Electronics > Projects, Designs, and Technical Stuff
Small engine magneto question
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Circlotron:
Small engines like on lawn mowers etc have a magneto setup to provide a high voltage spark. A moving magnet on the periphery of the flywheel tries to send flux through a laminated pole piece that has a coil of wire around it. This coil has a tapping at I think the 1% point and in times past at least, a set of mechanically operated contacts shorted out the coil at the tapping point. This shorting out of the coil discourages flux from starting to flow through the pole piece and consequently not making a high voltage pulse until the contacts open and remove the short, allowing magnetic flux to flow.

Now, the question is this - seeing the establishing of flux flowing through the pole piece is controlled by the contacts, why does the magneto output increase with rpm? The flux density is the same regardless of rpm. The passage of the magnet past the pole piece only puts the magnet in position. AFAICT this is not the same as a simple alternator.
tautech:
Well they must produce sufficient spark at cranking speeds in order to start and when running the speed at which the magnet passes the ignition poles increases the energy output. Unlike a CD ignition where energy is stored and then released so they are inherently easier to start.
AllTheGearNoIdea:
Would the core material not be more likely to be going into saturation at lower rpm. Transformers etc are frequency dependant would a magneto not be similar optimised.  Rate of change, I’m guessing wildly
Zero999:

--- Quote from: AllTheGearNoIdea on November 02, 2018, 07:39:20 am ---Would the core material not be more likely to be going into saturation at lower rpm. Transformers etc are frequency dependant would a magneto not be similar optimised.  Rate of change, I’m guessing wildly

--- End quote ---
I think you're confusing it with a transformer. In order to go into saturation the magnetic flux needs to stronger than a certain threshold.  It's true that in a transformer, as the frequency is reduced and everything else remains constant, the flux will increase until core saturation, but this is not a transformer. In the case of a magneto, the permanent magnet's field strength remains the same irrespective of the speed, so will never cause core saturation, no matter how slowly it's rotated.
AllTheGearNoIdea:
Im clearly out of my depth here. But clearly if the magnet is moving slow or not moving at all there is no rate of change and no voltage produced.

Chris
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