Comparative study of PMSG, EESG, and HESG for wind power application of different power levels

In this paper, permanent magnet synchronous generators, electrically excited synchronous generators, and hybrid excited synchronous generators are compared to evaluate the difference in performance and cost efficiency for 3 kW medium-speed-low-torque and 3 MW low-speed-high-torque wind turbines. These generators with outer rotor or inner rotor are optimized for maximum torque, and the optimal pole numbers are identified. The results of comparison show that the outer rotor generators have higher torque than the inner rotor ones while the difference between them decreases as the power level and radial dimension of generators increase. In the 3 kW case, the torque density and torque per material cost of the optimized electrically excited synchronous generator are 11.6% and 28.2% those of the optimized surface-mounted permanent magnet synchronous generators, and are increased to 51.0% and 211.3% in the 3 MW case, respectively, because electrically excited synchronous generators have better torque capability at heavy load scenarios. For the same reason, the torque advantage of hybrid excited synchronous generator over electrically excited synchronous generator is decreased from 149.1% in 3 kW case to 9.6% in 3 MW case. When considering lifecycle cost, however, electrically excited synchronous generators and hybrid excited synchronous generators have higher operational expenditures due to the maintenance of brush + slipring. Therefore, although the electrically excited synchronous generator is more than twice cost efficient (material cost) compared to the surface-mounted permanent magnet synchronous generator for low-speed-high-torque wind turbines, the surface-mounted permanent magnet synchronous generator is still an ideal choice for both wind turbines due to the highest torque and possibly lower lifecycle cost than the other two generators.