Direct Torque Control of Permanent Magnet Synchronous Traction Motor in Electric Vehicle Drive

Abstract: In order to improve the performance of permanent magnet synchronous traction motor(PMSTM)in electric vehicle(EV)drive,a novel direct torque control(DTC)strategy is proposed.Based on the operating mode of PMSTM,zero voltage vectors are chosen to obtain quick dynamic torque response and reduce inverter switching loss.An optimization control method about stator flux linkage is presented to improve system efficiency,the core of which is the real-time regulation of stator flux linkage by measuring inverter input power.Simulation and experimental results prove the proposed strategy.

Keywords: Electric vehicle,permanent magnet synchronous traction motor,direct torque control

1 Introduction

With high efficiency torque density and power density, permanent magnet synchronous motor(PMSM) is very attractive in EV drive,which has been in use especially in Japan[1].Quick dynamic performance and high efficiency are very important to traction motor system in EV drive.Vector control technique is a key control method to PMSM at present.However, variable PMSM parameters have an influence on the performance of vector control system due to the complicated operating mode of EV.With advantages such as high dynamic torque response and robustness, direct torque control(DTC)has become a competitive control technique to traction motor in EV drive compared to vector control technique. As a new technique,DTC was just introduced to PMSM several years ago.Owing to the different rotor structure,the DTC method is different in the application between PMSM and induction motor control[2,3].Up to now,the DTC to permanent magnet synchronous traction motor（PMSTM）has not beenstudied in detail.In this paper,the DTC strategy of PMSTM in EV drive including space voltage vector and stator flux linkage control is studied.

2 。Space voltage vector control

Space voltage vector and stator field orientation control technique are used in PMSM DTC, In order to control PMSM effectively,α-βplane is divided into six regions as shown in Fig.1,and the first voltage vector U1 keeps the same direction with αaxis.Thereby,DTC can be realized through the selection of voltage vectors based on changes of stator flux linkage and electromagnetic torque at any time.  Generally,only six voltage vectors without zero voltage vectors are applied in DTC to ensure the dynamic response of PMSM at the cost of switching loss of inverter[4].Nevertheless,zero voltage vectors play an important role in process of improving dynamic performance on basis of reducing inverter dissipation.  The reduced electromagnetic torque with zero

voltage vectors in one control period can be calculated from Eq.(1).Assuming the control period is 100μs,  the proportion of reduced electromagnetic torque vs.  rated torque at different rotational speed is calculated.  According to the calculation results,the conclusion can be drawn that zero voltage vectors can make electromagnetic torque reduce appreciably in one control period at different rotational speed.For example,the proportion is less than 2.2%at 3000r/min and the proportion reduces with a lower speed,even less than 0.37%at 500r/min.  In order to avoid undesired inverter switching loss and ensure necessary dynamic response,a new

space voltage control strategy based on operating mode of PMSTM is proposed.Speed error of PMSTM between the reference and real speed is monitored all the time,and the steady state can be determined if the error is less than a small reference value for several

measurements.According to the state of PMSTM system,different space voltage vector table is adopted,  i.e.Tab.1 is selected if PMSTM is in dynamic state,orTab.2 is selected. Where,?andτare output of flux linkage controller and torque controller,respectively.Further, indicates that reference flux linkage is more than the feedback,andτ=1 indicates that reference torque

is more than the feedback.Region I～VI denotes the position of stator flux linkage and the rotation is anti-clockwise.

3 Stator flux linkage control

3.1 Flux linkage basic control

The restriction of stator flux linkage satisfies PMSTM in EV drive often operates in constant power region,and then stator flux linkage in constant power region is deduced.The aptitude of stator flux linkage and the relationship between d-axis and q-axis currents in constant power region are expressed as follows[5].

3.2 Flux linkage optimization control

 Considering copper and iron loss,electrical loss model of PMSTM is shown in Fig.2[6,7].  where Ud,Uq——d-axis and q-axis components of  voltage id,iq——d-and q-axis components of armature current icd,icq——d-and q-axis components of iron loss

Further analysis shows that electrical loss is a concave function of stator flux linkage in steady state,  and the minimum electrical loss value can be obtained by adjusting stator flux linkage properly.The reduction of electrical loss means the improvement of PMSTM system efficiency.  According to the above analysis,a stator flux linkage optimization control approach in Fig.3 is proposed,which is realized by adjusting reference flux linkage based on the variation of measured inverter dc input power value on-line.When PMSTM system is in steady state,the inverter dc input power P(k)needs measuring at a fixed period,then a new reference flux linkage value

4 Simulation and experiment Based on the math model of PMSM[3]and proposed strategy,simulation results of electromagnetic torque and stator flux linkage are shown in Fig.4,which indicates that the good dynamic and steady state performance is obtained in both constant torque and power regions.PMSTM parameters in simulation are shown . A DTC system including stator flux linkage optimization controller is developed based on DSP TMS320F240 and a PMSM with 6.06?armature resistance,3000r/min rated speed,1.351mH d-axis

and q-axis inductance.  System efficiency in different control approach is shown in Fig.5.It can be seen that higher system efficiency is obtained by the optimization control,and the efficiency improvement is obvious at a low speed.  For example,the efficiency at 400r/min is more than 4% higher compared with the general control method.

The waveform of stator flux linkage components in Fig.6 fluctuates in a sinusoidal pattern,which illuminates that the locus of stator flux linkage is a good circular shape in space,and that is very effective to reduce the fluctuation of electromagnetic torque

and electrical loss.

   5 Conclusion

Special EV operating mode makes it difficult to acquire satisfied performance based on the usual DTC method.In this paper,the PMSTM DTC strategy combined with the operating mode of EV is presented.  Simulation and experimental results prove that the proposed strategy meets the requirements of EV drive.