Small engine magneto question - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: November 02, 2018, 05:07:09 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: November 02, 2018, 06:29:12 am
content: 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.

author: AllTheGearNoIdea
author_rank: Unknown
timestamp: November 02, 2018, 07:39:20 am
content: 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

author: Zero999
author_rank: Super Contributor
timestamp: November 02, 2018, 09:07:28 am
content: Quote from: AllTheGearNoIdea on November 02, 2018, 07:39:20 amI 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.

author: AllTheGearNoIdea
author_rank: Unknown
timestamp: November 02, 2018, 09:17:11 am
content: 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

author: xavier60
author_rank: Super Contributor
timestamp: November 02, 2018, 09:18:44 am
content: I don't understand magnetism so well, so my usage of terms might not be correct.With the magnetos that I deal with, the flux is allowed to enter the coil's core.The points are timed to close before the applied flux is reversed.With further rotation, the applied flux is reversed.Because the coil's primary winding is shorted by the points, a current flows in the primary winding that resists the flux reversal in the core. When the current path is broken by the points opening, the flux is allowed to rapidly snap from one direction to the other, inducing high voltage across the windings.At low speeds, the primary winding's electrical resistance causes the current to decay, allowing the flux to change somewhat before the points open, reducing the amount of rapid flux reversal.Also at low speeds, the points open slowly causing more arcing, wasting energy.

author: AllTheGearNoIdea
author_rank: Unknown
timestamp: November 02, 2018, 09:29:29 am
content: I guess you could be right about the points open speed.

author: xavier60
author_rank: Super Contributor
timestamp: November 02, 2018, 09:45:23 am
content: I have actually observed the primary current on an oscilloscope and noted the current decay at low speeds.With increasing speed, the HT quickly reaches a maximum and tends to stay the same for the rest of the speed range.

author: David Hess
author_rank: Super Contributor
timestamp: November 05, 2018, 11:20:18 pm
content: On mine the extra winding is for shorting the ignition to shut the engine off; it just produces a shorted turn when closed.

author: amyk
author_rank: Super Contributor
timestamp: November 06, 2018, 01:19:47 am
content: A magneto is simply an unregulated alternator and transformer (ignition coil) combined into one.

author: xavier60
author_rank: Super Contributor
timestamp: November 06, 2018, 12:44:05 pm
content: There are many poor descriptions of how magnetos work, some are totally wrong.I have found one that gets it right with this quote,"While the induced current is flowing in the primary circuit, it opposes any decrease in the magnetic flux in the core. This is in accordance with Lenz’s Law that states: “An induced current always flows in such a direction that its magnetism opposes the motion or the change that induced it.” Thus, the current flowing in the primary circuit holds the flux in the core at a high value in one direction until the rotating magnet has time to rotate through the neutral position to a point a few degrees beyond neutral. "https://okigihan.blogspot.com/p/low-tensionmagneto-system-high-tension.html