Topic Author
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Fri Apr 28, 2000 9:19 pm

I have asked this question before in the 'other' forum, but have not gotten the answer I was looking for. In simple terms, what makes a magneto spin by itself? Supposedly it is the same concept as how a lanwmower keeps going!!

Thanks for any help on this


RE: Magnetos...?

Sat Apr 29, 2000 12:42 am

While the engine is operating, the magneto is spun by the a shaft connected to the reduction gearing on the accessory section of the engine. But to get the whole process started, the magneto is equiped with an impulse coupling... basically a coiled peice of steel which acts like a spring. At engine speeds of between 400-600 RPM (during starter engagement), this impulse coupling coils until it reaches its full potentiality then releases, spinning the magneto shaft at a high enough speed for the electromagnet to produce enough electricity.

When the engine turns at higher than 400-600 RPM (after start), the impulse coupling is pulled away from the shaft by centrifugal force and the magneto is rotated normally.
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RE: Magnetos...?

Sun Apr 30, 2000 4:29 am

ATRPilot that is right only in some cases. And like you said in that case the magneto has an impulse coupling that consists of 2 spring loaded arms. Where you are wrong in your description is that the coil doesn't release when it reaches it full potential. During the normal rotation of the magneto the arms each contact a stop in the casing of the magneto. Two stubs which are spining with the magneto then come into contact with the arms and force them to shut which then releases the energy contained in the coil/spring and gives the temporary increase in speed.The reason this is done is because in order to get a spark of sufficient intensity to ignite the fuel in the cylinders the magneto has to be turning at a certain RPM. Obviously when starting the engine this doesn't happen so an artificial method is used to obtain this increase in RPM.Another method used to give the required spark intensity is the shower of sparks method where more than one spark is supplied by the spark plug thereby giving a lot of small heat sources over a larger area of the cylinder.As to what keeps the magneto spinning you were right in that the magneto is connected to the crankshaft of the engine via reduction gearing.Boeing 727 is this the answer you need? If not could you be more specific?PaNMAN
aaron atp
Posts: 517
Joined: Sun Mar 19, 2000 1:17 pm

Lets Get Even More Basic...

Sun Apr 30, 2000 8:36 am

panman is certainly right in his post, but I think he might be wanting to know how a magneto generates electricity on its own to create the spark. "Like a lawnmower" was his reference. That probably means he has no idea what an impulse couple is or why it's needed.

If you spin an armature with coils of wire around it inside of a permanent magnet, an electromotive force (voltage) is created by the coil passing through the lines of flux around the magnet*. This voltage created by the magneto is enough to create a spark that jumps across the gap between the electrodes in the spark plugs.

The magneto creates a lower voltage (and less current) at low rpm's like any DC generator. The starter doesn't turn the engine fast enough for the magneto to create a strong spark, so an impulse coupler is used to give it the boost it needs to get the engine running at sufficient RPM.

*a commutator is used to create DC voltage, or as the poles change a negative emf would be created


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RE: Magnetos...?

Fri May 05, 2000 12:07 am

Hi all, Buzz here. You've got good answers, now we need to see if we answered the question.
One advantage to a Magneto is that it's a self contained method of making the sparkplugs spark. If you spin it (bolted to the end of the engine) it generates a bit of electricity, tehn transforms it to a generous spark. Most piston engines have the mags bolted to the aft end of the engine, a few of the big radial engines (my favorite) have them on the nose case.
OK, Mr.B727. the ball is in your court now.
Friday i was checking mag timing on my favorite DC-3, and checking all the aft sparkplugs. A couple jugs were not firing when we went to Left mag, #1 engine.
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RE: Magnetos...?

Fri May 05, 2000 2:10 am

In simple terms, what makes a magneto spin by itself?

I always enjoy reading Buzz's answers 'cuz he makes continuous reference to the World's Greatest Airplane!

Back to the original question from my point of view, and I can remember this question using the same premise from some time ago.

The magneto, be it on an airplane piston engine or on a lawnmower, does not spin by itself!

In combination with the excellent answers above, I hope the originator of the thread now has a good understanding of these wondrous little things. (That's pilot talk for technical stuff)

Best Regards,

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RE: Magnetos...?

Wed May 10, 2000 4:55 pm

Magnetos are:
Rotating magnets that excite a low volt fixed coil. The magnetic force is transferred from the rotating magnet to the coil thru soft iron laminations that are connected to the ends of the coil. The connection transfers the magnetic force only to induce the low side.
They contain no brushes.
The manetos alternately fires plugs - ground to electrode and electrode to ground (which is why swapping plugs is critical to even out wear).
They have points to open the low voltage side field and cause an abrupt collapse of the low field and excite the high voltage outter coil. This happens at the highest magnetic resistence to rotation (the "E" gap also referred to as internal timing).
The high voltage is passed to a distributor that may be physically on top of the magneto or remotely located (as in a large round engine).
The Lawnmower is an excellent example of a magneto. The magnet is on the edgeof the flywheel and the coil assembly sits on the side. The magnet "passes" the coil to excite it. For one moment it is centered between the coil lamination and passes the magnetic field through the coil to excite it. The points break and fire the high side - BANG. (Lawnmower only fires in one direction.)

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