shorting coils on an alternator

[SparkyTris]

Pls excuse this posting which is somewhat off topic but purposeful to tap into a community without "accepted" bike-lore opinions. I am a stranger to the world of vintage bikes. An uncle who has become recently into nice old British bikes has asked me about wiring and converting alternators from the stock 6V systems to current 12V systems.
A lot of the theory appears to involve bunching the 3 phases coming out of the alternator together into one conductor and then feeding that lot to a rectifier/regulator unit....., against the "Common" point from the alternator (which I guess is the centre of a star wound coil arrangement) - as shown here:

present "modification" advice being as I have indicated with red pen. This strikes me as "Not Right" because it basically appears to short the 3 phases at the alternator. My limited practical experience of alternators is with small simple chinese wind-turbines, where the mfg-advised method of "braking" the fan is to short the 3 phases, which kills the output and puts max load on the fan in an attempt to stop it. If the wind power overloads this torque then the alternator cooks up, presumably because ALL the current from the peaking winding gets forced through its neighbours, many times a second.

perhaps I am missing nuance in the way that an alternator can be arranged?

there are stock drawings from the original bike manuals that indicate sets of coils can be shorted together, see here:

None of this seems right to me but I must be missing something. I just wondered whether the original stock "kludge" of shorting together the 2 pairs of windings was a known trick, then latterly the biking community has (with some ignorance?) blindly extended that trick to shorting all 3 coils together?
I must be misunderstanding sthgh because if this was genuinely an error there would be fried alternators all over the place.

 

[ericmark]

Each flywheel generator seemed to have its own wiring, I remember back in the 70s trying to convert to 12 volt, and to add extras to the system, and it not only depends on the coil arrangement but also the magnets, there was basic no standard between motor bikes.

The one I had there were 5 coils, the sixth place had the contact breaker for the ignition, and each coil had different gauge wire, and different number of turns, so one coil for side lights, two coils for headlights, one coil for stop lights, one for the horn, and one for ignition.

But another bike had 6 coils, and a magneto, as you show here, with the common not using the frame, we would take the common to the centre of the two batteries, and half wave pos to one battery and neg to the other, and a zenor diode across the battery to stop over charging.

But things have moved on, so today we have LED lights etc, and also switch mode power supplies which can convert nearly anything to anything, so I am sure there are devices now made which make it all easy.

I would look for a bike forum, and see what is made. Some things seem daft, my e-bike as a great headlamp but not tail lamp, I have to use a lamp clipped on at the back with its own batteries, but it comes from the 48 volt traction battery at the front.

So at what point it is simply easier to have a battery charged each day just for lights, the new batteries are so light, is it worth all the messing around to get good lights from the alternator?

Either way is not standard, so its down to what is easy.

 

[SparkyTris]

I guess in essence I am wondering in what situation it is "good practice" to short the coils on a generator?
I seem to be thinking of it as short-circuiting a current source: one would never join together 2 of the three phases at a 400V mains supply just to get a higher current for a singlephase purpose.

in the case of the cheap chinese windmill system, I am not surprised, you get what you pay for: the theory (I suppose) is that in shorting the alternator the load on the fan is massively increased so there is a natural braking effect. however, when the wind power exceeds the braking power then the coils cook up. the chinese windmill probably specifies "do not use in winds above 5m/s"

this theory (weak as it is) does not transfer to the case of the motorbike because the petrol engine will always spin the alternator possibly into destruction.

 

[ericmark]

On the Falklands we had two small generators to power floodlights, one was around 3 kVA and the other 6 kVA one with a single cylinder lister the other with a twin cylinder lister, and they looked the same, except one larger to the other, but in fact the 3 kVA could burn its self out, but the 6 kVA could not, if you overloaded the 6 kVA the field of the generator collapsed, and it could not be overloaded. Yet both of the same basic design.

We also had some 12 kVA with 12 windings, which could be arranged to give single phase or three-phase, and also different voltages. Single phase was called zigzag wiring.

Shorting out a winding may be OK, as at zero volts it may be also zero amps, but it would depend on internal resistance of the coils, and there is no real way to know. A lot depends on cooling as well.

So if the field of a generator is on the outside, and the generating part on the inside, there is less cooling surface to doing the reverse, which is why we moved from the dynamo to alternator in the car, and in the car, most could not produce enough amps to damage the windings. However not all vehicle alternators were like that, the ones designed for buses needed a high output at tick over, and the CAV AC 208 had a special 440 regulators with M1 and M2 terminals which went across a resistor to measure the output, and that alternator had current as well as voltage regulation.

There were so many alternators made, some were brushless, so even had a three-phase transformer built in and had a 12 and 24 volt output.

The French did a cheap single phase version with half-wave rectifiers, and the list goes on.

I had an outboard engine with electric start, which had no way to recharge the battery, you simply charged the battery at home.

So I look at my e-bike today, with a 48 volt battery, and one has to ask, at the size of modern batteries does it really need to be recharged by the motorbike?