International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering
MenuISSN ONLINE(2278-8875) PRINT (2320-3765)
ISSN ONLINE(2278-8875) PRINT (2320-3765)
Anjana Manuel1, Jebin Francis2
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This paper proposes design and simulation of a direct torque controlled induction motor drive system based on space vector modulation technique for ripple reduction.Direct Torque Control is a control technique used in AC drive systems to obtain high performance torque control. The principle is based on simultaneously decoupling the stator flux and electromagnetic torque. DTC drives utilizing hysteresis comparators suffer from high torque ripple and variable switching frequency.The proposed SVM basedDTC reduces torque ripples and preserve DTC transient merits suchas fast torque response. The basis of the SVM-DTC methodology is the calculation of the required voltage space vector to compensate the flux and torque errors and its generation using the SVM at each sample period.The performance of thismethod is demonstrated by simulation using Matlab/Simulink software. Simulation results presented in this paper show the torque,flux linkage and stator current ripple decreases with the proposed SVM-DTC algorithm.
Keywords |
| Direct Torque Control, Space vector modulation, Induction Motor |
INTRODUCTION |
| Induction Motors(IMs) are widely used in industrial, commercial and domestic applications as they are simple,rugged, lowcost and easy to maintain. Since IMs demands well control performances: precise and quick torque and flux response, large torque at low speed, wide speed range, the drive control system is necessary for IMs.Though DC motor is able to provide desired performance, its maintenance and unsafe in explosive environment restricts its use. In 1970s, field oriented control (FOC) scheme proved success for torque and speed control of induction motor. Decoupling of two components of stator currents (flux and torque producing components) is achieved as DC machines to provideindependent torque control. Hence the scheme proves itself superior to the DC machine. The problem faced by FOC scheme is complexity in its implementation due to dependence of machine parameters, reference frame transformation. Later DTC was introduced. The method requires only the stator resistance to estimate the stator flux and torque. |
| In conventional DTC, electromagnetic torque and flux are independently controlled by selection of optimum inverterswitching modes. The selection of optimum inverter switching modes is made to limit the electromagnetic torque and flux linkage errors within the torque and flux hysteresis bands. The basic DTC scheme consists of two comparators with specified bandwidth, switching table, voltage source inverter, flux and torque estimation block. Like every control method has some advantages and disadvantages, DTC method has too.Some of the advantages are lower parameters dependency, making the system more robust and easier implements and the disadvantages are difficult to control flux and torque at low speed, current and torque distortion during the change of the sector , variable switching frequency, a high sampling frequency needed for digital implementation of hysteresis controllers, high torque ripple. The torque ripple generates noise and vibrations, causes errors in sensor less motor drives, and associated current ripples are in turn responsible for the EMI. The reason of the high current and torque ripple in DTC is the presence of hysteresis comparators together the limited number of available voltage vectors. |
| If a higher number of voltage vectors than those used in conventional DTC is used, the favourable motor control can be obtained. Because of complexity of power and control circuit, this approach is not satisfactory for low or medium power applications. An another solution to minimize torque ripple is the space vector modulated DTC scheme. The objective of the DTC-SVM scheme, isto estimate a reference stator voltage vector V*and modulate it by SVM technique, in orderto drive the power gates of the inverter with a constant switching frequency, so that in everysampling time, inverter can produce a voltage vector of any direction and magnitude. Thatmeans the changes in stator flux would be of any direction and magnitude and consequentlythe changes in torque would be smoother. |
II. PRINCIPLE OF DIRECT TORQUE CONTROL |
| The basic functional blocks used to implement the DTC scheme in an induction motor is shown in Fig. 1. Three phase AC supply is given to the diode bridge rectifier which produces a DC voltage. A high value dc link capacitor is used to reduce the ripple content in the DC voltage. The filtered DC is the power supply to the inverter switches. The IGBT inverter switches are controlled by the direct torque control algorithm. The output of the inverter is connected to the stator terminals of induction motor. |
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C. Switching Table |
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III. PRINCIPLE OF SPACE VECTOR MODULATED DTC SCHEME |
| The basic functional blocks used to implement the DTC-SVM scheme is shown in Fig. 3. In the proposed system, flux and torque estimators are used to determine the actual value of the flux linkage and torque. Instead of the switching table and hysteresis controllers, a PI controller and numeric calculation are used to determine the duration time of voltage vectors, such that the error vector in flux and torque can be fully compensated. Two proportional integral (PI) type controllers regulate the flux and torque error. Since the controllers produce the voltage command vector, appropriate space voltage vector can be generated with SVM and fixed switching frequency can be achieved.. The output of the PI flux and torque controllers can be interpreted as the reference stator voltage components in d-q co-ordinate system. These dc voltage commands are then transformed into stationary frame (α-β), the command values are delivered to SVM block. The SVM block performs the space vector modulation of Vs to obtain the gate drive pulses for theinverter circuit. |
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A. Space Vector Modulation |
| Space Vector Modulation (SVM) Technique has become the most popular and important PWM technique for three phase Voltage Source Inverters(VSI) for the control of ACmotors. It has been shown to generate less harmonic distortion in the output voltagesand or currents applied to the phases of an AC motor and to provide more efficient useof supply voltage.The SVM technique is used to create a reference vector by modulating the switching time of space vectors in each of the six sectors shown in Fig. 4. The space voltage vector is produced by two active vectors, and null vectors. |
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IV. SIMULATION AND RESULTS |
| The DTC and SVM-DTC scheme for induction motor are simulated using Matlab/Simulink and their results have been compared. The motor parameters used for simulation aregiven. |
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A. Simulink Model of Direct Torque Control |
| The simulation of DTC was done in the discrete environment. The inverter switching pulses are obtained from the switching table which decides the pulses from the error signals of flux and torque. The flux position is also determined in the flux and angle calculation block. The estimation of flux and torqueis done from the motor parameters such as phase voltage and phase currents. The torque and flux estimator block is shown in Fig5. The overall DTC model is shown in Fig.6. |
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B. Simulink Model of Space Vector Modulated Direct Torque Control |
| The simulation block of the DTC-SVM control scheme is shown in Fig.7.The system is composed of the motor , three phase voltage source inverter, PI controllers, reference frame transformation blocks etc. The IGBT switches are controlled using space vector modulation technique. |
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C. Simulation Results |
| Both DTC and DTC-SVM circuits was simulated and obtained sinusoidal current waveforms.Fig. 8 shows the current waveform under transient conditions where at a time period of 2seconds when the torque varies the phase current also changes suddenly without any time delay. The electromagnetic torque and flux settles to reference value i.e 3.5 Nm and 0.8wb respectively. |
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| The torque waveform of basic DTC scheme contains large amount ofhigh frequency ripples and it is also reflected inthe current waveform. It is clear fromthe simulation results of DTC-SVM scheme, the torque and current ripples have beenreduced. While simulating the basic DTC induction motor drive, the ripple content obtained was about 50% and by the proposed SVM-DTC scheme, the ripple content have been successfully reduced to about 11%. |
V. CONCLUSION |
| This project has reviewed Direct Torque Control strategies for SVPWM inverter-fed ACmotor drives. The DTC represents a viable alternative to Field Oriented Control (FOC)being also a general philosophy for controlling the AC drives DTC strategies have been divided into two groups: hysteresis-based switching tableDTC, and constant switching frequency schemes operating with space vector modulators(SVM-DTC). Constant switching frequency SVM-DTCschemes improve considerably the driveperformance in terms of reduced torque and flux pulsations, reliable start up and lowspeed operation, well-defined harmonic spectrum as well as radiated noise. Therefore,SVM-DTC is an excellent solution for general purpose IM drives in a very wide powerrange. In conclusion, it is believed that the DTC principle will continue toplay a strategic role in the development of high performance drives. |
ACKNOWLEDGEMENT |
| The authors are thankful to Prof. K. R. Varmah, Senior Professor, Ms.Jayashri R. Nair, Head of Department and all other staffs of Electrical Department, Rajagiri School of Engineering and Technology for their valuable suggestion and facilities provided during the preparation of the paper. |
References |
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