An electric generator arrangement comprises a generator and a continuously-variable ratio drive, whereby the generator can be driven at a constant speed over a range of speeds applied to an input shaft. The drive comprises an epicyclic differential gear having a planet gear carrier driven by the input shaft, a sum gear coupled to coupled to the generator and a ring gear. The ring gear can be driven at varying speeds by hydraulic units to vary the ratio of the speeds of the input shaft and the sun gear. The hydraulic units are driven by the input shaft and the speed of the ring gear is controlled by an electro-hydraulic control system which is responsive to the frequency of an output signal from a part of the generator. Fluid under pressure for the hydraulic units and the hydraulic control system is provided by a combined pump and de-aerator on the shaft of another part of the generator.