Alternator rotor stator alignment

[Rick]

I'd like to develop a recipe for aligning a rotor and stator that doesn't rely on the factory witness mark- working with basket case engines I can't be certain that a rotor is marked for the static advance, auto advance unit, etc, that I'm using.
The best reference I've found is from a Lucas service manual. Before anyone feels compelled to add Lucas jokes, I have no doubt that Lucas engineers were well educated and understood analog electronics at least as well as today's engineers, and Ducati built electrical and electronic components before they built motorcycles- I trust they did good work with the technology of the day and that magnetism hasn't changed much since the 1950s.
Some of the terms get confusing, what Ducati calls a 'magneto' system is ofter called ET(energy transfer) by others, and Lucas calls it just AC- I think most people understand when someone is referring to a battery powered ignition system and the system that powers the ignition coil directly from the alternator- for now, I'm going to call the systems 'battery ignition' and 'alternator ignition'.
The alignment of the rotor and stator is critical with the alternator ignition system- to synchronize the peak amplitude of the alternator sine wave with energizing the ignition coil to generate a spark at the right instant. The Lucas manual has a great graph, and refers to the peak amplitude occurring when "... the interpolar gaps of the rotor are situated on the centre-lines of the stator limbs", or within a few degrees of that position. That agrees with what I learned about electric motors 45 years ago- a professor turning a loop of wire inside a horseshoe magnet that caused a galvanometer needle to deflect.
Here are pages from the Lucas manual:

Lucas AC page 1.jpg

Lucas AC page 2.jpg

The drawing on the left shows the magnetic force lines between the alternating North/South pole pieces.

rotor stator alignment.jpg

The center drawing(A) is what makes sense to me, and what I think the Lucas manual describes. But, I can't get that drawing to agree with the witness marks that Ducati stamped onto the rotors- the drawing on the far right(B) seems to be what Ducati used as the peak amplitude orientation in their chart of 'α' angular offsets.
So, can anyone point out what I'm getting wrong? What's the relationship between the rotor and stator at peak amplitude? At higher rpm the alternator output should increase enough to provide plenty of power, so is there any reason why the peak amplitude wouldn't be arranged to be at the 'static advance' point before TDC, to generate the best spark at kick starting speed?
Rick

[Jordan]

Could it be that the mark on the Ducati rotor is just to get it into the correct relationship with the stator, but doesn't mean it's necessarily the max voltage position?

[amartina75]

Rick this has been discussed before, here is one post to look at.
http://www.motoscrubs.com/forum/viewtopic.php?f=3&t=23&start=50

[Rick]

Jordan- that may be right, still, I don't know what the ideal orientation is
Amartina75- I think I read that before- I'll read the discussion again
Rick

[Jordan]

I didn't read the past discussions in depth. I stopped when my brain started to hurt.
I don't know if oscilloscopes were mentioned, and I'm no expert with these.
But, with a typical dual trace, would it not be possible to display the waveform of the alternator's output on one channel, and the ignition waveform on the other, and compare these?
It might confirm where ignition happens with respect to the alternator's output.

[DewCatTea-Bob]

Jordan- that may be right, still, I don't know what the ideal orientation is
Amartina75- I think I read that before- I'll read the discussion again
Rick

____ I'm sorry I missed noticing your new thread sooner after it was posted, Rick.
I may yet still get-around to picking-out some pieces of your original post-wording to reply-to or comment-on.



[quote= Rick ... " I don't know what the ideal orientation is "

____ It much depends on which engine-model & it's particular static-ign.timing (in relation to TDC).



" I'll read the discussion again "

____ Rick, if you happen to find anything within that older thread that's not clear enough and it's concerning something that's quite significantly related to that thread,, then please consider 'REPLYING' to any questioned post-wording over-there, rather from within that thread (instead of bringing it over to this-one).
However otherwise, please feel free to expand-on any fairly/clearly-understood related concepts found over-there, rather within this new thread (if you wish).


Duke-Cheers,
-Bob

[Rick]

Jordan- Using a scope to capture the alternator AC waveform wouldn't be hard, but there isn't much of an 'ignition waveform' to look at, or not that I can think of anyway- the ignition coil gets a short pulse from the alternator timed by the contact breaker, it seems like it would be complicated to trigger a scope to grab that pulse and display it.

DCT Bob- My problem is reconciling 'theory' and the Ducati witness mark- I read your description from the other topic, here's what you wrote:

____ On the stator there are two separate coil-windings, each wrapped around it's own magnetic-steel core. - (Those cores are made of several layers instead of one solid piece, so as to reduce stray eddy-flows of electrons which may create magnetic fields that are not coherent with that which is desirable).
As is obvious, the two ends (of each of the two cores) are convex-curved so as to closely clear their concave counter-parts within the rotor ! _
These curve-cut protrusions -(from the coil-windings), of the stator-cores are the 'poles' of the stator,, and of course the center of the core-protrusion, is the center of the pole!
__ In the rotor there are four magnets arranged so that the rotor's four concave-curved magnetic-steel poles are magnetically-charged (by both magnets on either side of each), with either North or South polarity. _ The center of these concave rotor poles is the point where maximum magnetic-flux is most concentrated, (thus the pole's equivalent of a 'peak'). ...
__ So when the rotor's four pole-centers become directly aligned with the stator's four pole-centers, that then is the point at which maximum/peak-inductance of electromotive-force occurs. _ And thus the alternator then produces the 'peak' of it's power-pulse !

That sounds like you're describing this orientation:

B.jpg

What I remember from motor theory, and several online descriptions is different- here's an Illustration from a Navy training manual, NAVPERS10622:

NAVPERS10622 illustration.jpg

The Ducati alternator isn't exactly like the illustration- the rotor spins ouside the stator, and the rotor rotation axis isn't in the center of the magnetic field, but I think the behavior of the Ducati alternator should be the same. So, I thought the peak amplitude of the EMF should have been with this rotor/stator orientation.

A.jpg

The lucas graph shows that peak EMF is offset a few degrees- probably from the induced current in the winding producing it's own magnetic field that interacts with the rotor's field.

So, it may not be obvious, but I'm trying to make this simple, not complicated- the first step toward a simple solution is to determine the rotor/stator orientation that will send the best pulse to the ignition coil.
I'll keep scratching my head...
Rick

[DewCatTea-Bob]

____ Well Rick, just as I was coming-back now to finally get started with responding to your very-first post (and the Lucas-paper as well), I've found this new post of yours to respond-to instead.
__ You've brought-up a particular concern which I can relate to, as I happen to be familiar with the same reasoning concept (concerning your two oppositely opposed alt.drawings). ...



[quote= Rick ...
" Using a scope to capture the alternator AC waveform wouldn't be hard, but there isn't much of an 'ignition waveform' to look at,
the ignition coil gets a short pulse from the alternator timed by the contact breaker, it seems like it would be complicated to trigger a scope to grab that pulse and display it. "

____ A little more complicated, yes,, but still doable, (with hopes of keeping the alt.rotor-RPM running steady).



" My problem is reconciling 'theory' and the Ducati witness mark- "

____ If you read-through most-all of the other thread, then you should've noted that it's been concluded that the 'timing-marks' (found on the face of many 4-pole mag.rotors) are quite often place-located without any particularly sensible rhyme&reason, (thus the need for the associated table of varied peculiar angles for the t.mark to be inclined away from the normally steadfast TDC.location) !
That's why I've always recommended that everyone should place their-own TDC.reference-mark on the mag.rotor before removing a factory-installed rotor from the crankshaft !



" That sounds like you're describing this orientation: "

____ That's correct, as I was indeed meaning to describe the alignment-orientation of the likes of your drawing 'B' !



" What I remember from motor theory, and several online descriptions is different-
So, I thought the peak amplitude of the EMF should have been with this rotor/stator orientation. "

____ You were no-doubt first trained same as I was - that magnetic flux-lines must past through a wire at a right-angle in order to induce current-flow ! _ And your drawing-A seems to best indicate such activity, (which is why I've stated that "I can relate to" such concern as you've brought-up). _ And-so I was once also a bit confused as to which orientation-position (A or B) actually generated the expected magnetic-induction action.
However the intended-purpose for the existence of the protruding-ends of the coil-cores (which otherwise wouldn't have-to have been constructed extended-out so far as they are), is to best become like-magnetized along-with the passing magnetic-poles of the mag.rotor. _ As the flux-fields induced into the stator-cores are the actual (twisted/tumbling) magnetic-fields which really (rather directly) induce current-flow through the surrounding coil, and not-actually the rather (indirect) remote mag.flux-fields of the rotor-magnets themselves !
__ I hope this clears-up your apparent disconcerted conception concerning the-issue.



" The lucas graph shows that peak EMF is offset a few degrees- probably from the induced current in the winding producing it's own magnetic field that interacts with the rotor's field. "

____ I'm not sure I gather exactly what that might actually possibly mean to really refer-to without further elaboration,, but within the other/old-thread, I believe I've touched-on why the ign.spark finally fully benefits only after 5 to 10 degrees later than the center-apex of the peak of max.magnetic-alignment...
Ya-see if the contact-points open right-at the very smack-dab center of the peak of a power-pulse,, then the mag.power-coil would only become charge-saturated with just the climbing-side of the current-waveform's peak that's induced before the peak's apex-center and-thus get no additional fortification from the falling-side as well, to help robustly-boost the establishing flux-field around the mag.power-coil (which ends-up FORCE-feeding whatever the established [short-circuited] current-flow has become, through the ign.coil !).
So it's more suitable to rather allow the entire peak (on both sides of it's apex-zenith) to get a chance to fully-establish top-end power-intensity, by having the points open 5 to 10 degrees AFTER the peak-center has gone-past.



" I'm trying to make this simple, "

____ Please go-ahead & try !
If you can come-up with simpler wording than I find to state everything correctly, then that'd certainly be appreciated !



" the first step toward a simple solution is to determine the rotor/stator orientation that will send the best pulse to the ignition coil. "

____ That's fairly simple... The ign.contact-points should open just-after maximum current, (not 'voltage' [as indicated by the Lucas-paper]) has become established (through the short-circuited alt.stator power-coil). _ And that would be most preferred at the static-timing point (for kick-starting purposes).
So all that then has to be worked-out, is to set the mag.rotor onto the crankshaft so that the mag-rotor is properly orientated with the stator, so that maximum-alignment has gone past by not much more than 2 to 5 degrees, (or-else the ram-intensity of the 'flywheel-effect' [from the established flux-field around the power-coil] will begin to become lost).



" I'll keep scratching my head... "

____ Hopefully you can now do-so with less intensity,, as now all that's left to be comprehended, is exactly how many degrees the maximum-alignment (of rotor & stator) ought-to be advanced in relation to TDC.
__ So have you yet read & fully comprehended this old thread-post : ( viewtopic.php?f=3&t=23&start=50#p9564 )
along-with it's presented picture showing various center-lines (representing various aspects of the mag.rotor) ?
If in any doubt, then please feel free to confer your interpreted conceptions for confirmation.


Hopeful-Cheers,
-Bob

[Jordan]

Jordan- Using a scope to capture the alternator AC waveform wouldn't be hard, but there isn't much of an 'ignition waveform' to look at, or not that I can think of anyway- the ignition coil gets a short pulse from the alternator timed by the contact breaker, it seems like it would be complicated to trigger a scope to grab that pulse and display it.

Do you mean the ignition pulse is too short to show up on the display?
If so, what about temporarily arranging a battery to supply the primary current? The resulting square wave would be easier to see.
The trigger would be from the ignition lead?
Sorry, I'm only guessing.

[DewCatTea-Bob]

[quote= Jordan ...
" The trigger would be from the ignition lead? "

____ No need to involve the high-tension output to the o.scope, as one of it's inputs can rather simply be set-up to display the ign.power-coil's AC.output as the engine-revs are kept steady.
The o.scope's other input could be fed with the lighting-coil's output for comparison (if so desired), since it's rather steady & unbroken AC.frequency-output is virtually the same as that of the other AC.output that's in question. _ And since that ign.power-coil's output will be dead while the contact -points are shut,, then when the points open and thus-then allow the o.scope to test it's then fully-available waveform for display, it can then be seen exactly what part of that waveform has become useable for the ign.system.
__ The real trick for getting a good/solid look at the resulting waveform, would be keeping the engine-revs steady long enough to realize what's being shown.
I imagine modern o.scopes ought have a flash-shot memory to hold-still a preferred snap-shot of the otherwise wavering display.
__ To get an idea of what ought to be seen on an o.scope display, I've added a pic of the expected waveform, (which is partial of actual, but at-least depicts the most pertinent section).
____ Concerning the captioned-wording underneath the pic.post... If it's word-lettering appears too small to easily read, then highlight it and drag & drop it to a word-pad page where it will then rather appear as normal print.
And that way, you could then also possibly read that text while looking-over the expanded-view of the (then caption-less) depiction of the detailed waveform.


Enlightening-Cheers,
-Bob