The dynamic torque of a wind turbine drive train with a direct-driven permanent magnet synchronous generator is analyzed in this paper. A simplified multi-mass model for the whole drive train — from the flexible turbine blades to the rotor of the generator — is presented. Dynamic torque is analyzed for both steady state and transient cases. In the mechanical model, torsional electromagnetic forces called “cogging torque” and “torque ripple” are used as excitation forces. The magnitude of electromagnetic excitation is highly related to the shape and placement of the magnets. Four different permanent magnet installations are analyzed. It can be noticed that during start-up only cogging torque affects the generator rotor. Allowable cogging torque in this case is 1.5–2 percent of the rated torque even when the corresponding resonance frequency does not occur in the operational speed range of the wind turbine. Furthermore, it was perceived that the resonance caused by the excitation torque should occur at the lowest possible speed to minimize dynamic torque during the start-up. Several excitations, both cogging torque and torque ripple harmonics, occur in the operational speed range. However, it was detected that the dynamic responses in the operational speed range are not critical to the operation of the wind turbine.

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