25 in the housing in this manner the stator 16 is centered in the housing the spacers 26 are bars extending in the longitudinal direction of the housing at least three such bars are provided but in the present embodiment 4 bars are arranged the spacers 26 are shorter than the axial length of a space 27 such that they do not impair the circulation of hydraulic oil in the space 27 the spacers 26 provide an annular space 27 between the stator 16 and the inner surface 11 of the housing said annular space 27 constituting the major portion of a reservoir 28 the reservoir is defined by the housing 10 the stator 16 and the rotor 17 are submerged in hydraulic fluid the motor shaft 19 of the motor 15 comprises an eccentric ring 29 driving a pump the pump is a multiple piston pump composed of a plurality of individual pumps arranged in a star like array about the motor shaft 19 the pistons of said pumps being driven by the eccentric ring 29 which is permanently connected in an eccentric manner with the motor shaft each individual pump takes in oil from the reservoir via a non return valve and supplies said oil to the pump outlet via another non return valve the multiple piston pump is a volumetric pump at the end of the housing 10 opposite the pump a fan 30 is located which comprises a housing 31 radially projecting beyond the housing 10.