34 research outputs found
Multi-phase Starter-Generator for 48 V Mild-Hybrid Powertrains
No full textTransportation 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
Thermal Conductivity Evaluation of Fractional-Slot Concentrated-Winding Machines
No full textThe use of fractional-slot concentrated windings (FSCW) in electrical machines allows more compact, efficient, and reliable design with respect to machines equipped with distributed windings. However, an electromagnetic design linked to a thermal analysis of the electrical machine is mandatory to achieve the desired performance, and to fulfill the requirements of efficiency and reliability. One of the most critical issues in thermal design of electrical machines is to assign fair values for the input parameters of the thermal simulation models, particularly those related to the stator winding insulation system. This paper deals with the assessment of the equivalent thermal conductivity of the insulation system of FSCW machines. For this purpose, three FSCW electrical machines for different applications were evaluated via an experimental method based on a dc thermal transient test. Whereas the investigated machines present different characteristics among themselves, different approaches were required to properly estimate the thermal conductivity
Experimental Validation in Operative Conditions of Winding Thermal Model for Short-time Transient
No full textThis paper presents the validation of a first-order winding thermal model in machine operative conditions. The proposed model can be used in motor control strategies for the winding temperature prediction during transient overload or viceversa, for the prediction of the maximum time duration of the overload maintaining the winding temperature within the limit imposed by the class of insulation. The thermal model has been validated using two different electrical machines. The first one is a 10 kW automotive starter-generator prototype for mini-hybrid powertrain equipped with distributed bar windings, while the second one is a 2.2 kW total enclosed fan cooled industrial induction motor equipped with conventional stranded wire windings. To both machines it is mainly required a short-duty transient operation in overload conditions. In particular, the automotive starter-generator must accomplish the engine cranking and torque assistance during the vehicle acceleration and braking, while the considered industrial induction machine has to operate in intermittent service in overload conditions for machine tool applications. As a consequence, an accurate stator winding temperature prediction is mandatory to fully exploit the machine performance. For both motors, the thermal model parameters have been evaluated by fast experimental approach and subsequently, the model has been validated during operative overload conditions
Thermal Parameters Evaluation of a Fractional-Slot Concentrated Winding Machine for Home Appliance Applications
No full textPrototyping experiences on 48V starter-alternators
No full textThis paper reports the design experience faced during the prototyping and testing stages of two multiphase induction starter-generators for 48 V mild-hybrid powertrains. The focus of the investigation is on the pole numbers and their influence on the electrical machine parameters and performance. According to the application and the usage profile in a real urban drive cycle, a 8 pole prototype has been initially designed. Since the designed magnetic circuit was also suitable for a 6 pole configuration, a second prototype with this pole count has been produced. In fact, the pole number reduction could lead incremental advantages with respect to efficiency and power factor as a result of the lower fundamental supply frequencies. The performance comparison in terms of torque, efficiency and power factor of the two prototypes have been investigated and validated both by means of simulations and experimental activities
Multi-scale testing of bituminous systems for additive evaluation
No full textThe Authors present the results obtained by using a system for the characterization of bituminous mixtures which was specifically devised to be employed for the evaluation of additives. The characterization system is based on the testing of multi-scale bituminous systems, progressively formed by an increasing number of components: binders, mastics, slurries and whole-mixtures. Specific tests which were performed at the various scale levels include rheological and empirical tests on binders, rheological and fatigue tests on mastics, volumetric and fatigue tests on slurries, volumetric, stiffness and fatigue tests on mixtures. The analysis system was applied to a specific case in order to evaluate the effectiveness of two additives: a silica reinforced vulcanised natural rubber and a patented granular product, basically constituted by ethylene-vinyl-acetate. Obtained results proved to be coherent at all scales and yielded information regarding the possible field performance of the mixtures containing the two additive
Stator Winding Thermal Models for Short-Time Thermal Transients: Definition and Validation
No full textIn this paper, four thermal models for the
stator-winding temperature prediction in short-time thermal
transient are presented and experimentally validated. The
lumped-parameter networks, based on physical representa-
tion of the stator components, are composed of multiple RC
cells in cascade. In particular, starting from a fourth-order
thermal network, the model complexity is progressively
reduced to a third-, second-, and then down to a first-order
system. The proposed models and their predicted temper-
ature evolutions are discussed in detail and validated by
means of a full experimental approach on an industrial 7.5-
kW induction motor. Finally, the behaviors of the proposed
thermal models are corroborated by some considerations
drawn by the analytical solution of the models
Winding Thermal Model for Short-Time Transient: Experimental Validation in Operative Conditions
No full textThis paper presents the validation of a first-order winding thermal model in machine operative conditions. The proposed model can be used in motor control strategies for the winding temperature prediction during transient overload, or vice versa, for the prediction of the maximum time duration of the overload maintaining the winding temperature within the limit imposed by the class of insulation. The thermal model has been validated using two different electrical machines. The first one is a 10-kW automotive starter-generator prototype for mini-hybrid powertrain equipped with distributed bar windings, while the second one is a 2.2 kW total enclosed fan cooled industrial induction motor equipped with conventional stranded wire windings. Depending on the application for both machines, a short-duty transient operation in overload conditions could be required. In particular, the automotive starter-generator must accomplish engine cranking and torque assistance during the vehicle acceleration and braking, while the industrial induction machine could operate in intermittent service in overload conditions when used in machine tool applications. As a consequence, an accurate stator winding temperature prediction is mandatory to fully exploit the machine performance. For both motors, the thermal model parameters have been evaluated by fast experimental approach, and, subsequently, the model has been validated during operative overload conditions
