Stator-permanent-magnet reluctance generator using ferrite magnet for small-scale renewable energy generation

This paper discusses a stator-permanent-magnet reluctance generator (PMRG) (Fig. I). The PMRG has a doubly salient pole structure, and has permanent magnets embedded in the stator yoke. Concentrated windings are arranged individually around each stator pole. On the other hand, the rotor has neither permanent magnets nor windings. Therefore, the PMRG has a simple and robust structure, and permits a multipolar structure. It is expected that the PMRG is applied into a small-scale wind-turbine and hydroelectric power generation. However, two major technical problems still remain. One is the cost of rare-earth magnets, and the other is torque ripple due to the doubly salient pole structure. First, this paper presents a PMRG using ferrite magnets. The ferrite magnet has about three times lower residual magnetic flux density as compared to the rare-earth magnet, but has about 10 times lower cost. It is demonstrated that the proposed PMRG using ferrite magnets has almost the same output power as that using rare-earth magnets by employing an outer rotor structure (Fig. 4). In addition, its material cost is lower by about 45 %. Next, it is proposed a method for reducing torque ripple of the PMRG by stacking two rotors (Fig. 10). One rotor is shifted by 180° in electrical angle from the other rotor. In addition, the rotor pole-tips are cut off so that the torque waveform consists of only odd-harmonics. In such a structure, the torque ripple of the PMRG can be completely cancelled in theory. Finally, a trial two-stacked outer-rotor PMRG has been manufactured. The maximum output power is 777 W at 1000 rpm, and the efficiency at the point is 84.5 %. In addition, the efficiency at less than 1000 rpm is improved by over 10 % in comparison with the present PMRGs with rare-earth magnets (Fig. 19).