12 Volt Field Coils

[Bob Culver]

Seems fully adequate magnetisation. Due the 3rd brush a full 6v not normally applied to the field.

[Winston Teague]

For general information, the new manufacture RB106 regulators are of such poor/erratic quality that 3hours of running before failure is quite normal, and that is installed on a vehicle for which it is intended! ie c40 dynamo. w

[Zetomagneto]

Winston, that's interesting.I guess you get what you pay for.I am using a DVR electronic regulator, not cheap at £71 plus post

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Hi Stuart, can you let me have the RS link please

[Special Bits]

Hi Z

Unfortunately the RS web site is down at the moment, but the stock number from my order is 158323, description HS50 AL house wirewound resistor, 4R7 50W.
I went delving in the spares boxes and came up with a NOS, genuine Lucas, 11A RB340. So I will fit that with the resistor and see what happens! The Lands End trial is less than two weeks away, and Im on family holiday duty for a week of that, so not much time for messing around. Im almost tempted to buy an accuspark unit and be done with it. Luckily in class 0 it would be allowed.

Winston, yes I suspected the modern things wernt much good. As I said it worked fine for quite a long time on an old one we fitted at first, but since then it has had the modern pattern ones fitted.

Its amazing how far an A7 will go on a part charged 12ah motor cycle battery, which has powered a map light all night!

[Zetomagneto]

Hi Stuart, have now used the resistance wire, cut a length to give 4.5 ohms, and used it to connect the field to the regulator, used it as a length of wire with heatshrink round it rather than making it into a coil.
The regulator is seeing 6 ohms from the field.
Now have 8 amps charge, the resistance wire gets warm only, not hot.Seems to have worked.

https://www.ebay.co.uk/itm/Nichrome-Wire...QDHMMcTZFw

[Stuart Giles]

For general information, the new manufacture RB106 regulators are of such poor/erratic quality that 3hours of running before failure is quite normal, and that is installed on a vehicle for which it is intended! ie c40 dynamo. w

Leaving aside any general build quality issues with the reproduction items, a period specification document I have seen stated that the genuine Lucas RB106 regulators had Palladium contacts. The ones in the repros are probably made of Chinesium as Palladium was and is a precious metal.

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.

[Bob Culver]

Hi Stuart 

It is armature heating what does the damage. Related to armature current squared times the (fixed) armature ohms. The current includes that to supply the field; not indicated by the ammeter. Normally 3 or 4 amps but with some 12v conversions seems likely to be much greater. Without lights, 8 amps charge on the ammeter, with coil current and field current is about 13 amps armature current.
The power produced and absorbed is related to volts times total armature current (including field supply). When struggling for 60 watts or so just for the headlights, 12v offers an advantage.  The timing gears have to transmit the extra power. 
The later original headlamps were only 18w each but this is not evident from most sources as bulbs were rated in candlepower. Dont know if 18 or 20 watt bulbs still available.
The use of larger bulbs with the dynamo adjusted to try and match was/is the cause of most cooking problems.

(I guess you know all that but it may be helpful to others)

[Mark McKibbin]

I run an electronic regulator designed for the purpose of running a 6V 3 brush at 12v 2 brush it was originally designed for motorcycles and it works flawlessly.  However the engine must to doing at least 1500rpm to get a charge and when idling  it goes negative 8 Amps.  I have been thinking of buying some 12V field coils available in the UK for about 60 GBP at https://www.eics-products.co.uk/shop/cla...eld-coils/ I think these would work to reduce the current needed to get a charge.

The regulator I use came from: http://aoservices.co.uk/car.htm the only downside is you have to remove the warning lamp as it is no longer needed so will be on all the time.

[andrew34ruby]

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.

[Special Bits]

a quick update

I fitted a 2,5 ohm resistor in the field coil wiring. This seems to have worked as it was still charging nicely after 250 miles on the Lands End trial. Thanks for every ones input.