| The downhole electric drilling system has high drilling efficiency,can significantly reduce the oil and gas production cost,is an important means to realize the high-efficiency and lowcost development of oil and gas resources in deep wells,and has a good application prospect.As the core component of electric drilling system,downhole submersible motor needs high torque density and high reliability.Permanent magnet vernier motor has simple structure and high torque density,but the traditional surface mounted permanent magnet vernier motor(SPMVM)has some problems such as low power factor and poor reliability of rotor structure.Low power factor will lead to an increase in the capacity of the frequency converter,which will increase the cost.However,there is a hidden danger of permanent magnet breakage or falling off during the assembly and operation of the surface-mounted rotor,and its reliability is difficult to guarantee.In this paper,based on the application background of electric drilling tools,the consequent pole permanent magnet vernier motor(CPMVM)is proposed to be used as the motor of downhole electric drilling tools,and systematic and in-depth design research is carried out around its slot pole matching,winding type selection and stator and rotor size optimization.Firstly,the structure and working principle of the CPMVM are expounded.By comparing with the traditional SPMVM,the advantages of CPMVM such as more reliable structure,less permanent magnet consumption and less inter-pole magnetic leakage are revealed.According to the simplified magnetic circuit model of magnetomotive force and permeance,the analytical expressions of magnetomotive force,air gap specific permeance,air gap magnetic density,back emf and electromagnetic torque of CPMVM are derived.Compared with the traditional SPMVM,the magnetomotive force of CPMVM contains even harmonics,and the harmonic content of air-gap permeability and magnetic density is higher.The electromagnetic characteristics of the air gap magnetic density,no-load back EMF,electromagnetic torque and power factor of the two are further compared.The results show that the CPMVM can achieve the same torque performance as the surface-mounted scheme with half of the permanent magnet consumption,but the power factor will become lower.Secondly,aiming at the problems of low power factor and structural reliability of CPMVM,further electromagnetic design and optimization are carried out.Under the condition of comprehensive consideration of torque and power factor performance,slot-pole matching and winding type are selected,and the influence of main stator dimensions on electromagnetic performance is analyzed.On this basis,the consequent pole rotors with three different structures are designed and optimized respectively.In order to solve the problem of large magnetic leakage of permanent magnet peripheral pole shoes in built-in consequent pole rotor,a design scheme of rotor with non-conductive pole shoes is proposed,which effectively improves the output torque.By comparing the comprehensive performance of three optimized consequent pole rotor schemes,it is found that under the same output torque condition,the built-in consequent pole rotor structure has the highest reliability,and the power factor of the motor is increased by about 12% compared with the traditional surface-mounted structure,which meets the performance requirements of the downhole electric drilling tool motor.Finally,a small prototype of built-in CPMVM with 18 slots and 4 poles of stator and 14 poles of rotor was designed and developed.The no-load back EMF,output torque,power factor and efficiency of the prototype were tested through no-load and load experiments,and compared with the finite element simulation results,which verified the feasibility of the proposed built-in CPMVM and the accuracy of electromagnetic design. |
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