached the end of travel when the crank was at about the 10 o'clock position, the plunger stopped but the crank carried on. This was a huge improvement and the engine ran on about 1/5 the voltage previously required.

These current experiments are an attempt to increase the torque and get smoother running of a solenoid engine by increasing the length of the power stroke.

Weston Bye
10-09-2011, 05:56 PM
....would there be full force available when the voltage reversed ... stroke.

Yes, but I would stop the armature just short of the midpoint.

Evan
10-09-2011, 06:29 PM
I would use a pair of coils that could be adjusted for spacing on the coil form. A single magnet would be sufficient as it would be attracted to the mid point of each coil. There would also be no need to reverse the flow of current so the switching can be simplified with one end of each coil connected together as a common and to power. All that is needed then is to switch a single lead between the free ends of the coils.

You may also wish to consider using a lever action to throw the crank. Magnetic attraction is subject to inverse cube law so reducing the throw of the solenoid by use of mechanical multiplication will pay off handsomely.

Weston Bye
10-09-2011, 09:35 PM
Evan may be referring to the crank linkage used on the Adkinson Cycle engine.

Evan
10-09-2011, 10:06 PM
Yes, that is a good example.

http://www.animatedengines.com/atkinson.shtml

darryl
10-10-2011, 12:55 AM
One of the enemies of efficiency is having long air gaps where magnetic flux must flow between magnets and coils. A helical solenoid has the interacting poles close together, which multiplies the torque but at the expense of a shorter stroke. You could probably simulate a helical solenoid by using multiple magnets spaced out along a rod, then a special winding method to interact with the several alternating poles generated by the magnets.

Round magnets with holes in them would be the choice, and you would mounting them all in repulsion. You would have very strong alternating magnetic field lines emanating radially from the rod, and they would remain very close to the diameter of the magnets, as the magnets would probably be fairly thin. The coil would be a close fit around the magnet stack, with some gap of course to allow free movement between the parts. If you surrounded the coil with iron, it would increase the flux crossing the coils, for an improvement in efficiency. This iron doesn't have to have a laminated structure because the magnetic field in it is static. It would just be a piece of steel tubing. This is pretty much a voice coil and magnetic circuit like a speaker has, but made into multiple but connected 'drivers'.

Now we get to my typical tendency to over-engineer- instead of just one coil, reversing direction every magnetic pole spacing, you have three coils, layered. As you energize them in turn, the magnet stack would go farther in a linear direction, but still have the same high torque due to the short and concentrated magnetic field. You can decide how far you want it to go before it reverses direction.

Yes, it takes some creative switching of current, but it does allow you a longer stroke without the losses that a longer stroke would normally create.

Chances are you will stick with a single coil, with simple on-off coil switching. If you set it up with double pole switching it could become double-acting, in other words push-pull.

When it comes to spacing of the magnets, and the length of each magnet (which will probably be a stack because these would normally be disc-type magnets) you will have to determine what you want the stroke to be first. Then you would arrange to have the magnetic poles spaced apart by about that same distance. You might go for a stroke of say 3/4 inch, which might mean that each magnet would be a stack of about 6 disc magnets, assuming they are 1/8 thick. Six magnets together in attraction, separated from another magnet stack in attraction, but put on the rod in repulsion. The spacing between the magnet stacks would be a factor, although I don't really have a 'proper' spacing in mind- possibly the spacing should be about half of the length of each magnet stack- Of course the reversal points of the winding direction would have to match. The outer end magnets would have a steel cap added, same diameter as the magnet and about 1/4 inch thick.

If you used four magnet stacks, you end up with three areas of radial flux, plus two areas where there is still flux but at half the strength. The coil could consist of one winding the length of three magnet stacks plus three spacings between magnets, and would reverse direction twice, evenly spaced along the coil.

Whether the coil moves or the magnetic assembly moves is up to you. It would be quite trick if the coil moved, getting it's current through a wire on each side of the push rod. The current would be fed into the crankshaft and contact points would come around and make connection at the appropriate times. If you can get this far, it's not much of a stretch to make it double-acting. Each crank pin would have a round pc board disc on each side of the connecting rod. Each disc would have two half-circle traces on it, with some space between the ends of the traces so the contact point on the connecting rod doesn't short them out. One trace is wired positive, the other negative, and it's reversed on the opposite pc board disc. A pair of slip rings on the crankshaft gets the current to the traces. You could probably hide all evidence of how the current gets to the coils.

Dang, I'd like to do this project myself, but I have so many on the go already-