Page - 250 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2017,10, 616 Forpractical application,wedefinean integration threshold sensitivity coefficient so that the actual integrationthreshold is: Ith= kIIlim= 1 2 kITemaxtint= 1 2 kIp0ψf ismaxtint. (19) InEquation (19),Temax is themaximumelectromagnetic torque in the currentwindingmode. For integration threshold sensitivity coefficient kI ∈ (0,1), the bigger kI is, the bigger integration threshold Ith isandthelesssensitivethejudgmentoftorquesaturation,whichcanreducethepossibility of improper switchingbutwill also increasedelays inwindingmode switching; the smaller kI is, the smaller integration threshold Ith is and themore sensitive the judgment of torque saturation, whichcanmaketorqueswitchingswiftbutmayleadto improperswitchingwhenworkingconditions changeunexpectedly. By changing ks and kI,we canmodulate the sensitivity and stability of the torquesaturation judgmentalalgorithm. It is important to note that if the currentmode is trianglemode, upswitching’s goal is only independentmode;whenthe torquesaturationthreshold is reached, thecurrentmodeisswitchedto independentmode.However, if thecurrentmode is starmode, thereare twooptions: triangleand independentmode;when the torquesaturation threshold is reached, terminals aredecidedby the degreeofsaturationofstatorcurrentvector.Whentheamplitudeofstatorcurrentvector is≥ ismaxΔ, it indicates that theamplitudeof thestatorcurrentvector in thecurrentmodeexceeds thecapacityof adjustmentof trianglemodeandupswitching’sgoal is independentmode;when is< ismaxΔ, thegoal is trianglemode. Downswitching is determined by the rotary speed threshold, which is the base speed of eachwindingmode.When themotor’s rotary speeddecreases to the star or trianglemode’s base speed,downswitching is triggered. This strategy staggers theworkingpointsofupswitchingand downswitching to avoid frequent switching. Switching is not onlydecidedby themotor’s speed, likeupswitching,butalsobythedegreeof saturationof thestatorcurrentvector.Whether thecurrent mode is triangle or independent, as long as themotor’s speedhas the relationship nr < nbY the motor’s speed is lower than the starmode’s base speed, and the currentmode is switched to star mode. However, if the currentmode is independent and themotor’s speed has the relationship nbY<nr<nbΔ, ithas tobedecidedwhether toswitch to trianglemodeornot.Whentheamplitude ofstatorcurrentvector is≥ ismaxΔ, this indicates that theamplitudeof thestatorcurrentvectorhas exceededthecapacityofadjustmentof trianglemodeandcurrentwindingshouldstay in independent mode. If is< ismaxΔ, thecurrentmode isswitchedto trianglemode. Switchingprinciplesamongthe threewindingmodesaresummarized inTable2. Table2.Switchingalgorithmofeachwindingmode. StartingMode SwitchingGoal Star Mode TriangleMode IndependentMode Starmode NA Positive torquesaturationdecisionand is< ismaxΔ Positive torquesaturationdecision and is≥ ismaxΔ Trianglemode nr<nbY NA Positive torquesaturationdecision Independentmode nr<nbY nbY≤nr<nbΔ and is< ismaxΔ NA 3.Dual Inverters’CurrentModulationMethod Thispaperadoptsahysteresiscontrol-basedcurrentcontrolmethod. Insingle-sourcedstarand trianglemodes, inverterscouldonlyprovidetwopotentials foreachwinding. Thus, the traditional hysteresiscurrentcontrolmethodwasadopted. Indual-sourcedindependentmode,dual inverters, whencontrolledcoordinately, couldprovidethreeor fourpotentials.Hence,amulti-levelhysteresis currentcontrolmethodwasemployed. 250