Design And Optimization Of Direct-drive Permanent Magnet Motor For Downhole Electric Drilling System

As the main equipment of oil exploitation,the drilling tool plays a vital role.Therein the electrical drilling tools are widely used due to their advantages over other downhole power drilling tools.And the electric motor is an important part of the drilling system.In order to avoid a series of problems caused by the reducer,the electric drilling tool is driven by the permanent magnet motor directly.However,in view of the technical requirements of low-speed and high-torque in the underground working conditions,the torque density of traditional permanent magnet synchronous motor(PMSM)is still not high enough,besides it is easy to bring problems such as excessive consumption of permanent magnets and volume.In this paper,a permanent magnet vernier motor(PMVM)is proposed to be used as an electric drilling motor and studied.Compared with PMSM of the same diameter,the advantage of high torque density of PMVM is highlighted.Firstly,as a kind of magnetic field modulation motors,PMVM satisfies the basic principle of magnetic field modulation.This paper analyzes the principle and operating principle of PMVM combining its structural features,and summarizes the design criteria of PMVM served as an electric drilling motor.Considering the size limitation and design indexes,a topological structure of surface-mounted permanent magnet vernier motor(SPMVM)whose pole-ratio is 5,with 18 stator slots and 15 pole pair number,is designed in this paper.Secondly,this paper discusses and analyzes the important dimensional parameters affecting the electromagnetic characteristics of PMVM,and their optimized scopes are confirmed.Under the consistent thermal load,the finite element model of the motor is optimized aiming at the maximal torque density,including finite element parametric scanning and the genetic algorithm(GA).Then,after the two optimizing ways are compared and meanwhile the actual level of manufacturing technology is taken into account,an optimal motor scheme is obtained.Then,the performance of PMVM is analyzed and compared with traditional PMSM.Combined with the finite element method,it is proved that the torque density of PMVM is about 10.8%higher than that of PMSM.In terms of material consumption,there is not much difference between the amount of copper,while the iron core of PMVM decreases about12.9%,and the magnet consumption of PMVM is only 42.2%of that of PMSM,which greatly saves the total manufacturing cost.Finally,a small size prototype of PMVM and a same size prototype of PMSM are designed and developed.The no-load and load tests of the two prototypes are carried out.By comparing the experimental and simulation results,the accuracy of the electromagnetic design of PMVM and its characteristics of high torque density are verified.