EL022 ELECTRICAL

I have recently joined the ranks of Velo owners having aquired a Venom first registered Nov 1962 (although the numbers show that engine and frame were built in1960.The electrics are 6v-the dynamo has D6V stamped on the end.According to information gleaned elsewhere in Q and A this would indicate a60w output.However, the ammeter shows a discharge of a couple of amps or so with the lights on and a charge of about 4 amps with them off.The bike had stood for about 18 months before I bought it and the battery was low on charge. The bike was extensively restored 4 yrs ago apart from the bottom end which was soundand a new battery fitted along with the VOC toothed belt conversion for the dynamo.The dyno sprocket has 32 teeth i.e. 1.25/1 ratio.Is this acceptable or could the higher running speed be causing a problem in the charging circuit?
Otherwise the bike looks and goes superbly-looking forward to the better weather. - Dave Disney 19/1/2007

The higher running speed shouldn't cause a problem.
In effect you are driving the dynamo about 14% faster than standard.
So when the engine is revving at 5,500 the dynamo is doing 5,500 x 1.25 rather than 5,500 x 1.1. But on a standard bike it was designed to survive up to 6,500 x 1.1 or more anyway.
Unless you hold the engine revs above 6,000 the dynamo still isn't doing more than it was designed for.
Even if it was, it would probably only risk causing mechanical damage to the armature, not electrical problems.
It does sound as though your dynamo is not producing its full power.
A good 60w dynamo without lights on and with a low battery should show a charge of about 8 or 9 amps at medium revs.
With the headlight (35w bulb) and tail-light and speedo light on this would drop to about 2 to 3 amps.
If someone has put in a 45w bulb then maybe only 1 amp at medium revs.
Of course if the battery is fully charged then the ammeter should show only about half an amp because all the dynamo is being asked to do is balance the load of the lights and keep the battery topped up.
One useful test of a good charging setup is if the ammeter is showing the dynamo just nicely balancing the lights and your bike has the stoplight and/or horn connected directly to the battery (like as standard) and not through the ammeter, then press the horn and/or brake pedal and the extra load should make the ammeter jump up a couple of amps as the dynamo gives its last few watts to balance the extra load.
It seems your dynamo can only produce about 24 watts, so shows only 4 amps charge with no load other than a battery that needs a charge, and minus 2 with a lighting load of about 40 watts.
It could be the brushes are worn or stuck, or the commutator is very dirty, or the armature has a defective winding, or the voltage regulator is faulty or needs adjusting. - TW 19/1/2007

Was out on the bike the other day(bone dry roads/lovely sunshine).Noticed that the charge rate was getting better,even to the stage where the charge was neutral/slightly positive when the headlight(35w) was on.Dabbing the rear brake caused a charge increase.I have done nothing to the set up since we last spoke except loosen the dynamo drive belt-Ithought that it was too tight.Maybe there was muck on the comm.. which has now worn off(the previous owner did 60 mls in nearly 4 yrs)or ,and this is something I have recently thought about,maybe the armature had lost some of its residual magnetism and this has been restored with use-I have done 200 mls in the past month.Hence,since it does not now appear to be broken I’m not going to fix it!Thanks for your help and advice. - Dave Disney 8/2/2007

Sounds like you've got a good, self-repairing, Venom.
The armature doesn't have any magnetism, only the field coil pole piece, and that needs only a tiny bit since it is an electro-magnet so I think you may well be right - just the brushes getting back in contact. - TW 8/2/2007