Franz, if you take a functional motor/generator setup like the coffee grinders into a motor shop, ask for a  "dip and bake" on them. They will tear the motor & generator down to the winding, test them with a hi-pot, then bake them in an oven for a half day to cook off the moisture, dip them in new varnish, then bake again.

Makes the windings last for a good long time if you do this for any motor (or generator/alternator) winding before it arcs through the aging varnish.