Thermal modeling of hollow conductors for direct cooling of electrical machines

A direct cooling design using hollow conductors with the coolant flowing inside can significantly improve the heat dissipation in an electrical machine. To predict the thermal performances of an electrical machine with such cooling configuration, this paper proposes a computationally efficient thermal model of hollow conductors with direct cooling features. The hollow conductor is modeled using four equivalent solid cuboidal elements with a three-dimensional thermal network and internal heat generation. The heat transfer coefficient between the coolant and conductors is determined by an empirical model considering fluid dynamics behaviors. Axial discretization is performed to take into account the nonuniform temperature distribution along the axial direction. Experimental validation is performed with a U-shaped hollow conductor test rig. Compared to computational fluid dynamics analysis, the proposed thermal model is much more computationally efficient, and thus can be incorporated into design optimization process and electrothermal simulations of the electrical machine over a driving cycle

Numerical Modelling and Analysis of a New Rotor Cooling Technique for Axial Flux Permanent Magnet Machines

An efficient thermal management is essential for an electrical machine because it determines its durability and performance; particularly the continuous power output. Without good thermal management, the operational temperature will exceed the machine’s temperature threshold limit, which may possibly lead to catastrophic failure. YASA Motors Ltd. specialise in the design and development of high efficiency electric motors specifically aimed at the automotive industry. However, the current Yokeless and Segmented Armature (YASA) machine has limited performance due to the sealed or confined design that limits the heat transfer on the rotors and the permanent magnets. Therefore, this thesis presents a new cooling technique for the YASA machine but which can also be adapted to any Axial Flux Permanent Magnet (AFPM) design in order to maximise its continuous performance for automotive and motorsports applications. The work begins with a detailed review on the issues of thermal challenges for electrical machines (i.e. efficiency, reliability and performance), the derivation of an AFPM machine and then the heat sources from which the electric machine losses are produced. Utilising the Computational Fluid Dynamics (CFD), the losses of a 50kW sealed YASA machine has been studied in order to understand the thermal characteristics and thermal distribution. The novel secondary cooling strategy of the rotor has been implemented by attaching several fan designs on the rotor including other design iteration to assess its cooling performance. The idea is to allow the fan to drive the coolant (air) in the machine and become a heat exchanger at the same time. At this stage, only a single side of the rotor has studied under secondary cooling design, while the other side remained sealed. In order to aid the design assessment, a novel dimensionless number named Cooling Performance Index (CPI) has been proposed. The CPI number helps in comparing the cooling performance, apart from the comparison in the flow and thermal characteristics of each design change. The dual rotor cooling technique for the YASA machine is subsequently presented, where the backward curve fan has been selected as the best option based on its higher CPI number. The air outlet of the non-drive-end rotor that has an attached fan, was channelled to the drive-end to cool the other side of the rotor. The CFD analysis prove that the dual rotor cooling technique is able to maintain the rotors and magnets temperature with an increase up to 300% (150kW) continuous power compared to the 50kW on the existing sealed machine. The work presented here is not limited to the YASA machine case; rather it can be extrapolated to any other disc-type AFPM machine

High Performance Cooling of Traction Brushless Machines

The work presented in this thesis covers several aspects of traction electric drive system design. Particular attention is given to the traction electrical machine with focus on the cooling solution, thermal modelling and testing. A 60 kW peak power traction machine is designed to achieve high power density and high efficiency thanks to direct oil cooling. The machine selected has a tooth coil winding, also defined as non-overlapping fractional slot concentrated winding. This winding concept is state of the art for many applications with high volumes and powers below 10 kW. Also, these have been proven successful in high power applications such as wind power generators. In this thesis, it is shown that this technology is promising also for traction machines and, with some suggested design solutions, can present certain unique advantages when it comes to manufacturing and cooling.The traction machine in this work is designed for a small two-seater electric vehicle but could as well be used in a parallel hybrid. The proposed solution has the advantage of having a simple winding design and of integrating the cooling within the stator slot and core. A prototype of the machine has been built and tested, showing that the machine can operate with current densities of up to 35 A/mm^2 for 30 seconds and 25 A/mm^2 continuously. This results in a net power density of the built prototype of 24 kW/l and a gross power density of 8 kW/l with a peak efficiency above 94%. It is shown that a version of the same design optimized for mass manufacturing has the potential of having a gross power density of 15.5 kW/l which would be comparable with the best in class traction machines found on the automotive industry. The cooling solution proposed is resulting in significantly lower winding temperature and an efficiency gain between 1.5% and 3.5% points, depending on the drivecycle, compared to an external jacket cooling, which is a common solution for traction motors

Experimental study of a switched reluctance motor stator tooth with slot and end winding cooling

This paper presents an experimental study of direct coil cooling applied to a stator tooth of a switched reluctance motor where a direct contact is realized between the winding and fluid. Experiments were performed with a setup consisting of one tooth of a SRM without rotor, but including stator iron and one preformed winding. Three configurations of the cooling method were investigated: slot cooling, end winding cooling and a combination of the two by pumping an Automatic Transmission Fluid (ATF) over the designated sides of the winding. The setup is equipped with 17 thermocouples integrated within the components to determine the temperatures. Three inlet temperatures (21, 33 and 44°C) and four flow rates (1.5, 2, 3.5 and 5 l/min) of the coolant were tested at four different heat losses in the winding (10, 30, 50 and 70W). The results show that the maximum temperature is always located in the centre of the winding and is the lowest for the combined cooling (73.0°C), followed by the slot cooling (79.7°C) and then by the end winding cooling (91.6°C) for the lowest ATF inlet temperature and the highest heat losses and flow rate. With a determined current density in the range 13.8A/mm² to 19.5 A/mm², all three direct coil cooling methods show a great potential in increasing the power density of electric motors