If one debate over a specific design feature stands out amongst all the others in the submersible waste water pump field, it must be, “Which is better, air-filled or oil-filled motors”? With The introduction of premium efficient motors by several manufactures, this topic has come to the forefront once again. Each design has specific advantages. However, the advantages of an oil-filled design out-weigh those of an air-filled design.
The NEMA MG-1 Standard specifies that a submersible totally enclosed non-ventilated motor (TENV) with IP68 rating is cooled by its surrounding pumped media (NEMA MG1 5.2, 6.2.5 and 6.2.6). For the heat generated by the motor windings to get to the cooling medium, it must pass from the inner working parts of the motor to the outer shell. Both air-filled and oil filled motors rely on a great extent upon convection to transfer heat from the rotor core, motor windings and laminations to the outer shell. The heat convection process uses oil as the transfer media in an oil filled motor and air as the transfer media in an air-filled motor.
Since oil has from 5 to 7 times the thermal conductivity of air, it follows that oil filled motors pass internal thermal energy to the cooling medium much more efficiently than do air-filled motors.