[1] Dr. Ing. Herbert Rentzsch, Elektromotoren – Electric Motors, ABB

1. Table 1: comparison table AN INNOVATIVE TYPE OF SYNCHRONOUS GENERATOR WITHOUT RARE EARTH MATERIALS AND WITH HIGH EFFICIENCY Diego Artioli, Engineering R&D ManagerSicme Motori SRL PO. ID xxxx Abstract Advantages of the Synchronous Reluctance solution Since 2002 Sicme Motori started the project SICMEWIND for the development and construction of synchronous PM generators for wind applications at low and medium speed, up to 3 MW rated power. The cost of rare earth materials dramatically increased since last spring, Sicme Motori faced a challenge to find a cost-effective solution in order to continue proposing synchronous machines for wind turbines. Synchronous reluctance machines can be used as generator, even though they are mainly known for motoring applications. This is a fact which doesn’t require to be proven. The originality of Sicme Motori’s offer is in a new type of assisted synchronous reluctance machine (ASR) for generator applications and without rare earth materials. Sicme Motori, after acquiring a patent license for the design and production of SR machines, continued to develop these concepts, specifically investigating and prototyping the possibility to have Ferrite assisted machines, so getting rid of rare earth materials, whose cost suddenly increased last year and is always affected by finance speculating operations and, furthermore, facing the difficult task to design SR machines for multi-polar solutions, such having a product suitable for medium speed operation (hybrid machines and mini-wind applications) and currently investigating the further development to low speed applications (direct driven multi-MW generators). SR is combining the advantages of permanent magnets and induction machines, having both a “cold” rotor (thus meaning the rotor has no fundamental losses, joule or iron components) and the field regulation, which allows a certain range of regulation of the voltage, which is no more “fixed” by the permanent magnets’ flux. It has the robustness of an induction motor and the size, efficiency, and synchronous speed operation benefits of permanent magnet motor technology. From the user’s point view, SR can be seen as a synchronous machine with magnetizing current. This means that its operative potential and flexibility can be compared to the performance of an induction motor, but without rotor cage, therefore with a rotor which is without fundamental losses (which typically are about 30-40 % of the total losses). FASR Ferrite Assisted Synchronous Reluctance generators Assisted SR can be realized starting from a properly designed SR machine and installing permanent magnets in order to improve the power factor and the performance; in more technical words, the PM flux compensates the magnetizing current effect and moreover gives an additional torque contribution. This is the ASR machine (Assisted, with permanent magnets, Synchronous Reluctance), or simply ASR, which has the following features: - No rotor windings, thus no rotor Joule losses, resulting in HIGH EFFICIENCY - Very high torque density (almost the same as brushless motors) - Good range of constant power speed regulation - Low torque ripple, if properly designed (patented design) - Very low inertia (very light rotor) - Smaller dimensions compared with an induction machine Permanent magnets could obviously be rare earth magnets, but Sicme Motori’s train of thought was ‘can we use this solution with different materials?’ ‘Can we find a way to improve the design of SR machines so that we have a performance which is really close to the one of the PM generators, in terms of efficiency and power factor, but eliminating rare earth materials?’ Sicme Motori’s Research and Development Department, together with the Politecnico of Turin, demonstrated that a proper quantity of low cost permanent magnets made of iron ferrites, without rare earth materials, can be installed in the rotor “slots”, such giving a great opportunity to produce machines with performance really close to that of standard synchronous PM generators, but without using rare earth materials. The following table shows the comparison for an application already designed by Sicme Motori. Working Principle of the Synchronous Reluctance machine The synchronous reluctance machine (SR) is made by a conventional three-phase AC stator (the same as an asynchronous motor, for instance) and by an anisotropic rotor. As stated in [1], the principle of the “reluctance” is based on the unwillingness of the magnetic flux to overcome the large air-gap in the spaces between the salient poles. This is a very well known principle, which is obviously considered and quantified even hen discussing salient rotor poles, such as in [4], where a “reluctance torque” is added to the “cylindrical torque”, which is a peculiar term to refer to the torque given by the excited rotor field. A good design for the SR machine requires a peculiar rotor shape, which maximizes the ratio between the reluctances of the magnetic circuit 90 electric degrees away from each other, such maximizing the “anisotropic” component of the torque. The rotor is designed to produce the smallest possible reluctance in one direction and the highest reluctance in the “perpendicular” direction (referring to the electric axis), with shapes as shown in figure 1, where a visual comparison between a (copper) squirrel cage and an SR rotor is shown. Conclusions Figure 1: visual comparison between induction motor’s rotor and SR rotor . Sicme Motori has currently designed and engineered a few ASR solutions for small wind turbines and bigger hybrid generators (at approximately 200-500 rpm), and has produced a first prototype at 300 rpm which is going to be tested and compared in back to back electric and mechanical operation with a catalogue product of the series SWCn which is just the same size. These products can be now offered to clients as competitive solutions, in order to face the important issue of the cost of rare earth materials. Very important is to underline that this new solution is very cost-effective and would even be competitive against traditional PM machines in case the rare earth’s cost came back to the level of three years ago. References • [1] Dr. Ing. Herbert Rentzsch, Elektromotoren – Electric Motors, ABB • [2] Vagati, Synchronous reluctance electrical motor having a low torque-ripple design, 1996 • [3] Nicola Bianchi, Design, Analysis and Control of Interior PM Synchronous machines, IEEE 2004 • [4] Stephen J. Chapman, Electric Machinery Fundamental, Mc Graw and Hill EWEA 2012, Copenhagen, Denmark: Europe’s Premier Wind Energy Event