R. Camilleri

Predicting the Temperature and Flow Distribution in a Direct Oil-Cooled Electrical Machine With Segmented Stator

This paper presents a computationally efficient thermo-fluid model to predict the temperature and flow distribution in an oil-cooled electrical machine with a segmented stator. The Yokeless and Segmented Armature axial flux machine was used as a case study in which a numerical model was set up and validated to within 6% of experimental results. The model was adapted to predict the temperature distribution of the segmented stator of a machine, identifying the hotspot temperatures and their location. Changes to the flow geometry on the stator temperature distribution were investigated. It was shown how by carefully controlling the flow distribution in the stator, the temperature distribution is improved and the hot spot temperature is reduced by 13 K. This benefits the machine by doubling the insulation lifetime or by increasing the current density by approximately 7%.

Prediction and Measurement of the Heat Transfer Coefficient in a Direct Oil-Cooled Electrical Machine With Segmented Stator

The heat transfer coefficient (HTC) is a critical parameter that is required for accurate thermal modeling of electrical machines. This is often achieved from empirical correlations of ideal geometries or computational fluid dynamics (CFD) simulations. This paper presents a novel technique using double-sided thin film heat flux gauges for measuring the HTC from a direct oil-cooled electrical machine with segmented stator. While thin film gauges are often used in transient measurements of the HTC on gas turbine components, their application to electrical machines has been largely unexplored. This is the topic of this paper. Due to the large viscosity of the coolant, the transient technique was found to be inadequate and a steady-state adaptation for oil-cooled machines was developed. This paper explores the challenges linked with this measurement technique when applied to oil-cooled machines, and develops new nondimensional correlations of the Nusselt number with Reynolds number. These correlations are applicable to machines with different geometries, flow, and coolant properties. The experimental results were compared to CFD simulations and existing pipe flow correlations. It is shown that these underpredict the HTC by approximately 60% at Re = 20. The discrepancy gradually decreases to around 10% at Re = 200.

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.