Windmill Car Dynamos
Dynamos need regular maintenance, at intervals of about one year, perhaps longer if well treated. This involves removing the armature from the body, cleaning out the carbon dust, cleaning the surface of the commutator (taking great care to keep it concentric; a lathe is sometimes used to skim it), cleaning and greasing the bearings as required. The brushes must be able to move freely in their boxes, and the commutator surface must be completely free of grease. It’s a dirty, fiddly job, but very necessary. If the commutator is not functioning properly, the dynamo will fail to self-excite, and the windmill will consequently overspeed.
You can almost halve the rated speed of a 24 volt dynamo by using it to charge a 12 volt battery. The field coils need to be rewired in parallel rather than series to work properly at the lower voltage. Figure 5.18 shows a typical layout.
If you are lucky enough to find a low speed dynamo, take care not to overload it. High currents will lead to sparking at the brushes and accelerated wear. In the worst case it will burn out the armature, which is tricky to rewind.