Jump to content

Unofficial Home of Old Simplicity & Allis-Chalmers Garden Tractors

Sign in to follow this  
bob1959

Magneto Guru

Recommended Posts

bob1959
I know a good bit about electrical system and wiring a house but am very confused in getting spark in this 10 HP brigs. Last night I did continuity cks on the total system. The thing that confuses me is the fact that the mag grounds to the block that in turn grounds the whole system. The wire that goes from the back plate to the condenser and pts becomes grounded from the mag wire. In looking at the way that I'm thinking the system should work. There should be a break in continuity for the pt so that when the pt armatures makes contact that should complete the circuit. If the armature is grounded and the Pt is grounded all to the block then there is no way for the current to flow. Again why would you have an insulated post on the back plate and insulated connection at the pts. If someone with more wisdom than me can give me their number so I can call and discuss it would be greatly appreciated. Thanks

Share this post


Link to post
Share on other sites
firefoxz1
Let me try to explain, and believe someone else may know more. First a magneto is basiclly a transformer. The magnets on the flywheel drives the primary circuit which in turn drives the secondary circuit. You know electric so you know there has to be a complete circuit for power to flow. When the points close and the magnets pass the magneto current flows through the primary circuit then the points open collapsing the primary circuit which releases the secondary field to fire the plug, again there has to be a complete circuit back to the magneto. There should be continuity between the block and the points (primary circuit) and also continuity between the block and the plug wire (secondary circuit). These two circuits only have a ground in common and are not connected inside the magneto.

Share this post


Link to post
Share on other sites
B.Ikard
Well said Thom- The flywheel magnets are the source of voltage. This is induced into the armature or coil by rotation. The coil is nothing but a step up transformer with a primary and secondary winding. The primary winding connected to the points and ground on the other end, and the secondary being connected to the spark plug and ground on the other end. Seperate circuits with a commpn ground. Current flow stops when the points open (notice the arcing-why a condenser/capacitor is in the circuit), and the voltage is induced into the secondary step up winding in that instant to the spark plug. It is explained a lot better in the older Briggs and Stratton repair manual. Brent

Share this post


Link to post
Share on other sites
ka9bxg
It might not be a briggs but I was able to pick up a dixie 2 cyclinder mag today for my eagle engine and best of all it is hot.And only $75 dollars. What a deal and am I happy YOU BET.Bob

Share this post


Link to post
Share on other sites
Al
Hi, Some awful good posts here. I have started a series about electrical theory and how electric current flows and how energy is measured etc. I have gotten the theory series done and was starting to write some articles on how to apply the theory to troubleshoot etc. I have not had time to do the rest of the articles yet, but hope to get back at them before long. If you have any interest, go to my site. edensltd.com and go to the tech notes. I hope if you go there, it is interesting and helpful to you. Thanks, Al Eden

Share this post


Link to post
Share on other sites
bobjohn
The flywheel has two permanent magnents that rotate past the ignition coils. This induces a magnetic flux in the primary coil (the one that is grounded by the points) by the first magnet and then the second which is opposite poled (north-south) which produces a large single neg-positive change in the magnetic flux. This in turn causes a high frequency oscillation within the primary coil and almost instantly the points open by the cam shaft lobe. Since the capacitor is parallel to the points, the capacitor completes the circuit for the alternating current throught the primary coil and replaces the points as the source to ground. The coil inductance in microhenry and capacitor capacitance in microfarad must be of such value to resonate at the proper frequency to produce an alternating current in the primary coil to induce a current in the secondary coil, which is a step-up transformer. The AC continues to oscillate through the primary inductor and series capacitor and produces a relative long series of oscillations in the primary coil that is coupled to the secondary coil to produce an electric arc across the spark plug gap as long as the points are open. This is the reason that proper point gap setting is a must for proper igition. This entire unit is a tuned electronic circuit and must have the proper capacitance and inductance with proper plug gap to work properly. A leaking capacitor will change the resonant frequency of the circuit and reduce the voltage at the spark plug, as will using the wrong value of capacitor. A worn flywheel to shaft key will change the timing of the permanent magnents movements across the coils in relation to the point opening and reduce the voltage across the spark plug. As will loose or weak magnets in the flywheel, or hairline crack in the flywheel will change the magnetic flux generated. bob

Share this post


Link to post
Share on other sites
Al
Bob, Well explained. Right on. The matching of the capacitive reactance and the inductive reactance is the thing that makes it work and something that is passed off in most explanations as stopping the arcing of the points. When the points open and the collapsing field of the coil starts charging the capacitor is where it all begins. You are absoulutely right the primary winding and the capacitor are a parallel resonant circuit. One can display this very well on an ignition scope. Again well done. Al Eden Al Eden

Share this post


Link to post
Share on other sites
bobjohn
Thanks Al I am shocked at the number of service manuals and printed articles, and service techs that think that the only purpose of the capacitor is to reduce the arcing across the points, even in conventional battery powered ignition systems. One point of interest, the capacitor and primary coil are series tuned, and not parallel tuned, since they are in series after the points open. The inductance of the primary coil is determined by the self induced reactance of the secondary coil and changes dramatically when the plug fires, almost at the same instant as the points close. The hysteristic losses in such a circuit is tremendous and is the reason all components must be within mfr.tollerance to get the correct voltage to the plugs at the right instant to fire the combustion BTDC at proper rotational angle. Rebuilding an engine from parts combed from several used engines can result in an engine that sounds good. But, when parts from different ignition systems are mix/matched, seldom does a properly firing engine result. bob

Share this post


Link to post
Share on other sites
Al
Bob, Isn't the capacitor, the points and the primary winding all connected to ground and the points, capacitor and other primary winding and points all connected in parallel. As I look at it the primary winding would be a parallel tank. This combo would determine the resonant frequency. The secondary is wound over the primary and it is connected in series with the primary creating an auto transformer. When the magnets are passing over the coils, and the points open the capacitor is discharged and instantaneously looks like a direct short across the primary. It instantly starts to charge due to the circulating current in the primary. As it becomes charged the collapsing field reverses direction and the capacitor is then charged at the wrong polarity and begins to charge in the other polarity. This time constant is controlled by the inductive and capacitive reactance and the resistance of the wire in the primary. Since there is so much core iron in the coils, the Q of the circuit is relatively low. This is how I see the oscillation developed in the most basic form, disregarding the flywheel speed and the rest of additional factors. Is this how you see this happening? I think I agree with everything you say, except I see it as a parallel tank. Thank you very much for your post. It brings a lot to this club. Very Respectfully, Al Eden

Share this post


Link to post
Share on other sites
D-17_Dave
Well, no matter who is right here this thread is as good of an example of how great this club really is. We should all be proud of the collective minds we have participating here. If only the minds at Briggs could see all of this.8D;) Thank you both for the level of thought and the time putting this to words for all of us.

Share this post


Link to post
Share on other sites
bobjohn
Hi Al et al. Thanks for the reply. To simplify this discussion, lets limit to the primary coil, points, engine block and condenser. Fist, lets consider the engine block as a circuit bus, just a connection block and part of the circuit (most call it 'ground' which is misleading). If we look at the coil, we will see two wires from the coils, one small and one larger, that connect to the coil lamanation with a rivet, the larger one being the primary winding, the smaller the secondary. The rivet electrically connects the wire to the lamanation steel. The lamanation steel is connected to the engine block (bus) with a couple of screws (which also adjust the spacing of the flywheel magnets and the coil lamanation). This completes the electrical circuit from one end of the coil to the bus. Continuing along the bus, we find the points connected to the bus with a screw and is electrically connected. One of the points set is connectd to the bus at this point. Therefore we have a series of one end of the coils, the bus, and one of the points set. The second point set is insulated from the bus. The other end of the primary coil connects to the insulated point set. When the points are closed, both ends of the coil are connected together via the points. When the points open, the primary coil has an open circuit. Next, the condenser, or capacitor: A capacitor can only carry changing voltage/current. It will not carry direct current (DC). It is an oppen circuit to dc voltage. The capacitor is connected to the bus with a screw near the place that the points are connected to the bus with a screw. This connects one end of the capacitor to one end of the coil and the point that is connected to the bus. The other end of the capacitor connects to the other point with the coil wire. So the coil and the points are in series. The capacitor is parallel to the points. When the points open, the coil and capacitor are in series, since the capacitor is now where the points were before they opened. We now have a series tank circuit until the points close, which short-circuit the capacitor and we only have the primary coil in the circuit, awaiting the next pass of the flywheel magnets. Am I confused or what? lol KD4LV bob

Share this post


Link to post
Share on other sites
Al
Bob, I agree with you that this is true in most battery ignition coil set ups. It is also true in any magneto coil system that the primary winding is not grounded to the laminations. In a Briggs coil, one of the leads from the primary winding is grounded to the laminations and then to the engine block. this connects the mounting tab on the capacitor, the primary lead of the coil, and the points all to ground. The insulated point, "the tap", aka the other primary lead and the one secondary lead, and the other capacitor lead are all permanently connected together in parallel, creating a parallel resonant tank. If one primary winding is not connected to ground, then you would have a series resonant circuit as you describe. It all depends on how the magneto is constructed. You are correct regarding the battery ignition coil being a series resonant circuit and also in a magneto that the primary winding is not connected to ground. I have very great respect for your posts and how well you describe things, and in no way mean to "challenge" you. I really appreciate you pointing out that I might be in error. I actually never thought about the series theory on the 12 volt system before you brought it up. Thank you for making me take a deeper look at something I had in error assumed. Al Eden

Share this post


Link to post
Share on other sites
bobjohn
Thanks Al. Your post are great and very detailed. I, in no way will challenge your knowledge or your theories. I wish that you would consider this: considering only the primary coil, the points and capacitor; open the points so that they cannot contact. What is left? we have the primary coil in series with the capacitor. That is, one end of the coil is connected to the lamanation, along with one end of the capacitor. (the lamination is only a connection point) The other end of the coil is connected to the other end of the capacitor at the insulator on the point spring. (the points have no part in the circuit since they are open) I believe this to be a series L/C circuit, if you call it a parallel circuit, then there is no problem as long as we understand what we each call it. I think that we have bumped into an obscure definition, and I appreciate your helping me through it. bob

Share this post


Link to post
Share on other sites
DMedal
I suspect the math is the same whether it is serial or parallel tuned circuit in this case. I tried doing a quick freehand sketch but it was too crude even by my standards, so please bear with the verbal description. Consider the circuit during the time between when the points open and the spark ends. The only things in the circuit now are the coil and capacitor. With only two components there's only one way to make a circuit, unlike the types from which we derive the series or parallel resonant circuit discussion. The collapsing magnetic field is the power source but it is within the coil, not externally applied. By the way, without the capacitor in the circuit we completely lose the resonant effect and points arcing is not only doing tough things to the points, it is wasting much of our spark energy by arcing in an undesired place.o Question for those of you who've watched a magneto system on a 'scope: I'm sure you get a nice "ring" with the plug out of the circuit. But once the plug fires, does this not damp out the oscillations quickly? Now we've introduced a third element - a near short - on the high side of the coil. How long does the spark duration last? -Don

Share this post


Link to post
Share on other sites
bobjohn
Hi Don Thanks for the input. You are correct in that there are only two components in the circuit, the primary coil and the capacitor. Of course, the secondary coil is playing a part, but at this point in our discussion, it is part of the primary coil because it is coupled by induction to the primary. The current flow in the primary coil produces the magnetic flux in and around the coil, and vice versa. We cannot have one without the other. Since we can only have a magnetic flux with a change in the current/voltage in the coil, we can only deal with the alternating current within the coil. (BTW, the capacitor will not pass direct current anyway). When the current increases in the primary coil, it must pass through the capacitor, as it is the only thing attached between the ends of the coil. When the magnetic field collapses, a current is produced in the coil which, again must pass through the capacitor to complete the circuit, since it is the only device attached between the ends of the coil. If the value of the capacitor is properly chosen, then the capacitor will pass the current with very little loss. The current and voltage will be in phase and the circuit will be 'in tune' and will oscillate. This oscillating current produces the magnetic flux that is transfered to the secondary winding and produces a high-voltage spark. The spark drains the stored energy of the magnetic flux stored in the coils and we will see a tremendous spike in current in the coils and through the capacitor, along with a sudden drop in voltage. So, the current must flow through the coil then the capacitor, back into the coil, and back through the capacitor with each cycle of alternating current. This constitutes a series circuit. If the current traveled through the coil and capacitor at the same time, then they would be in parallel. A sure way of testing is to use a dual trace scope and look at the voltage across the cap. and the primary coil. if they are in phase, then it is a parallel circuit, if not, it is a series circuit. Thanks for listening to my ramblings. bob

Share this post


Link to post
Share on other sites
DMedal
I should have said at the start: it is a series circuit, but it really doesn't matter. It was something like 4am when I wrote that and I've been out of engineering school for a while. The secondary doesn't really enter in until a current flows in it. No current flows until the plug fires and then the whole thing collapses with (we hope) most of the energy dissipated in the spark. It may take a couple of cycles for this to happen but my guess is the spark dampens the coil pretty quickly. That was really my question, what does this look like on an ignition scope? In a battery ignition we'd expect to see the ringing fall off even without a spark as the energy in the coil is at a peak right before the points open. But a magneto is a bit different, I don't have quite as precise knowledge of the timing and phase of the energy input since that comes from the movement of the flywheel in the magnetic field of the coil. In either case it would seem peak voltage, maximum rate of change of current, and start of spark happen pretty quickly after the points open. In a carbon rich environment one would expect the spark to dampen oscillations quickly. My question was how quickly on a running engine. How many oscillation cycles of spark do we get? So in summary: 1) you need a hot energetic spark for best ignition of the fuel/air charge and resulting power. We all know that. 2) That spark is generated when the magnetic field in the coil collapses, transferring energy to the spark plug. In battery ignition the energy is stored in the coil by the battery when the points are closed. In a magneto ignition the energ comes from part of the magnetic circuit rotating with the flywheel and causing a major change in magnetic flux as the pole passes the magneto. 3) you need a good capacitor(condenser) of the right size for hottest spark. Since the current can't stop instantly when the points open, the capacitor absorbs that current somewhat like a spring, then kicks it back. In layman's terms, with the right sized capacitor that "kick" doubles the spark voltage. A shorted capacitor or one with internal arc will in effect short out the points. Without an effective capacitor the dropping current is maintained in an arc at the points for a while, losing coil energy and hurting the points. For maximum spark the capacitor has to match the coil, in both mag and battery ignition circuits. I'm thinking not everyone knew this. 4) gaps between points, between the mag coil and the flywheel and plug gap all are critical to best energy transfer, as is the all important timing of the flywheel passing the coil. we knew that too. 5) if all these conditions are met and connections are low resistance, the circuit is incredibly simple and effective. There's not a whole lot to go wrong. Did I leave something out? -Don

Share this post


Link to post
Share on other sites
bobjohn
Hi Don. I think we were making a mountain out of a mole hill, to use a cliche older than I am...which is pretty old... Next June will be my 40th aniversary for graduating EE school and simple things dont seem simple sometimes....lol You are absolutely correct in your statement. And to try to answer your question; I'm not sure that I can with the space of this forum. There are just too many variables. But the damping will be much quicker than that of a battery powered ignition system, for sure. I think the eddie currents in the lamination is one of the most contributing factors. Since most of these systems use open lamination cores, the eddie currents are very high, so the collapse of the flux field will be slowed somewhat. But, since we have only a one-shot engergizing force, the flywheel magnets, the current developed in the primary coil is not a smooth curve. It looks more like a pine-cone....with a very wide frequency width. So the spark is not a single pulse or a series of sine waves, but a burst of frequencies ranging from near DC to microwave. I think that if we were to apply Fourier analysis, or just look at the spark with a scope, we would see the voltage increase and the length of the spark increase for a few miliseconds while the ions super-heat, then a sharp decrease in voltage, almost a vertical drop as the magnetic field makes its last few swings of oscillation. I think that the voltage trace would swing upward in a parabolic curve to a sharp point, then a downward vertical drop to almost zero, with the normal lobes from the flux fields swinging causing the normal voltage side lobes. So the short answer is that the current/voltage is not a cyclic form that we can see with a scope, but a shower of waves and pulses of different frequences and voltages that fall off differently at different frequencies. bob

Share this post


Link to post
Share on other sites
Ronald Hribar
So why doesn't Bob have spark. I am only jokeing. It just goes to show that even the simpliest systems are complex. Just think of the problems that faced the person who made the first motor.

Share this post


Link to post
Share on other sites
DMedal
Bob1959: You can check continuity on this as you did but unlike the circuits you are used to all the good stuff here happens in the moment of transition from points closed to points open. As already stated the electrical energy gets into the coil by the generator effect of the flywheel moving past the coil. In this instant your wire from coil to plate and points is anything BUT grounded and i wouldn't put a DVM on it. In fact, it is by grounding that wire that the switch kills the engine. The insulated plate is a connection point, if I understand you right. (this is where we LOVE pictures). The points, the coil wire, the condenser and the kill wire from the switch all join here abouts. So to diagnose your lack of spark: 1) I'd disconnect the kill wire from the ignition switch to simplify troubleshooting. Or disconnect it long enough to check that it doesn't go to ground. 2) check points and plug gaps. Are points actually closing fully (and opening)? 3) if points are good then replace the condenser with a known good one of the right type. We probably have 10 years time as a group spent on bad condensors. Cheapest you can get, out of dusty box may still be bad. If points are worn, replace points AND condenser. 4) If after all this you STILL don't have any spark then come back and we'll go from there. Possibilities would be sheared flywheel key, a pinched wire under the sheet metal, bad coil,.... good luck with your tractor! -Don

Share this post


Link to post
Share on other sites
Al
HI, In these days of peoples feelings getting hurt and going out shooting people etc, isn't it wonderful that as friends we can set down and discuss things like this without any malice? I think it is wonderful we can all stimulate each others thought processes and we all benefit from it. This such a wonderful group. I am not an engineer, I went to country school and a small high school. When I was about 10, I wanted my folks to try to pay and sign for me to get a National Schools Diesel correspondence course. We we were poor (had a sawmill) and they couldn't afford it. Got interested in electronics when I was about 12 and a freshman. Learned my electronics from a borrowed used correspondence (plus devouring a lot of other books and a lot of experiments) course at that time. At that time tubes were everything and the transistor had just been discovered. About 14 I got a tax permit and opened a small radio repair shop at hone and fixed radios and B+W tvs for spending money. Graduated from a small high school (45 students total)at 16 and at 17 got hired at Collins Radio and was a technician there for 38 years. When I started there I was testing 600 volt dynamotor aircraft transmitter power supply,s. I was under the law allowed to work in test, but was too young to run a freight elevator, (not 18) so I had too push carts full on the elevator and then walk down stairs and push the button and bring them down and take them off and test them. As I think of a parallel tank, I think of a tuned tank, like in the output of a transmitter or the input of a receiver with the capacitor across the coil. This tank has infinite (theoretically) impedance at resonance. I think of a capacitor hooked to the end of a coil and looking at the two ends having 0 impedance (theoretically) at resonance. Now after this discussion I see I see how the tank can be a series circuit also. Thanks so much for making me think. I just have this thing, I have to understand how everything works. Next I want to learn about something I don't understand. Why are the laminations brought above the coil and have a gap. The new Briggs coils have a strip of mylar between these poles. I know it is for hysterisis. I would like someone to explain the effect. I believe it is sort of a "balancing effect" for the magnetic field induced on the magnet ends of the laminations. Thanks again guys for the stimulating and thought provoking discussions. Looking forward to learning about the magnetic issue. Thanks again, Love Ya All. Merry Christmas. Al Eden

Share this post


Link to post
Share on other sites
DMedal
Al- Picture, Please? <grin> Laminations are insulated (varnish or thin insulator between) in motors, transformers and ignition coils in order to reduce eddy currents. I'm not sure that's your question but it is a lot easier to explain than hysteris effects so I'll start there. As the field changes and induces current in the coils, it also induces currents in the iron core. These currents are called eddy currents and are much like the eddy you get with a canoe paddle, a swirl of current in the iron core. Because this current never makes it to the wires but is lost in resistance and heat, it is a bad thing. By isolating the eddies to the individual laminations we can greatly reduce these losses. Not sure I understood your question however, so maybe I didn't answer it. For my question, have you looked at a mag ignition on a scope? How long does the spark persist vs the ringing you see? -Don

Share this post


Link to post
Share on other sites
bobjohn
This is getting better and better...!! Al, I grew up on a farm and my father owned two sawmills. I have worked the 'bull pen' many hot days.... Don, while the engine is running, the ringing never stops. As I mentioned before, the ignition system goes into oscillation and with various harmonics and sub-harmonics, the ringing will look something like the smokestack of a coal-burning locomotive. A large pulse during charging/discharging of the coils, and a flurry of smaller pulses caused by the harmonics. The inductance of the coils is so large, the magnetic field never completely discharges before the next surge comes along. The engineer's job is to put the smaller pulses into the large pulse to increase the spark.sm01 I wonder if a narrow-band dynamic filter would help?XX( bob

Share this post


Link to post
Share on other sites
DMedal
quote:
Originally posted by bobjohn
Don, while the engine is running, the ringing never stops.
Even when the points are closed? I'm learning lots. The engineers do the stuff that requires math, but engineering is just about no help in repairing. I always thought engineers should first have to be fixers. Got that idea while working as a Cadillac mechanic in 69. (Just who had the idea of needing to remove the A/C compressor to change plugs?) -Don

Share this post


Link to post
Share on other sites
bobjohn
Sounds about right Don. Engineering is mostly math. But the math is no good without a good understanding of all the values, both fixed and variable. An engineer understands things from the inside out, while technicians understand things from the outside in.sm06 Had a '73 Ford that number 8 plug was under the exhaust manifold, above the starter and behind the steering column. Easiest way to change it was to remove hood, disconnect engine mounts, raise engine 4 inches, and remove exhaust manifold. I have yet to meet the engineer or tech that cannot learn something new during each conversation with his/her fellow man.sm04 bob

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Who's Online (See full list)

    • FloridaRich
    • Sovereignm88
    • Bluevista
  • Today's Birthdays

    No users celebrating today
  • Recent Status Updates

    • Paul M.Murphy

      Paul M.Murphy

      Hello glad I got on this site. I hope to upload a couple of photos of my Simplicity Broadmoor 
      Model 707 Tractor. I have replaced all the belts on it and installed a new ignition switch.
      Have a mower deck which was in poor shape , repaced a couple of bushings and welded the
      deck where there was some holes. The tractor and mower deck came from a friend and the 
      mower deck metal was getting pretty thin. Some new metal was put in it and we also undercoated the underside of the deck. This was done by my friend Greg Toulman.
      I have not put the mower deck on the tractor yet, waiting for Greg to come home and give me a hand. Can you please tell me if there is a manual for the mower deck and where I could get one.
      I would also like to get some moon wheel discs as well.
      I will try and down load some photos of the tractor when my wife shows me how to. 
      I really love driving this tractor and also how well it is made compare today's tractors, especially the flexible rear end and the rollers.
      Tell the next time 
      Paul
       
      · 8 replies
    • jbrooks

      jbrooks  »  Talntedmrgreen

      Josh
       
      folowing  this from a link at the downloads.  I just rescued an AC 920 with the lambodini diesel. I can' t download  yet .  Where did you find the PDF copy you posted. . I would like to figure out how to adjust the injection pump. 
      I assume it's a bosche, correct
      · 0 replies
  • Adverts

×