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{draw:frame} {draw:frame} I. INTRODUCTION *II. *BASIC CONCEPT OF SVPWM Space Vector Modulation treats the two level inverter of fig. 1 as a single unit which can be driven to eight unique states that each state creates a corresponding voltage vector. An electric-motor control system, comprising: *a two level voltage source inverter. III. *IMPLE*MENTATION OF SPACE VECTOR PWM For the three phase two-level PWM inverter as shown in Fig. 1, the switch function is denoted by (1) ,shown below. here x=R,Y,B;”1″denotes Vdc/2 at the inverter output (a,b,c)with reference to point neutral;”0″denotes ???Vdc/2;O is the neutral point of the dc bus. {draw:frame} Fig. 1. Three phase two level inverter {draw:frame} Fig. 2 eight switching states {draw:frame} Fig. 3 Voltage vector space. In the vector space, according to the equivalence principle the following conditions are obtained. The following steps are carried out to implement SVPWM Step. 1 Determine the possible switching vectors in the output voltage space. Step. 3 Calculate the time variables Step. switching time calculation to each sector There are eight possible switching states for the inverter at any instant of time. The eight states of the inverter and the corresponding space- phasors of the output voltages are shown in fig 2 and 3 respectively. It can be observed that at any instant of time there are only eight possible positions for the voltage space phasor. *2. DEFINING V*REF VRN=13 (VRY-VBR)=13 (2VRO-VYO-VBO) VYN=13 (VYB-VRY)=13 (2VYO-VBO-VRO) VBN=13 (VBR-VYB)=13 (2VBO-VRO-VYO) (3) Given a set of inverter pole voltages (VRO, VYO, VBO) the vector components (V? , V? in this frame are found by the forward Clarke transform Equation 4 denotes the magnitude of the reference vector The phase angle can be evaluated by The continuously moving reference vector VREF is sampled at a sampling frequency f s. During this interval Is =1fs between samples the reference vector VREF is assumed to be constant. 3. CALCULATION OF TIME VARIABLES Now consider VREF is situated in sector 1 as shown in fig 4. The angle ? represents the position of the references vector each respect to the beginning of the sector Assume that the sampling period IS is divided into the three sub-intervals T1, T2 and T0.
The inverter is turned ON to produce the vector V1 for T1 seconds. V2 for T2 seconds. And zero (i. e. either V7orV8) for T0 seconds. From fig. 4 {draw:frame} Fig. 4 Sampled Reference Vector Located in Sector 1 Where ??VS*??is the amplitude of the reference vector. From equation (5) It is important to note that each sector has different pattern and also different value of ta, tb and to as the value of phase (or) angle differs from time to time. {draw:frame} Fig. 5 Output signal based on Symmetrical Sequence algorithm in sector 1 Fig. shows the reason for some distortion in the phase voltage in addition that the ideal maximum realizable modulation index for low distortion phase voltages is described by a circle of radius Tp ???T min. The maximum achievable modulation index is thus 4/3(1-Tmin/Tp). Where Tp is the PWM period. {draw:frame} Fig. 6 A space vector diagram showing aspects of the operation of the system. The space vectors are calculated using standard space vector modulation technique and we obtained corresponding results as follows: The alpha and beta voltage are calculated and normalized with respect to half of he DC link voltage. The SVM sector that the voltage demand vector lies in is determined mathematically. The nominal duration, Ta, Tb, of the two non-zero SVM adjacent to that sector are calculated to produce a compound vector equal to the demanded voltage vector to control the speed of the BLDC motor. *IV. IMPLEMENTATION *OF SVPWM BASED INVERTER FOR BLDC MOTOR. The following circuit and a method for controlling the rotating speed of the brushless direct current (BLDC) motor are provided, whose aim is to control the speed of the motor by space vector pulse width modulation.
The circuit includes a open loop control mechanism for adjusting modulation index value thus controlling the rotating speed of the motor. Control circuit drives the BLDC motor by turning on and turning off the metal oxide semiconductor field effect transistors (MOSFET) according to the output patterns from SVPWM algorithm. {draw:frame} Fig. 7 control circuit for BLDC motor. From the above control circuit diagram the speed control apparatus comprises a plurality of hall sensors, a plurality of switches, and a turn on control circuit and gate driver logic.
The hall sensors are configured to detect magnetic rotor sections of s poly ???phase brushless DC motor at different positions. The turn on control circuit generates s conduction time variables for each sector based on the SVPWM algorithm. However, the conventional motor with PWM speed control has some draw backs. If the duty is too short, for example, 0. 5 or lower, the motor will have a low starting torque such that the motor may not start properly or it may cause the motor to stall. But these kinds of problems are overtaken by SVPWM as it is having linear wide range of modulation.
A motor driver circuit, comprising A plurality of output circuits each having an upper side switch and a lower side switch connected in series for supplying the current to a motor from a connection point between said upper side switch and said lower side switch of each output circuit. *V. *SIMULATION RESULT The new software technique has been implemented in Matlab/Simulink software packages. The two level voltage source inverter circuits for BLDC motor (fig. 8) is drawn using Matlab/Simulink package. The switching pattern also generated by Simulink package.
The switching time duration for each sector are given as signals to inverter circuit are modeled. It has been suggested two different techniques of PWM for decreasing THD in the speed control of BLDC motor. Both sinusoidal PWM (SPWM) and Space Vector PWM (SVM) techniques are simulated in the MATLAB 2007 software and results of wave forms and its FFT is shown in fig 9 to 14. {draw:frame} The simulation parameters are as follows Input DC Voltage V = 230 volts Modulation Index = (0. 1 ??? 1) Switching Frequency = 2 KHz. Supply voltage: 230V Stator resistance (Rs):2. 750? Stator inductance (Ls):8. 5e-3H Back EMF (Trapezoidal): 120V Rotor inertia (J):1e-3Kgm^2 Flux induced by magnet: 0. 175wb Flux distribution: Trapezoidal Simulation results are given below for the performance characteristics of the BLDC motor . Table 2: Comparison of PWM Techniques. {draw:frame} Fig. 10 Line voltage and Line current of BLDC motor {draw:frame} {draw:frame} {draw:frame} Fig. 14 FFT Analysis of load current. (SPWM) VI. *CONCLUS*ION From the simulations, we can conclude that SVPWM provides a superior performance in speed control of BLDC motor . e can observe that the harmonic distortion is lower at higher frequencies. From this paper we also conclude to the following: Space Vector PWM can be used to generate a sinusoidal voltage. The phase-to- center voltage (VRO, VYO, VBO) of the space vector PWM is not sinusoidal. REFERENCES [2] Keliang Zhou and Danwei Wang, “Relationship between Space- Vector Modulation and Three-Phase Carrier-Based PWM: A Comprehensive Analysis”,_ IEEE__ Trans. Ind. Applicat. , Vol. 49, NO. 1, FEB. 2002_ [3] Joachim Holtz, “Pulse width Modulation-A Survey,” IEEE Trans.
Industrial Electronics, Vol. 39, NO. 5, Dec1992, pp-410-420. [6]Xiyou Chen and Mehrdad Kazerani, “Space Vector Modulation control of an AC-DC-AC Converter with a front-end diode rectifier and reduced DC-link capacitor”. IEEE Transactions on power electronics, Vol. 21, No. 5, SEP 2006 [8]D. Rathnakumar, J. Lakshmana perumal and T. srinivasan, “A new software implementation of space vector PWM”. 0-7803-8865-8/05/$20. 00 2005 IEEE [9] S. C. Persyn, M. McClelland, M. Epperly, and B. Walls, “Evolution of digital signal processing based spacecraft computing solutions,” in Proc. 0th DASC Conf. , vol. 2, 2001, pp. 8C3/1???8C3/10 [10] Y. Enomoto, M. Ito . H. Koharagi, R. Masaki, S. Ohiwa, C. Ishihara and M. Mita, “Evaluation of Experimental Permanent-Magnet Brushless Motor Utilizing New Magnetic Material for Stator Core Teeth”, IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 11, NOVEMBER 2005 ACKNOWLEDGMENT The authors are grateful to the Correspondent and the Principal of Mepco Schlenk Engineering College at Sivakasi, for their constant encouragement and support in providing the resources for this research work.