mad_jock . .. .Don't know for sure about the thyristor but I would assume there would be circuitry to protect other equipment in the event of the voltage trying to "wind up" as would probably happen if you were relying on a residual field to establish an output which would then be used to excite the alternator. The logistics of power normally being provided from the battery to supply the field would mean that the weak output initially gained from a residual field would be presented on the ancillary equipment bus and could do considerable damage as it fluctuates it's way up.