Windings Indirect Liquid Cooling Method for a Compact Outer-Rotor PM Starter/Generator With Concentrated Windings

Due to the larger diameter of effective air-gap for torque production, outer-rotor permanent magnet (PM) machines exhibit the distinctive advantages, such as high torque density and large rotor inertia. This article presents a compact outer-rotor permanent magnet starter/generator (OR-PM S/G) to match diesel engine for extended-range hybrid electric vehicle. In order to solve the serious temperature rise problem resulting from the copper losses under constrained slot space in the inner stator, a new winding indirect liquid cooling technique by inserting heat pipes in the center of slots is proposed. It facilitates a path from the center of the coils to the cooling medium with a higher thermal conductivity, thus increasing the tolerable current density of coils. By optimizing the geometry and dimensions of heat pipe and coil, the temperature rise in slots are achieved within an acceptable limit according to the computed results of FEA and computational fluid dynamics (CFD). In order to validate the cooling design, a 30kW OR-PM S/G prototype with rectangular-shaped PM and the proposed indirect liquid cooling windings is manufactured and experimented. The power capability and efficiency of the proposed OR-PMS/G are validated by the measured results with calculation results. A good agreement between the predicted and measured temperatures indicates the validity of indirect cooling method for concentrated winding with heat pipe.