AC Coil Operation

brewsky · Post by **brewsky** » Mon Jul 05, 2010 6:39 pm

Hate to display my ignorance, but how would an AC coil work with points/condenser system.
Here's my current thinking, someone please tell me what I'm missing:
In a DC coil, the constant voltage creates a magnetic field around the primary windings which collapses when the points open and the primary coil loses its current.
The collapsing field induces voltage in the secondary winding which is amplified by the increased # of windings and the plug fires.
In an AC coil though, the field would be expanding, contracting, and reversing each time the rotor magnet passes across the primary winding. This should create a constant spark, except for the time the points are open? Does this mean the spark occurs when the points CLOSE?
If not, how does it work?
I'm working on an old 2 stroke (non honda) that evidently has an AC coil setup.

48lesco · Post by **48lesco** » Tue Jul 06, 2010 2:53 pm

OK - As an old Lawn Boy service tech, I'll give this a shot! What you have is a magneto ignition system that operates the same as the DC system in your Superhawk except that a magnet passing by the primary windings induces a current instead of the battery. As the magnet passes by, the induced current flowing in the primary circuit creates a magnetic field around the coil that collapses as the points open. As you correctly stated, this collapsing field induces the high voltage in the secondary windings that fires the plug. The current induced in any coil is proportional to the speed at which it is moving with respect to the magnetic field. The speed of the flywheel isn't nearly sufficient to produce enough voltage in the secondary coil, but the collapsing field moves much faster (limited by the speed at which the points can break the circuit). Any arcing across the points due to self-induced current reduces the speed of collapse, and the effectiveness of the scheme. That's why the condenser is so important. Arcing is also bad for the points contact surfaces.

brewsky · Post by **brewsky** » Tue Jul 06, 2010 4:55 pm

Ok, maybe I'm beginning to see how it could work....

If the magnet position on the rotor is at the exact spot necessary to produce a positive (relative term) voltage at the time the points open, then there would be a collapse in the secondary coil that would fire the plug.

If there is a single magnet on the rotor this would happen once per revolution and make sense.

I believe the condenser prevents arcing across the points by "absorbing" the echo current produced in the primary when the secondary coil magnetic field collapses across the primary coil as the points fire.

The problem I was having was that AC produced in the primary coil automatically produces AC in the secondary coil (as long as the points are closed), and that means the magnetic field is expanding, collapsing and reversing each time the magnet passes a primary winding. This should mean the plug would fire each time it happens even with the points closed.....if the rpms are fast enough to produce enough voltage. Maybe the rate of collapse is greater when the points open than when the AC reverses field.

Thanks for the reply!
Hopefully there is just a single magnet on a lawnmower engine flywheel....that should ease my mind!
If not, I'll just stick to connecting red to red, green to green, and black to black and not worry about the rest.

48lesco · Post by **48lesco** » Tue Jul 06, 2010 10:12 pm

Yep, it's timed so that as the magnet approaches the coil and the current induced in the primary reaches a maximum, the points open to collapse the field created by the current (different than the magnetic field of the permanent magnet).

The direction of the current would flip as the magnet passed, but who cares? We already sparked by that point.

Its the speed of the field collapse that's important. If the current were stopped instantaneously by the points, the field would collapse at nearly the speed of light (relatively fast compared to the rotational speed of a flywheel).

The condenser is just a big capacitor that absorbs the self-induced current in the primary to prevent arcing, and is sized to form a resonant circuit with the coil (you'll have to Google that one). Basically, it adds back to the current in the primary for the next cycle.

Hope that helps! If its still not clear, Google magneto and you can read all night...

brewsky · Post by **brewsky** » Wed Jul 07, 2010 4:45 am

Thanks, 48, that clears it up! I did the google search previously, but I never saw any clear explanation of the phasing of the AC current. I was hung up on seeing multiple magnets on the rotor on previous bikes.

I remember playing with AC coils in science class in High School and they could make one heck of a continous spark without a set of points!

I'm getting ready to tackle a seized 12 HP Briggs motor for a neighbor to see if it is worth repairing...might need some advice later!

I guy gave me a "siezed up" push mower a while back. After cleaning out the dried grass under the deck which fouled the blade it ran like a new one! Don't think this one will be that easy.

Thanks again!

48lesco · Post by **48lesco** » Wed Jul 07, 2010 7:24 am

Good - Happy to help - I'll bet the AC coils in high school had 120VAC to start with, not a passing magnet!

Now... Good luck with your seized 12HP Briggs!

brewsky · Post by **brewsky** » Thu Jul 08, 2010 5:58 am

YEP!
I remember playing with doorbell transformers at home also......ouch!