Abstract: An electrical alternator system is capable of being driven by either of two prime movers and of being automatically transferred therebetween without substantial variation in the electrical output of the alternator. A planetary gear system having two mechanical drive inputs and one output is interposed between the normally used prime mover and an alternator. One mechanical input to the planetary gear system is provided with a controlled variable speed input N.sub.R from the prime mover which is controlled so as to compensate for speed variations in the other mechanically input N.sub.E from the prime mover thus maintaining the output speed N.sub.S substantially equal to a desired reference speed N.sub.O.