Hybrid Excited Synchronous Machines

Abstract

I. IntroductionHybrid excited synchronous machines (HESM) combine permanent-magnet (PM) excitation with wound field excitation. The goal behind the principle of hybrid excitation is to combine the advantages of PM excited machines and wound field synchronous machines. The principle of hybrid excitation allows responding to many problematic related to electric machines: flux weakening [1], energy efficiency [1], and fluctuations of permanent magnets price [2].Due to their advantageous characteristics, this type of machines is gaining more and more attention [1]–[8]. They have been identified as one of the emerging technologies of modern energy conversion systems [3]–[5]. They have been the subject of many review papers [6]. Figure 1 shows the evolution of the number of contributions dedicated to HESM in the IEEE Xplore Digital Library, between 1989 and 2019. As can be seen this number is in constant evolution, going from one contribution in 1989 [6], to more than 350 contributions in 2019.In this contribution, after a description of the operating principal of HESM, a brief literature review of scientific and technical literature dedicated to HESM will be established. Then, the use of this type of electric machines for different applications (constant and variable speeds applications) will be discussed. The design and operation of three particular structures will be presented. Two of them have been designed as generators for transportation applications [7] [8], and the third one has been designed as generator for renewable energies conversion [2]. All of them are flux switching structures.II. Hybrid Excitation Synchronous MachinesThe hybrid excitation principle allows a wide variety of structures to be realized. Figure 2 shows three different structures of rotating HESM. All of them are flux switching hybrid excited synchronous machine (FSHESM). Due to their advantageous characteristics FSHESM are the subject of increasing attention. This figure illustrates the variety of HESM structures. More structures will be presented in the full version of the contribution.In most references dedicated to HESM, it is mainly rotating machines which are studied. Nevertheless, many linear HESM are also studied. Examples will be provided in the full version.III. Study of Three HESM PrototypesHESM characteristics make them well suited for transportation applications [1] [5] [7] [8]. Nevertheless, HESM could also be used as renewable energy conversion generators [2], even if, in these applications, it is not all of their advantageous characteristics which are exploited. In fact, it is mainly economic considerations which are driving the use of an electric machine technology [2].The use of HESM in renewable energies applications is mainly driven by fluctuations of rare earth PM price and their availability [2]. Another important reason is the fault tolerance characteristic which could be brought by HESM. Considering the evolution of wind energy market toward the use of large power rated offshore wind turbines, the fault tolerance characteristic is very important. In fact, the HESM allow controlling the air-gap magnetic field, even with the presence of permanent magnets, and they therefore offer a protection against faults as stator short circuits, for example. The fault tolerance allows the reduction of maintenance requirements, and consequently exploitation costs. The fault tolerance they bring increases their attractiveness in many applications.The design studies of three HESM prototypes (Fig. 2) will be presented. The first one [Fig. 2(a)] has been studied for an aircraft DC electric power generation [7]. This prototype weight 3.5 kg, and was designed to deliver 6 kW at 6000 rpm. It is a 2D flux switching structure. The second prototype can be used as a car alternator [Fig. 2(b)] [8]. Instead of having a distributed excitation windings, the excitation coil is annular without end windings. The excitation flux has a 3D nature, which requires the use of massive ferromagnetic material to channel it. This prototype was able to deliver 2.5 kW at a speed of 11 krpm. The third prototype [Fig. 2(c)] was designed to be used as a generator for renewable energy conversion (10 kW at 300 rpm). As compared to transportation applications, where the speed is relatively high, renewable energy conversion systems operate at relatively low speed, which requires the development of high torque machines. This prototype includes the Vernier effect in order to be able to cope with the torque production requirement. It is a water cooled machine. It has a 2D structure. The full version will include more details about the three prototypes.HESM machines may seem more complicated as compared to classical machines, as PM machines for example, but regarding the worldwide shift toward a greener economy, economic considerations will be in the favor of the development of even more complex electric machines technologies [5]. Indeed, considering the number of pieces required for the construction of even the simplest IC engine, the construction of even the much complex electric machines requires much more less operations, and simpler production lines. **