| The permanent magnet synchronous machine(PMSM)has become one of the dominating type of machie used in the medium-and-small power industrial applications.However,the PMSM exists inherent shortcomings of demagnetization risk,and the air-gap flux is difficult to be regulated.In addition,rare earth materials are increasingly expensive.It is a challenge of long-term supply of rare earth materials since they are non-renewable strategic reserve resources.Therefore,developing machines with electric excitation scheme or hybrid excitation scheme which using none or little permanent magnet materials has significant values for both industries and academics.Traditional synchronous machine excited with brush and slip-ring.This excitation scheme has low reliability.For electrically excited synchronous machine and hybrid excitation synchronous machine,brushless electric excitation technology is a key problem.Independent exciter usually used in large generator is not suitable for small-and-medium size synchronous machine due to the limit of the volume and cost.Brushless electric excitation schemes in existing hybrid excitation machines also have deficiencies,such as large leakage flux and complex structure.On this background,new topologies of Zero-sequence Spatial Harmonic Field Brushless Excitation Synchronous Machine(ZSHF-BESM)are proposed in this paper.The harmonic magnetic field is set up in the air gap for realizing brushless excitation.Those excitation schemes provide solutions to improve the reliability and flux regulation characteristic of the machine.In this paper,thorough theoretical analysis and experimental research have carried on the harmonic current brushless excitation machine(THC-BESM)and the dual harmonic winding brushless excitation machine(DHW-BESM),respectively.The research work focused on ZSHF-BESM as follows.Firstly,in order to realize brushless electrical excitation,the THC-BESM excited with harmonic magnetic field is proposed in this paper.The harmonic excitation magnetic field is established by adding harmonic current with same frequency and same phase in the open-end stator windings.Analysis results show that the excitation efficiency of high-frequency current is higher than that of third harmonic current and dc.However,the third harmonic current has the ability to compose the saddle-shaped armature current,which can improve the power density while having better controllability,so it is more suitable for using as the excitation current.The mathematical model of THC-BESM is proposed and the structure of THC-BESM system is given.For improving the excitation efficiency and the flux regulation performance of the THC-BESM,the machine structure is optimally designed.The design rules of the harmonic winding is put forward.Three-phase winding coefficients of the fundamental current and the third harmonic current have been defined.On this basis,the harmonic fields produced by the two currents have been analyzed.The air-gap coefficient has been calculated based on the equivalent decomposition analysis method of the special rotor tooth.Tooth harmonics in the air-gap are studied.The orders of tooth harmonics suppressed by the fractional-slot THC-BESM are studied.The regulation performance of the excitation current greatly improved due to the weakening of the tooth harmonics and high order harmonics of armature magnetic fields.Secondly,for further improving the performance of the THC-BESM,the flux coupling rule of parallel equivalent magnetic circuit and influence factors on excitation performance and power factor of THC-BESM are studied.In order to analysis the coupling problem of the harmonic flux and the main flux,a parallel equivalent magnetic circuit model of the THC-BESM is established.The calculation method of the magnetic circuit with coupling flux is given,and the flux regulation characteristics are obtained.For further improving the excitation performance of the THC-BESM,methods of improving the excitation performance from changing structural parameters,such as polar arc coefficient,width and radial length of the harmonic windings slot,are studied and given.Aiming at the problem that the harmonics has a great influence on the power factor,the main influence factors on power factor of the THC-BESM are analyzed.Methods of improving power factor by changing turns of field windings and the width of stator slot are researched.The efficiency of THC-BESM at zero d-axis current load condition is tested and analyzed.Test results show that the efficiency of THC-BESM is slightly lower than that of the same size traditional synchronous machine excited with brush and slip-ring,and the efficiency can be further improved by optimal design.Thirdly,in order to improve the reliability and reduce the cost of ZSHF-BESM,the DHW-BESM is put forward.An equivalent third harmonic magnetic field is set up in the air-gap by the specially designed stator harmonic winding.Because of the decoupling of stator harmonic current and armature current,the full power converter can be removed.Therefore,the cost of the machine system is reduced,while the efficiency and reliability are improved.Design rules and distribution schemes of the stator harmonic windings are analyzed.EMFs of rotor harmonic winding and field winding are calculated under different distribution scheme of stator harmonic windings.The optimum winding structure is obtained.The relationship between rotor harmonic EMF and turns of harmonic winding is studied.Electromagnetic interferences between power windings and harmonic windings are analyzed,including the influence to the saturation situation of the stator core.The magnetic flux density is also calculated,and the regulation curve of the magnetic flux density is obtained.And then,a fractional-slot topology of DHW-BESM is designed to improve the output ability of DHW-BESM,as well as weaken the interference harmonics still exist in the machine.The design rule of the fractional-slot stator harmonic winding is analyzed.Unequal-pitch distribution scheme of the armature windings make higher harmonics partially offset and greatly weakened the higher harmonic content.The winding EMFs and MMFs of the machine have calculated,and the distortion influence of the harmonic magnetic flux to the armature EMF is analyzed.Analyze result indicates that the three phase EMFs of the unequal-pitch distributed armature windings are symmetry.The ratio of the armature winding slots to the stator slots significantly increased in the fractional-slot topology of DHW-BESM,so the output power is also increased.Considering the rotor slot structure,reactance parameters of d-axis and q-axis are analyzed.The required excitation MMFs of the machine with no-load and load conditions are calculated.Finally,for verifying the correctness and effectiveness of the excitation principle and working characteristics of the DHW-BESM,two 1-k W prototype machines are developed,and a test platform is also built for testing the prototype machines.Key parameters of the prototype machines are given,and electrical parameters,such as d-axis and q-axis inductances are tested.A 3-k W dc machine is used as an engine to test the prototype machines,respectively.Excitation performance and regulating characteristics of machines with different stator harmonic winding distribution scheme are tested and studied.The field current regulation performance under no-load and load condition are analyzed.The no-load characteristics and the output characteristics of the machine with resistance-load condition are tested,respectively.Experimental results show that the fractional-slot DHW-BESM has the nearly linear field current regulation characteristic.And it has better output capacity under the same excitation current.The efficiency of DHW-BESM is slightly lower than the traditional machine excited with brushless and slip-ring.The additional iron loss,which can be reduced by further optimization design,is mainly caused by harmonic fields in the machine.The experimental results are consistent with the 2-D FEA calculation and theoretical analyses,which verify the validity of the DHW-BESM,and also provide the theoretical and experimental basis for further improvements and industrial applications of this machine. |
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