FAN DRIVE SYSTEM

Hydraulic fan system and method for controlling the speed of a hydraulic fan system. The fan system comprises a fan operated by a hydraulic motor which is driven by a hydraulic variable displacement pump comprising a displacement volume adjusting element. A tilt angle of the displacement volume adjusting element can be adjusted by controlling a tilt current supplied to an electronic displacement control unit. The hydraulic fan system further comprises means for determining the volumetric flow rate of the pump and a control unit comprising: a signal connection to the means for determining the volumetric flow rate of the pump, a signal connection to a fan speed setting device, and a fan speed calculating unit for calculating a fan speed from the determined volumetric flow rate, a fan speed error determining unit for determining a fan speed error by comparing the calculated fan speed to the fan speed set by the fan speed setting device. The control unit can supply an adapted tilt current to the electronic displacement control unit in order to adjust the tilt angle of the displacement volume adjusting device, such that the volumetric flow rate can be adapted in order to reduce the fan speed error.