Another important aspect of the problem is that a dynamo or alternator produces a current, not a voltage. Thus, if the thing is spun by whatever mechanical means, it pushes a certain current, whether or no. If you unload / lightly load it, the voltage climbs, trying to force the current produced through the load. This can damage the load by overvoltage, unles some shunt path is available. In the old days, one simply moved the bicycle dynamo's friction wheel away from the tyre to stop current production....