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Detailed description of the restoration of a magneto

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Magnetoman

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Some of you might be interested in the detailed description of the restoration of a magneto I posted to BritBike Forum. It is the most extensive such description I am aware of on the web or in print, requiring 20 posts over four months to complete. Although it is about a Bosch ZEV for a 1923 Harley-Davidson, essentially everything in it is directly applicable to a Lucas KVF. You can find the thread at:

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=446733#Post446733

My obsession with magnetos was, ahem, sparked in the mid-1990s when I restored a BSA Gold Star that came with a nonfunctioning magneto. Rather than taking the time to figure out how to fix the magneto myself, I decided to send it to a specialist. That was a mistake. Although the specialist came highly recommended, had a very well equipped shop (where he also specialized in Vincents), and seemed quite sure of his abilities when I spoke with him, the magneto failed after 90 miles with what I recognized as a bad replacement condenser.

Unfortunately, before I had run the Gold Star I already had sent him my Vincent's KVF to restore, and it has been sitting on the shelf ever since. However, as I posted here at the end of October, I am now (re)embarking on the restoration of my own Black Shadow, so one of the components I will be restoring using the instruments described in that thread is the KVF. If only I knew then about magnetos what I know now… Anyway, as a result of that unfortunate experience with a highly recommended magneto rebuilder, as I wrote in the Epilog to that thread, "Although I have no way of knowing for sure, I seriously doubt even the most heavily-equipped professional magneto rebuilder has the range of equipment and facilities I do for diagnosing and restoring magnetos."
 

johnmead

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Non-VOC Member
Where is Al Gore?

I have a magneto theory question for you. The primary circuit of a magneto creates AC power. Does the secondary pass the AC through it all the way to the plug(s) or does it turn into DC some where along the way?

John Mead
 

Magnetoman

Well Known and Active Forum Website User
VOC Member
I have a magneto theory question for you. The primary circuit of a magneto creates AC power. Does the secondary pass the AC through it all the way to the plug(s) or does it turn into DC some where along the way?
It's not quite as simple as pure AC or pure DC. Here's the short version of what happens:

--As the armature rotates in the field of the alnico magnet a sinusoidal AC current initially begins to flow in the primary.

--But, because of the way it is shaped, as the armature rotates it compresses the field lines from the extended pole pieces of the alnico magnet so before it has rotated very far the field lines have become very compressed.

--Just before the "wings" of the armature pull away from the extended pole pieces of the magnet the field lines have become so compressed that they find a lower energy path to the other side.

--This new path for the field lines is in the opposite direction in the core of the armature, causing the magnetic field to abruptly reverse itself in the core (the compression of the field lines is what you feel when you rotate an armature by hand -- it becomes very difficult to turn just before the point of field/flux reversal).

--This rapid change in the magnetic field (actually the magnetic flux) creates a very large current in the primary (actually a voltage, but that directly results in a current), which itself creates its own magnetic field (the armature is acting as an electromagnet, whose field combines with the static field of the fixed alnico magnet).

--Just at the moment all of this has happened the points open, causing the current in the primary to cease.

--Since the current in the primary is what creates the magnetic field from the primary, the field instantly collapses along with the current.

--This creates a very large voltage across the primary (here the condenser plays its critical role, providing a low impedance bypass to the points for a critical msec or so, quenching the arc across the points that otherwise would take place; by the time the current that flowed into the condenser reverses itself, the points have separated by too much for an arc to be re-established).

--Since the secondary is wound around the primary it has the same configuration as a transformer, so the voltage across the primary is stepped up by the turns ratio of ~50:1 (although transformer losses result in an actual voltage increase of ~35:1).

As this simplified description shows, the magneto is designed to create abrupt changes, which in turn results in it being a nonlinear device. Although the power is "AC" in the sense it alternates every revolution, it doesn't do so with a simple sinusoidal output that most people usually think of as AC.

Disclaimer: I wrote the above extemporaneously late in the evening, so I wouldn't be surprised if someone points out it could be clearer.
 

redbloke1956

Well Known and Active Forum Website User
Non-VOC Member
It's not quite as simple as pure AC or pure DC. Here's the short version of what happens:

--As the armature rotates in the field of the alnico magnet a sinusoidal AC current initially begins to flow in the primary.

--But, because of the way it is shaped, as the armature rotates it compresses the field lines from the extended pole pieces of the alnico magnet so before it has rotated very far the field lines have become very compressed.

--Just before the "wings" of the armature pull away from the extended pole pieces of the magnet the field lines have become so compressed that they find a lower energy path to the other side.

--This new path for the field lines is in the opposite direction in the core of the armature, causing the magnetic field to abruptly reverse itself in the core (the compression of the field lines is what you feel when you rotate an armature by hand -- it becomes very difficult to turn just before the point of field/flux reversal).

--This rapid change in the magnetic field (actually the magnetic flux) creates a very large current in the primary (actually a voltage, but that directly results in a current), which itself creates its own magnetic field (the armature is acting as an electromagnet, whose field combines with the static field of the fixed alnico magnet).

--Just at the moment all of this has happened the points open, causing the current in the primary to cease.

--Since the current in the primary is what creates the magnetic field from the primary, the field instantly collapses along with the current.

--This creates a very large voltage across the primary (here the condenser plays its critical role, providing a low impedance bypass to the points for a critical msec or so, quenching the arc across the points that otherwise would take place; by the time the current that flowed into the condenser reverses itself, the points have separated by too much for an arc to be re-established).

--Since the secondary is wound around the primary it has the same configuration as a transformer, so the voltage across the primary is stepped up by the turns ratio of ~50:1 (although transformer losses result in an actual voltage increase of ~35:1).

As this simplified description shows, the magneto is designed to create abrupt changes, which in turn results in it being a nonlinear device. Although the power is "AC" in the sense it alternates every revolution, it doesn't do so with a simple sinusoidal output that most people usually think of as AC.

Disclaimer: I wrote the above extemporaneously late in the evening, so I wouldn't be surprised if someone points out it could be clearer.

Hi Magneto(charlie)man, I am an Electrical Trades Teacher in a college and it made absolute sense to me....well done (and thanks) on this highly compressed version of events.

Batman (Kevin)
 

Magnetoman

Well Known and Active Forum Website User
VOC Member
Hi Magneto(charlie)man, I am an Electrical Trades Teacher in a college and it made absolute sense to me....well done (and thanks) on this highly compressed version of events.
I'm glad to hear you like the description. Magnetos are surprisingly complex mechano-electromagnetic devices, and it's amazing they were able to figure out how to design them over a century ago without the benefit of an oscilloscope.
 
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