| Line-start permanent magnet synchronous motor(LSPMSM)is a permanent magnet synchronous motor with self-starting capability,which is started by the asynchronous torque generated by the interaction between stator rotating field and cage rotor.Compared with the three-phase induction motor,LSPMSM has the advantages of high efficiency,power factor,torque density and wide economic operation range,so it has been widely applied in textile machinery,pump,oil field pumping machine,fan and other industries,and has achieved remarkable energy saving effect.However,due to the limitations of its working principle,existing structure and starting mode,the poor starting ability is a prominent problem.During the start process of the conventional LSPMSM,due to the equal number of stator and rotor poles,the fundamental magnetic field generated by the rotor permanent magnet interacts with the stator winding to generate the braking torque.The fundamental magnetic field generated by the rotor permanent magnet has the same rotation direction,the same number of poles,and different rotating speeds as the fundamental field generated by the stator winding,the interaction produces large pulsating torque.The larger pulsating torque and braking torque make the torque in the start process fluctuate greatly,which has a great impact on the motor and the load,and make the minimum torque in the start process very small,which in serious cases leads to the motor being unable to complete the start.In order to improve the starting capacity of the motor,it is generally realized by increasing the rotor resistance,reducing the number of series turns per phase of stator winding or optimizing the structure of the motor.However,the braking torque and pulsating torque are inevitable,it cannot fundamentally solve the poor starting capability of LSPMSM.To fundamentally improve the starting capability of LSPMSM,this paper studies a novel pole changing LSPMSM by relying on the National Natural Science Foundation of China "Study on the Novel Pole-changing Line-start Permanent Magnet Synchronous Motor"(5 1 577107).The main research work is as follows:1.Analysis of structure,working principle and electromagnetic characteristics of the novel pole changing line-start permanent magnet synchronous motor.The structure of the pole-changing LSPMSM is the same as the conventional LSPMSM,and the difference is the stator winding.A pole changing winding with two different pole states,with less required switches and high reliability is adopted.At the beginning of starting,the stator winding operates in the state of few poles,due to the difference of pole numbers between stator winding and rotor PM,the braking torque and pulsating torque during starting process are greatly weakened,thus the starting capability obtain significantly improved.When the motor reaches a certain speed,the stator winding is switched to the state of multi-pole to complete synchronization and finally run stably.Through adopting the proposed pole changing starting strategy,the poor starting capability of the LSPMSM obtains solved.According to the difference of pole numbers between stator winding and rotor PM during starting process,the calculation formula of the no-load induced electromotive force(EMF)when the pole numbers between stator winding and rotor PM are different are derived.And the characteristics of the induced EMF at different ratios of rotor/stator poles are summarized.During the starting process,the pole numbers between stator winding and rotor PMs are different,through analyzing the induced EMF when rotor PM s cut the stator winding can fundamentally translate the decrease the braking torque by using the pole changing strategy.Through calculating the induced EMF in a wire,a coil,and a coil group in stator winding under multi-pole rotor PMs' magnetic field,and by calculating the per pole magnetic flux corresponding to each coil group connected in series with one phase winding,the calculation formula of the induced EMF in one phase winding is derived.The results show that,when the polar ratio of rotor/stator(p2/p1)is even and fractional,the induced EMF is zero,when p2/p1 is odd,a certain magnitude of induced EMF is generated.To further analyze the results,the characteristics of the induced EMF when p2/p1 are different odds(3k1(k1?1,3,5...)?6k2-1(k2=1,2,3…).?6k2+1)are studied.And by using the finite element method(FEM).and taking the motors with p2/p1 are respectively 6/2?6/4?6/6?8/4?8/8?1 0/2?1 0/10 as examples,the correctness of the calculation method and result analysis are verified.The results show that,when p2/p1 is 3k1,an induced EMF with the same phase in space is generated,when p2/p1 is 6k2-1.a three-phase symmetric induced EMF in which the phase sequence is opposite to the stator winding is generated,when p2/p1 is 6k2+1,a three-phase symmetric induced EMF having the same phase sequence as the stator winding is generated.Through the calculation and analysis of the no-load induced EMF when the pole numbers are different between stator winding and rotor PMs,the theoretical basis for the research of the pole-changing permanent magnet synchronous motor is provided.Combined the study of the on-load induced EMF and the corresponding induced currents when the pole numbers are different between stator winding and rotor PMs,the magnetic field components produced by stator winding,rotor winding and rotor PMs are analyzed during few pole number stator winding is working when the p2/p1 is even,fractional and odd.Then the torque components produced by the interaction between the fields and the mechanism are obtained.Through the comparison with the conventional LSPMSM,the results show that the braking torque during starting is effectively eliminated and the pulsating torque is greatly weakened.According to the generation mechanism of each torque component,the calculation method of torque during the pole-changing process is proposed.The correctness is verified by taking the 6/8 pole-changing permanent magnet synchronous motor as an example.2.Aiming at the small power LSPMSM,a pole-changing starting strategy with different number of poles/slots combination is proposed.The pole-changing winding with two different pole-number states is adopted in the stator,however,since the same stator is shared,the number of stator slots needs to meet the needs of two pole-number windings for the stator slots.So there is the problem of pole/slot matching.Firstly,the selection of the minor pole numbers of the winding is studied.Taking the conventional 4-pole,6-pole,8-pole LSPMSMs as examples.The 2/8,4/8,6/8,2/6,4/6,2/4 pole changing stator windings,which respectively correspond to the 8-pole motor with 2-pole starting,the 8-pole motor with 4-pole starting,the 8-pole motor with 6-pole starting,the 6-pole motor with 2-pole starting,the 6-pole motor with 4-pole starting,the 4-pole motor with 2-pole starting,are designed.The structure and work mode of each pole changing winding are analyzed,and the two pole states,fundamental winding factors are calculated.The results show that the 2/4,4/6,6/8 pole-changing starting strategy with near-pole ratio requires less switches,high reliability and high fundamental winding factor.Based on this,the pole numbers of the winding that used for starting is determined when the motor with different pole numbers adopts pole changing starting strategy.Then,according to the different frame size,different stator slots are selected to design the 2/4,4/6 and 6/8 pole-changing windings,so as to obtain the feasible pole-changing slot coordination and determine the corresponding pole-changing starting scheme.The results show that when the 4-pole motor starts with 2-pole,24 stator slots with frame sizes of 63,71,80,90,and 36 stator slots with frame sizes of 110,112,132,160,and 48 stator slots with frame sizes of 180,200,225,250,and 60 stator slots with frame sizes of 280,and 72 stator slots with frame sizes of 315,355 can be selected.When the 6-pole motor starts with 4-pole,36 stator slots with frame sizes of 110,1 12,132,162,and 72 stator slots with frame sizes of 315,355 can be selected.When the 8-pole motor starts with 6-pole,72 stator slots with frame sizes of 250,280,31 5,355 can be selected.Based on the design and analysis of the pole-changing windings with different number of stator slots,the feasible pole-slot matching principle is deduced and the correctness is verified.The results show that in order to make the number of stator slots meet the requirements of both pole winding,the common multiples of 6p1 and 6p2 should be selected.By studying and summarizing the feasible pole-changing strategy when the number of poles of different motors is matched with the number of stator slots,it lays a foundation for the research of the pole-changing LSPMSM.3.The study of the novel 6/8 pole changing LSPMSMTo analyze the performance of the novel pole changing LSPMSM.and verify its superiority a 380V,30kW,6/8 pole-changing LSPMSM is designed,and take the motor as the research object to carry out in-depth research.The finite element model is built to analyze the armature field and no-load back EMF,the results verify the reasonableness of the designed pole changing winding and the correctness of the calculation method of the no-load EMF when the pole numbers are different.By using FEM,the 6-pole starting process is studied,mainly including locked-rotor torque,locked-rotor currents,the braking torque,the pulsating torque,the starting process with rated load torque,the largest load torque and the minimum applied voltage that motor can start-up.Through comparing with the conventional 8-pole motor,the results verify that using the pole-changing strategy can significantly improve motor starting capability.Through calculating the parameters of the motor,and combining with the theoretical analysis,the torque components during starting process are respectively calculated.Compared with the finite element results,the two results are basically consistent,which validates the correctness of the torque calculation method.The influence of rotor resistance on the ability of pull-in synchronization is analyzed,and on the premise of significant improvement of starting ability by pole changing starting strategy,the rotor resistance is appropriately reduced to improve the synchronization capability.The steady-state performance under the 8-pole state is analyzed,and the power angle characteristic is obtained.Through the prototype test,the working characteristic curve is obtained and compared with the finite element simulation.The results are basically consistent and the results show that high efficiency and power factor can be obtained in a wide operating range.4.Study on the switching process and the control strategy for pole-changing LSPMSM.As the motor accelerates to a certain stage with minor pole numbers winding,it needs to switch to the multi pole numbers winding to complete the pull-in synchronization and stable operation.By analyzing the winding switching process under different conditions,the switching control strategy is determined,which has little impact on the power grid,the motor and the load and is beneficial to the motor starting.A dynamic simulation model of bipolar number is established,and based on this model,the influences on the fluctuation of speed,impact current,impact torque and pull in synchronization when the winding switching with different speeds,phase difference,load torque,rotational inertia and rotor resistance are analyzed.By using FEM,the demagnetization of PM when switching with different conditions is studied.The results show that the motor can start-up with any switching condition,but different switching time cause different effects.The impulse currents is mainly affected by the switching phase difference,the larger the phase difference,the larger the impulse currents.In addition,the larger the voltage difference,the more serious the PM demagnetization is.However,through the comparison with the conventional LSPMSM,the demagnetization of the PM during winding switching process cannot more serious than starting process of the conventional LSPMSM.In order to reduce the impact of winding switching on motor and load and the demagnetization effect of permanent magnet,and to facilitate the synchronization of starting and pulling,the corresponding switching control strategy is proposed.Choose to switch when the phase difference is small.When the motor starts with rated or less than rated load torque and moment of inertia,choose to switch at synchronous speed.When the motor starts with larger load torque and moment of inertia,choose to switch at higher synchronous speed.5.Prototype testingBased on the electromagnetic design scheme,a 380V,30kW,6/8 pole-changing LSPMSM is manufactured as a prototype.The no-load back EMF,the braking torque,no-load starting process with the rated voltage,the minimum voltage that motor can be started,and the 6/8 pole-changing starting process are respectively measured and calculated.The correctness of theoretical analysis,analytical calculation and finite element simulation results has been effectively verified through prototype testing.Meanwhile,the feasibility and advantages of the 6/8 pole-changing LSPMSM have also been further verified. |
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