A wind machine turns a shaft to activate a multistage pump having intake and discharge means respectively coupled to intake and discharge fluid lines. Control means regulates the operative positions of the intake means or discharge means to remove selected stages from effective pumping while at the same time fluid passes into and from the selected stages. The stages can be of different capacities and selected groups or all of them may be periodically sequentially idled. Torque on the shaft is thereby adjusted to retain the shaft speed in a range for effective wind energy conversion. During starting the stages are idled until an operational speed is reached. Power assisted starting can be achieved by initially reversing the fluid flow through selected stages. Constricting means is located in the common discharge line from all stages and can be additionally located in each discharge line from each stage. Slowing or stalling of the wind rotor is effected against overspeed by constriction of the flow from the stages to increase the torque on the shaft. Pressure downstream of the constricting means is maintained substantially constant without the need for energy accumulation. The discharged fluid drives a turbine coupled to a generator wherein the fluid power is converted into electrical power.