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Vincent Magneto Article, might be interesting for someone

Spqreddie

Well Known and Active Forum Website User
VOC Member
http://www.themagnetoguys.co.uk/page4.php

Quote:
However, only one of the bumps on the Vincent cam ring can be made to coincide with a break point. As a result, only one cylinder receives a good spark whereas the other cylinder receives a much weaker spark because the points open long after the break point by which time the magnetic field has been collapsing for an appreciable time. A situation which is far from ideal and is, perhaps, one reason why starting engines fitted with Lucas KVF magnetos is not easy.
End Quote
 

Kansas Bad Man

Well Known and Active Forum Website User
Non-VOC Member
Good information , the GUYS think like I do, agree with every thing they say and there approach to dealing with spark INTENSITY AND TIMING .

THANKS---- MAX
 

Magnetoman

Well Known and Active Forum Website User
VOC Member
Quote:
...the other cylinder receives a much weaker spark because the points open long after the break point by which time the magnetic field has been collapsing for an appreciable time. End Quote
The part about the #2 spark being weaker is correct, but the reason they give isn't right and indicates they don't understand how a magneto actually functions. A proper description is more lengthy than the one I'm about to give, but here goes anyway:

Very briefly, a voltage is generated across any conductor moving in a magnetic field. This voltage is proportional to the rate of crossing the field lines (e.g. if the field changes twice as fast, the generated voltage is twice as much). If that conductor is formed into a coil two additional aspects of physics come into play. One is that a current flowing in a coil generates a magnetic field of its own (i.e. it is an electromagnet), and the other is that the current flows in such a way as to oppose any external changes in magnetic field. Actually, to oppose changes in the magnetic flux, which is field x area of coil, but I'll ignore that in the interests of hoped-for clarity of a semi-technical description. Because of these properties, if the external magnetic field starts to decrease, a current is set up in the direction that is necessary to add some field to try to keep it the same as before (again, it acts to conserve flux, but if the area of the coil is fixed it does that by changing the field).
What this means is just rotating the primary coil on an armature between N and S poles of a magnet would generate an oscillating current in the primary. If a secondary coil is wound over the primary coil, and if the current through the primary is interrupted by opening the points, the collapse of its current (and hence the field created by that current) would generate a large voltage in the secondary.

The genius of Herr Bosch was realizing he could do a lot better than just spinning a coil in a horseshoe magnet. By designing the pole pieces and the armature the way he did, the magnetic field (flux) threading through the core of the coil doesn't have to be limited to changing sinusoidally in time. First, his design -- shamelessly stolen by Lucas, BTH, and others at the time of WWI -- results in the the density of field lines keep getting compressed (this is the strong resistance you feel as you try to rotate the armature). At one moment in time all of these compressed magnetic field lines are threading their way through the coil of the primary in one direction. But a moment later the armature has passed the break in the pole pieces and the magnetic field lines (again, actually it's the magnetic flux) abruptly reverse direction. So, instead of the current in the primary of the armature being limited to a slow sinusoidal variation, it is forced by this design to have a very large change in a very small time. This in turn generates a very large voltage in the secondary that is wound directly on top of the primary.

Returning to the above quote from the web page, it's isn't that the second cylinder gets a weak spark because the field has "been collapsing for an appreciable time." In fact, that entire collapse happened within a few degrees of engine rotation at the time of spark #1, so it is entirely done well before spark #2 is needed. The reason the second cylinder gets a weak spark is it can't take advantage of the genius of Bosch's pole/armature design and instead is limited to the much slower sinusoidal (sort of; again more complex than this) change of magnetic field seen by the primary.

As the web page shows, early magnetos for V-twins had specially shaped pole pieces and armatures. However, we have to live with the compromise design Lucas used with when designing with the then-new alnico, which is optimum for vertical twins and "good enough" for V-twins. The consequence of this compromise for Vincent owners is you must have your magneto properly magnetized in order that the weak #2 spark not be so weak as to be useless.

If this post sounds even somewhat coherent it's a miracle. I'm typing this as a way to help keep awake for a few more hours after a very short ouzo-aided sleep last night followed by ~24 hours in transit back to the U.S. from Athens.
 

highbury731

Well Known and Active Forum Website User
VOC Member
If the K2F gives poor results due to being made for 360 degree twins, and the second Vincent cylinder firing 25 deg off peak flux, are there other magnetos which will do it better?

Wonderful things are claimed for the magneto with its secondary wiring in the same armature as the primary. How does the SR1 and 2 magneto compare for spark quality, and ability to cope with V twin sparking? I do wonder if a more compact rotating magnet magneto could be created, with a generator and contact breaker in the body, and remote coil for converting to high voltage. I have to admit that my knowledge of magnetos is a bit sketchy. Now I'm ducking for cover....
 

Chris Launders

Well Known and Active Forum Website User
VOC Member
I have seen dynamo's with a points mounting on the end for coil ignition, I think some lightweight panthers used them among others, but they must run at engine speed whereas the Vincent twin dynamo doesn't.
As regards SR1 and SR2 magneto's, all the aftermarket Harley magneto's are of a similar type so should be ok.

Chris.
 

Magnetoman

Well Known and Active Forum Website User
VOC Member
I do wonder if a more compact rotating magnet magneto could be created, with a generator and contact breaker in the body, and remote coil for converting to high voltage.
In fact, this is how the new BTH magnetos are made. They use a body that looks like a KVF which disguises the fact they are actually an E.T. ignition system, with the low voltage generated in the housing and remote HT coils, exactly as an ET's rotor/stator system does it.

The Fairbanks-Morse rotating magnet magnetos for Harleys (and clones made by Hunt, ARD, and others) cast the magnets asymmetrically to give the optimum timing of the magnetic field for the 45-deg. V of that engine. But, while a more compact design could be made today using modern SmFeCo rare earth magnets, a properly magnetized old fashion KVF will do just fine.
 
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