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The objective of this paper is to compare the permanent magnet synchronous generator (PMSG) systems for wind turbines with the different drive trains and power ratings by design optimization. The comparison is mainly based on the cost and annual energy production (AEP) for a given wind climate. Firstly, the analytical models of the wind turbine, the 1-stage and 3-stage gearbox, the 3-phase radial-flux PMSG and the back-to-back converter are presented respectively. Then the direct-drive PMSG (DDPMSG), the PMSG with 1-stage gearbox (PMSG1G) and the PMSG with 3-stage gearbox (PMSG3G) are optimized by the genetic algorithm for the minimal system cost respectively. The optimized results of DDPMSG are validated by the finite element method. Furthermore, a range of wind turbines from 750 kW to 10 MW are compared to get the tendency. The suitable range of gear ratios of PMSG1G in the different power ratings is also investigated. At last, the efficiency and loss characteristics of the different wind generator systems are discussed. The results show that DDPMSG has the highest AEP, but it is more expensive than PMSG1G and PMSG3G with the increase of power rating. PMSG1G seems more attractive in terms of AEP per cost. PMSG3G has no significant advantage compared with others.