The Research Of Controllable Flux Composite Rotor Permanent Magnet Synchronous Motor

Permanent magnet synchronous motors(PMSM) have been widely used in many applications because of their high power density,high efficiency and high power factor.But since 1920s when the first permanent magnet synchronous motor appeared, one of main design guides is to prevent magnet material from demagnetization,which means the magnetization state of permanent magnet is difficult to regulate after being assembled and flux in air gap could not be changed largely with load.When motor speed arrives at certain value,the EMF induced in stator windings balances out the impressed voltage.If higher speed is needed,a d-axis current is usually applied to reduce flux in air gap,which is called vector control.Unfortunately,the small d-axis inductance of PMSM limits the flux-weakening capability and the speed range. Moreover,in flux weakening operation,the stator current is limited—when d-axis component increases,the q-axis component cuts down.As the result,the electromagnetically torque will decline fleetly as well as the stator joule loss increases and efficiency decreases.If inverting in transducer fails,the over voltage induced in stator winding due to the high speed rotating magnetic field will lead the destruction of components in transducer.This shortcoming limits the use of PMSM in area of wide speed.So it has great significance to combine the wonderful operation character and flexible speed regulating into PMSM.For a long time,flux-weakening is one of the main problems in PMSM.In 2001, Vlado Ostovic first proposed a new class of controllable flux permanent magnet machine for wide speed operation—memory motor.Different to normal flux weakening way,memory motor obtains needed air gap flux density through magnetizing magnets in the motor.A prototype was designed and tested by the author. The rotor of the prototype is built following the sandwich of permanent magnets,soft iron,all of them being mechanically fixed to a non-magnetic shaft,and the stator is the same as that of normal induction motor.Magnetic equivalent circuit was used to get the current to magnetize the PM and the flux per pole.The test results verify the author's design.The efficiency and power factor of permanent magnet motor is higher than that of induction motor.But the configuration of the sandwich-type rotor is complicated,and construction strength is low.Moreover,magnetic equivalent circuit with neglecting the core magnetic saturation and assuming the permanent magnet either magnetized to saturation or fully demagnetized after magnetization results to very large warp from truth.Based on Vlado Ostovic's research,the main study works of this dissertation are as follows:1.In order to solve the shortcomings of sandwich configuration rotor,novel composite-rotor is applied.It consists two rotor segments with different permanent magnet materials.The NdFeB rotor segment could provide high air-gap flux in low speed area and the torque is high enough,while AlNiCo rotor segment could change its magnetization state to adjust the air-gap flux.The former adopts surface-mounted rotor,simply and widely used.The latter is similar to interior-type rotor,which pole shoes shield the q-axis armature reaction,thus the load current will not influence the state of magnet material.Parts between poles of each segment and interspaces of the two segments are filled with non-magnetic material.In the normal operation,control strategy i_d = 0 is put in practice.In wide speed operation,a certain current pulse i_d is applied to remagnetized or demagnetized Alnico while the character of NdFeB will not be changed.2.The initial magnetization model and remagnetization model of permanent magnet are suggested.In Vlado Ostovic's research,it assumed that all the permanent magnets in the rotor were either saturation magnetized or fully demagnetized.In fact, it is impossible.In this paper,the initial magnetization model is improved and remagnetization model of permanent magnet is built up.Simulate the magnetic hysteresis by nerve net technique.Then determine the state of magnet by combining the magnetic hysteresis and magnetization model.3.The performance calculation of composite-rotor permanent magnet synchronous motor is studied.Due to the air gap magnetic field determined by magnetization state of two rotor segments,the paper proposed equivalent air gap magnetic density to compute the EMF according to that produced by Alnico and NdFeB planar finite element methods,respectively.Then formulas on operation characteristics are deducted at the state of i_d = 0.4.Design and make a 4-pole,36-slot prototype.The magnetizing test and demagnetizing test are done.