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.

[Dave Mann]

I changed to 12 volt in the 1980s primarily because the flashing rate of 6 volt flashing indicators varied considerably and as I used my Seven to commute into work I needed reliable indicators at night. A local alternator repairer put me onto the 12 volt idea when he sold me an electronic alternator voltage regulator for £4.50, so I removed the third brush, fitted a 32 ohm resistor to the CF3 cut out and I've run 12 volts for nearly 40 years. At that price I could afford to buy a second regulator as a spare which lives under my seat. On a long run to Beaulieu the ammeter seems to be glued on zero rising to 5 amps for a minute or so if I've been sat in stationary traffic. At night with 25 watt quartz halogen headlights (which draw the same current as the original 12 watt 6 volt bulbs) and LED rear lights the dynamo is producing about 7 amps well within it's capability.

[Bob Culver]

It beats me. In conditions of low system voltage ie flat battery and lights on would expect the regulator to apply near continuous field feed which at 12v, would result in field current more than double normal (4X heating effect). How hot does field get on steady 12v? Maybe can take it (I have tried a steady 9v and does not get very hot.)
The “field” of an alternator is very low resistance so, unlike e.m. generator regulator,  it happy with the original low resistance dynamo field. The alternator regulator probably has a maximum current sensing function but this likely far above anything a Seven dynamo could produce.
Maybe the conversions have never encountered a situation of very low battery and conventional high current lights on for a lengthy period after?
An additonal 1.5 ohm or so in the field lead would seem prudent. If this function is inside the regulator it would run warm. 

We have some  more clued in electrics than me. What do they think?

And Dave, I would be interested to know what the regulator is please? As far as I can fathom voltage is now often controlled by a signal from the car computer so a self contained regulator may be somewhat rare.

[andrew34ruby]

It beats me. In conditions of low system voltage ie flat battery and lights on would expect the regulator to apply near continuous field feed which at 12v, would result in field current more than double normal (4X heating effect). How hot does field get on steady 12v? Maybe can take it (I have tried a steady 9v and does not get very hot.)

An additonal 1.5 ohm or so in the field lead would seem prudent. 

Bob, if the additional 1.5 ohm resistor is just external, and heating up the world, it is not a very efficient use of that precious electricity we are trying to produce. Better is to have higher resistance field windings. That way less current flows to create a field, but none of it is wasted.

Higher resistance field coils are now available, though not cheap...   
https://www.eics-products.co.uk/shop/cla...eld-coils/

and a suitable electronic regulator is here... https://www.eics-products.co.uk/shop/cla...eld-coils/

Apparently this regulator will cope with the original low resistance field coils, but works better with higher resistance field coils.

[Dave Mann]

Unfortunately the regulator has no makers mark or other ID so I can't tell you what it is Bob, although the supplier said it was for a Volvo alternator and somewhere I have a data sheet. Unfortunately the alternator repairer is no longer in business having retired years ago. Given that I have 6 of them, two in each car, one fitted one under the driver's seat as spare I think it will be a long time before I need another.