DYNAMIC MODELING AND DESIGN OF A RADIAL HYDROSTATIC PISTON PUMP FOR INTEGRATED PUMP-MOTOR

There is a current trend towards the electrification of high force/torque density machines that have traditionally been dominated by diesel engine driven hydraulics. Power dense electric machines tend to favor high operating speeds whereas a hydraulic pump is more efficient at low speed and high torque conditions. The power density of a pump can be increased by decreasing the displacement and increasing the operating speed to provide the flow demand. This miniaturization of the pump allows it to be directly integrated into an electric motor inside a single casing. This integrated pump-motor is free of shaft seals and eliminates a set of bearings otherwise required when coupling an electric motor and pump with a shaft. Additionally, the leakage from the hydraulic pump can be used as coolant for the electrical machine, thereby improving the power density. In this paper, a hydrostatic radial piston pump has been evaluated for integration with an axial flux PM machine. The proposed hydrostatic piston pump uses a spherical head piston that can tilt while reciprocating inside the cylinder, eliminating the need for a joint at the slipper. To reduce the frictional loss between the slipper pad and the cam at high operating speeds, the cam freely rotates. A detailed model of the pump, with focus on the hydrostatic piston slipper, has been developed and a grid search approach has been utilized to select the critical parameters of the pump. Finally, an efficiency map has been presented for this pump at different operating conditions which shows around 86% efficiency at the 12500 rpm speed for 7 MPa pressure differentials.