Background: The safe and stable operation of large-capacity hydro-generators is of great significance to the power grid. However, due to the armature reaction and the stator slots of the hydroelectric generator, the torque ripple of the generator is usually large. Long time running under the condition of large torque ripple will cause damage to the generator components and even threaten the safety of the power grid. Objective: To study the factors affecting the torque ripple of the hydro-generator and reduce the torque ripple. Methods: A 24 MW bulb tubular hydro generator is taken as an example, a two-dimensional transient electromagnetic field model was established, and the correctness of the model was verified by experiments. The causes of armature reaction and cogging effect on motor torque are analyzed. The effects of these two factors on torque ripple are studied respectively. The torque ripple under different working conditions is calculated, and the main factors causing and increasing torque ripple during normal operation of hydro generator are determined. Results: The torque ripple increases approximately linearly with the increase of the 7th and 11th harmonic contents, while it increases nonlinearly with the increase of the 5th harmonic contents. When the excitation current increases from 720 A to 960 A, the torque ripple is increased by 11.4kN•m. In addition, when the excitation current increases from 720A to 840A (Rated excitation current of the generator is 840A), the torque ripple increases greatly, the torque ripple increases by 6.6kN•m. When the excitation current exceeds the rated excitation current, due to the influence of magnetic saturation, the increasing extent of torque ripple gradually decreases. When the excitation current increases from 840A to 960A, the torque ripple increases only by 4.8kN•m. Conclusion: Due to the armature reaction, the harmonic magnetic potential will be generated in the generator, which will affect its torque ripple. The torque ripple of the generator will increase obviously with the increase of harmonic content. When the generator is in no-load operation, the slot effect is the main cause of the torque ripple. In the actual operation of the generator, it usually needs to adjust the excitation current to make the generator meet the requirements of various working conditions.

The bearingless motor with dual-winding embedded in the stator can not only produce electromagnetic torque and can also produce suspension force to support the rotor suspension, and the design of the dual-winding will directly affect the torque and suspension performance of the motor. It is a complex problem of multi-objective optimization, multi-performance coupling, and multi-parameter collaborative optimization, which is great significance to improve the comprehensive performance of the motor. Firstly, in order to solve the problem of slot space ratio and turns design of dual-winding under the restriction of stator slot space and thermal loading, the calculation formulas of the electromagnetic torque and the suspension force are derived. The influence of dual-winding design on electromagnetic loading, thermal loading, torque and suspension performance are analyzed. The linear variation of electromagnetic torque with the slot space ratio of torque winding is obtained, and the influence of the dual-winding magnetic motive force on suspension force is clarified. Secondly, in order to obtain the dual-winding design scheme with the optimal torque and suspension performance, the performance parameters of the motor are calculated by finite element method and the influence of the slot space ratios of the torque winding on the torque and suspension performance is analyzed. Based on the coupling analysis of the electromagnetic torque and the suspension force in the case of different thermal loading distributions of dual-winding, the dual-winding optimal design of the bearingless motors is given. Finally, the suspension force of a prototype is tested by experiment.