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					                                                                    International Journal of Advances in Science and Technology,
                                                                                                               Vol. 4, No.4, 2012

  Simplified SVPWM Algorithm Based Direct Torque
Controlled Induction Motor Drive For Reduced Harmonic
                      Distortion
                            Chalasani Hari Krishna1, J Amarnath2, and S Kamakshiah3
         1
          Department of Electrical and Electronics Engineering, Mother Teresa Inst. of Sci. and Tech., Sathupally, AP,
                                                             India
                                               chalasaniharikrishna@gmail.com
                      2
                       Department of Electrical Engineering, JNTUH, Hyderabad, Andhra Pradesh, India
                                                amarnathjinka@rediffmail.com

                                         JNTUH, Hyderabad, Andhra Pradesh, India
                                             kamkshaiahsatram@yahoo.co.in



                                                         Abstract

          In conventional direct torque control (CDTC), the stator flux and torque are directly controlled by
       the selection of optimal switching modes. The selection is made to restrict the flux and torque errors
       in corresponding hysteresis bands. In spite of its fast torque response, it has more flux, torque and
       current ripples in steady state. To overcome the ripples in steady state, a space vector based pulse
       width modulation (SVPWM) methodology is proposed in this paper. In the proposed SVPWM
       method, To avoid the requirement of reference voltage vector, sector identification and angle
       determination by a concept of imaginary switching times. This simplifies the computational burden
       and improves the total harmonic distortion. To strengthen the described one, simulation results are
       presented and compared the two-level SVPWM based DTC drive to CDTC drive.

       Keywords: CDTC,SVPWM,MATLAB/SIMULINK..

       1. Introduction

                         In the middle of 1980’s, a technique for the torque control of induction motors was
       developed and presented by I.Takahashi as direct torque control (DTC) [1], which is preferable for low
       and medium power range applications. Depending on one of the voltage space vectors of voltage
       source inverter the torque and flux are controlled independently in CDTC which keeps the stator torque
       & flux within the limits of hysteresis bands. CDTC consists of adaptive motor model, torque and flux
       hysteresis comparators, optimal switching table and a voltage source inverter. CDTC is able to produce
       quick torque response hence this control algorithm is increasingly being used in the industry and is
       considered to be next generation motor control method [2]. Due to the torque and flux hysteresis bands
       variable switching frequencies are produced and leads to large ripples in torque, flux and current during
       steady state operation.
                         In order to improve the performance of CDTC in terms of torque, flux and current
       ripples, a discrete space vector modulation (DSVM) is proposed in [3]. Where higher number of
       voltage vectors is generated and sensible reduction in current, torque and flux ripples in entire speed
       ranges are possible.
                         In proposed SVPWM-DTC consists of adaptive motor model, speed and torque
       proportional plus integral (PI) controller’s, reference voltage vector calculator and VSI. A voltage
       reference generated based upon the errors of torque and flux and reference voltage vector is realized by
       using the principle of space vector pulse width modulation (SVPWM) to achieve constant switching




  April Issue                                            Page 31 of 83                                        ISSN 2229 5216
                                              International Journal of Advances in Science and Technology,
                                                                                            Vol. 4, No.4, 2012
     operation [4]-[5]. And also torque and current ripples are eliminated. To avoid the requirement of
     reference voltage vector, sector identification and angle determination are determinate by using the
     concept of imaginary switching times in [6] and this concept is used for different switching sequences.
                       The main objective of this paper is to develop SVPWM algorithm for direct
     torque controlled induction motor drive based on imaginary switching times to reduce the
     steady state ripples, complexity and computational time.



     2. Modeling of induction motor

                       To study the dynamic performance of induction motor, a motor model has been
     developed in stationary reference frame by using (1)-(5) written in terms of space vectors:

                                              d s
                                Vs  Rs is                                           (1)
                                                dt
                                            ds
                                0 = Rr ir        j r r                            (2)
                                             dt
                                s  Ls is  Lm ir                                    (3)


                                r  Lr ir  Lm is                                    (4)


                                Te 
                                       3P
                                       22
                                           
                                           s X is  
                                                     3P
                                                     22
                                                         dsiqs  qsids            (5)



     3. Conventional DTC

                        The block diagram representation of CDTC is shown in fig 1. The stator currents and
     DC bus voltage are sampled at every sampling inverter of time. The d-q components of stator voltage
     space vector are calculated by using inverter switching position and DC link voltage. Speed, torque,
     stator flux and flux angle are estimated in the adaptive motor model by considering voltages, currents
     to the drive. The estimated torque and flux are compared with their corresponding hysteresis
     comparators respectively. The number of sector where the stator flux space vector is located and the
     outputs of hysteresis comparators are fed to optimal switching table to select an appropriate voltage
     vectors. In next sampling time this voltage space vector is applied to inverter.

     4. Proposed DTC

                       The block diagram of proposed DTC is shown in fig.2. Reference stator flux vector’s
     speed is derived by adding actual rotor speed and additional slip speed. After each sampling inverter
     actual stator flux vector s proposed by the adaptive motor model is corrected by error and it tries to
                                               *
     attain the reference flux space vector s , and the flux error is minimized in each sampling interval.
                        Reference and actual values of d-q axes stator fluxes are compared in the reference
     voltage vector calculator block and the error in the d and q- axes stator flux vectors are obtained as
                                                   ds  ds  ds
                                                           *                  (6)

                                                    qs  qs  qs
                                                            *
                                                                              (7)
     The appropriate reference voltage space vectors due to flux error and stator ohmic drop are given as




April Issue                                        Page 32 of 83                                     ISSN 2229 5216
                                                      International Journal of Advances in Science and Technology,
                                                                                                 Vol. 4, No.4, 2012
                                                                    ds
                                                 vds  Rsids 
                                                  *
                                                                                 (8)
                                                                    Ts
                                                                    qs
                                                 vqs  Rsiqs 
                                                  *
                                                                                 (9)
                                                                    Ts




          Reference
          Speed +
                                           Te*        ΔT
                                 PI       +                           Optimal
                                                  -                  Switching
                      -                    Te                          Table
                Motor
                Speed        Ψs*      Δψ
                             +
                                      -
                                 Ψs                                                     Vds, Vqs
                                                             Sector                     Calculator

                                                           Calculation


                                                                 Adaptive
                                                                  Motor                              3
                                                                  Model                       2

                                                                                                                    IM


                                        Fig. 1 Block Diagram of CDTC
                        The reference voltage vectors from d-q form are transformed to three phase reference
     voltages in SVPWM block from which actual switching times of each inverter leg are calculated.
                        In space vector approach, the reference voltage vector is synthesized by the time
     average over a sampling time period of two adjacent active states and two zero states. The conventional
     switching sequences, which uses reference frame transformation and it needs longer calculation time as
     a result increase the complexity of algorithm. To reduce the complexity involved in conventional space
     vector approach and memory size, simple SVPWM algorithm has been developed using the concept of
     imaginary switching times. The proposed approach is based on the instantaneous value of reference
     voltages Van, Vbn and Vcn which are transformed from d-q components of reference voltage vectors.
     Then the imaginary switching time period’s proportional to the instantaneous values of reference phase
     voltages are
                     T                              T                          T 
               Tan   s Van ;
                     V                        Tbn   s Vbn ;
                                                      V                    Tcn   s Vcn
                                                                                   V                   (10)
                      dc                             dc                         dc 
     Where Ts is the sampling time and Vdc is dc link voltage.
                       The maximum and minimum values of imaginary switching times are calculated in
     every sampling time are given as
                                    Tmax  Max(Tan,Tbn,Tcn, )     (11)
                                          Tmin  Min(Tan,Tbn,Tcn, )                    (12)
     Then the active vector switching times T1 and T2 are expressed as
                                          T1  Tmax  Tx : T2  Tx  Tmin (13)
     Where    Tx  (Tan ,Tbn ,Tcn ) and is either maximum or minimum switching time.




April Issue                                                  Page 33 of 83                                      ISSN 2229 5216
                                              International Journal of Advances in Science and Technology,
                                                                                         Vol. 4, No.4, 2012
     The zero vector switching time is calculated as TZ = Ts – T1 – T2.     (14)

     The CSVPWM uses 0127-7210 clamping sequences and employs equal division of zero state times,
     which results in least harmonic distortion due to unequal divisions.




Reference
 Speed                     Te*                   ωsl     ωe         θ                    Vds*
                      PI   +               PI                   ʃ           Reference           S
      +                                          +
              -                                                              Voltage            V
                               -   Te                +   ωr                  Vector             P
                  Actual                                                    Calculator          W
                  Speed                                       Ψs*
                                                                                                M
                                                                                         Vqs*
                                                                        Ψ
                                                                        ψ                           Vds,Vqs
                                                                                                    Calculator


                                                                        Adaptive
                                                                         Motor
                                                                                                     3
                                                                         Model                  2


                                                                                                                         IM




                                        Fig.2.   Block Diagram of proposed DTC.

     5. Simulation results:

               A numerical simulation has been carried out by using Matlab/Simulink. Sampling time of
     125µs and ode4 (runge-kutta) methods are used for a fixed step size of 10µs. the reference flux taken
     as 1wb and starting torque is limited to 45 n-m. A 4 Kw, 1200 rpm, 4 pole 3-phase induction motor
     with parameters as Rs= 1.57Ω, Rr= 1.21Ω, Ls=0.17 H, Lr=0.17H, Lm=0.165H, J=0.089 are considered
     for the proposed scheme.




April Issue                                              Page 34 of 83                                           ISSN 2229 5216
                                   International Journal of Advances in Science and Technology,
                                                                              Vol. 4, No.4, 2012




               Fig. 3. Starting Transients under No-Load condition of CDTC.




              Fig.4. Steady State Transients under No-Load condition of CDTC.




April Issue                             Page 35 of 83                                  ISSN 2229 5216
                                  International Journal of Advances in Science and Technology,
                                                                             Vol. 4, No.4, 2012




                               Fig.5. Flux Locus of CDTC.




              Fig. 6. Starting Transients under No-Load condition of SVPWM.




April Issue                            Page 36 of 83                                  ISSN 2229 5216
                                    International Journal of Advances in Science and Technology,
                                                                               Vol. 4, No.4, 2012




              Fig. 7. Steady State Transients under No-Load Condition of SVPWM.




               Fig. 8. Steady State Transients in SVPWM under Load of 30N-m.




April Issue                              Page 37 of 83                                  ISSN 2229 5216
                              International Journal of Advances in Science and Technology,
                                                                         Vol. 4, No.4, 2012




              Fig. 9. Speed reversal SVPWM from 1200 to -1200 rpm.




                         Fig.10. Flux Locus of SVPWM.




April Issue                       Page 38 of 83                                   ISSN 2229 5216
                                               International Journal of Advances in Science and Technology,
                                                                                          Vol. 4, No.4, 2012
     6. Conclusions
     The conventional direct torque control is simple and produces fast torque response, beside’s torque,
     flux and current ripples in steady state operation. In this paper, the switching sequences are developed
     by using the imaginary switching times, where sector calculation and angle information are not
     required and simplifies computational burden. From the simulation results, it is observed that torque,
     flux and current ripples in steady state are reduced and system performance is improved on compared
     with CDTC. And with the proposed method, harmonic distortion in line current is also reduced.

     7. References

     [1] Isao Takahashi, Toshihiko Noguchi, “A new quick-response and high-efficiency control strategy
         of an induction motor” , IEEE Trans Ind Appl, Vol.1A-22, No.5, pp.820-827, sep/oct, 1986.
     [2] James N. Nash, “Direct Torque Control, induction motor vector control without an encoder”,
         IEEE Trans. IND. Appl., Vol 33, pp.333-341, Mar/Apr 1997.
     [3] Domenico Casadei, Giovanni Serra, Angelo Tani, “ Impementation of direct torque control
         algoritm for induction motors based on discrete space vector modulation” IEEE Trans. PE, Vol.15,
         No.4, pp. 769-777, July 2000.
     [4] Thomas G. Habetler, et al, “ direct torque control of induction machines using space vector
         modulation”, IEEE Trans. Ind. Appl., Vol.28, No.5, pp 1045-1053, Sep/Oct. 1992.
     [5] L. Tang, L.Zhong, M.F. Rahman, Y.Hu, “ An investigation of a modified direct torque control
         strategy for flux and torque ripple reduction for induction machine drive system with fixed
         switching frequency” IEEE proc. Ind. Appl. Society, pp 837-844, 2002.
     [6] Dae-Woong Chung, Joohn-Sheok Kim, Seung-Ki Sul, “Unified Voltage Modulation Technique
         for Real-Time Threee-Phase Power Convertion” IEEE Trans. Ind. Appl. VOl.34, No.2 pp374-380,
         Mar/Apr, 1998.


     Authors Profile


                                Chalasani Hari Krishna was born in 1982. He graduated from Jawaharlal Nehre
                                Technological University, Hyderabad in the year 2003. He received M.E degree
                                from Satyabama University, Chennai in the year 2005. He presently Associate
                                Professor in the Department of Electrical and Electronics Engineering at Mother
                                Teresa Institute of Science and Technology, India. His research area includes DTC
                                and Drives.




                                Dr. J Amarnath graduated from Osmania University in the year 1982. M.E from
                                Andhra University in the year 1984 and Ph.d from J.N.T.University, Hyderabad in
                                the year 2001. He is presently Professor in Electrical Engineering Department,
                                JNTU College of Engineering, Hyderabad, India. He presented more than 80
                                research papers in various national and international conferences and journals. His
                                research areas include Gas Insulated Substations, high Voltage Engineering, Power
                                Systems and Electric Drives.




April Issue                                         Page 39 of 83                                        ISSN 2229 5216
                            International Journal of Advances in Science and Technology,
                                                                       Vol. 4, No.4, 2012


              Dr. S. KAMAKSHAIAH is from JNT University, Hyderabad and worked
              as Professor, Head of the Department & Chairman of Electrical Sciences.
              He annexed Gold Medal for getting 1st rank in SV University at B.Tech
              level. He was also awarded Gold Medal for Ph.D. work from
              I.I.Sc.Bangalore for its industrial application. He was the Best Teacher
              award winner of the year 1997-98 which was promoted by A.P.State
              Government. He has got 40 Technical Publications in International level. He
              is also an author of three text books Introduction to Electrical Engineering,
              Basic Electrical Engineering and Electrical Technology with his co – author
              Prof. Naidu, and HVDC Transmission published by Tata McGraw Hill. He
              carried research work in the area of Vaccum arcs in Triggered Vaccum
              Gaps (TVG). At present he is working on the problems of Gas Insulated
              Substation (GIS) ,HVDC and Electric Drives. Presently he is working as
              Director, Krishna Chaitanya Institute of Technology & Science, Markapur,
              He visited various countries like Canada, Germany and Doha and presented
              papers at IEEE International Conference at ATLANTA, USA in the year
              June 2010..




April Issue                      Page 40 of 83                                    ISSN 2229 5216

				
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