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Visible in this cross section of a water cooled motor are the hollow motor shaft, water jacket, oil-mist system bearing thermocouples and bearing heat shield that differentiate these devices from air-cooled motors. The heat shields are pressed on the motor shaft to divert rotor-cage heat from the bearings. Heat flows through the shaft, heat shield, air gap, and end bells to the cooling water.

The motor shaft is designed with the high thermal resistivity and usually is hollow to further minimize the heat transmitted to the bearings. Generally comprised of heat-treated stainless steel for strength, it is formed by two segments attached by a coupling and welded together.

The thermal design of a water-cooled motor is interesting because cooling by convection and radiation is negligible. Thermal-circuit calculations ignore them. Heat generated from motor losses raises the temperature of the rotor, air gap, stator teeth, stator winding, stator end coils, stator slots, stator yoke, as well as the housing and water.

Heat flows to the water through four main paths. The first is from the stator copper loss through the end coils of the winding, slots, slot insulation, stator teeth, and yoke, to the housing and water. The second is from the rotor copper loss through the air gap, stator teeth, and yoke, to the housing and water. Third, part of the rotor copper loss passes through the shaft, bearings, and bearing heat shield through the front-end bell to the water. Finally, some rotor copper loss passes through the shaft, bearings, and bearing heat shield through the rear end bell to the water.

A comparison of frameless water-cooled and air-cooled motors that have the same size and weight illustrates the dramatic difference in horsepower ratings for these devices.

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