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A modular motor drive (MMD) is composed of multiple identical pole drive units (PDUs), which consist of a concentrated stator winding fed by a dedicated power electronic converter module. To exploit the full flexibility and reliability of these MMDs, they are equipped with multiple controllers, operating as synchronized peers, and each driving only a limited set of PDUs. An MMD can hence be considered as a multiagent system, in which all the agents cooperate to deliver a certain torque demand. The goal of this work is to distribute this torque demand among the agents of a modular drive in a completely decentralized way, in order to comply with the flexibility and reliability of the hardware, while still taking into account the present condition of each agent. A deterministic consensus algorithm is used for this purpose, in combination with decentralized proportional integral (PI) current controllers. Communication is only allowed between neighboring agents, to limit the communication load. It is shown in this article that the total torque demand is delivered, under healthy conditions as well as during agent malfunctions and with nonidentical agents. The concept is demonstrated on a modular axial-flux permanent magnet synchronous machine with 15 PDUs that are assigned to five agents. [ABSTRACT FROM AUTHOR]

The electric drive power density and fault tolerance capability are of fundamental importance in many applications such as aerospace and traction applications. The modularization and the physical integration of the electric motor and the power converter components can lead to a high power density and a high fault tolerance drive system. The power density of a highly modular and integrated drive based on an axial flux permanent magnet synchronous machine and GaN converter is investigated in this paper. Several power density boosting techniques are provided and investigated using CFD simulations. These techniques incorporate optimization of the converter topology, the geometry of the shared cooling structure of the electric machine and the power converter and optimal selection of the materials in the path of heat transfer from the machine and the power converter to the cooling ambient. The power density of the reference integrated design is increased from 1.12 kW/ $l$ to 2.14 kW/ $l$. The CFD computations are validated by extensive measurements on a modular integrated setup. [ABSTRACT FROM AUTHOR]

In this article, a circulating-current-excited switched reluctance generator system using a standard diode rectifier is proposed and examined. It features a dc power source connected in series with the ring-connected windings, which provides the dc-bias current as the excitation current in the windings. Using a series-connected dc power source, a dc-bias current is easily realized in the ac winding. Compared with the existing machine having dc-bias in the ac winding, the proposed generator can provide dc power with a standard diode rectifier. Less active legs and no rotor position sensor are needed for the control of the circulating current of the proposed generator, which increases the reliability of the generation system. The system configuration and the operation principle are introduced in detail. Then, the electromagnetic performance of the proposed generation system is analyzed at both open-circuit and load conditions. The asymmetric current distributions are deduced and verified by the simulations. At last, the prototype is tested under varied operation conditions and the experiments agree well with the analysis. [ABSTRACT FROM AUTHOR]

Multiphase machines with a prime number of phases (five and seven) have shown better torque density than machines with other phase numbers. Unfortunately, these machines require a special stator design. Hence, this article proposes general rewinding techniques to obtain a five-phase winding from the existing standard three-phase stator frames. The proposed rewinding techniques are applied to construct a five-phase star-connected synchronous reluctance machine (SynRM 1) and a novel combined star-pentagon winding (SynRM 2). Simple mathematical formulations are introduced to compute the equivalent winding factor. The performance of five-phase SynRMs is analyzed using two-dimensional Ansys Maxwell transient simulations under the same copper volume for both healthy and faulty cases. SynRM 1 and 2 provide 6.56% and 13.35% higher torque compared to the three-phase SynRM at rated current and at optimal current angle. The torque ripple is decreased by 17% and 30%, respectively. Moreover, SynRM 1 and 2 offer a better performance at the faulty case; e.g., at one phase opened, the average torque of SynRM 1 and 2 is 82% and 108% higher compared to the three-phase SynRM. In addition, the torque ripple is reduced by about 63% and 81%. Finally, experimental results are done to validate the proposed winding techniques. [ABSTRACT FROM AUTHOR]

In this paper, a circulating-current-excited switched reluctance generator system with the diode rectifier is comparatively studied with a field-current-separated switched reluctance generator. By using a DC power source connected in series with the winding to provide the circulating current, a controllable excitation field is established in the proposed generator. To highlight the influence of this DC voltage source, the injected field current of the field-current-separated generator is kept the same as the circulating current of the circulating-current-excited generator. Then, both advantages and disadvantages of the generators are summarized. The circulating-current-excited switched reluctance generator system inherits both the advantages of good reliability of the separated-winding generator by using the diode rectifier and low copper loss of the integrated-winding machine by combining the AC and DC current in one set of windings. Furthermore, two more practically conventional field-current-separated generators are also compared with the proposed generator in terms of the external characteristics. These two conventional generators have different winding configurations. At last, the analysis is verified by experiments. [ABSTRACT FROM AUTHOR]

This article compares the performance of an existing three-phase synchronous reluctance machine (SynRM) with an identical machine that is upgraded to a five-phase machine. Two different methods are possible to convert the existing three-phase machine to a five-phase one with minimum cost. In the first method, a five-phase SynRM (SynRM 1) is obtained by replacing the existing stator by another one that has an integer number of slots per pole, i.e., a multiple of five. In the second method, a five-phase SynRM (SynRM 2) is obtained by rewinding the existing stator with a special type of winding. This is because the slot number of the existing stators is not a multiple of five. The novelty of the article is that two five-phase SynRMs are compared with the original three phase reference machine in terms of torque and torque ripple, in healthy and fault conditions. At rated condition and optimal current angle, SynRM 1 performs better than SynRM 2 in terms of rated torque: SynRM 1 and 2 have 11.8% and 6.6% higher torque than the three-phase machine. But, SynRM 1 has 25% more torque ripple than the three-phase reference machine, while SynRM 2 has 17% lower torque ripple. At faulty case, SynRM 1 and 2 work at 74% and 79% of the healthy rated torque of the three-phase SynRM, while the three-phase SynRM works at only 43% with a very huge torque ripple. Finally, experimental measurements are obtained to validate the theoretical work. [ABSTRACT FROM AUTHOR]

In this article, a circumscribing polygon integrated modular motor drive topology with shared cooling for the power converter and the electrical machine is proposed and benchmarked with the nonintegrated version. The proposed topology is applied on an axial flux permanent magnet synchronous machine. The topology is capable of mechanically mounting the power converter, sufficiently cooling both the power converter and the electrical machine, and combining both of them into the same housing. The mechanical and the thermal design are done to ensure thermal decoupling between the power converter and the motor winding, and to provide a low thermal resistance from the power converter and the electrical machine to the ambient. Due to the limited space available for the power converter module, wide bandgap semiconductor technology is chosen for the implementation of the converter module, thanks to their small package size and low power losses. A highly modular, integrated, and compact drive is achieved compared to the nonintegrated one. A computational fluid dynamics (CFD) model is developed for one module of the proposed integrated drive to evaluate the maximum current that can be injected by one converter module without exceeding the junction temperature limit of the switches and the maximum winding temperature. An experimental setup is built to validate the results of the introduced multiphysics models. [ABSTRACT FROM AUTHOR]

Due to the significant advantages of cost savings, low temperature rises and high efficiency, synchronous reluctance machines (SynRMs) have attracted growing interest from academia to industrial applications. Numerous articles have been published to obtain a SynRM with improved performance (i.e., a high torque density and efficiency and a lower torque ripple) using different focusing points at healthy and faulty conditions such as rotor design, magnetic steel grade, stator winding, and fault tolerance control (FTC) strategy. Hence, this article reviews the scientific researches about SynRMs aiming to improve their performance. These scientific researches can be categorized into four sections: 1) optimal rotor design; 2) magnetic steel grade; 3) winding configurations; and 4) FTC strategies. [ABSTRACT FROM AUTHOR]

This paper proposes a method to design an electrical variable transmission (EVT) of which the stator and inner rotor contain a distributed three-phase winding, and the outer rotor is equipped with permanent magnets and a dc-field winding. The main problem with the design of EVTs is that the modeling relies on time-consuming finite-element (FE) calculations. Studying the effect of design changes is, therefore, a cumbersome task. To minimize the need for FE calculations, scaling laws are applied, which reduce the computational effort to a matter of milliseconds per design. Therefore, the losses of multiple scaled designs can be analyzed for a given load cycle while taking constraints into account such as maximum torque. By presenting this data in a performance map, the optimal design regarding the losses is easily deduced. To validate the presented methodology, FE calculations, measurements, and available literature are used. [ABSTRACT FROM AUTHOR]

Increasing performance requirements for electrical machines such as energy efficiency and torque density require a multiphysics design approach. In these designs, many machine parts very often combine multiple functions, such as electromagnetic, thermal, and mechanical, to keep the design compact and reduce the overall weight, or introduce nonconventional materials in electrical machines. In this paper, epoxy potting techniques are used to keep the different stator parts of a yokeless and segmented armature axial flux PM machine together, and it focuses mainly on the multiphysics aspects of the epoxy resins as well as on their implications on the overall stator design. In this multiphysics design approach, the effect of specific epoxy resin parameters such as maximum service temperature, thermal conductivity, Brookfield viscosity, tensile strength, and coefficient of thermal expansion, on the electromagnetic, thermal and mechanical behavior of the stator are examined. Epoxy resin parameters are evaluated in detailed experiments on small test samples, and finally, measurements on a full prototype 4 kW, 2500 rpm stator are performed. [ABSTRACT FROM AUTHOR]

Despite the use of state-of-the-art thermal modeling tools in the design stage, the measured thermal behavior of prototype electrical machines can differ significantly from the modeled ones. This paper shows how a thermal model, based on the finite-element method, of an electric machine can be improved using inverse modeling techniques. In a thorough study, a forward high fidelity finite-element thermal model of a 4-kW axial flux permanent magnet (PM) machine is introduced and improved using inverse modeling techniques via noncollocated thermal sensors. Parametric model order reduction of the high fidelity finite-element thermal model based on the moment matching method is performed to make the recovery of the actual thermal parameters characterizing the thermal behavior of the axial flux PM machine tractable. Furthermore, the same reduced order model is used to identify the different power loss components in the machine. Experimental results confirm that the presented two-stage approach is capable of identifying the thermal parameters and losses with high accuracy. [ABSTRACT FROM AUTHOR]

This paper investigates the performance of synchronous reluctance motors when the stator is equipped with a combined star–delta winding layout. The conventional star winding is used as a benchmark in this study to compare different possible single-layer winding layouts. Among these different winding layouts, those which maximize the fundamental magneto-motive force component are selected. A simple mathematical formula is then derived to calculate the equivalent winding factor for different shares between star and delta subwindings. It has been proved that for the same copper volume and line current magnitude, the star–delta connection can offer an enhancement in the torque density of approximately 5.2% over the conventional star case under rated conditions. However, this gain is affected by the employed number of poles and stator slots. On the other hand, the effect of the winding layout on either power factor or core loss can be merely neglected over a wide range of speeds and currents. Nevertheless, the machine efficiency under a combined star–delta connection is relatively improved under light loading as a result to the machine torque/current ratio enhancement. The theoretical findings are experimentally validated using two identical 5.5 kW prototype machines, having star and combined star–delta winding connections. [ABSTRACT FROM AUTHOR]

This paper describes how an electrical variable transmission (EVT) can be used as an alternative for mechanical continuously variable transmission (CVT) systems. An EVT is an electromagnetic power split device having two mechanical and two electrical ports. The speed ratio between both mechanical ports (rotors) can be varied in a continuously variable way, so that this machine can be used as an electromagnetic alternative for the mechanical push-belt and half-toroidal CVT. Its operating principle is based on splitting the power into an electromagnetic direct path and an electrical path involving two inverters connected back to back. Its potential use as a CVT is investigated by comparing experimental data against toroidal and belt CVT data from literature having comparable power ratings. To this end, measurements are performed on a prototype \text120-kW permanent magnet EVT. [ABSTRACT FROM AUTHOR]

An electrical variable transmission (EVT) is an electromagnetic power-split device with two mechanical and two electrical ports. It can be used in hybrid electric vehicles to split the power to the wheels in a part coming from the combustion engine and a part exchanged with the battery. Although crucial for the EVT design and evaluation, no papers are found to give a detailed overview of the different loss components in such a machine and how they can be calculated and measured. In contrast to conventional electrical machines, this machine has more degrees of freedom, which can be exploited to measure the different loss contributions separately. This paper proposes a methodology to identify and measure the different loss components in this kind of machines. The proposed method is able to identify the iron losses in stator and inner rotor, the copper losses, bearing losses, and slip ring friction losses separately. To this end, measurements of both torque and speed sensors in different operating points are combined. The methods are applied to identify the different loss contributions in a prototype permanent magnet assisted EVT, both in no-load operation as under load where its functionality as power-split device is evaluated. [ABSTRACT FROM PUBLISHER]

An electrical variable transmission (EVT) is an electrical machine consisting of a stator and two concentric rotors. It is a competitive alternative for power split devices used in current series–parallel hybrid electric vehicles (HEVs). Although tending to have high power densities, permanent magnet (PM) machines suffer from no-load iron losses. Since the torque interaction between stator and outer rotor is low during large time intervals, the EVT in this paper possesses a flux bridge with dc-field winding to modulate the stator field, while maintaining the inner rotor flux linkage. In this paper, a torque controller is proposed that optimally combines the stator, dc-field, and inner rotor currents in order to minimize the copper and iron losses in every operating point of torque and speed on both rotors. Also the electromagnetic coupling between both rotors is considered. It turns out that there are two optimal load angles for the stator currents, of which one of the two is dominant depending on speed and torque. The control is tested on a 120 kW prototype EVT, and measurement results are given to illustrate the optimal performance obtained with the proposed torque control. In addition, efficiencies are measured in order to support the statements made concerning the proposed method. [ABSTRACT FROM PUBLISHER]

The use of power electronics has led to a growing importance of higher time-harmonic content in electrical machines. To gain insight in phenomena related to these higher harmonics, such as noise and losses, a good understanding of the magnetic field’s harmonic content is mandatory. Moreover, the development of fast and accurate, harmonic-based, analytical models requires a qualitative knowledge of the machine’s time- and spatial-harmonic content. Although the harmonic content of electric machines is an extensively studied topic, previous publications tended to focus on one type of synchronous machines and often did not consider higher time-harmonic orders. This paper complements the existing theory by providing a more general approach, thereby covering machines and operating points that were not covered until now. It considers both three-phase and multi-phase machines with an odd number of phases. The winding distribution can either have an integer or a fractional number of slots per pole and per phase, and higher time-harmonic content is regarded as well. Note that the saturation is neglected. Despite its general validity, this paper succeeds at providing one simple equation to determine the machine’s time-and spatial-harmonic content. Moreover, this paper also extensively discusses the physical causes of the harmonic content. The combination of this general validity, the simple result, and the insight in the physics makes that this paper is a strong tool to both study harmonic-related phenomena in electric machines and to develop harmonic-based analytical models. [ABSTRACT FROM AUTHOR]

In this paper, a complete sensitivity analysis of the optimal parameters for the axial flux permanent magnet synchronous machines working in the field weakening region is implemented. Thanks to the presence of a parameterized accurate analytical model, it is possible to obtain all the required parameters of the machine. The two goals of the ideal design are to maximize the power density: P\text{density} and the ratio of maximal to rated speed: n\max/n_r, which is an inductance related parameter keeping the efficiency at the target speed above 90$\%$. Different slots/poles/phases combinations are studied to reveal the optimum combination for each phase. This paper has studied the effect of the ratio of number of stator slots to number of rotor poles on the P\text{density} and n\max/n_r. It is shown that a low value of this parameter results in a better P\text{density} and n\max/n_r. The effect of the outer diameter, and the inner to outer diameter ratio are studied with respect to the two design goals. In addition, a comparison between the finite and the theoretical infinite speed designs is implemented. A complete 3D finite element validation has proven the robustness of the analytical model. [ABSTRACT FROM AUTHOR]

An electrical variable transmission (EVT) can be used as a power splitting device in hybrid electrical vehicles. The EVT analyzed in this paper is a rotating field electrical machine having two concentric rotors. On the outer rotor, permanent magnets (PMs) are combined with a dc-field winding, being the first implementation of its kind. The magnetic field in the machine as well as the electromagnetic torque on both rotors are a function of the q- and d-axis currents of the stator and inner rotor, as well as the dc-field current. To describe and fully understand this multiple-input multiple-output machine, this paper gives an overview of the influence of the different current inputs on the flux linkage and torque on both rotors. Focus is given to the hybrid excitation in the d-axis by combining the dc-field current and the alternating currents. This has the advantage compared to other EVT topologies that unwanted stator torque can be avoided without stator d-axis current flux weakening. Results of the analysis are presented by means of the torque to current characteristics of a double rotor PM-assisted EVT, as well as the torque to current ratios. The machine characteristics are finally experimentally verified on a prototype machine. [ABSTRACT FROM PUBLISHER]

This paper analyzes and compares the synchronous reluctance motor (SynRM) performance for a conventional star winding and for a combined star-delta connected winding under healthy and faulty conditions. The finite element method is used to simulate the SynRM characteristics in case of normal star and in case of combined star-delta connected windings. For the healthy case, it was found that for the same current, the SynRM output power increased by about 5% for the star-delta connected windings compared with the conventional star connection. Moreover, the efficiency increased slightly by 0.20%. Probably the strongest advantage of the combined star-delta winding is observed when a phase fault occurs. When one phase is missing, for the same current, the star-delta connected motor keeps 80% of the rated torque in the healthy case, while the star connected motor only keeps 40%. Furthermore, the torque ripple of the star wound machine is extremely high in case of a missing phase, compared to the star-delta wound machine. Simulation results were validated by experimental results to show the effectiveness of the star-delta connected windings on the SynRM. [ABSTRACT FROM PUBLISHER]

This paper deals with iron losses caused by fringing fluxes generated by the concentrated stator windings of an axial-flux permanent-magnet synchronous machine. Fringing fluxes enter the laminated stator stack perpendicularly to the plane of the laminated silicon steel sheets. They cause additional eddy currents and losses in the sheets. Three approaches for reducing these losses are compared experimentally with a dedicated setup. The setup has no moving parts and makes accurate iron loss measurement possible. The first method directly restricts the eddy-current losses by segmenting the lamination surface. The second method deflects the fringing flux by using soft magnetic composite. The third method magnetically short circuits the fringing flux using ferromagnetic wires. The first method gives the best results, with a reduction of 6% of the total iron loss at an induction level of 0.6 T and a frequency of 200 Hz. [ABSTRACT FROM PUBLISHER]

In this paper, a combined wye-delta connection is introduced and compared with a conventional wye-connection of a concentrated winding. Because the combined wye-delta connection has a higher fundamental winding factor, the output torque is higher for the same current density when a sinusoidal current is imposed. As the combined wye-delta connection has only a minor influence on the losses in the machine, the efficiency of the machine is also increased. The combined wye-delta connection is illustrated in detail for an axial-flux permanent-magnet synchronous machine with a rated power of 4 kW at a fixed speed of 2500 r/min, using finite element computation and measurements on a prototype machine. [ABSTRACT FROM PUBLISHER]

In this paper, the efficiency optimization of an axial flux permanent-magnet synchronous generator with concentrated pole windings is examined for a 3.6 kW/2000 rpm combined heat and power application. Because the efficiency of the machine is important, specific measures are taken in order to reduce losses in the machine: thin laminated grain oriented material in the teeth, concentrated pole windings, and segmented magnets. A study of the influence of a limited set of geometry parameters on the efficiency of this type of machine is done, using both analytical and finite-element methods. In the analytical as well as in the finite-element model, the inherent 3-D geometry of the axial flux machine is approximated by multiple 2-D models at different radii in circumferential direction. Afterwards, the influence of mass on the optimal values of the geometry parameters and the efficiency is considered, and it is found that mass can be seriously decreased with only a small reduction in efficiency. Finally, the results of both methods are compared with measurements on a prototype to evaluate their validity. [ABSTRACT FROM AUTHOR]