International Journal of Scientific & Engineering Research, Volume 6, Issue 12, December-2015 946 ISSN 2229-5518 Modeling and Simulation of Direct Torque Control of Induction Motor Drive Nikhil V. Upadhye Mr.Jagdish G. Chaudhari Dr. S.B.Bodkhe th 4 sem student, M.Tech(PED) Research Scholar Professor Dept. of Electrical Engg. Dept. of Electrical Engg. Dept. of Electrical Engg. GHRCE, Nagpur(India). GHRCE, Nagpur(India). RCoEM, Nagpur(India). Abstract— The main focus of this paper is towards the analysis of Direct Torque Control (DTC) scheme with Space Vector Modulation (SVM) technique. Dynamic performance of the induction motor is improved by the DTC-SVM technique. Both motor and inverter are controlled in most efficient way by DTC. The fast control of torque and flux in Induction Motor (IM) without complexity is the feature of DTC. The selection of voltage vector for the desired resultant voltage vector is described and the IM is simulated for both DTC and without DTC system. Index Terms— Direct Torque Control (DTC), Induction Motor Modeling, Space Vector Pulse Width Modulation (SVPWM) —————————— —————————— Rotor voltage has zero magnitude as rotor windings are 1 INTRODUCTION short circuited. Modeling of induction motor in stator co- A wide range of speed and power is covered by squirrel ordinates by its voltage equations is given as cage rotor type induction motor and is generally used in = many industries. Use of vector control allows the control of 푑 푠 푉 induction motor same as that of separately excited dc 푉 � 푞 � motor. Two methods of Induction Motor Drive (IMD) = 푉 control are scalar and vector control method. The angular 휓푑푠 푠 � � speed of current, flux linkage and voltage in space vectors 휓 휓푞푠 are controlled by scalar control operating in steady state. = 푑푠 Instantaneous positions of current, voltage and flux linkage 푠 푖 of space vector are controlled using vector control [2]. 푖 �푖푞푠� AC motors are used in most of the electrical drive due to = development of low cost converters with progress taken 휓푑푟 place in power electronics and other advantages of ac 푟 � � IJSER휓 휓푞푟 motors over dc motors like absence of commutator, less = maintenance cost, volume, weight making motor 푑푟 푟 푖 inexpensive. Induction motors performs better in high 푖 �푖푞푟� torque and speed and are robust. AC motor’s high level of The above equations give stator voltage, stator flux linkage, speed control can be achieved with the help of the trans- stator current, rotor flux linkage and rotor current vector theory but for a wide range of applications its respectively. controllers are too complex hence Direct Torque Control Where, (DTC) is used as alternative. In AC motor drives DTC ωr – Rotor speed in elect.rad/sec become more popular technology. DTC is simple in Rr – Rotor resistance structure and has good dynamic behavior giving high Rs – Stator resistance performance and efficiency [3]. The stator and rotor flux linkages in the form of stator and High torque and flux ripples, variable switching frequency, rotor currents are as = + problem during starting and low speed operating = + conditions and current and flux distortion caused by stator 푠 푠 푠 푚 푟 Where, 휓 퐿 푖 퐿 푖 flux vector changing with the sector position are the 푟 푟 푟 푚 푠 Ls – Stator self inductance휓 퐿 푖 퐿 푖 drawbacks of conventional DTC. Lr – Rotor self inductance Lm – Mutual inductance Equations of Induction Motor in d-q axis reference frame 2 INDUCTION MOTOR MODEL are given as The stator and rotor voltage of induction motor are given as = + = + 푑푠 푠 푑푠 푑푠 = + =푉 푟+푖 푝휓+ 푠 푞푠 푠 푞푠 푞푠 푠 푑휓 푠 푠 =푉 푟+푖 푝휓 0 = 푉 . 푅 +푖 푑푟 푟 푑푟 푑푟 푟 푞푟 푑푑 푉′ 푟′ 푖′ 푝휓′ 푗 휓′ 푟 푞푟 푟 푞푟 푞푟 푟 푑푟 푑휓 푟 푟 푟 푉′ 푟′ 푖′ 푝휓′ − 푗 휓′ − 푗푗 휓 푅 푖 IJSER © 2015 푑푑 http://www.ijser.org International Journal of Scientific & Engineering Research, Volume 6, Issue 12, December-2015 947 ISSN 2229-5518 Zero sequence components do not take part in the Where, production of torque hence can be avoided. Flux linkages σ – Leakage factor are given as γsr – angle between stator and rotor flux and is similar to = + torque. = + 푑푠 푠 푑푠 푚 푑푟 By changing the angle between the two fluxes it is able to 휓 = 퐿 푖 +퐿 푖′ 푞푠 푠 푞푠 푚 푞푟 control the torque. The motor may saturate if the value of 휓 = 퐿 푖 +퐿 푖′ the flux should not kept constant and motor can’t deliver 휓′푑푟 퐿′푟푖′푑푟 퐿푚푖푑푠 rated power. Hence γsr controlled to control the torque. 휓′푞푟 퐿′푟푖′푞푟 퐿푚푖푞푠 ωψqs L L’lr (ω-ωr)ψ’qr ids rs ls r’r i’dr 3 DIRECT TORQUE CONTROL DTC has simple control structure, robustness, fast dynamic Lm Vds response etc. By selecting the optimum inverter voltage ψdr V’dr=0 ψds vector the control of the stator flux linkage and electromagnetic torque has been done in DTC. The switching look-up table provides fast torque response, low harmonic losses and low inverter switching frequency. The ωψds L L’ (ω-ωr)ψ’dr iqs rs ls lr r’r i’qr direct torque control (DTC) scheme is represented in figure. Ref. Torque Inverter Torque Torque Comparator Status Lm Optimal DC Vqs Switching ψqs ψqr V’qr=0 Hystersis Flux Logic Window AC Comparator Flux Status Ref. Flux Fig.1 d-q Equivalent Circuit of IM. Flux Position q-axis Switching Position Actual Flux Adaptive AC Motor Motor ψ DC link Voltage s Model ωe Actual Current ψqs Torque Fig.3 Basic DTC Scheme. It consists of torque comparator and flux comparator block IJSERalong with optimal switching logic. The main part of the DTC is accurate adaptive motor model. The actual torque, ψr ωe stator flux and actual aped are estimated by adaptive motor ψqr γ model. The actual values are compared with the reference sr values of stator flux and torque which gives error values. The two level and three level hysteresis blocks are fed with stator flux and torque errors respectively which are then θfs fed to the optimal switching logic block and then switching θfr d-axis logic is given to the inverter. The main objective of the DTC ψds ψdr technique is to control the flux linkage directly. Fig.2 Vector Diagram for IM. 4 PRINCIPLE OF DTC In three phase induction motor, the electromagnetic torque Traditionally field oriented control (FOC) technique has is given as been used for the control of Induction Motor torque which 3 P involve transformation of stator currents into d-q reference T = ψ . i ψ . i 2 2 frame aligned. Both torque and flux errors are determined by the instantaneous value of flux and torque calculated e ds qs qs ds 3 P L � − � from the terminals of induction motor. The selection of T = ψ . ψ ψ . ψ 2 2 LmL proper voltage vectors to drive induction motor with the e � qs dr − ds qr� help of switching look-up table is based on these errors. 3σPs Lr T = ψ ψ sin The estimation of flux is based on the stator voltage vector 2 2 LmL (stator resistance neglected) and is given as e � s�� r� γsr σ s r IJSER © 2015 http://www.ijser.org International Journal of Scientific & Engineering Research, Volume 6, Issue 12, December-2015 948 ISSN 2229-5518 give the required value of flux. Hence we are able to ψ = ( ) = control stator flux. The angle between the stator and rotor ∆푡 �s flux is automaticallyψ being controlled and hence the torque �푠 � 푉�푠 − 횤̅푠푅푠 푉�푠∆푑 The output voltage is0 regulated using PI controller for is controlled. Let the flux vector at t=0 is in sector I and both achieving desired stator current and therefore torque. The flux and torque have to increased i.e. dψ=1 and dm=1 then transient response of the torque controller is limited by PI from Fig.4 we can analyze that the vector V2 should be controller. To achieve a desired output torque in DTC an applied so as to get the resultant flux vector. effective induction motor has to be used. The values of V3(- + -) V2(+ + -) stator flux and output torque can be achieved by current and voltage measurements with the help of induction II I motor within fixed time. This desired voltage is then synthesized by Space Vector Modulation (SVM). III ψ ̅s V7(+ + +) V̅s ∆t ψs V4(- + +) V1(+ - -) 5 DTC WITH SPACE VECTOR PULSE WIDTH MODULATION Space Vector Pulse Width Modulation (SVPWM) is one of IV V (- - -) ψr VI the best and advanced techniques used in variable 0 frequency drive. The main objective of this technique is to have variable output with minimum harmonics and V maximum fundamental component. It gives minimum total V6(+ - +) harmonic distortion (THD) and higher voltage which is V5(- - +) further fed to motor. The transformation of the three phases to their equivalent two phase quantities has been done Fig.4 Output voltage vectors. using this modulation technique in stationary or synchronously rotating frame. The reference vector If only flux has to be increased keeping torque constant magnitude is derived from these two components which then the angle between stator and rotor flux should not be modulates the inverter output. The three phase sinusoidal changed for this we have to apply the zero voltage vector voltage component of stationary frame is given as hence for dψ=1 and dm=0 the vector V7 has to be applied. V = V sin t The voltage vector should be decided such that the inverter V = V sin( t ) a m has minimum number of switching.