Conversion - 6v to 12v

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
Groily
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Re: Conversion - 6v to 12v

Post by Groily »

You have choices here on all levels.
To obtain the same rpm for dynamo cut-in as original you can fit finer-wound so-called 12v armature (AND field coil, essential); or you can continue with the heavier wound parts. Both are available new. There are two qualities - UK-wound or something else. You get what you pay for.
As to regulation, you also have choices. A DVR2 will work fine, and there are others - JG, V Reg2 etc. There are cheap parts also out there - but again, you get what you pay for. If you use a JG, the field coil wiing has to be altered at the dynamo to connect between F and D, the others work with the Lucas wiring of the field coil between the F terminal and Earth. All the above can be had Negative earth (I think), and the first two work at 6v or 12v with one wire disconnected for 12v operation
Whichever regulator, using the original type windings at 12v you will need higher revs to get charging as the dynamo needs to be generating c. 14v to support a 12v battery. That's not a comment on the regulator - they have to have something to regulate and it takes revs to get to the higher voltage. City riding is therefore more stressful on the battery etc at 12v whatever regulator you use.
With the finer-wound parts you are very strictly limited on output - 60W is the outer limit (well under 5 Amps at a system voltage of approx 14v) and overloading will melt the wiring on the commutator. With the heavier windings you can safely get 75W from the thing, using a decent regulator. I run three twins with original type armatures and field coils and DVR2s at 12v, and haven't ever had a regulator problem in 15 years with them. But they do need more revs to cut-in than originally.
All are easy to fit - and forget mostly. If the current allowed into the field is properly controlled, the dynamos should last for years. That is where the DVR2 is particularly good, as well as in terms of prompt cut-out on descending rpm (no discharges on the ammeter at tickover).
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sunnybob
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Re: Conversion - 6v to 12v

Post by sunnybob »

Groily, thank you so much for your exhaustive summary. I don't know about the overloading problem going 12v. Can you better explain to me how real is the risk of overloading the dynamo with a finer-wound armature? If the risk of damaging the dynamo is very high I'd better stay at 6v ...
Thanks again
Roberto
Andy51
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Re: Conversion - 6v to 12v

Post by Andy51 »

Hi Roberto, you can also overload a 6v Dynamo! These things were designed to give 45watt (E3N, used on singles) and 60 watt (E3L, used on twins). If you keep the load (total watts of headlight, tail light and speedo light, and anything else you have added and is on most of the time) below the design limit you will not cause damage at either 6 or 12 v - make sure you should leave a little spare capacity to ensure the battery is kept charged. You can ignore the horn as it is only used for short periods (although drivers in Naples think differently!) and the battery can usually cope with indicators as they are also on intermittently. You can also go LED as they draw very little current, and will not cause any Dynamo problems. The main issue with 12v systems is that some people start using powerful modern bulbs such as 55/60 watt, and draw too heavy a current leading to damage. As far as I know you cannot get such wattages in 6v, so few have melted their Dynamo on 6v. Hope this helps, Andy
Groily
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Re: Conversion - 6v to 12v

Post by Groily »

As Andy says!
The 12v part are made of thinner wire, with a higher resistance field coil and far more turns on the armature windings. They cut in at the same rpm as the original parts did and are perfectly OK if not overworked - but as Andy says, people get carried away . . . Amps are not our friends and there are no free lunches!
aobp11
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Re: Conversion - 6v to 12v

Post by aobp11 »

More precisely but roughly speaking, for 12 V armature you need twice the number of turns (--> wire length doubled) of half the wire cross sectional area to fit in the same space (--> resistance per meter doubled). Hence the resistance gets 4x higher. But at 12 V only half the current is needed for the same power output. The heat losses in the copper wire are given by resistance times squared current, so increase 4X because of the increase in resistance, but also decrease by a factor 4 because of the decrease in current. So these losses remain the same. So when heat losses are the limiting factor you cannot draw more power from a 12 V armature than originally designed for 6 V.
Heat losses in the brushes will decrease, so that's an advantage of 12 V.
I assumed here that a 6 V armature is rewound for 12 V. I don't know if the 12 V market armatures have the same physical pole piece dimensions and magnetic properties.

Albert
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