Page - 135 - in Hybrid Electric Vehicles

The design of a high-power density in-wheel motor is a complex optimization problem which will conclude to the most suitable candidates according to some criteria. There are several requirements that have to be met. Some of them are related to the motor’s placement and physi- cal constraints, such as its outer rotor radius and active length, whereas others are imposed by the motor’s desired operation. The efficiency, for example, is of great importance considering the energy consumption. Efficiency higher than 90% will be an appropriate choice. Despite that, since the motor is mounted inside the wheel, as depicted in Figure 5a, its weight must be as low as possible in order to reduce unsprung mass and eliminate vibrations. Recently in-wheel motors with power-mass ratio of approximately 1 kW/kg have been implemented in commercially available HEVs. In this study, it will be investigated if this value can be exceeded. Thus, the objective function chosen for the case study is a compromise of motor’s weight and power losses minimization. The desired SPMSMs characteristics are given in Table 2. Furthermore, there are more than 15 design variables that have to be optimized simultaneously (under certain constraints) by the applied algorithm. Apart from the geometrical parameters that are presented in Figure 5b, variables such as the number of poles (2p), the number of slots per pole per phase (q) and the number of conductors per slot (nc) are also involved. Table 3 sum- marizes the upper and lower bounds of all these quantities that will be considered as problem constraints. At this point, it must be mentioned that for sake of space, the analytical equations that describe the electromechanical and magnetic behaviour of the specific machine are not given here. The reader can refer to [18, 36] for more details. Concerning the materials used for different motor’s parts, a high quality silicon steel (M19-24G) has been selected both for stator and rotor, according to NEMA’s instructions for super premium efficiency motors. Moreover, high energy NdFeB magnets have been chosen, as they have been proven efficient and reliable Component Parameter Value Vehicle Mass (kg) 1200 Frontal area (m2) 2.16 Tire radius (m) 0.30 Total wheel inertia (kg m2) 0.10 Aerodynamic drag coefficient 0.26 Transmission inertia (kg m2) 0.50 Transmission friction coefficient 0.001 Engine to wheel gear ratio 1.30 In-wheel motor (×2) Rated power (kW) 15.3 Rated speed (rpm) 850 Rated torque (Nm) 170 Rated power (kW) 57 Internal combustion engine Maximum speed (rpm) 5000 Torque (Nm) @ 4200 rpm 115 Table 1. HEVs under study main components specifications. Design, Optimization and Modelling of High Power Density Direct-Drive Wheel Motor for Light Hybrid Electric Vehicles http://dx.doi.org/10.5772/intechopen.68455 135