Investigation of an Integrated Evaporative Cooling Mechanism for an Outer-Rotor Permanent Magnet Machine

This paper investigates an integrated evaporative cooling mechanism for an outer-rotor, axial flux permanent magnet machine. The success of the cooling mechanism relies on two parts: an internal means to transports heat from the windings to the casing, and an efficient heat sink to reject the heat from the casing to ambient. The paper demonstrates how a heat sink on the casing dictates the size of the active components in the machine and influences the choice of the working fluid. An experimental investigation on a wick-assisted cooling mechanism was performed. This mechanism was found to be adequate for an evaporatively cooled electrical machine. The effects of the wick were compared with immersion cooling and it was found that the wick prevents a temperature overshoot and keeps the winding at the boiling temperature once a self-sustaining steady-state condition is reached. However, the fluid dynamics inside the machine move across different flow regimes as the motor is accelerated. This plays an important role in the design of the wicking geometry. The paper presents a general systems integration and shows how different limiting factors come into play during different operating points of the machine.