Ignition coils?

Discussion in 'General Electronics Chat' started by MikeML, May 15, 2015.

  1. MikeML

    Thread Starter AAC Fanatic!

    Oct 2, 2009
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    Older-style Kettering automotive point/condenser ignition coil:
    Primary resistance?
    Primary inductance?
    Secondary resistance?
    Turns Ratio?


    I have in-hand some ignition coils from an old Natural-Gas powered, 6 cyl, industrial stationary engine that I am try to get running. It used one coil per spark plug. The spark source is a low-voltage Magneto, with a 1:6 low-voltage distributor that drives the primary side of the six coils.

    The original coils have a primary resistance of ~5.7Ω, a secondary resistance of ~10K, and a turns ratio of 1:100. These coils are obsolete, and are no longer made. Cant find any NOS replacements, so am asking about automotive coils on the chance that they may work for the magneto output?
     
  2. Hypatia's Protege

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    Yes, however inasmuch as ignition coils are, in essence, representative of 'flyback' topology, primary pulse shaping will likely be necessary... Of course you'll need to incorporate sufficient 'adjustability' to satisfy 'dwell' (i.e. coil 'charge' time) and spark timing...

    FWIW, that (high) value 'smacks' of an internal series (ballast) resistor --OR-- a 'standard' transformer --- In any event an automotive coil will need to be driven 'flyback style'...

    Best of luck!:)
    HP
     
    Last edited: May 15, 2015
  3. crutschow

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    Mar 14, 2008
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    According to this a typical Kettering coil has a 100:1 turns ratio and an inductance of 8mH.
    The primary resistance is about an ohm but during normal operation a resistor of a few ohms is connected in series to limit the current to about 5A.
    No info for the secondary resistance but that likely is not a significant factor.

    Based upon that info I would think a standard coil would work to replace the coils you have with perhaps an added resistor in the primary.
     
  4. Hypatia's Protege

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    If the magneto's waveform is a pulse (i.e. marked by rapid transition times) I agree with @crutschow --- On the other hand, should said output be sinusoidal, disappointing results will ensue...

    Best regards
    HP
     
  5. Hypatia's Protege

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    --- emphasis added

    Another thought: Is the engine of Fairbanks Morse, Cooper-Bessemer or Babcock & Wilcox manufacture? Most of these are readily adaptable to Diesel-mode (i.e. compression ignited) operation by design...

    Best regards
    HP
     
  6. MikeML

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    The Magneto is made by Fairbanks Morse. The engine is Cat, originally was plumbed for LPG, with a heat-exchanger to evaporate the LPG into gas. I converted it to run on Natural Gas. It ran all last summer, but one of the coils has developed an intermittent breakdown, and it misses on one cylinder.

    Cat makes a similar engine that is diesel, but I have no interest in diesel because of the expense. Due to lack of parts, Cat will only talk about a several thousand $ conversion from the old FairBanks magneto+coils to a newer ignition system... The coils for the new system are not compatible with the Fairbanks magneto.

    I am very familiar with aircraft magnetos, which have points and capacitor in the primary circuit (ala Kettering) but use a single, internal 1:100 step-up auto-transformer to feed 4 to 6 cylinders. The distributor is done on the high voltage side like a car. Aircraft mags have an impulse coupling to snap the rotating magnet past the pole piece to aid starting when the engine is turning slow during cranking.

    I do not know what is going on inside the Fairbanks mag. It has an impulse coupling for starting, just like the aircraft mag. It even looks and feels like an aircraft mag, except for the high voltage.

    It has points, capacitor, and a low-voltage inductor (not transformer, there is no high-voltage inside) which is parallel with the external coil primary? The rotating commutator connects the coil to each individual external coil on a round-robin basis. Firing order is established by the order that the distributor feeds the six external coils.

    I worry that the primary pulse timing/shaping depends on the external coil as much as the internal one. I would have to get a battery-powered O'scope out to where the engine is to find out. Unfortunately, there is 120Vac within a 1/4mile.
     
    Last edited: May 15, 2015
  7. alfacliff

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    Dec 13, 2013
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    6 coils, low voltage distributor, seems that all 6 would not fail at the same time. ahve you checked the "condensers" in the ignition circuit? magneto systems do not have ballast resistors.,
     
  8. MikeML

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    Only one coil is bad; the other five are ok.
     
  9. alfacliff

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    how about motorcycle coils? not the kind that drive two spark plugs, they have no common ground.
     
  10. crutschow

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    I would just try an old type standard automobile coil and see if it works.

    A standard magneto operates similar to a battery ignition except the magneto rotor (typically a rotating magnet) generates the primary coil current rather than a battery. In either case this primary current is then interrupted by points (or other switch) to generate the high voltage in the secondary by flyback action.
     
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  11. MikeML

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    Yes, I agree. What bothers me is that the Coil used with the Fairbanks Magneto has a significantly higher primary-side resistance than an automotive one. That external coil effectively is in parallel with the internal coil, so what does the do to the flyback tuning? I will have to try it and see...
     
  12. crutschow

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    I would think the two coils are in series.

    You could add a few ohms external series resistance to the coil if that's a concern.
     
  13. alfacliff

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    the magneto coils are run off a higher ac voltage than the car coils. typicly 35 volts or so.
     
  14. ian field

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    AFAIK: the magneto was the forerunner of the energy-transfer type ignition used on almost all 70s/80s Japanese mopeds.

    No idea for sure - but those type of coils could get you out of a bind.
     
  15. crutschow

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    Where did you get that information?
     
  16. ian field

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    AFAIK: the magneto was the forerunner of the energy-transfer type.

    A magnet rotates next to a coil and the points open and close.

    Nothing in particular regulates the voltage - its just whatever you get from lines of flux cutting the inductance at whatever speed its going at the time.
     
  17. alfacliff

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    my grandfather rebuilt magnetos. some magnetos are more complicated, since they dont generate as much voltahe at lower rm, they have a spring loaded "impulse" mechanism that whips the rotor over faster at lower speeds. the seperate type with the magneto and coil seperate usually run at higher voltage than the battery type ignition. the forerunner of the "capacitive discharge " ignition. most also have the "condenser" to help boost the spark and reduce burning of the points. without the "condenser" the spark is weak.
     
  18. MikeML

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    The magneto has an impulse coupling, check post #6.
     
  19. debe

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    What you describe with 300v a condencer & points sounds like a crude CDI ignition system. 300V generated & stored in the condencer then the points fire the capacitor voltage across the Ign coil. I would try a coil on plug that only has 2 conections & no internal switching. They are found on 6 cylinder Fords here in Australia.
     
  20. crutschow

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    The condenser is just to prevent arcing across the points, it's not to store energy for the spark.
    The spark energy is provided by the inductive energy from the current in the coil.
     
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