The principle of electronic commutation makes brushless DC motors suitable for deep-sea application by sealing the motor in an oil-filled housing. However, if the oil-filled actuator is not designed properly, it will malfunction in the deep sea in spite of its excellent performance on land. In this paper, oil viscosity variations with pressure and temperature are reviewed because both factors vary with the operating depth, and a practical approach to estimate the viscous torque is advanced based on a viscous drag model of the selected motor and the properties of the oil. An experimental rig that can simulate the deep sea environment was developed, by which a series of experiments to study the motor efficiency and dynamic performance in different conditions were conducted. The values of viscous torque obtained in the experiments agreed well with our estimation. The low efficiency in the 2°C experimental group confirmed the influence of temperature, while the dramatic difference in the dynamic performance of the motor when filled with different oils verified the importance of analyzing the properties of the oil and of making a deliberate selection.NomenclatureT0Normal temperatureP0Atmospheric pressureμDynamic viscosityμ0Dynamic viscosity at given temperature T0 and pressure P0ρDensityPPressureTTemperatureαPressure-viscosity coefficientηKinetic viscosityTaylor numberMvViscous drag momentMCViscous drag moment acting on cylinder flankCMMoment coefficientRiRotor diameterCAir-gap clearanceLRotor lengthBClearance between housing and end face of the motorωRotor angular velocityReReynolds numberkCoefficient relating to the motor geometryPeLInput electric powerPirIron power lossPmeMechanical powerPjJoule power lossRPhase resistanceUInput voltageIInput current
