Cooling of a synchronous electric motor using an axial air flow

Abstract

The current work investigates the rate of cooling of an axial air flow through the gap between the stator and the rotor of a four-pole synchronous electric motor. The axial air is passing though the rotor's four longitudinal slots. The stator is considered insulated. The effect of the axial air flow rate, the rotor rotational speed and the slot size on the average Nusselt number has been investigated numerically using ANSYS-CFX. These effects have been studied considering a motor radius ratio (RR) in the range of 0.65–0.85, an axial Reynolds number (Rea) between 1750 and 40,000 and a rotational Reynolds number (Rer) of 1750 up to 35,000.Numerical results have been validated using published experimental data. The maximum deviation was less than 15 %. The present numerical results showed that increasing the axial air flow rate always enhances the rate of cooling. The effect of the axial Reynolds number was found to be more dominant than that of the rotational Reynolds number. Increasing the rotor rotational speed (Rer) enhanced the rate of heat transfer up to a certain limit beyond which a revered effect was observed. Such limit was investigated and found to depend on the motor radius ratio and the relative magnitude of the axial and rotational Reynolds numbers represented by X = Rea/Rer. The enhancement due to increasing Rer was reversed at X = 0.33 and 0.12 when RR was at 0.85 and 0.75, respectively. A new correlation of the average Nusselt number as a function of the axial and rotational Reynolds number has been developed.