How Does a Magneto Work in Aviation?

[EdgarW]

TL;DR Summary

Magneto operation explained

Not sure this is the correct forum; if a different forum would be more appropriate please advise: where is there a complete explanation of a magneto starting from the Physical Magnet's AC current thru to the Secondary Winding output. Some of aviation is totally dependent on a magneto and it would be nice to have a reliable explanation. Thanks for any help.

 

[Drakkith]

What level of explanation are you looking for? Do you want to get into the details of the circuit with math and all that, or a math-less explanation that just goes over the workings in general?

A few sources:

https://science.howstuffworks.com/question375.htm
https://en.wikipedia.org/wiki/Magneto
https://pilotinstitute.com/how-do-magnetos-work/

 

[EdgarW]

What level of explanation are you looking for? Do you want to get into the details of the circuit with math and all that, or a math-less explanation that just goes over the workings in general?

A few sources:

https://science.howstuffworks.com/question375.htm
https://en.wikipedia.org/wiki/Magneto
https://pilotinstitute.com/how-do-magnetos-work/

Thank you for your reply.
Initially without the math so I can be comfortable with the concepts, hopefully.

Beginning, this is what I think I understand?
To start the Physical Magnet (Pm) on the Mag shaft provides ac current to the primary winding; the PhysicalMagnet's change in the Number of MagneticLinesOfForce (MLof) going thru the PrimaryWinding (Pw) induces a current in the Pw. The current going thru the Pw INDUCES Mf around the Pw. That's ok when the Pm's SouthPole (Sp) is on the Left (L) and the Pm's NorthPole (Np) is on the Right(R).
And the Points are on the L.

Pm'sMLof always go from N to S, so it's reasonable the Pm INDUCED current would flow N to S; ie: R to L toward the Points.
PRIMARY WINDING
POINTS Left end of Pw S Right end of Pw N PwGround

1- The Pw is grounded on both ends; one end is grounded thru the Points to ground and the other end grounded to the Mag case. When the Points open why doesn't the Pm INDUCED current simply go to the other end of the Pw that is also a ground?

PRIMARY WINDING
POINTS Left end of Pw N Right end of Pw S PwGround

When the Sp is on the R and the Np is on the L; now the current is going in the opposite direction thru the Pw. And the Opposite direction Current is INDUCING a Mf in the opposite direction.

2- The Pw is grounded on both ends; one end is grounded thru the Points to ground and the other end grounded to the Mag case. The Pm INDUCED current is already going toward the R end of the Pw that is a ground, why is the current stopped by the Points opening?

Does this make any sense??? Hope the abbreviations make things simpler.

 

[berkeman]

Welcome to PF.

Beginning, this is what I think I understand?

Is there a diagram that goes with your explanation? Use the "Attach files" link at the lower left of the Edit window to upload the diagram.

 

[Drakkith]

I'm not an expert in this area, so I've attached a diagram I found on google to reference. The diagram doesn't do a good job of showing it, but note that the secondary and primary are either grounded together on one end or both simply attach to the common ground at separate points. The other end of the secondary is grounded through the rotor and distributor, while the primary appears to be grounded through the condensor/contract breaker part of the circuit.

1- The Pw is grounded on both ends; one end is grounded thru the Points to ground and the other end grounded to the Mag case. When the Points open why doesn't the Pm INDUCED current simply go to the other end of the Pw that is also a ground?

According to this diagram and several more, the primary winding has one end directly grounded and the other end grounded through the condenser/breaker circuit, as shown above. Assuming that 'points' means the contact points on the distributor, then they have nothing to do with the primary winding.

2- The Pw is grounded on both ends; one end is grounded thru the Points to ground and the other end grounded to the Mag case. The Pm INDUCED current is already going toward the R end of the Pw that is a ground, why is the current stopped by the Points opening?

Looks to me like it is the secondary winding that connects to the points on the distributor.

 

[EdgarW]

I'm not an expert in this area, so I've attached a diagram I found on google to reference. The diagram doesn't do a good job of showing it, but note that the secondary and primary are either grounded together on one end or both simply attach to the common ground at separate points. The other end of the secondary is grounded through the rotor and distributor, while the primary appears to be grounded through the condensor/contract breaker part of the circuit.
View attachment 349974



According to this diagram and several more, the primary winding has one end directly grounded and the other end grounded through the condenser/breaker circuit, as shown above. Assuming that 'points' means the contact points on the distributor, then they have nothing to do with the primary winding.


Looks to me like it is the secondary winding that connects to the points on the distributor.

Let me see if I can simplify my concern:
As the drawing clearly indicates, the Pw has 2 Grounds: 1st ground (Direct Ground) is connected directly to (as you say) a common ground; the other Pw ground is thru the Points (Contact Breaker) to a common ground. So the question is when the Points open, the first ground is lost, why doesn't the Pw current simply go to the Direct Ground? For some reason, that I don't understand, when the points open the Pw current stops.
If you have a lite bulb on with two grounds and for some reason you disconnect one ground, the bulb is still on.

 

[Drakkith]

So the question is when the Points open, the first ground is lost, why doesn't the Pw current simply go to the Direct Ground? For some reason, that I don't understand, when the points open the Pw current stops.
If you have a lite bulb on with two grounds and for some reason you disconnect one ground, the bulb is still on.

When the contact breaker is opened, there isn't a complete circuit anymore. The current can't go through the capacitor (condensor) and it can't come from or go to the other ground, as there would quickly be a buildup of charge in the disconnected side that would stop it.

I'm not sure what you mean about the light bulb having two grounds. There needs to be a complete circuit for current to flow through. If there is still a complete circuit after disconnecting from one ground, then the light bulb will light up. If there isn't, then it will not.

 

[EdgarW]

Thanks for the response.
Let me explain my thoughts: the Magneto gets it's initial current from a physical magnet mounted on the Magneto Shaft, because the physical magnet is turning the Magnetic Lines of Force (MLof) is Inducing Ac current; initially, lets say the MLof are going thru the Pw from L2R and then as the shaft turns the MLof are now going thru the Pw from R2L. With both ends of the Pw grounded it seems that the direction of the MLof would use the ground at the opposite end of the Pw; ie: when MLof are going thru the Pw R2L the MLof would go to the L ground and when the MLof are going thru the Pw L2R the MLof would go to the R ground. Your last comment would suggest this is not correct? Or am I not understanding something?

 

[sophiecentaur]

TL;DR Summary: Magneto operation explained

a complete explanation of a magneto starting from the Physical Magnet's AC current thru to the Secondary Winding output.

That diagram is missing something, I think. The magneto on my old motorcycle had a physical linkage (same shaft) between the magnet and coils and the breaker 'points'. This times the breaking of the circuit with the appropriate part of the varying field around the coil. Breaking the circuit at the right time produces a far higher voltage than just having 'AC'. Automatic advance of the spark was achieved by flying weights that changed the angle of the plate holding the points. (No vacuum advance, though)
The magneto system was used by early aircraft (& cars) because they had no on-board appropriate electrical system. You would start by turning the prop - and could lose an arm if you weren't too careful.

 

[Drakkith]

With both ends of the Pw grounded it seems that the direction of the MLof would use the ground at the opposite end of the Pw;

What exactly do you mean by, "use the ground"? Current will flow in a loop, going into/out of either ground in order to make that loop. So when current flows into the ground at one end of the primary winding, it will flow out of the ground at the other end. Same for the secondary.

 

[sophiecentaur]

What exactly do you mean by, "use the ground"?

Is this just a matter of semantics? "Ground" is a term often used to describe the nearest large conducting object - for example, the car, plane or equipment chassis in which the Potential Difference between all connected points is (near enough) 0V. The assumption can easily fail when alternating voltages are involved.

There are many 'amusing' cartoons of electrical equipment using a bucket of soil in place of the planet Earth.

 

[Janus]

The magneto system was used by early aircraft (& cars) because they had no on-board appropriate electrical system. You would start by turning the prop - and could lose an arm if you weren't too careful.

Small single engine aircraft use the magneto system because it allows the plane to keep flying even if the rest of the electrical system fails. In fact, they have two magnetos. You typically use both, but the engine will run on just 1, allowing you to make it to an airport. One of the pre-flight checks is to switch between running on 2 and 1 magnetos, to insure both are working.

 

[sophiecentaur]

it allows the plane to keep flying even if the rest of the electrical system fails

Too right; the electric charge budget can be really stingy.

I made a parachute jump from a Cessna and its battery was FLAT! They had to go elsewhere and get one which was installed by a man with 'certificates'. The battery was for all the electrics and was only lunch box sized. The pilot said he wouldn't start it with the prop because it was high risk of taking an arm off. I guess that a proper sized start battery for a car would have weighed too much. At least the twin spark plugs + magneto is a basically more predictable system than a tiddly little battery.

 

[Averagesupernova]

The basics of an aviation mag is little different than any other mag with the exception of better reliability and of course a distributor. Last I checked, Google still works. After that more specific questions would be appropriate. Incidentally, certain racecars will use a mag for multiple cylinder engines.

 

[Drakkith]

Is this just a matter of semantics?

No, I don't think so. The phrasing used by the OP suggests that he thinks that only one ground is being used at a time, and I want to make sure I'm understanding what he is saying.

 

[sophiecentaur]

The phrasing used by the OP suggests that he thinks that only one ground

We can't know but I look at the thread title with the word 'aviation'. That implies the OP could be thinking there's something special about the Earth, other than the term 'earth' meaning a common reference point - with an abritrary zero origin. Planes can't be connected to The Earth but the magneto works the same wherever it is.
Engineers can be very sloppy in their use of terms.