Magneto reftifiation

[dleligdon]

Hello from St Louis,

I am trying to convert my 1981 Honda Passport scooter from AC to DC operation in order to upgrade the ignition and charge the battery. I am planning on installing a Shorai 6 volt battery.

When I measure the coil I get a nipple shaped AC wave form. When I add a GBU6J full wave recitifier I loose the negative side of the AC wave and the DC output measured on channel 2 looks like the input on channel 1.

I checked the diodes in the bridge and the appear to be "OK".

Comments?? Thanks for your help. Dave

 

[shortbus=]

Most motorcycle alternators are three phase. You are using a single phase rectifier. Google for three phase bridge rectifier.

 

[Mr RB]

That's true on larger bikes. On very small and simple bikes from the early 80's it is likely a single coil used for the "6v AC generator".

To the O.P; First you need to make sure both wires of the AC alternator output are fully disconnected from ground. It's possible at the moment that one wire is connected to chassis ground, and that will stop the bridge rect working properly.

 

[dleligdon]

Mr RB,

You are correct. The magneto has two bar stators with both windings connected to ground. I will disconnect the ground on the one I am working on and see what wave form I get. Attaching schematic for others to look at. Please note that all measurements are made with the points and condenser disconnected.

Dave

 

[Mr RB]

That's the same system as used in a lot of dirt bikes. One bar is only for the ignition (its the magneto bar).

The other bar is the single phase alternator. You need to disconnect one side from ground, then the two end wires (disconnect the one to the headlight) go to the bridge rectifier. After that it will probably make enough voltage to charge a 12v battery, many people make that mod. You won't get a lot of current, maybe 4-5A max, but it's enough to run a 30W (3A) headlight and charge a 12v battery and run turnsignals and brake light etc.

You can also buy 12v LED tail/brake lights and LED turn signals too, which reduce current required.

 

[dleligdon]

Mr. RB,

Thanks for the information. FYI I am working with an engine/transmission assembly only. I have a new battery coming next week and I will resume my experiment. I am doing this to find a good modification to make my daughters (and others) Passport more realiable. The magneto coil is hard on the points. I intend on balancing the battery charge by replacing turn singal bulbs with LEDs as required as to not over charge the battery. The electronic ignition will take about 5 ampers at 6 volts. No regulator in the system and I will try to abandon the Ignition winding.

Dave

 

[Mr RB]

Cool project!

If you are abandoning the magneto coil, you could rewind it and use it as another alternator coil (with another bridge rectifier). That should give you plenty of power to run a full 12v system (which is much better than a 6v one). They are a straight bar (easy to rewind!) with not that many turns on.

You can get 12v "points-less" electronic ignitions that run about 3-4A to the coil at about 40% duty cycle, so in use they only consume about 1.2-1.6A average from the charging system.

 

[dleligdon]

Mr RB,

My original plan was to rewind the stators after I determined what their capacity was. Being a PM magneto I don't know what the magnetic flux is hence the preliminary testing. The system is a 6 volt negative ground and I would rather keep it this way as could keep the starter motor.

Honda went to a 12 volt CDI ignition in 1982 and when they did they redesigned the magneto case with different side cover and starter motor. None of the newer parts fit the older engine. I appreciate the fact that a 12 volt system would make life a lot simpler as there are all kind of 12 volt ignition parts available.

I read up on rewinding the bar stators and I would do it in a heart beat if could deternine the number of turns.

Thanks for your comments, they keep me thinking. Dave