help with motorcycle alternator

setsquar

Joined Oct 11, 2012
34
hi all
i have a early motorcycle which has a wipac 6volt permanent magnet alternator which i would like to convert to 12 volts with a regulator/rectifier.
this has 3 coils, two are used to power headlight and one to charge battery
the two are wired in parallel and the single coil is connected to a rectifier
the two coil are at 7 and 5oc on the photo and the single coil is at 6oc
(the top one is the HT ignition coil)
but this is where im stuck, the rotor has 6 magnets each cover two coils at a time when rotating.
so how can i wire the 3coils together to get a reasonable AC sine wave to feed the R/R ?

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MikeML

Joined Oct 2, 2009
5,444
Since there are six magnets, the voltage induced in the odd winding will be out of phase with respect to the other two. I would wire the two larger coils in series (which should have the same phase), and ignore the odd one.

crutschow

Joined Mar 14, 2008
26,721
Perhaps you know this but permanent magnet alternators don't work well with a standard series regulator, since the alternator output voltage can become dangerously high if you try to restrict the output current.
Most motorcycle alternator regulators short the excess current directly from the output windings to ground to regulate the voltage as described here.
This is one such design for a single-phase alternator.

ian field

Joined Oct 27, 2012
6,539
hi all
i have a early motorcycle which has a wipac 6volt permanent magnet alternator which i would like to convert to 12 volts with a regulator/rectifier.
this has 3 coils, two are used to power headlight and one to charge battery
the two are wired in parallel and the single coil is connected to a rectifier
the two coil are at 7 and 5oc on the photo and the single coil is at 6oc
(the top one is the HT ignition coil)
but this is where im stuck, the rotor has 6 magnets each cover two coils at a time when rotating.
so how can i wire the 3coils together to get a reasonable AC sine wave to feed the R/R ?
Usually, the DC circuit on those types only has half-wave rectification. If you can isolate the ground wire that goes to the frame, you can try full wave bridge rectification - you might even get away with charging a 12V battery.

If it uses AC lighting, you're probably stuck with 6V for that. You could try isolating the ground wire on all the windings and use as many bridge rectifiers as the windings can deliver to - but that's putting all your eggs in one basket (battery).

Another trick is the charge pump voltage doubling rectifier - but you'll need an awfully big charge coupling capacitor.

The other option is rewind all the coils with more turns of thinner wire - not a job for the faint hearted!

setsquar

Joined Oct 11, 2012
34
yes it's the out of phase bit that i can't fathom
the reg/rec used on this type of bikes is a shunt type with the rectification (full wave) in series the shunt
i was wondering if the single odd coil could be rectified and then connected to the input of the reg/rec
the 12volt conversion is popular on old bikes as it allows easy to obtain bulbs inc LEDs and fitting of turn flashers
unfortunately with this bike just the two coils are not quite enough to power lights and charge battery

crutschow

Joined Mar 14, 2008
26,721
If the single odd coil is isolated then you should be able to rectify its output and add it to the other coils rectified output.
If it's output is relatively low current you may not need to regulate it, just connect the bridge output to the battery.
Regulating the other coils output should be sufficient as long as the bike's circuits are always drawing more current than the single coil can generate.

ian field

Joined Oct 27, 2012
6,539
If the single odd coil is isolated then you should be able to rectify its output and add it to the other coils rectified output.
If it's output is relatively low current you may not need to regulate it, just connect the bridge output to the battery.
Regulating the other coils output should be sufficient as long as the bike's circuits are always drawing more current than the single coil can generate.
Certain Hondas like the CB250G5 etc switched out certain windings if the main beam lights were off. There were basically 3 alternator windings, but 2 of them were hard wired in parallel - that's the bit that got switched out. With no lights on the single winding did all the work.

The regulator was basically a SCR that shorted one AC arm of the bridge rectifier to ground, there was just the SCR, a zener diode and a few resistors.

In later models; the rectifier and regulator were combined in a single unit. Later still, the altenators were 3-PH so a couple more diodes in the rec/reg pack.

One of those units could be a neat solution for the TS - there are 6V and 12V versions. But it would only work well if the ground wire from the generator windings can be isolated.

The generator pictured is a flywheel type - most of my experience is with PM alternators, but flywheel generators probably aren't *VERY* different.

The PM alternators are almost constant current, if you rev the engine with no load on an alternator, it will probably reach about mains voltage, and if you grab the wires - it could well shift enough current to kill.

On very small bikes; flywheel generator regulation was pretty much gassing the distilled water out of the battery to absorb the excess current - as I found out to my cost on a bike that tapped the rectifier off the lighting coil so it also regulated that. For some reason the battery fuse blew - and then so did all the lights!

Power zeners started turning up on later models.

setsquar

Joined Oct 11, 2012
34
yes there is no problem isolating the ground wires as they are connected to posts on the other side of the stator
this got me thinking about the possibility of using a 3 phase regulator, but bearing in mind that two coils are and need to be in parallel how could i connect the odd coil.

ian field

Joined Oct 27, 2012
6,539
yes there is no problem isolating the ground wires as they are connected to posts on the other side of the stator
this got me thinking about the possibility of using a 3 phase regulator, but bearing in mind that two coils are and need to be in parallel how could i connect the odd coil.
Don't parallel any coils unless you're sure they have identical windings - otherwise the one with more turns will push some of its current through the other coil instead of the load. Any difference in phase as the magnets spin past will also cause a mis-match.

The 3-PH rectifier has 3 alternator connections, I'd join the 2 isolated grounds and connect them to one of the wires - then you have the 2 original wires left and 2 terminals left on the rectifier - it doesn't matter which way round any of the 3 wires go

Some wire colour arrangements have all 3 alternator wires the same colour - it doesn't matter which coil what wire goes to..

setsquar

Joined Oct 11, 2012
34
hi ian
ok thanks for you help will give that a go and see how it works out

ian field

Joined Oct 27, 2012
6,539
hi ian
ok thanks for you help will give that a go and see how it works out
If you only have a single phase rectifier/regulator, you can add a couple of rectifier diodes externally.

The rectifier/regulator pack doesn't use a series pass element - the positive lead and earth are direct to the internal bridge rectifier. Regulation is done by thyristors shorting one or more AC inputs.

I'm guessing anything old enough to have Wipac electrics, probably has positive earth - you probably won't find anything other than negative earth in modern regulator packs.

If you want to retain original positive earth, you can just buy a 3-PH bridge rectifier. The Lucas system just used a dirty great 15V zener for regulation, they put it on a neat little cast heatsink that usually mounted between the forks under/behind the headlight.

As far as I could tell; the zener was a basic 100W type, at the time Philips offered it with a choice of either stud polarity.