Electromagnet for "Football Game

Discussion in 'The Projects Forum' started by Michigan Joe, Aug 24, 2010.

1. Michigan Joe Thread Starter New Member

Aug 24, 2010
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Hello all. I am new to this forumn and I was really impressed with the sharing of ideas and information here. I am doing a project that has brought me to a point where I need some outside help and I was hoping to get some ideas/feedback from people here.

First to give you a little back ground on my project, I am a hobbyist that is building a super sized ELECTRIC FOOTBALL Board. Yes thats right, an old school game from the 60s-80s where the metal board vibrates and the football players scoot across it. What I want to drive the vibration of the metal field is an electormagnet run on AC power (similar to how the old Tudor company did it in the past). However, I need a more powerful electromagnet than what was used back in the day because my supersized field is much larger and more structural. After many various types of motor sources, I have come to the conclusion that the linear vibration I get from an electromagnet (on AC power) is the absolute best thing for the job (for vibration characteristic purposes). I've had some early success of vibrating my field with a coil that is ~550 wraps, 24 guage wire (~2.5 ohms) around a spindle with a cold rolled steel core. My main problem is [1] overheating...when power is supplied by a rail power 1370 (19VAC, 18VA) I get the magnetic power I want but the coil gets really hot to touch within 90 seconds. It doesn't seem to cause major damage (yet) but it seems too hot. FYI, I measured the voltage and current across the leads of the coil is ~16VAC and 2.5A (which surprised me because isn't that higher than the 18VA power source??). Then I got a 9VAC/1A power wall transformer and then my second problem arrose...I seemed to [2] damage the transformer which I believe may be due to inductive kick back voltage that my low voltage transformer couldn't handle.

Now on to my questions:
(1) The duty cycle on this electromagnet is 30 seconds ON (max) and 30 seconds off (MIN). Do you have any suggestions on what max current I should strive for in the coiled 24 guage wire so that I do not overheat and cause serious damage.
(2) Should I be worried about inductive kickback damaging my power supply? If this truely is a concern then what would be the best recipe to disipate that energy coming from the coil when the switch is turned off. I've read about a diode and an RC snubber...I could really use some specific values on these circuits if its not too difficult.

I am a mechanical engineer not electrical engineer so forgive me if any part of my question seems dumb, or incorrect in any way. Having said that, please do not refrain from being very technical in a response as I am trying to be as resourcefull as I can to best understand a good solution to the excess heat and potential inductive kickback problem. If you find my project interesting and can my questions seem simple than I would appreciate any feedback you could give me.
Thanks!
Joe

2. beenthere Retired Moderator

Apr 20, 2004
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It may be that any old vibrations is good enough. I seem to recall the players motion came from two thin metal vanes that were angled backwards so the net motion was always forward.

If that is the case, an electric motor will make the field vibrate when it is securely attached to the underside and has an unbalanced mass on the shaft. The shaft should turn at a right angle to the playing surface (straight down). If you have a motor, try it with an off-center weight. A small AC fan motor might be enough. It might even work attached to one end.

3. Michigan Joe Thread Starter New Member

Aug 24, 2010
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Thanks for the initial feedback. The mechanics...now this are is more my expertise . The rotary style electric motor was my first attempt and while it works okay it does not product the most consistent vibration patterns which are optimal for performance. First off the rotary motor with counter weight is slow to start and slow to stop which is bad for game play. Secondly the rotary motor also creates wave like vibration thru the metal and that flow is not optimal to the game play (running into the motor or away from is like running into the wind and against the wind) and there are many hot spots and slow spots on the board. Thru experiementation I've determined that the linear vibration from the electromagnet is really supreme in terms of having the most consistent measurable vibration pattern across the entire surface.

4. Bernard AAC Fanatic!

Aug 7, 2008
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How large is the field & weight? What shaking amplitude is needed? Does the magnet have a magnetic outer return path? What is the coupling between magnet & field? Maybe a drawing or picture of the magnit. One more- does field have a resonate frequency? Some form of Ledex rotary solenoids, minus the balls, have a verry strong pull, @ a short stroke. For a snubber, I would use 5Ω in series with a .1μF cap.

5. Michigan Joe Thread Starter New Member

Aug 24, 2010
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thanks for the interest and the questions. Attached are some pictures showing the electromagnet and the field setup so you can understand how the electromagnet couples to the field. The electromagnet and the (steel) field are seperated by a very small gap. The AC current thru the electromagnet creates a varying magnetic field at 60HZ and the magnetic pull force excites the thin metal board surface. I do not know the exact amplitude that is desired, but I can tell you the electromagnet provides an easy path to tweak the right amplitude by increasing the proximity of the electromagnet to steel opposite it. And absolutely yes the various fields each have a resonant frequency...dependant on how the field is mounted and screwed into place. The size of the field is 2' x 4' and weighs ~6-8 pounds.

I can tell you from MUCH experiemenation that the electromagnet is superior for smoth vibration on the tin metal surface (you will have to trust me on this ) and now I just need to figure out how to run it at optimal power and with protection circuit?? (RC snubber) to make sure it is safe.

Thanks for reading and showing interest. I hope the pictures spark some more interest.
Joe

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6. Bernard AAC Fanatic!

Aug 7, 2008
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For kicks, might try 1/2 wave rectification, might get more bounce at 60 Hz than 120. Tried a line matching transformer on 2000Ω, with 60 Hz line @ around 20V, bar removed from E core by 1/16 in. Runs cool. Could not raise picture-unknown file type or something.

7. MMcLaren Well-Known Member

Feb 14, 2010
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Wouldn't the metal panel have some resonant frequency where you could drive the solenoid or magnet at reduced power at that frequency?

Food for thought. Regards, Mike

8. Michigan Joe Thread Starter New Member

Aug 24, 2010
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Here are some pictures of my setup in .jpg format if thats easier to pull up. thanks for the feedback so far. I agree that the ideal would be to run the electromagnet at the natural frequency of the metal plate ("field") structure but honestly designing the electromagnet to operate at differnet frequencies sounds really hard to do . Bernard, thanks for responding twice forgive me though as I don't fully understand a line matching transformer. I will do my homework though to try to better understand.

I am a actually quite happy with the way the electromagnet excites the thin metal structure at 60Hz, and for now I am happy with controllig the amplitude of the vibration by turning the screw to change the gap betwen the electromagnet and the metal (magnetic) counterpart (see picture 4). I just need to figure out how to run the electromagnet so it won't overheat and still has enough power. Also, I'm trying to figure out how much damage this inductive kick back can cause my power source. My first power source had a built in "protection circuit" but when I used a 9VAC power transformer it no longer works after connecting it to my coil and plugging it in. I am very mechanically inclined, just not very electrically inclined, but at leat I'm trying. Heck, I've got my multimeter out making voltage and current measurements across my coil to characterize the system that works. You may laugh, but just having two coils in series makes the coils run much cooler and the magnetic force is acceptable. The voltage across one coil (when put in weries with another coil) is about 9.5V and ~1.0A so I think this is how I determined a "ball park" range for a power source. I think I may be close and I really appreciate the various feedback. You see, not only do I have to make one field, i need to make some for friends now too!

Joe

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Jul 26, 2010
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10. Michigan Joe Thread Starter New Member

Aug 24, 2010
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I really like the idea of the tactile transducers, especially the bass shaker (20-80Hz). I would just need to find one that "fits" from a physical size and force standpoint. I'm hoping to keep this solution under \$20 as I know I am going to have to repeat it a few times for friends. Great lead...THANKS!

As far as my current \$3 coil goes, should I just experiment with the power source to find the max voltage I can run it at without producing too much heat (for my expected duty cycle)? My problem is that I don't have a power source capable of controlling the AC power. Or duh, a simple variable resistor in series will work won't it? Then I can just measure the voltage/current across the coil to know the exact power conditions and then go buy a small AC transformer to meet that approximate power condition. What do you guys think?

11. marshallf3 Well-Known Member

Jul 26, 2010
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Either way. There's some pretty cheap tactiles on that page and they usually put out quite a surprising output for their size.

Sure would be an advantage when you have friends lined up all wanting one, probably cut your assembly time in half.

Two for \$5 total? What can you lose? Worst case you may need to use more than one per board.
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=309-148

Last edited: Aug 26, 2010