Research On Air-gap Magnetic Field Distribution Characteristic And Its Influence On Key Performances Of An Induction Motor With The Dual Skewed Rotor

The single skewed rotor is normally adopted in small and medium-sized three-phase induction motors.The long-term research and engineering experience have shown that using a single skewed rotor is beneficial to improve many performances of the motors.In recent years,with the gradual improvement of the motor performance requirements and the rapid development of other new permanent magnet motors,the optimization of the single skewed rotor has a limited improvement of the motor performances,which hinders the promotion of the three-phase induction motors and the development of the national energy transformation.Based on the study of the traditional single skewed rotor,the development of the novel rotor topologies and the relevant performance analysis and calculations are the important issues to be solved urgently.In this disseration,an induction motor with the dual skewed rotor is considered as the research object.By building the global linear and local nonlinear air-gap magnetic field models,three kinds of key performances related to the air-gap harmonic magnetic field are researched theoretically.Aiming at weakening the electromagnetic noise,reducing the stray loss and improving the starting performance,the relevant optimization strategies of the dual skewed rotor are proposed.Finally,the verification methods include the finite element simulation and the prototype test.The specific content and innovation points are as follows:(1)Based on the deduced skewing coefficient of the dual skewed rotor,the collaborative design method of the structure parameters is proposed.The design scheme of the dual skewed rotor could be preliminarily confirmed to weaken the harmonic magnetic field.According to the phasor relations of the rotor current,the general phasor diagram of the rotor current is proposed to calculate the current amplitudes of two kinds of intermediate ring(IR)conductors.(2)The air-gap mangetic field including the dominant harmonics of the induction motor is modelled with an analytical model.Based on the finite element simulation and the two-dimensional Fourier decomposition method,an axial variation model of the saturation magnetic field due to the skewing is proposed.The basic characteristics of the dominant air-gap magnetic field could be verified.The electromagnetic forces are calculated with the Maxwell tensor method.By establishing the relationship between the theoretical value of the radial electromagnetic force and the experimental value of the peak noise,the optimization design scheme of the dual skewed rotor could be proposed to reduce the overall noise of the induction motor.(3)The magnetic density distributions of the IR and the adjacent air-gap regions are calculated with the Schwarz-Christoffel transformation method.Based on the assumption of the slot leakage magnetic field,the distribution of the rotor eddy-current loss could be calculated with the layered method.Two design principles of the IR conductor are proposed,and the rationality of this design scheme could be verified by the regional temperature-rise test.By establishing the unit model of the transverse current in the dualskewed rotor,the transverse current losses in two kinds of contact resistivity ranges are calculated respectively.The difference of the contact resistivities between two kinds of rotors could be verified by removing the end ring.The attained conclusions in this section provide a reference for the rotor optimization design to reduce the stray loss.(4)In the case of the induction motor with the dual skewed rotor,the design range of the slot combination should be modified to reduce the additional torques.Based on the model of the air-gap magnetic field,the asynchronous additional torque is calculated with the chain equivalent circuit,and the synchronous additional torque is calculated with the virtual displacement method.Thus,two parameters could be concluded respectively to describe the relative magnitudes of the additional torques.The slot combination and the skewed rotor could be determined according to the assumed parameter ranges.In order to verify the weakening effect of the dual skewed rotor on the two kinds of additional torques,the experiments of the prototype motors are carried out to test the minimum torque and the starting torque in the case of the equivalent slot combination.