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Low tension mag

Benny Mckheean

Registered
Age
72
On my associated mag I put a digital meter on the output while spinning the flywheels with a crank as fast as I could(wire unhooked from igniter)and getting 2.5 Volts.Is this enough to start the engine and if not would recharging the magnets bring it up higher.The mag.is four bolt with a brass body.Thanks for any input.
 

Keith Smigle

Registered
Benny~

As far as I know, a digital meter is a little slow to report the max reading on an Associated low tension magneto. I believe that an analog meter would give you a higher reading. As far as running, it is VERY important to take advantage of timing the ignition trip to the exact time of the highest moment of output which is indicated by the buttons on the front of the magneto. Any amount of fluctuation will result in a weaker spark at ignition.
 

Elden DuRand

In Memory Of
Age
78
Last Subscription Date
12/22/2017
Benny Mckheean said:
On my associated mag I put a digital meter on the output while spinning the flywheels with a crank as fast as I could(wire unhooked from igniter)and getting 2.5 Volts.Is this enough to start the engine and if not would recharging the magnets bring it up higher.The mag.is four bolt with a brass body.Thanks for any input.
Rotary low tension mag output is dependent on the speed of rotation. 2.5 Volts (Alternating Current) is a reasonable figure to read on a digital meter.

Low tension ignition depends on the principle of "inductive kickback". If the output of the magneto is shorted (ignitor points closed) as the mag passes the point of maximum output (current in the coil), a fairly large magnetic field is generated in the armature. If, while the current is high, the short is removed, the field rapidly collapses and causes a relatively large voltage pulse to be generated.

If you don't short the mag, there's some voltage but no current and no generated pulse.

I studied the Webster setup on an Aermotor engine and made a bunch of oscilloscope pictures. They're at:

http://home.cybertron.com/~edurand/Aermotor/scope pictures.html

So......more than you needed to know about Alaska, eh?? :rolleyes:

Take care - Elden
http://home.cybertron.com/~edurand
 

Harvey Teal

Subscriber
Last Subscription Date
07/22/2019
Interesting 'scope work, Elden. Looks like some pretty fast rise times on some of those spikes. Makes you wonder how the old varnish is holding up on those mag windings.
 

Benny Mckheean

Registered
Age
72
Elden, Great info,I just added it in my favorites.Going out to the garage and get my analog meter and see what it reads per Keith.Wish I had a scope now like the one I used at work for 20 years. Thanks all
 

Elden DuRand

In Memory Of
Age
78
Last Subscription Date
12/22/2017
Benny Mckheean said:
Elden, Great info,I just added it in my favorites.Going out to the garage and get my analog meter and see what it reads per Keith.Wish I had a scope now like the one I used at work for 20 years. Thanks all
Benny:

I'm glad I could clear up some of the questions. Thanks for the compliments. I guess that now I can claim to be a know-it-all and be obnoxious. :rolleyes:

If you use an analog meter and spin it fast enough to damp out the 'wiggle' in the meter, you will most likely see a somewhat higher voltage indicated than with a digital.

In a private email, Chuck Parcher asked about adding a capacitor across the contacts of the ignitor (across the low-tension mag coil).

In actuality there is unavoidably built-in to the magneto what is called "parasitic" capacitance. This is due to the combined capacitave effect of parallel turns of the windings. When these magnetos were designed, I don't think the engineers really knew much about it and didn't have instruments like oscilloscopes to see the effect of it, which was the high frequency oscillation.

In any spark ignition system be it low or high tension, the oscillation is necessary to produce a hot "spark", actually a series of sparks spaced very close together time-wise. In high tension systems, in addition to making the oscillations slower (for a hotter spark), the capacitor -also- keeps the points from arcing badly.

If you put more capacitance across a low tension magneto, the oscillations will be slower but the energy will be about the same. As a matter of fact, once I get the correct rocker arm and trip on the Aermotor, I might just hook up the scope and try adding various values of capacitance across the ignitor points to see what happens.

My guess is that, as I add capacitance up to a point, operation of the engine won't be affected. After that point, the oscillation frequency will be low enough so the rise of voltage will be slower than the opening of the ignitor contacts, i.e., the voltage won't build fast enough to make much of a spark.

Boy! This is fun!!!!

Take care - Elden
http://home.cybertron.com/~edurand
 

Steve Wright

Registered
Last Subscription Date
10/10/2009
Elden DuRand said:
Benny:

the capacitor -also- keeps the points from arcing badly.
On a low tension system the arcing of the points is the ignition. Wouln't puting a capacitor accross the the points in a low tension system (The ignitor itself) reduce the arcing and be a poorer ignition?
Steve in Nebraska
 

Elden DuRand

In Memory Of
Age
78
Last Subscription Date
12/22/2017
Steve Wright said:
Elden DuRand said:
Benny:



On a low tension system the arcing of the points is the ignition. Wouln't puting a capacitor accross the the points in a low tension system (The ignitor itself) reduce the arcing and be a poorer ignition?
Steve in Nebraska
Steve:

You are right. But since there is already some parasitic capacitance in the magneto, you get the damped oscillation. My unproven theory is that a certain amount of added capacitance may cause the arc to sustain a little longer on each oscillation. This should give a more effective spark.

It's all theory on my part. I'll have to do some scope testing to see if it really means anything or if I'm full of it. :D

Take care - Elden
http://home.cybertron.com/~edurand
 
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