The output of the dynamo depends on how fast it is rotating and the strength of the magnetic field generated by the field coils. Some of the output is fed to the field coil windings to maintain the magnetic field and this is controlled by the voltage regulator which ensures that as the speed of rotation increases the field is reduced to limit the output to the desires level (6V).
As the engine speed varies the field coil voltage is adjusted to maintain a constant output.
If the voltage regulator isn’t doing it’s job the field coil voltage can increase and you have a feedback situation where the voltages shoot up. Then the dynamo overcharges, overheats and becomes an unhappy dynamo.
Have a look at this website for more/better explanations and under the rewinds & repairs sections it lists common problems.
Usually there is no need of a fuse on the regulator line,
And originally there is no fuse in the system.
Anyway, a fuse would not prevent overcharging.
(As Leon mentioned, an overcharge at the past probably caused the commutator soldering to melt)
If you want to install a fuse, I would put one on the main lead to the battery.
(Can be the negative line to the chassis or main positive- will work the same)
Th correct value would be a bit over the maximum normal current (Amp) consumption
By the formula:
I=P/V
I= Current in Amps
P=Power in Watts
V=Voltage in Volt units.
So if your headlight is 18W, for example, and rear lamp is 21+5W,
You are already at 44W, which is 44W/6V=7.4Amp
So for this EXAMPLE 10A is OK, (We did not consider the horn, if it fused)
And anyway,
If the same fuse is also going to the feed from regulator to battery,
You need at least 10A fuse, to cover the maximum theoretical charge.
So a 10A fuse should be OK, and to be on the safe side, I guess 15A will also protect from a short-current.
(We do not add the consumption value to the charge value,
As they flow in different directions- one to the battery and one from the battery,
so the difference is deducted)
