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Possible to convert a three phase motorcycle stator to single phase?

  1. Jan 15, 2017 #1
    I have an engine that has three phase 18 pole stator for DC fired cdi system .

    I want to install it this engine in another bike frame the issue is that that frame has ac fired cdi wiring system. Where there is a special coil for the Ignition, lightening and charging.

    Can i covert the 18 pole one be such as that of ac fired system?

    The three phase stator is exactly like this

    I want it to have a pole for ac ignition. And some poles for battery charging and lightning.
  2. jcsd
  3. Jan 15, 2017 #2


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    I would be inclined to use a different ignition unit.
    That stator is probably the crucial part of the Power Generation so, if you really want AC then why not use an inverter? We'd need to know more details of the system. Is the ignition timing dependent on the phase of the original stator output? (Old fashioned 'Magneto' style)
  4. Jan 15, 2017 #3


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    Welcome to PF.

    A 3PH alternator will usually have three AC terminals and be wound as a Y, without any connection to the middle of the Y. It is normal to connect the three AC terminals to a 3 input, 6 diode, bridge rectifier to produce the B+ and B– for lights and for battery charging. The rectified DC will probably have the B– side connected to the chassis.

    I would expect a coil dedicated to AC power the cdi to be wound for a higher voltage than those for lights or battery charging. But don't give up hope. If you need a single AC phase to charge the cdi, then you might take one of the AC outputs from the alternator before it enters the 3PH bridge rectifier. If the above assumptions are correct the voltage on any one of the alternator terminals will oscillate between the chassis and B+. Depending on the input to the cdi, that offset AC may be sufficient to charge the cdi internal capacitor, but it might need an AC coupling capacitor if the cdi input is a transformer primary.

    If you need more information you will need to provide a circuit for the AC powered machine and the cdi module it uses. We need to know the AC voltage required to drive the cdi module.

    It may well be easier to replace the AC cdi with a DC powered cdi module.
  5. Jan 16, 2017 #4
    I work on these machines. IMHO, this is not the best way to go about this project. The source coils for AC-CDI systems are usually large bobbins wound with wire between 30-40ga (it varies from model to model). If you try to wind an AC source coil onto a 3-phase DC-powered system's metal core you're going to need to go across several poles to maintain the resistance, and you're going to have to get the directionality right to make sure they're not cancelling each other. I've done projects like this before and it is MUCH easier to get a cheap stator for your vehicle off ebay; find the cheapest one possible and re-wind it keeping the source coil the same and winding the lighting/battery poles for whichever configuration you prefer (generic single-phase, single-phase with a tap at one point similar to the small Polaris ATVs, etc). As the previous posters have noted, can you provide specifics about which vehicle you're working on?

    Also just a note, the stator shown is almost certainly not an OEM piece. Depending on the manufacturer you may be dealing with a low-grade metal core to begin with.

    Just my .02
  6. Jan 16, 2017 #5

    jim hardy

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    I'd experiment with a step-up transformer connected phase to phase across the stator on upstream side of the rectifier/regulator..
    An old TV vertical output transformer seems right flavor. Maybe the 70 volt audio output transformer from an old PA amplifier.

    But that'd require experimentation and i dont know what is chamjisky's capability. Do you have a good voltmeter and soldering skill? How about an oscilloscope?
  7. Jan 17, 2017 #6
    I don't have oscilloscope but am good at multimeter and soldering.

    The DC fired three phase engine which I am going to install in the ac fired single phase bike is from Chinese Cg200 water cooled version. The stator shown in above video is exactly like mine. All 18 coils wound with same gage of wire in three phase configuration.

    The frame of the bike is a Piaggio 125cc. Engine is bolt on fit.

    Google shows me some coils that gives me the feelings that existing stator can be converted to ac.

    Look at below coils seems like they have reserved the space of three coils for ac ignition coil where they removed the core for two coils to and wound one core to the thickness of three coils.

  8. Jan 17, 2017 #7


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    That is certainly an interesting design modification. Remove three poles from the low voltage, high current 3PH, to make the space needed to replace them with two poles wound with more turns of thinner wire to provide the high voltage AC needed to fire the cdi. I expect those two HV poles are connected in series.
  9. Jan 17, 2017 #8
    They are. It's actually very common across dirtbikes, ATVs, snowmobiles, etc. that run AC-CDI systems with lights (and possibly a battery). The ignition source coils are usually wound with 30-40ga, the three-phase winds are usually 18-19ga. The problem the OP is going to experience in adapting the metal core shown in the first post to this style AC-CDI circuit is it's going to be across three poles now instead of two (all in series). He might even need to go across 4 poles to get a high enough resistance to match the OEM spec. This 4-pole configuration needs to be wound in such a way relative to the flywheel that the peak Vac across the two ends matches the OE spec as closely as possible. It can be done without an oscilloscope but the scope makes it much easier.

    Also don't forget that charging the capacitor is only the first stage. If the timing is significantly different between the two engines it won't work. Crank position sensors are almost always in fixed locations and can't be adjusted. If the stroke of the new engine is significantly different from the original you're going to end up with a mis-timed engine. Depending on the engine (I have very little experience with the Chinese scooters) you may be able to account for this mechanically, you may not.
  10. Jan 17, 2017 #9

    jim hardy

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    My experience is with outboard motors.
    A coil makes around 400 volts to charge the capacitor for a CD ignition, a small trigger coil senses crankshaft position and fires the thyristor for spark.
    So all one needs is a higher voltage and something to sense crankshaft postion.

    It looks to me like a tricky job to rewind coils on that stator, and it's an irreversible step.
    but It's easy to wire in a transformer to make a couple hundred volts out of that fifteen or so stator volts from the charging coils.

    If the bike has a voltage regulator it should clamp stator volts to around fifteen peak so the 220 to 24 volt transformer from an airconditioner or furnace system thermostat might work. I suggested an audio transformer because frequency will be a lot higher than 60 hz.

    Creative adaptation ? Trial and error.

    just thoughts, i've never done it.
    Last edited: Jan 17, 2017
  11. Jan 17, 2017 #10


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    I do not see the problem. In summary …

    An alternator with the high voltage AC windings (for use with AC fired cdi), can be used as a simple 3PH alternator by insulating and not connecting the high voltage winding.

    If you have a simple 3PH stator but have an AC fired cdi, you can;
    1. Replace the 3PH stator with a spare part of the same size having the extra HV AC windings;
    2. Change to a DC fired cdi module. Or;
    3. Take Jim's advice, use a voltage step-up transformer with a ferrite core, drive it from two of the three alternator phases.
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