12 Volt Field Coils

[Stuart Giles]

Interesting reading about 12v conversions on the A7 dynamo; I have come across so many of these converted dynamos that have died having thrown the solder off the commutator wiring. I've always thought that the issue is basically one of maximum power output. i.e. 6v dynamo max 10A output =60W; but a 12v converted dynamo would only make 5A  to produce the same power.; i.e. at that output it's still a 60W dynamo. Running it at 8 or 10 amps @ 12v means it's potentially a 120W dynamo -does look like trying to get a quart of power from a pint pot of dynamo to me -running the dynamo without the steel strap over the brushes might help with heat dissipation though.

Stuart, it doesn't quite work like that. The solder in the commutator is melted by the power, or heat, produced in those windings. Heat generated (in watts)  = resistance x current squared. The resistance of the windings is almost constant, so heat generated is proportional to amps squared. The more you can reduce the amps the better. Increase the volts and you decrease the amps for the same power output. Higher voltage really is like getting something for nothing, when you are concerned with the windings overheating. It isn't something for nothing, as the engine still has to do more work to turn the dynamo, but the higher the volts the less heat will be generated in those windings.
That's why the overhead powerlines on pylons is at a very high voltage. Higher voltage means lower amps to power the same brightness of headlamps. Lower amps through the same windings means less heat generated in those windings. With higher volts you get more power at the lamps, and less heating in the windings. Going to 12v headlights means headlights with a higher resistance and that means the same light but with reduced current. 
So the best way is higher voltage, higher resistance bulbs, and less current. As for eliminating the chance of melting the solder on the commutator, the best way would seem to be a fuse.

Yes, I thought that was the point I was making in my post. I used V x I to arrive at the power output rather than I²R but unless ohms law has changed since I was at college all those years ago the calculation should have the same result.

I don't have any doubt that all the burnt out 12v converted dynamos I've seen have been running at significantly higher output than 5A or 6A @ 12v and which results in overheating and failure of the soldered commutator connections.

It's certainly true that the reduced effect of voltage drop on lighting circuits etc. is a benefit of running at 12v, allowing for brighter lighting with bulbs of a reduced wattage; but I don't think that is the reason that most of these conversions were carried out.