Research On Torque Density Of Ultra-low Speed And Large Torque Permanent Magnet Motor

Transmission system matched with large reducer and permanent magnet motors caused some problems such as long transmission chain,huge volume,low efficiency,and complex structure of the large reducer.Recently,this transmission mode has been replaced gradually by the system of permanent magnet direct drive motors.Due to the large outer diameter,the ultra-low-speed and high-torque permanent magnet motor is flat.A lot of unused space inside the rotor is caused by this structure.Therefore,an internal and external composite stator structure is proposed to improve the utilization of the rotor’s internal space in order to increase the torque density of the ultra-low-speed and large-torque permanent magnet motor.The composite stator motor is a new structure of permanent magnet motor with high torque density which can be seen as a running outer rotor motor nested inside the ultra-low speed and large torque motor,synchronously.The inner and outer rotors are mechanically connected which can improve the torque characteristic and utilization of the internal space of the rotor.In this paper a new idea is proposed to improve the torque density of the ultra-low-speed and high-torque permanent magnet motor.The advantages of the ultra-low-speed and high-torque direct drive permanent magnet motor with the internal and external composite stator structure is analyzed in this paper.The structure and working principle of the internal and external composite motor are introduced.Then,the equivalent magnetic circuit model of this structure is established.Due to the size constraints of the inner and outer motors,the size equations and constraints of the inner and outer motors are derived.On this basis,the torque density of motors with internal and external composite stator structure is improved obviously because of the use of inner space of rotor.Taking a 2500 kW16r/min cement mill with ultra-low-speed high-torque direct-drive permanent magnet motor as an example for electromagnetic design,the design methods and principles of the composite structure are studied.During the design process,it needs to be prioritized the selection of poles and slots.According to simulation analysis,the output torque and fluctuation amplitude of the motor,the sine of the no-load back EMF waveform,and the cogging torque are significantly different under different pole slots.When 120 poles and 144 slots of the motor are determined,the winding power assignation,electromagnetic load and main size,air gap length,and magnetic pole parameter are needed to be designed respectively.The relationship between torque density and the internal and external motor split ratio is deduced in order to analysis the influencing parameters of torque density such as poles,the outer and inner split ratios and magnetic flux density.The simulation and analysis are taken to verify the relationship which is deduced between torque density and stator split ratio.The formula of cogging torque is deduced by analyzing the torque fluctuation of the ultra-low-speed and large-torque permanent magnet motors.The impact of pole arc coefficient,magnetic pole stagger angle,and eccentricity on torque fluctuation are discussed in this paper which can provide a certain reference for designing composite motor to suppress torque ripple in the future.