Fan revolution speed control method

The invention relates to a fan revolution control method for controlling a pump-motor system (19), thereby controlling the fan revolution speed of a cooling fan (17) that is adapted to be rotated by a fan motor (15) so as to cool cooling target fluids. The pump-motor system (19) serves to drive the fan motor (15) by means of hydraulic fluid fed from a fan pump (13) driven by an engine (11). The pump-motor system (19) is controlled so that the fan revolution speed starts from the minimum fan revolution speed (Nmin) when the engine is started up (Step 1); the minimum fan revolution speed (Nmin) is maintained for at least several seconds (Step 2); after the elapse of at least several seconds, the fan revolution speed is gradually increased from the minimum fan revolution speed (Nmin) (Step 3); and that the fan revolution speed reaches a target fan revolution speed (Ntf) over a period of at least several seconds (Step 4). The method prevents generation of peak pressure or pressure hunting, which may otherwise result in damage to the pump-motor system (19).