Research on an Asymmetric-primary Axis-flux Hybrid-excitation Generator for the Vertical Axis Wind Turbine

Abstract

I.IntroductionThe vertical axis wind turbine has been widely concerned because of its advantages such as multiphase wind, stable force, easy installation and maintenance, and small starting wind speed. Vertical axis wind turbines mostly use permanent magnet synchronous generators or doubly fed induction wind turbines, and most of them are axial flux permanent magnet generators[1]. Compared with doubly fed induction wind turbine, permanent magnet synchronous generator (PMSG) has the advantage of high power density and is the main research object of vertical axis wind turbine. However, the direct coupling of the high power axial flux permanent magnet synchronous generator and wind turbine causes the starting of the wind turbine to become more difficult. In order to improve the starting performance of vertical axis wind turbines, maglev technology is considered to reduce the starting resistance as the development direction of vertical axis wind turbines. Therefore, an asymmetric-primary axis-flux hybrid- electricity generator is proposed in this paper, and the decoupling control of levitation force and output power is implemented using asymmetric primary and hybrid excitation.II.operation principleThe proposed asymmetric-primary axis-flux hybrid-excitation generator for the vertical axis wind turbine structure is shown in the Fig.1.The generator is composed of double stator primary and single actuator. The double stator primary has an asymmetric structure design, and the rotor shaft is connected with the wind turbine shaft to realize direct drive wind power generation. The rotor adopts the spacing design of permanent magnet and modulating tooth to reduce the consumption of permanent magnet. Both the excitation winding and the armature winding are located in the primary stator, so the mixed excitation and magnetic field can be adjusted without adding any mechanism such as brush slip ring, which reduces the complexity of the structure.In general, the proposed generator has the following characteristics(1) The flat structure shortens the axial length of the generator and is suitable for direct drive of vertical axis wind power generation system.(2) The levitation force is provided by bilateral unbalance design of axial flux. The two sides adopt different primary sizes, and then the axial electromagnetic force imbalance of the bilateral air gap forms the suspension force, which avoids adding active and passive levitation mechanisms and reduces the complexity of the overall system.(3) The levitation force and output power are controlled by hybrid excitation. The primary tooth DC excitation is adopted to realize zero crosslinking of the excitation with permanent magnet and armature, and the levitation force regulation does not affect the output power. The system can start at low wind speed, and the levitation force and total output power can be adjusted according to the change of wind speed. Through free distribution of bilateral power to achieve the total power following the wind speed and the levitation force following the lift force of the wind turbine.(4) Using magnetic field modulated axial magnetic field structure, the rotor structure is simple and reliable. If the pole pair numbers of rotor and armature are pr and ps, and the number of the stator tooth is pn, there is pr = pn±ps. The design makes it have the magnetic field modulation effect, which can increase the magnetic field change rate under the low speed, thus increasing the generation power.(5) The use of concentrated armature windings and excitation windings reduces the end, thus reducing magnetic leakage and losses.III.simulation and resultsA three-dimensional finite element model of the asymmetric-primary axis-flux hybrid-excitation generator is established, and the no-load characteristics and load characteristics of the generator are obtained. The no-load characteristics under different excitation current are simulated, and the flux linkage of armature coils and DC excitation coils are shown in Fig.2.We can see that when different excitation currents are applied, the crossing flux linkage between DC excitation winding and secondary permanent magnet is almost 0, and DC flux has little effect on the armature flux. Therefore, When the excitation changes, the unilateral levitation force changes only in amplitude, but there is no harmonic phase shift, and positioning torque is almost unaffected. he proposed generator has good decoupling performance of levitation force, output power, and DC excitation. In addition, the optimization design of the generator is further studied such as pole arc coefficient, positioning moment and pole slot ratio. IV.conclusionIn this paper, we proposed an asymmetric-primary axis-flux hybrid-excitation generator, studied the no-load characteristics, load characteristics and excitation characteristics, and optimized the structural parameters of the generator. The simulation results are obtained, and state that the generator can realize the decoupling of the levitation force of excitation and power, which is suitable for vertical axis wind turbine. **