Multi-phase Starter-Generator for 48 V Mild-Hybrid Powertrains

Transportation electrification has experienced a significant growth in recent years, and the electrification of the powertrain – namely hybridization – is considered the most viable solution seen by car manufacturers to achieve the challenging emission targets. Among the hybrid electrical powertrain topologies, the mild-hybrid configuration with the 48 V battery system offers the best ratio cost versus CO2 improvements. In particular, the 48 V technology does not require electrical shock protection whilst allows to leverage a variety of fuel saving functions such as electrical boost and regenerative braking. The thesis is focused on the electromagnetic and thermal design of a Belt-driven Starter Generator, BSG, for 48 V mild-hybrid powertrains. In the BSG layout, the starter-generator replaces the conventional alternator with a low impact on the engine compartment layout, even if a redesign of the belt tensioner is required. It is noteworthy to keep in mind that the electrical machine shall provide high starting torque and wide constant power speed range, both in motor and generator mode. Furthermore, the application imposes the adoption of low cost materials and the electrical machine is located in a harsh environment. As a consequence, the design is challenging from the electromagnetic, thermal and mechanical point of view. The novelties of the research lie in the 48 V automotive applications, by describing the practical difficulties to fulfill the design specifications through a suitable material selection, the identification of the cooling system and the available technological solutions. The first section of the thesis reports results from a literature review on electrical machine for mild-hybrid application aiming to highlight different criteria for the selection of the electrical machine. In this context the advantages in terms of fault tolerance and stator current splitting of multiphase drives are investigated. Furthermore, in this section the required performances and the constraints imposed by the specific application are analyzed. Among the different motor technologies, a dual three-phase induction machine having two stator winding sets shifted by 60 electrical degrees is selected as a suitable candidate. The second part of the thesis reports electromagnetic and mechanical issues addressed during the design stage, with special focus on stator winding layout, pole number and rotor slot. The adopted six-phase machine uses a four-layer bar stator winding that has been demonstrated as a good solution to improve the slot fill factor and thermal behavior. In addition, the thesis reports a comparison supported by experimental tests between open and closed rotor slots solutions; the focus is to maximize the machine electromagnetic performance according to the mechanical limits imposed by the rotating speed. Finally, predicted and measured performance of the prototypes are reported and discussed for validation purposes. The third part of the thesis deals with the thermal assessment of the BSG with particular emphasis on accurate winding temperature prediction as well as the cooling system selection. Since the stator-winding region is very sensitive to thermal issues and is usually attributed as being the main heat source within the machine body, its thermal modeling is of major importance. In these regards, a simplified stator winding thermal model was developed for the temperature prediction during transient condition. Moreover, considering that the driving cycle is characterized by time variable loss distribution, an effective cooling system must be mandatorily adopted together with high temperature class insulation material. In this context, the development of heat extraction through forced convection is experimentally investigated on the BSG prototype. As a main outcome of this research activity, it has been demonstrated the feasibility of the proposed design solution with respect to electromagnetic and thermal requirements.