Experimental Direct Torque Control Induction Motor Drive with Modified Flux Estimation and Speed control Algorithm.

Transcription

1 Experimental Direct Torque Control Induction Motor Drive with Modified Flux Etimation and Speed control Algorithm. Bhoopendra ingh, Shailendra Jain 2, Sanjeet Dwivedi 3 (RGTU, Bhopal), 2 (MANIT Bhopal), 3 (Danfo Power Electronic, Denmark) ABSTRACT An enhancement in dynamic performance of a traditional direct torque control (DTC) drive can be achieved by a robut peed control algorithm while the teady tate performance depend upon an efficient flux etimation algorithm. In thi paper a conventional PI controller in the peed control loop i replaced by a liding mode peed controller and a modified low pa filter baed digital integration algorithm with tunable cut off frequency i propoed for flux etimation. The propoed tudy i invetigated through imulation and experimentally validated on a tet drive. Keyword - Direct torque control, Induction motor, modified low pa filter, liding mode control I. INTRODUCTION In a direct torque control induction motor drive, the baic concept i to control both tator flux and electromagnetic torque of the machine imultaneouly by the application of one of the ix active full voltage vector and two zero voltage vector generated by an inverter. The tator flux and torque track their reference value within the limit of two hyterei band with two hyterei comparator and a heuritic witching table to obtain quick dynamic repone []-[3]. The teady tate a well a the dynamic performance of the drive i cloely related to the efficient implementation of flux and peed control algorithm. There are few well-known method to etimate the tator flux. Mot of them are voltage model baed [3], where the flux and torque are etimated by ening tator voltage and current. The method baed on voltage model are mot preferable for enor le drive ince thee method are le enitive to the parameter variation and doe not require motor peed or rotor poition ignal. However, the etimation of tator voltage when the machine i operating at low peed introduce error in flux etimation which alo affect the etimation of torque and peed in cae of enor le drive [4]-[]. In a conventional DTC drive the baic voltage model baed flux etimation i carried out by integrating the back emf of the machine. A pure integrator ha the following limitation.. Any tranduction error in meaured tator current due to offet introduce DC component and hence reult in integrator aturation. 2. Integration error due to incorrect initial value. A commonly employed olution i to replace a pure integrator with a low pa filter [] [2], however it i achieved at the expene of deteriorated low peed operation of the drive, when the operating frequency of the drive i lower than the cut off frequency of the low pa filter. Flux etimation baed on the current model i mot uitable for low peed operation [3], however it i a parameter dependent method, which require rotor peed or poition. Thu parameter independent operation, which make a DTC drive more robut and reliable compared to a FOC drive, get affected when current model baed flux etimation i implemented. In thi paper a low pa filter baed digital integration algorithm with tunable cutoff frequency i implemented for flux etimation and a liding mode peed control algorithm i propoed to improve the dynamic repone of the drive [4]. Thu the aim of thi paper i to enhance the teady tate a well a the dynamic performance of the drive while maintaining the implicity of control algorithm. The propoed control trategy i illutrated by imulation and validated through experimental reult. II. DTC OPERATION According to the DTC principle, an independent control of torque and flux can be achieved by the application of appropriate voltage vector in uch a way that the error between the etimated torque and flux with their repective reference value remain within the limit of hyterei comparator. The deired voltage vector to compenate the error are elected baed on the output of the torque and flux hyterei comparator a well a the locu of tator flux vector. From the baic equation governing induction motor operation tator flux λ i given by λ = V R I dt Neglecting the drop in tator reitance, λ = V t Where t i the time interval of application of the applied voltage vector. Electromagnetic torque in an Induction motor i given by () T e = P L m σl L r Where σ = L m 2 L L r λ λ r in δ () ISSN: P a g e

2 It can be concluded from () that an increment in torque can be achieved by increaing the angle between tator and rotor flux vector. Splitting the vector λ into horizontal and orthogonal component it can be concluded that orthogonal component of λ i reponible for torque control and the horizontal component control the flux a hown in Fig.. q Torque increae r V t Flux increae Fig. Flux and torque control by the applied voltage vector in a DTC drive. III.FLUX ESTIMATION A well-known olution to the dc-offet and initial value problem with a pure integrator i to replace it with a low pa filter (LPF) with an appropriate cut off frequency. The mathematical expreion of the low pa filter with a cut off frequency of ω c can be given by (2) (5) λ α = E +ω α (2) c λ β = +ω c E β (3) d The value of the cutoff frequency ω c ha to be judicially choen, becaue a cutoff frequency higher than operating frequency of the drive lead to flux ditortion at low peed. A poible olution to thi problem i a modified LPF baed flux etimator with cutoff frequency proportional to the ynchronou frequency, a hown in Fig.2.The relation between the cutoff frequency and ynchronou frequency ω e can be given by a imple relation ω c = kω e.the typical range of k lie between. and.5 and the ynchronou frequency can be given by (6). ω e = E β λ α E α λ β λ 2 (6) IV. SPEED CONTROL A liding mode (SM) peed controller i propoed for obtaining a robut dynamic performance of the DTC drive. The chematic diagram of the peed controller i given in Fig.2.The equation for modeling the SM peed controller are given by equation (7) (9). The rotor peed error can be given a X = N r (k) N r (k) (7) Where N r (k) i the reference peed at k th ampling intant and N r (k) i the actual rotor peed. The rate of change of peed error in dicrete form i given by (6) X 2 = X k X (k ) Δt (8) Which can be expreed in dicrete form a λ α k = + t ω c t E α k + λ α (k ) (4) λ β (k) = + t ω c t E β k + λ β (k ) (5) E S c c ab K E E 2 2 E S c X * N r + - N r + t - X2 Sliding Mode Controller Tk ( ) * T /Z limiter Unit delay Fig. 2 Flux etimation by modified low pa filter liding mode peed controller ISSN: P a g e

3 The output of the liding mode controller i given a T k = C 3 X k Y k + C 4 X 2 (k)y 2 (k) (9) Reference torque command T*(k) i obtained after limiting the output of the liding mode controller. In the eqn. (9), C 3 and C 4 are gain of liding mode controller and Y (k) and Y 2 (k) can be given a: Y (k) = + if ZX (k) Y (k) = - if ZX (k) < And, Y 2 (k) = + if ZX2 (k) Y 2 (k) = - if ZX2 (k) < Z i witching line on which the liding mode action occur and i given by () Z=C X (k) + C 2 X 2 (k) () Where C and C 2 are adjutable parameter. V. RESULTS AND DISCUSSION The teady tate a well a dynamic performance of the propoed drive are invetigated through imulation uing Matlab/Simulink and are further validated experimentally. A tet drive et up developed in the laboratory i hown in Fig.3.The experimental tet drive etup conit of the following element: ) Machine unit; a.75 kw, 4 V, 5-Hz quirrel-cage induction motor with a haft mounted tachogenerator for peed ening coupled with dc generator for loading. 2) A power module with MOSFET baed voltage ource inverter with Hall Effect enor and gate drive circuitry. 3) dspace DS4 control board. The parameter of the motor for experimentation are a follow.r =.75 Ω,R r = 9.28 Ω, L = L r = 5.9 mh, P=4 and L m = mh. The ampling time of the DTC experiment i taken a µ while the dead time for the witche i µ. The value of torque and flux hyterei comparator bandwidth i take a.5 Nm and.5 wb. All experimental reult are recorded uing the Control Dek platform of dspace DS4 by aving the target variable a mat file. 3 phae ac ource Hot PC with dspace 4 controller v dc PWM Switching ignal Connector panel for interfacing ia Fig.3 Experimental tet drive et up. i b 3 phae I.M. with loading and peed feedback The dynamic repone of the drive wa invetigated for peed reveral and tep increment in operating peed at full load.fig.4 how the experimental and imulated reult for a peed reveral at rated peed (-49 rpm to +49 rpm). It can be verified from the experimental reult that the time m Ref.peed (rpm) Me. Speed (RPM) Me. Speed (RPM) Ref.peed (rpm) (c) (d) Fig.4 Experimental and imulated peed reveral dynamic ref. peed (experimental) meaured rotor peed (experimental) (c) ref. peed (imulation reult) (d) rotor peed (imulated) ISSN: P a g e

4 peed(rpm) flux peed(rpm) flux peed (rpm) 4 2 ref.peed mea. peed mea. peed refrence peed Fig.5 Experimental dynamic repone for a tep change in peed command input. dynamic repone for a two-tep reference peed input in forward direction dynamic repone for a continuou variation in reference peed in revere direction (c) (d) fluxd and fluxq (e) (f) Fig.6 Experimental reult to validate the effectivene of the flux etimation algorithm peed in rpm flux repone for tep reduction in operating peed for modefied LPF method (c) peed in rpm (d) flux repone of conventional LPF method (e) teady tate Stator flux component (λ α, λ β ) (f) flux trajectory. taken by the drive to attain teady tate peed after reveral i approximately 2 ec. Similar performance i alo verified from the imulation reult hown in Fig.4(c) and 4(d). The experimental drive repone for a tep input peed command i hown in Fig.5 and it can be verified from the experimental reult that the actual peed reache the reference peed with minimum overhoot and teady tate error. To further tudy the peed tracking performance of the peed controller, the drive i ubjected to a continuou change in reference peed hown in Fig.5, which i achieved by the control dek platform of dpace having the feature of online change in an input parameter. From Fig.5 it can be een that the actual rotor peed track the reference peed from - rpm zero rpm.thu a fat dynamic repone validate the uperiority of the peed control algorithm. The invetigation on the propoed DTC drive are ummarized in Table. Table. ummary of the propoed invetigation Performance parameter Flux Repone Speed control Remark. Improved flux repone due to the modified flux etimation method having tunable cutoff frequency during udden variation in operating peed 2.Circular tator flux trajectory A fat dynamic repone with liding mode peed controller. ISSN: P a g e

5 CONCLUSION An enhancement in teady tate and dynamic performance of a DTC drive i achieved by implementing a modified integration algorithm baed on a low pa filter with tunable cutoff frequency. An improved flux repone during low peed operation with a circular flux trajectory ha alo been achieved by the propoed technique. Implementation of liding mode algorithm in the peed control loop ha reulted into a fat dynamic repone of the drive. ACKNOWLEDGEMENTS Thi work wa funded and upported by the All India Council of Technical Education, reearch promotion cheme (AICTE-RPS). REFERENCES [] I.Takahahi and T.Noguchi, A new quick-repone and high efficiency control trategy of Induction Motor, IEEE Tranaction on Indutrial application, vol. 22, no 5, pp ,986. [2] M. Depenbrok, Direct elf-control (DSC) of inverterfed induction machine, IEEE Tran. Power Electron. vol. 3, no. 4, pp , Oct [3] G. S. Buja and M. P. Kazmierkowki, Direct Torque control of a PWM inverter-fed AC motor-a Survey, IEEE Tran. Ind. Electron., vol. IE-5, no. 4, pp , Aug. 24. [4] M. Shin, D.S. Hyun, S.B. Cho, and S.Y. Choe, An improved tator Flux etimation for peed Senorle tator Flux orientation control of induction motor, IEEE Tran. Power Electron.,vol. 5, pp.32-38, [5] E. D. Mitronika, A. N. Safaca, An Improved Senorle Vector-Control Method for an Induction Motor, IEEE Tran. Ind. Electron., Vol. 52, No. 6, Dec. 25. [6] J. Holtz, Senor le poition control of induction motor An emerging technology, IEEE Tran. Ind. Electron., vol. 45, pp , Dec [7] J. Holtz, Drift and Parameter Compenated Flux Etimator for Peritent Zero-Stator-Frequency Operation of Senorle-Controlled Induction Motor, IEEE Tran. Ind. Electron., vol. 39, no. 4, pp. 52 6, Aug. 23. [8] K. D. Hurt, T. G. Habetler, G. Griva, and F. Profumo, "Zero-peed tachole IM Torque control: Simply a matter of tator voltage integration", IEEE Tran. Ind. Application, vol. 34, pp ,998. [9] B. K. Boe and N. R. Patel, "A programmable cacaded low-pa filter-baed Flux ynthei for a tator Flux-oriented vector-controlled induction motor drive", IEEE Tran. Ind. Electron., vol. 44,pp.4-43,997. [] J. Holtz and J. Quan, "Senorle vector control of induction motor at very low peed uing a Nonlinear inverter model and parameter identification", Conf. Rec. IEEE-IAS Annu. Meeting, vol. 4, pp ,2. [] J. Hu and B.Wu, New integration algorithm for etimating motor Flux over a wide peed range, IEEE Tran. Power Electron., vol. 3, pp , Sept [2] M. Hinkkanen, J Luomi, Modified integrator for voltage model Flux etimation of induction motor, IEEE Tran. Ind. Electronic, Vol. 5, No. 4,Aug. 23. [3] Bertoluzzo, M.; Buja, G.; Meni, R, A Direct Torque Control Scheme for Induction Motor Drive uing the Current Model Flux Etimation, Conf. Rec. IEEE Int. ympoium, pp. 85-9, Nov. 26. [4] Singh.B, Singh.B.P. and Dwivedi.S, DSP baed implementation of Sliding Mode Speed Controller for Vector Controlled Permanent Magnet Synchronou Motor drive, in Proc. IICPE-26, 26, pp ISSN: P a g e