Voltage control Circuit design wanted

Discussion in 'The Projects Forum' started by oldbikeles, Dec 29, 2009.

  1. oldbikeles

    Thread Starter New Member

    Dec 29, 2009
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    Hi all. I am new here, so not sure if this is the best place to ask my question.
    I ride a 72 year old "utility" motor cycle (1937 New Imperial of 150cc). It has a "curious" ignition/ battery charge system, called a "Maglita". It was originally manufactured by "M.L.", later by "Lucas".
    Two flat plate permanent magnets on front and back of unit "feed" the field to laminations.
    An armature with a 4 segment commutator rotates within this field. There is only one winding, so two of the segments are dummies. A DC output is taken from the two brushes, for power to a six volt battery, with a maximum lighting load of 20 watts.
    At the top of the unit, in an area unoccupied by the laminations, is an ignition coil. It has a primary winding energised as the laminations of the rotating armature cross this top part. In conjunction with a contact breaker and capacitor, a high voltage to the spark plug is developed in the secondary.
    This "two in one" unit was a low cost, low performance unit, whose principal drawback is interaction between the two functions. There is an optimum point at which the contact breaker should open for best spark intensity, but when current is drawn from the DC generation side, the spark weakens and the engine misfires. This also appears to happen when the unit gets hot, possibly as a result of the approach towards the curie point of the magnets. (Having said that, also I think when it is very cold it plays up! As it did 2 days ago!) Retarding the manual ignition control will bring back the spark intensity, but to the detriment of the engine's performance.( I think for my particular unit, retarding ignition merely brings it closer to the optimum intensity position. However, it is not possible toalter this point within the unit.) Furthermore, this weakening of the spark is most evident at higher engine speeds. If the headlamp switch stays in the "No charge/ no lights" position, the spark is always satisfactory. Sometimes it gives no problems whatsoever with lights demanding maximum DC output.
    I think that a simple voltage control circuit would help reduce this to a considerable degree. (Obviously not a "shunt" system). I could probably draw up a traditional series circuit, but heating / transistor breakdown would be a likely problem. Whilst the design output is about 3.5 amps, at high speeds it can probably produce 6 amps due to the permanent magnet design.There must be a suitable modern and hopefully inexpensive switch mode circuit I could adopt. I am OK at troubleshooting circuits, and also constructing them, but NOT at design.
    Any suggestions please.
     
  2. SgtWookie

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    Jul 17, 2007
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    Interesting project - and that is perhaps where the thread belongs; in the Projects forum, as in the General Discussion forum it will quickly gallop down the page.

    Reliable ignition has only been achieved over the last 30 years or so.

    One of the problems you're facing is the stratification of the air/fuel charge vs the plugs' spark gap and low system voltage. The charge is most dense when the cylinder is at top dead center. Before TDC the charge is more rarefied; thus harder to ignite. If the spark plug gap is too small, it may not ionize enough of an air/fuel mix to reliably ignite the charge.

    If your spark gap is too large, or voltage across the primary too low, there may not be enough voltage developed in the ignition coil secondary to ionize the air/fuel mix in the plug gap, thus creating a spark and igniting the air/fuel mix.

    Adding a linear voltage regulator in this mix would only make the problem much worse, due to the additional voltage losses incurred.

    One of the earlier cures for ignition woes was "CD" ignition, or capacitive discharge ignition. It still used the ignition points as a timing trigger, but I believe that the units used a thyristor to provide the ground/earth to the coil. A large capacitor (or several of them) were used on the + side of the coil. I don't remember all of the specifics offhand, but it may be possible to adapt something like that to your motorcycle.

    However, your motorcycle being pre-WWII vintage - is it wired with a positive earth/ground?
     
  3. SgtWookie

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    Jul 17, 2007
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    I think I might've answered my own question...
    Googling around looking for "Maglita", I found this schematic:

    [​IMG]

    There is no date on the drawing, but it does show a negative earth/ground.

    Anyway, back to the stratification problem - this happens primarily under idle and light throttle conditions, where the manifold vacuum is quite high.

    I don't know if you would want to modify the original Maglita unit, as that may spoil it from a collector's viewpoint. However, as long as you don't drill extra holes physically change the unit itself, some minor interior re-wiring might be accomplished without the spoilage potential.

    In the schematic, there appears to be a switch mounted in/on the armature labelled "Automatic cut out", which might have been for a sort of rudimentary voltage regulation or prevention of engine overspeed?

    At any rate, it looks like the supply for the coil is taken directly from one of the armature brushes. It might help a good deal to simply add a capacitor on that side of the coil. You might need to use a diode between the brush and the coil to prevent the capacitor from discharging back through the armature, or though the compensating winding.

    [eta]
    Found this scan of a 1926 Maglita manual:
    http://www.barnstormers.co.nz/wp-content/uploads/2009/08/1926mlmag.pdf
    Note that there is a caution on page 12 regarding the removal of the armature and possible demagnetization.
    Troubleshooting procedures given starting on page 14.
     
    Last edited: Dec 29, 2009
  4. oldbikeles

    Thread Starter New Member

    Dec 29, 2009
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    Hi, thanks for replies. I already have an original manual such as your link, and the later FD manual.
    You are slightly misunderstanding some of what I said. When I spoke of "retarding the ingition", I meant in the context of the operating cable out of the Maglita. This rotates the part which carries the contact breaker points. If you look at p 13 of that manual, you will see two holes, one either side, via which the cable can rotage the carrier. I assume there are two depending upon whether the specific maglita was for clockwise or anti clockwise rotation. This rotation of the carrier will presumably allow you to move around the optimum point for field at the moment of opening of the contacts. There is actually a "zero adjuster" (an eccentric screw) at the bottom of the carrier, but I think it is just a little out of range to get the optimum field with the control in the maximum advance position.
    When I start the bike, I can generally move the switch to charge, main or pilot light position with no problems. When the bike warms up, there may be some slight misfiring, which can be cured by slightly retarding the control lever. When I went out for our local VMCC post Christmas run, with freezing temperatures, moving the switch to any of the charge positions caused the misfire. I have the feeling that the point of armature rotation when the spark is of greatest strength varies not only with any loading on the LT side, but also with the engine's speed. If you read the cited manual, there is an explanation of the "compensating winding", which only confuses me.
    No, I do not wish to seriously modify the maglita, but I have done some internal changes. The "cutout" you referred to, I have shorted out, but I have added an avalanche diode in series with the LT+ out lead, so no risks of demagnetising the unit, or discharging the battery.
    What I think is that a voltage controlled charge circuit (NOT shunt type) would give the minimum drain on the LT side with a fully charged battery, whilst preventing over charge if I leave it in "charge" position when it is behaving (not misfiring).
    Is that all clear? The maglita was well known for being "uncertain", and I have improved mine by careful adjustments since I first commissioned it, but it was also well liked because it would "always get you home, even if you had no lights". It was fitted to the New Imperial as an optional extra, at a time when the standard setup was a rudimentary dynamo, with a contact breaker mounted on the end outboard of the commutator. In those days the dynamo was a most unreliable "third brush" type, hence the preference of the pathetic maglita.
     
  5. SgtWookie

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    Jul 17, 2007
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    If your have a later device, it would be very helpful to have reference material available that covers your model

    I see. I refer to the part that holds the breaker points as the "breaker plate", as that is what they were called in the States when they were in common use.

    It would be logical to have two cables, no matter which direction the maglita rotated. Pulling one cable would advance the timing, pulling the other would retard the timing.

    How do you know if it is related to the field strength rather than spark gap vs air/fuel concentration?

    Did this misfire occur before you shorted the automatic cut out switch and installed the rectifier diode?

    After more reading in the manual, it looks like it is a 4v system! I'd never seen anything less than 6v used on an auto or motorcycle.

    Do you mean the one I posted a link to, or the one you have? I didn't see a reference to the "compensation winding", but I didn't take the time to read the manual from cover to cover.

    From what I've read in the manual, overcharging the battery was not a big problem with these units. It's undercharging that's the problem. Unfortunately, using a diode in series with the LT out adds a voltage drop across the rectifier diode. The voltage drop would not be of great concern with a higher voltage system; automotive alternators have had rectifier bridges in them for many years. However, with the very low output capacity of your Maglita, every fraction of a volt counts.

    It's actually fairly clever, considering how relatively primitive the technology of those days were.
     
    Last edited: Dec 30, 2009
  6. SgtWookie

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    Jul 17, 2007
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    re: compensating winding - OK, it's somewhat explained on the top of page 4 of the .pdf I linked to.

    Not quite certain how that works yet.
     
  7. SgtWookie

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    Jul 17, 2007
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    I have a feeling that your permanent magnets are simply losing their field strength.

    You might consider replacing them with neodymium magnets. Neodymium magnets are far more powerful than any other types that I know of, and have become very plentiful in recent years; available in a myriad of sizes and shapes.

    I've never used this supplier; just linking to it so that you have a rough idea of what you can get:
    http://www.kjmagnetics.com/products.asp?cat=11

    Replacing the original magnets won't exactly be a trivial process - however, the enormous relative strength of the neodymium magnets will surely cause your generator to output a greater flow of current.
     
  8. oldbikeles

    Thread Starter New Member

    Dec 29, 2009
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    Hi, Sgt.,I will try to fill a few more things in, Oh! and thanks for the transfer here.

    The later version maglita is essentially the same as the 1926 model, except that it IS 6Volt. The max rated output is to power a headlight bulb of 12 watt, plus speedo, rear light, and sidecar lamp, each of 3 watt. i.e., 21 watts total.

    My series diode is only 200mV drop, and rated at 20 amps (ex. computer PSU.)

    There is in fact a torsion spring aiding the "return to retard" when the lever is moved back.

    I agree the maglita is "fairly clever", but it was a low cost option against the alternative of separate magneto and dynamo. The "standard option" on my 'bike was the dynamo with a contact breaker "tacked on" to the end, but this was not well liked.

    I don't know if the magnets are losing srength, but obviously it is a real possibility.
    I have previously thought of trying to augment them with some "stuck on" bits of rare earth magnets, and I have loads of small bits taken computer hard drives. However, I have done nothing about it, due to lack of time.

    The two magnets are about 4" x 2" x5/8" from memory. Obviously magnetised N - S along thier length, I had thought that I could attach a piece of rare earth at each end, but of course all the rare earth magnets I can see listed, like my "hard disc" magnets, are all magnetied "through the thickness", and as magnetism was never my strongest subject, I have just avoided the issue of how to fit them.

    You ask did the misfire occur before I added the series diode. The Maglita was "dead" when I bought the bike. I carefully dismantled it (using a keeper) and found that the lead to one commutator segment was unsoldered. Obviously, I resoldered this, and at the same time I removed the cut out, bi-passing it with a wire link.
    I generally overhauled everything at the time, including commutator skim, new brushes, replacement of the (leaky) 0.22 mike capacitor with a modern one, replaced the contact ponts, and fitted the diode.

    How does the misfire occur?
    Some days, I can go out on the 'bike, and even with headlights on, no misfiring at all occurs. Another day, I ride in "charge" position, and no problems untill about 15 minutes, then a slight misfire will begin if I accellerate at low revs, but which can always be cured by moving the advance lever slightly to retard.

    Last week when I went out (temperature around freezing point), the misfiring in charge position was quite bad over a wide rev range, only cured by either turning charge "OFF", or slightly retarding lever.

    I think the problem is related to the "compensation winding". I have the idea that as the engine speed increases, the point for optimum ignition (contact opening) moves, but this effect should be masked by the greater generated voltage resulting from the speed increase. However, drawing current on the LT side for charging must "rob" the ignition coil's LT side. The enigma to me is the fact that the misfiring only occurs at higher speeds.
    Does this last bit make sense?

    My original request for advice on a control circuit is to ensure some charging at all times, which would not be "all or nothing" as in the present circumstances.
    An "off load" voltage of up to 20 volts can be measured at higher speeds, which into a 6Volt battery means either big charging current, or an overloaded generator. Clearly this is not a "hard" power supply, so rather than a damaging overload, we merely "sink" down the output load line. I reason that a low-loss regulated voltage control circuit would give me the best chance of optimising my setup.
    Obviously I could simply put a series resistor across the "charge" switch position to give me a trickle charge, but I was hoping for something a bit more sophisticated.
    Les.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    I can't help but wonder if your compensation winding is either intermittently open or shorted.

    It might interest you to know that the Lucas ignition on the V12 Jaguars made from about '78 to '88 used dual ignition coils. One of the coils had the HV tower plugged with a greenish clear plastic substance (like Lucite or Perspex), and was located between the grille and the A/C evaporator. I believe it performed a very similar function to your "compensation winding". If this 2nd coil was malfunctioning, the spark intensity was greatly diminished; the engine ran quite poorly indeed - if at all.

    The idea of this 2nd ignition coil was to cause the current in the primary coil to surge back and fourth a number of times; basically a resonant tank circuit. This caused the plug to fire several times instead of just once. It made ignition much more reliable.

    Nowadays, ignition systems fire the spark plugs for an extended duration at a much higher voltage, using a much wider gap than was previously possible.
     
  10. SgtWookie

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    What is the voltage at your battery when the motorcycle's engine is running at moderate to high RPM?

    About 6.8v to 7v should be optimal for charging your battery. If it is less than that, you'll need a higher voltage from your Maglita. If it's significantly more, like 7.2v, you may benefit from a regulator.

    I'm basing these voltages from a typical 12v automotive electrical system, and just cutting them in half. Since the battery chemistry is identical between 12v automotive and your 6v motorcycle electrical system, it's a pretty valid assumption.

    What is of concern is that there is little "extra" power available from the Maglita unit, if any. A typical linear regulator circuit would not do, as you would lose voltage across the regulator, and the voltage loss would result in significant dissipation of power in the regulator as heat, the combination of which would be undesirable.
     
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