Assessment of fluid drag loss in a flooded rotor electro-hydrostatic actuator motor

For a permanent magnet (PM) motor used in an electro-hydrostatic actuation system, fluid drag loss in the air gap can be as high as 60% of motor internal losses and affects the motor efficiency; especially at low temperatures where the viscosity of the hydraulic fluid increases significantly. A PM motor has been designed and built to assess electromagnetic, fluid drag loss and dynamic performance. The design process utilised a theoretical equation for the fluid drag loss estimation which assumes a laminar flow. Assumption of the laminar flow has been validated by computational fluid dynamic analysis. A dummy motor was built and the fluid drag losses were measured for various speeds and temperatures. The test results show reasonable agreement with the theoretical calculation although the self-heating effect of the fluid made measurements at constant temperatures difficult.