Tedwood
Two Stroke Sniffer
Joined: 08 May 2014
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hmmmnz
Super Spammer
Joined: 22 Aug 2006
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Tedwood
Two Stroke Sniffer
Joined: 08 May 2014
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craisin
Derestricted Danger
Joined: 25 Jan 2014
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Teflon-Mike
tl;dr
Joined: 01 Jun 2010
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 Posted: 01:41 - 10 Jun 2014  Post subject: |
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| Tedwood wrote: | But then I don't understand why they sell 12vac and 12vdc ignition coils. |
OK, working back through the system; your front end is the spark-plug; you have a gap, typically around half a mm or so, and you need electric to jump through the stuff in the middle, hopefully setting it on fire. To jump that far you need a big voltage, bigger than most generators are ever likely to make, so you need some sort of step-up transformer. So we get to the coil.
The coil is essentially a big step-up transformer; two coils of wire around a common iron core; you put volts on one coil and that creates a magnetic field through the iron core; magnetic field then 'induces' a voltage in the second coil. Ratio of input coils to out-put coils gives you the ratio of voltage magnification.
However; the efficiency of the system is dependent on a 'rate of change of magnetic field'.. if you put DC volts on a coil, that don't change so DV transformers tend not to be as efficient as AC ones.
However, within that little nugget, lies the 'special' bit of an ignition coil....
In operation they are essentially a DC device; and on a more 'usual' battery fed ignition system; the low tension winding would be given 12v straight off the battery. That might 'induce' a DC voltage in the High-Tension 'secondary' winding, of maybe 600v; that would be from a winding ratio of 50:1, BUT you want a LOT more volts than that to get a spark to jump a gap even only a half mm one.
Now, a property of a coil in an AC circuit is that it will strongly resist any change to an applied voltage; and does so with an opposing voltage... BUT that reverse voltage is proportional to the rate of change of voltage. So faster voltage changes, more reverse volts you get.
SO... if you put 12v on a 50:1 step-up transformer... then instantly remove the 12v supply... the coil reacts to oppose the change... and the voltage change happening so suddenly, the 'reverse' voltage on the HT windings, can go from 600v to 6,000v... and bang... you have a spark.
Its called 'field collapse'... and its how ALL ignition coils work.
So AC or DC 'coils' are generally reffering not to how the coil works, which is a DC device, but to the supply system...
In the example, I said that the LT windings were 'energised' by being fed straight from a 12v battery, the spark inducing 'field' collapse created by turning 'off' that supply voltage... wither with a pair of points, or a more elaborate transistorised switch in teh form of a CDi unit.. but same deal either way.
But, what if we took volts straight off the generator? As many older bikes do, like yours.
Generator is simply a flywheel with strong magnets glues to it, that rotate around a bunch of windings, so that as the magnet passes a coil, it induces a current in it. and it will generally make AC current...
As magnet passes over one end of a winding; volts get sent one way.. when the magnet has gone half a turn on the crank it passes the other end of the winding, and sends the volts the other way.
So you have an 'AC' source... BUT... if you have a winding just for the ignition coil..... well, you are only going to demand a field collapse when you open the points... and you are going to do that at a few degrees before top-dead centrer... so you actually don't care whether you are getting AC volts or DC volts out the generator, or much give a hoot whether the voltage trace is a full sign wave or a square wave or a tidal wave, REALLY.... only bit of the generator output you are bothered about is the bit for perhaps 30 degrees before you want to open the points...
You would like the cold to have been fully energised, and reached 'peak' AC volts, just before you open the points, so you have as many volts on it to take off as possible... BUT in actual fact, doesn't really matter, what's more important is the rate of change of voltage, remember, so IF the voltage was increasing before you took it off... there would already be an opposing voltage being generated by the coil; and if you opened the points at that point, that opposing voltage would negate some of the reaction volts you want from creating a field collapse... IF however, you wait until the supplied AC voltage is just starting to decay... then there's already an opposing voltage being induced by the coil, trying to 'hold' the peak volts.. and when you stimulate field collapse opening the points, that is going to get magnified....
So, the positioning of the 'ignition winding' in the generator will be arranged such that the magnet generating current in it has just over centered the winding at about the point you want to open the points, and as far as the ignition is concerned, its got a DC voltage on it, just as if it came from a battery... except, its coming straight from the genny... and that small amount of 'decay' in the wave form voltage is timed to be most beneficial to get biggest effect from field collapse.
So the coils cont work differently; ones sold for AC ignitions work the same way as ones sold for DC ignitions; the difference is in the 'tuning' of them; same as the 'tuning' that differentiates coils for CDi ignitions compared to the ones for points ignitions.
It's a fancy step-up transformer, remember, and it works from having two windings around a common iron core, and the ratio of turns on each winding deturmins the amount of voltage magnification.
So; if you have a 50:1 turns ratio; 12v gets turned into 600v. BUT, you can get a 50:1 ratio from having 10 turns on the LT and 500 on the HT, or from having 20 on the LT and 1000 on the HT... same winding ratio, same step-up voltage, BUT, the more windings, the more current the coil will draw, and the stringer the magnetic field that will be produced, and the 'slower' the coil will react.
On a DC system, the supply coming from the battery is essentially unlimited; so the coil can draw a fair few amps... you have more than one charge winding in the generator, and with a rectifier, it can store the electric generated on each winding, in either direction, when the magnet passes it.
On an AC system, you ONLY have the 'power' from the volts made by one winding, and even that only briefly, from the portion as the magnet over centres the winding core.
So an Coil for an AC system, is likely to have a lower resistance, suggesting fewer turns on each of the windings, so that it can react much more quickly when the volts from the magneto are put on it, and it doesn't draw too much current, as the magneto isn't making very much.
So back to top..... Yes you CAN put 12vCD on a coil for AC, BUT.... low resistance coil, 'tuned' to only have voltage briefly applied, you can easily burn it out,
And it may not tell you very much anyway, as the 12v AC coil is designed to work on an 'average' 12v delivered in a pulse, that could peak up near 17v or so, not a constant 12v as from a battery.
AND you have already answered the question, testing on an alternative winding... bike revs out fine... even though that winding is likely not in optimum position to give volts at the best time for best field collapse...
Indicating your ignition winding is duff, and or, the stator is grossly out of alignment...
So, having found the fault, in the stator, what are you hoping to achieve? You have already diagnosed the fault; so get the stator replaced or re-wound.
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?' |
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Tedwood
Two Stroke Sniffer
Joined: 08 May 2014
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Posted: 18:56 - 10 Jun 2014  Post subject: |
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| Teflon-Mike wrote: | Indicating your ignition winding is duff, and or, the stator is grossly out of alignment... |
Many thanks for your detailed and informative explanation of how the AC ignition system works.
I can't believe that is the first time it has been explained to me, but then it is the first old bike that I've had that has AC ignition.
1985 Honda CB125RS 125cc Single Cylinder with SOHC.
Further discovery tonight, I treated the bike to a new set of points, set the gap and moved on to also ensure that the timing was correct.
On setting the crank to the "F" mark, I was horrified to find that the contact breaker cam was some 40/45 degrees away from where the points would open. So the spark would occur quite a way past TDC. With the valve/cam timing out that much it's unbelievable that the engine actually run as well as it did.
I REALLY CAN'T BELIEVE THAT SOMEONE HAS ACTUALLY SET THE ENGINE UP LIKE THIS! I've heard that this does happen, but never ever experienced it before. The moral of this story is, when buying an old bike: CHECK EVERYTHING.
Therefore, hopefully, my exciter coil might be OK because this stator misalignment might actually be my revving problem.
I removed the contact breaker plate and advance mechanism (how do you lubricate this? HMP grease?). The camshaft timing marks didn't line up and confirmed my findings when trying to set the static timing.
Loosened the camshaft chain tensioner locknut (the one nearest the crankcase) and tried to lift the chain with a screwdriver to move it around the teeth to the correct position but it seemed reluctant to move and I didn't want to use too much force. Do I need to remove the camshaft sprocket to get the cam in the correct position? Also, is there any danger of the chain coming off the sprocket on the crank and causing problems?
Thanks for reading and hope that you can help. |
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