Research On Direct-drive Permanent Magnet Synchronous Motor For NC Machine Tool Turntable

Direct-drive Permanent Magnet Synchronous Motor(DD-PMSM)is used to drive the turntable of direct-drive NC machine tool directly.Compared with the traditional NC turntable of "mechanical transmission mechanism and High-speed Servo motor",because the mechanical transmission chain of DD-PMSM is zero,there are no inherent friction,wear and lubricant leakage problems in the mechanical transmission chain.The operating efficiency is also much higher than that of the traditional transmission NC machine tool turntable.When the existing permanent magnet synchronous motor is directly installed in the turntable of NC machine tools,the following two problems will arise:(1)the cutting ability of NC machine tools is insufficient due to the low torque density(currently the torque density of permanent magnet synchronous motor is only 50 kNm/m3);and(2)the closed NC machine tool turntable will cause the internal heat of permanent magnet synchronous motor to be unable to dissipate,which will directly lead to the stator winding burnout and rotor permanent magnet loss of excitation.To solve the problems above mentioned,this paper designs a DD-PMSM with a torque density up to 65 kNm/m3 and water-cooled turntable for NC machine tools,which can effectively solve the above shortcomings of existing permanent magnet synchronous motors.Firstly,this paper summarizes the current research status of permanent magnet synchronous motor at home and abroad,expounds the electromechanical structure,temperature field calculation and cooling system design method of permanent magnet synchronous motor.Through the electromagnetic field mathematical model of permanent magnet synchronous motor in ABC three-phase coordinate system,the torque equation of permanent magnet synchronous motor in rotor synchronous rotating coordinate system is established,and the optimal magnetic circuit structure suitable for DD-PMSM is given.According to the design requirements,the key parameters of the designed DD-PMSM electromechanical structure(including stator size,slot structure,winding type and permanent magnet size)are preliminarily analyzed and calculated.The parameters are imported into RMxprt module.The maximum slot depth,permanent magnet thickness and air gap length are optimized by controlling variable method,and high torque density and high operating efficiency are obtained.The permanent magnet synchronous motor model can be directly used in the direct drive NC turntable and then the DD-PMSM model needed in this paper can be formed.The DD-PMSM model generated by RMxprt is imported into Maxwell.Finite element method(FEM)is used to simulate and analyze the no-load,load and loss of RMxprt.Compared with the results obtained by magnetic circuit method,the rationality of the above parameters optimization is verified.Based on the principle of equivalence,the equivalent winding model is established,the equivalent thermal conductivity of insulating layer and the effective thermal conductivity of air gap are calculated.Combined with the boundary convection heat transfer coefficient,a three-dimensional magneto-thermal coupling model is established under Workbench environment,and the temperature distribution nephogram of the core components of DD-PMSM,winding and permanent magnet,is obtained.Aiming at the problem of high temperature of DD-PMSM winding and permanent magnet,an axial water cooling system suitable for DD-PMSM is presented.The flow velocity is calculated and optimized,and the trace nephogram of flow field is obtained by CFX module.The above work shows that:(1)the designed torque density of DD-PMSM can meet the requirements of NC machine tool turntable;and(2)the water cooling system can ensure the good heat dissipation of DD-PMSM,so that the temperature of permanent magnet and winding can be reduced to below the allowable temperature.