Abstract: In order to improve the start-up ability of speed adjustable magnetic coupling, the maximum starting torque of the magnetic coupling was studied. We analyze the magnetic circuit structure of the variable speed magnetic coupling, study the method of enhancing the air gap magnetic field, use the three-dimensional finite element software to compare and analyze the influence of the opening of the magnetic gathering slot on the starting torque of the magnetic coupling, and discuss the cogging torque caused by the uneven distribution of the air gap magnetic field caused by the magnetic gathering slot. We use different methods to suppress the cogging torque, analyze the number of cogging, the shape of cogging, and the influence law of the tooth slot width on the operating characteristics of the magnetic coupling. The results show that the magnetic gathering slot can effectively improve the starting torque of the speed adjustable magnetic coupling, and the minimum common multiple between the slots and magnet poles has definite effect on the cogging torque suppression. At the same time, the inclined slot can improve the starting torque while maintaining the rated torque of the magnetic coupling. The width of the slots needs to be selected after considering the actual needs.