A high demand for electrification of the transport sector has resulted in a need to provide compact and reliable electric power trains where the electric machine is a key element. The cooling system has a strong impact on the specific torque and power capability of the machine. The selection of the cooling solution should not be based on the steady-state cooling performance only but also on the transient characteristics owing to the typical operational load nature of the propulsion system, where the load is not continuous (as it usually is in industrial applications), but it frequently varies in time depending on the vehicle type and the driving style. This article investigates in detail the cooling capabilities of a direct winding liquid cooling solution and compares it with a traditional frame liquid cooling arrangement in the transient condition. As a result of the comparison, it was found that the location of heat removal close to the heat source makes the temperature transient time faster, which allows to apply overload operating points more frequently.

Direct liquid cooling (DLC) of electrical machine windings is a clearly effectual cooling method for high-torque and high-power-density applications needed, especially, in heavy vehicles. Usually, in electric vehicles, the stator copper winding losses are the most dominating losses during the accelerating or decelerating (recuperation) modes, and therefore, traditional windings may heat up significantly during high-torque periods. Removing the heat directly from the winding in which most of the heat is generated, allows the most straightforward and effective way of cooling. This paper investigates the feasibility of the direct liquid cooling in a radial-flux permanent-magnet traction motor. This case provides experimental information of the behavior of the direct liquid cooling of the windings. The prototype motor equipped with directly liquid-cooled windings is installed in an electric bus so that typical bus load cycles can be tested.

In this paper, a direct liquid cooling method is proposed for a radial-flux permanent-magnet motor. To demonstrate the feasibility of the cooling method, a test motor with a rated output of 205 kW was designed, constructed, and tested in an actual vehicle application, an electric city bus. The energy consumption tests were conducted by applying a heavy-duty chassis dynamometer capable of simulating the inertia, weight, and road loads that the buses are subjected to in the normal on-road operation. The electricity consumption on the real bus route of Espoo line 11 in Finland was 0.61 kWh/km. Test results of the cooling solution show that the motor is capable of meeting the most challenging requirements of the load cycle even with a full payload. The highest winding temperature rise in the test driving cycles was only 26 °C, which proves the effectiveness of direct-liquid-cooled coils in a vehicle motor.

High-speed electric machines aim for compact, direct-driven elevated speed applications and highly efficient operation, especially when a gearbox can be avoided. The design of these types of machines is highly iterative, combining multiphysics optimization and leading to custom types of machines that fulfill the application-specific requirements. The Axially Laminated Synchronous Reluctance Machine (ALASynRM) with a solid rotor is one of the motor types that can be considered for high-speed applications. An axially laminated solid rotor structure combines magnetic and nonmagnetic layers rigidly bonded to each other by vacuum brazing, hot isostatic pressing, soldering, explosion welding, or even additive manufacturing. In this study, six nonmagnetic materials and nine magnetic materials are cross-compared. The results show clear differences in performance, efficiency, and physical properties of the rotor when made of different material combinations and can thereby suggest the best pairs when the application-specific performance criteria are known. The study is carried out on a 12 kW machine with a maximum speed of 24000 rpm.

High-torque-density electrical machines applied to traction operate at a varying torque and speed, and therefore, when minimizing the torque ripple, a wide variety of possible loads must be considered. Different loads need different current vectors, which affect the magnetic state of the machine. Thus, the torque ripple must be analyzed by using appropriate current vectors as it is proposed and described in this paper. The torque ripple values were estimated also for a skewed machine using a new method of torque ripple estimation of a skewed machine. The method is computationally less heavy and allows computing the torque ripple at all possible load points. Using the proposed method in the analysis of the machine parameters, the rotor pole geometry was adjusted to minimize the torque ripple without degradation of the other properties of the machine. Finally, the best machine performance was compared with the measured results.