Author Topic: Magneto spark output vs RPM  (Read 626 times)

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Online CirclotronTopic starter

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Magneto spark output vs RPM
« on: September 03, 2020, 02:18:47 pm »
Conventional wisdom and internet lore has it that the faster a magneto is rotated on an engine, the greater the spark voltage and energy it produces because as everyone knows, a magneto is just a specialised generator.

Thought about this for quite a while. I've come to the conclusion that up to a point a magneto output does depend on rpm, but not for a same reason a generator does. In a traditional magneto a small percentage of the winding is short circuited by a set of contacts, and when the rotating magnets on the flywheel are aligned with the magneto coil pole pieces the contacts open circuit and a high voltage is produced by this coil. So far, so good.

When the points are closed and the magnet comes into position, it tries to put magnetic flux through the coil pole piece but the short circuited coil has a current induced into it that creates a magnetic field in the pole piece that opposes the rotating magnet's flux. If the short circuited portion of the coil had superconducting wire with zero resistance, the rotating magnet could be positioned stationary next to the pole piece and it's flux would never be able to enter the pole piece. And in fact, if the short was then removed a single high voltage pulse would be produced despite the magnet being stationary.

Because the short circuited portion of the coil has a small value of resistance, the result is the magnet's flux will gradually creep in to the pole piece even though the coil is shorted. The longer this is allowed to happen before the contacts open and the short circuit is removed, the less flux change is available to generate a high voltage.

What I am suggesting is that above a certain rpm a magneto will produce a more or less constant output voltage into an open circuit, not increasing indefinitely with rpm, and as rpm drops the output voltage decreases because of less available change in flux when the contacts open, not because of slower rate of change of flux as in a conventional generator.

Comments invited.
« Last Edit: September 03, 2020, 11:47:39 pm by Circlotron »
 


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