Design Of A High-Speed Radial Hydrostatic Piston Pump For Integration With An Electric Motor

Abstract

Off-highway mobile machinery requires high force/torque and power density operations, for which hydraulic power transmission plays a critical role. The conventional hydraulic power transmission uses an engine as the prime mover to run a pump and throttle-based flow control valves to control the motion of the actuators. The throttle-based control and inability to regenerate energy results in poor efficiency, and the prime mover contributes to greenhouse emissions. The efficiency can be improved by electrifying the drive train, where an electric motor can drive a pump, and the flow control is directly achieved by controlling the speed of the motor. However, the issues with the electrified system are slow dynamic response due to the high inertia of the separate electric motor and hydraulic pump, frictional losses at the shaft seal and multiple bearings, pump leakage, undesirable noise and vibration, larger space requirements, limited power density of the electric motor and pump, and bulky auxiliary cooling needed for the electric motor. This work proposes a radial hydrostatic piston pump for direct integration with an axial flux electric motor developed at the University of Wisconsin-Madison to solve these challenges. Direct integration of the electric motor and pump inside a single casing requires less space and improves efficiency by eliminating redundant bearings, couplings, and seals. The integrated machine is designed to run at high speeds (~ 12,500 rpm at 7MPa pressure differential) to achieve a power density of 5kW/kg. In addition, the pump uses hydrostatic piston-slippers that contact a freely rotational cam ring to minimize the viscous energy losses at high speeds. The circulating fluid in the oil-filled casing is used to directly cool the electric rotor and stator, enhancing heat transfer over conventional water jacket cooling. This work demonstrates machine design studies followed by a mathematical model-driven design of the radial hydrostatic piston pump to build the actual pump prototype, which is then built and tested at the University of Minnesota- Twin Cities- TWIN CITIES facilities. The experimental testing shows a 49% peak overall efficiency of the pump prototype at a 2723 rpm speed with a pressure differential of 1.5MPa, which is promising as a first prototype. There is still room for design improvements to hit the target pump efficiency of 90% at 12,500 rpm speed and 7MPa pressure differential, which is discussed in this work. The compact design of the pump also allows it to fit inside the hollow space of the stator, tightly integrating it with an axial flux electric motor. A detailed design of the integrated machine is also provided in this work. The building and testing of the axial flux electric motor are done at the University of Wisconsin-Madison WEMPEC facilities separately. From the standalone experimental testing of the high-speed radial hydrostatic pump and axial flux electric motor, the projected efficiency of the integrated machine is around 63%, with a power density of 5 kW/kg. In the actual integrated machine prototype, powering up the hydraulic unit generates three-phase voltage and current at the electrical unit, demonstrating the proof-of-concept integrated machine. A lumped parameter thermal model is also developed to predict the thermal cooling efficiency of the integrated machine being directly cooled by hydraulic oil. The thermal model is validated by further CFD and steady-state thermal FEA analysis. The simulation results show that the integrated machine can self-cool itself at the peak power (12,500 rpm speed with 7MPa pressure differential) operating conditions, keeping the temperatures of the components below 1800C and PM temperature below 800C.