Power generating system including gearing allowing constant generator torque

A power generating system in which a fluid driven turbine drives an alternating current generator comprises a rotor to be driven by a fluid and a transmission assembly for coupling the rotor to an electrical generator. The transmission assembly includes a hydraulic pump (16) which is a positive displacement hydraulic pump which has a linear relationship between fluid pressure and output torque. The pump is connected in a hydraulic circuit which includes a valve (22) which in a first state prevents flow of operating fluid from the pump through the hydraulic circuit and in a second state permits such flow. With the valve in the first state, torque is transmitted from the rotor input to the generator with substantially zero slip and with the valve in the second state, the transmission assembly slips so as to permit the rotation speed of the rotor to increase relative to the rotational speed of the generator so as to maintain a substantially constant torque at the generator. A wind turbine preferably drives a shaft (6) through fixed ratio step-up gearing (3). This shaft (6) drives an annulus (13) of a second set of epicyclic gearing and the hydraulic pump (16). In a second embodiment the second set of epicyclic gearing is omitted and the pump is mounted integrally within a rotating fluid reservoir. The pump station is connected to the output shaft and the rotor to the shaft from the step-up gearing.