A Novel Four Switch Three Phase Inverter Controlled by Different Modulation Techniques A Comparison

Transcription

1 Volume 2, Issue 1, January-March, 2014, pp , IASTER Online: , Print: ABSTRACT A Novel Four Switch Three Phase Inverter Controlled by Different Modulation Techniques A Comparison 1 Somendra Banerjee, 2 Dr. S. Chatterjee, 3 Shimi. S. L 1 Pursuing M.E, 2 Professor, 3 Assistant Professor National Institute of Technical Teachers Training & Research, Chandigarh, India. DC motor is one of the most commonly used speed control drive system, but in recent time three phase induction motor is replacing DC motor as an speed control drive system due to its various advantages over DC motor. The only disadvantage of induction motor is that the speed control of induction motor is difficult as compared DC motor due to the fact that the range of speed variation in case of induction motor is very less. Mainly cage rotor type induction motor is used for speed control drive system. There are many methods used by researchers to control the speed of induction motor, among those methods one of the most commonly used method is V/Hz control method. In this method the speed of induction motor has been controlled by keeping the voltage by frequency ratio constant, to get that researchers have to use three phase voltage source inverter, and to control the output voltage and frequency of the inverter researchers have to use different modulation techniques. In this paper researcher apply two different modulation techniques to control the switching of the switches of four switch three phase inverter and compare the result. Keywords: Four Switch Three phase Inverter (FSTPI), Pulse Width Modulation (PWM), Sinusoidal Pulse Width Modulation (Sin M/SPWM), Space Vector Modulation(SVM), Six Switch Three Phase Inverter (SSTPI), Total Harmonics Distortion (THD). I. INTRODUCTION Induction motors are being used for many industrial and commercial applications because it is easy build, high robustness, and generally satisfactory efficiency. Induction motors, which contain a cage, are very popular in variable speed drives system. They are simple, rugged, inexpensive and available at all power ratings. Progress in the file of power electronics and microelectronics enable the application of induction motors for high performance drives. There are many techniques for different types of control of induction motor. Each technique has its own importance according to the applications. For the applications like rolling mills, paper mills, lathe machines etc. where low power ratings motors are implemented, the technique which is mainly use in industry for speed control of induction motor is V/Hz technique. In this technique the speed of the induction motor can be controlled by varying its input ac voltage and frequency using a three phase voltage source inverter. A conventional six switch three phase voltage source inverter has six switches in three legs with a pair of complementary power switches per phase. A reduced switch count voltage source inverter that is four switch three phase inverter uses only four switches in two legs and two DC link capacitors in third leg. The advantages of this inverter due to the use of four switches instead of conventional six switches is lesser switching losses, lower electromagnetic interference, less complexity of control algorithms and reduce interface circuits [1]. Now the main drawback of four switch three phase inverter is that it has more harmonics distortion in its output voltage and current as compared to conventional six switch inverter. The harmonics distortion will increase in four switch inverter because in each cycle the voltage of each phase become zero for some duration, which arises due to two dc link 14

2 capacitors, which use in the circuit in place of two switches. Many Investigators suggest different techniques to reduce the THD in the output voltage of voltage source inverter; among those techniques most commonly used techniques are different types of modulation techniques. To control the speed of induction motor by V/Hz method and to reduce the harmonics distortion in voltage of inverter output, researchers suggest many types of modulation techniques, but among those techniques the modulation schemes which are mainly used by engineers are: Single Pulse Width Modulation Multiple Pulse Width Modulation Sinusoidal Pulse Width Modulation Including these PWM techniques there is some more modulation techniques such as 60-degree PWM, third-harmonics PWM and space vector PWM techniques. The techniques mentioned above are mainly belonging to scalar method of speed control but SVM technique is belongs to space vector method of speed control. Here researcher mainly taken SPWM technique and SVM technique into account, as SVM technique is very much similar to PWM technique but the main difference is that in PWM technique three phase inverter is considered as three single phase inverter but in case of SVM technique a three phase inverter is considered as a single unit. So, as SVM technique is also considered as modulation technique that s why author here apply both SVM technique and SPWM technique on four switch three phase inverter and then analyzed the result that which technique is easier to apply for FSTPI and which one is give better result from THD reduction point of view. But before that researcher discuss the different modulation schemes in brief. II. DIFFERENT MODULATION TECHNIQUES In an ac motor control system, in which a constant V/Hz has to maintain to avoid saturation of the motor, during the speed control, step-less control of inverter output voltage is necessary. And to get this type of control the technique which are mainly used are different Modulation techniques. Pulse Width Modulation (PWM) techniques and other modulation techniques are extensively used for eliminating harmful loworder harmonics in input and output voltage and current of static power converter. In modulation control techniques, the converter switches are turned on and off several times during a half cycle and the output voltage is controlled by varying the pulse widths. In this section, the discussion is mainly focused on the application of PWM techniques and space vector technique for harmonic elimination in voltage-source inverters, and makes the speed control of induction motor drive smoother. The most commonly used methods in PWM techniques are:- Single Pulse Width Modulation. Multiple Pulse Width Modulation. Sinusoidal Pulse Width Modulation. Among these techniques, sinusoidal PWM technique is most commonly used for speed control of three phase induction motor through VSI. Another method which is most commonly used for getting sinusoidal output from VSI is space vector modulation scheme. Here author firstly discuss about SPWM technique then discuss about SVM technique and after that implement both the technique on FSTPI and then analyzed the result. 15

3 III. SINUSOIDAL PWM TECHNIQUE In this technique, the pulse width is a sinusoidal function of the angular position of the pulse in a cycle. For realizing sin M, a high frequency triangular carrier wave Vc is compared with a sinusoidal reference wave V r of the desired frequency. The intersection of V c and V r waves determines the switching instants and commutation of the modulated pulse. V c is the peak value of triangular carrier wave and V r that of the reference or modulating signal. The carrier and reference waves are mixed in a comparator. When sinusoidal wave has magnitude higher that the triangular wave, the comparator output is high, otherwise it is low. The comparator output is processed in a trigger pulse generator in such a manner that the output voltage wave of the inverter has a pulse width in agreement with the comparator output pulse width. When triangular carrier wave has its peak coincident with zero of the reference sinusoidal, there are pulses per half cycle. In case zero of the triangular wave coincides with zero of the reference sinusoidal, there are (N-1) pulses per half cycle, has that are four pulses per cycle. The ratio of V r /V c is called the modulation index (MI) and it controls the harmonic content of the output voltage waveform. The magnitude of fundamental component of output voltage is proportional to MI, but MI can never be more than unity. Thus the output voltage is controlled by varying MI. Fig 1: Pulse Generation in SPWM Technique In above discussion author discussed about SPWM technique for single phase inverter. The similar method can be used for three phase inverter also. In case of three phase inverter, instead of considering single reference wave one has to consider three sinusoidal reference waves (v ra, v rb, and v rc ) each shifted by A carrier wave is compared with the reference signal corresponding to a phase to generate the gating signals for that phase. Comparing the carrier signal v cr with the reference phases v ra, v rb, and v rc produces g 1, g 3, and g 5 respectively. The instantaneous line to line output voltage is: V ab = V s (g 1 g 3 ). (3.1) The output voltage is generated by eliminating the condition that two switching devices in the same arm cannot conduct at the same time. All phase voltage (v an, v bn, and v cn ) are identical, but out of phase without even harmonics, moreover at frequencies multiple of three are identical in amplitude and phase in all phases. For instance, if the ninth harmonics voltage in phase a is 16

4 The corresponding ninth harmonics in phase b will be, V an9 (t) = V 9 sin (9ωt) (3.2) V bn9 (t) = V 9 sin (9(ωt )) = V 9 sin (9ωt ) = V 9 sin (9ωt) (3.3) Thus, the AC output line voltage v ab = v an v bn does not contain the ninth harmonics. In this way one can use SPWM technique to reduce the harmonics in the output of VSI. This technique is not only affective but also easy to implement, so this method is frequently used by engineers and researchers to control the output of VSI. And output of VSI use to get constant V/Hz control method for controlling the speed of three phase induction motor. Including this PSWM technique there exist many other different modulation techniques also which are very helpful to get smooth output from VSI. As discussed above by author, among those techniques SPWM technique is frequently used in industries, but one another method of modulation which is frequently used in industries is Space Vector Modulation (SVM) technique. So here author not only apply the SPWM technique on FSTPI but also used SVM technique, but before apply these techniques on FSTPI, here author discuss about the concepts of SVM technique. IV. SPACE VECTOR MODULATION TECHNIQUE Space vector modulation is a digital modulating technique where the objective is to generate PWM load line voltages that are in average equal to a given (or reference) load line voltages. This is done in each sampling period by properly selecting the switch states of the inverter and the calculation of the appropriate time period for each state. The selection of the states and their time periods are accomplished by the space vector (SV) transformation. According to space transformation, any three functions of time that satisfy u a (t) + u b (t) + u c (t) = 0 (4.1) can be represented in a two dimensional space. The coordinates are similar to those of three phase voltages such that the vector [u a 0 0] T is placed along the x-axis, the vector [0 u b 0] T is phase shifted by 120 0, and the vector [0 0 u c ] T is phase shifted by This is shown in fig 2. The space vector in complex notation is then given by where 2/3 is a scaling factor. Equation 4.2 can be written in real and imaginary components in the x-y domain as (4.3) using equations 4.2 and 4.3, one can obtain the coordinate transformation from the a-b-c axis to the x-y axis as given by (4.2) (4.4) which can also be written as (4.5) 17

5 (4.6) Fig 2: Three Phase Coordinate Vectors and Space Vector U (T) The transformation from the x-y axis to the α-β axis, which is rotating with an angular velocity of ω, can be obtained by rotating the x-y axis with ωt as given by (4.7) Using equation 4.2, author has find the inverse transform, which is as follows = Re (u) (4.8) = Re (u ) (4.9) = Re (u (4.10) Here author considered u a, u b, u c as the three-phase voltages of a balanced supply with a peak value of V m, and so author write them as u a = V m sin (ωt) (4.11) u b = V m sin (ωt - 2π/3) (4.12) u c = V m sin(ωt + 2π/3) (4.13) then, using equation 4.2, author get the space vector representation as u (t) = V m = V m (4.14) which is a vector of magnitude V m rotating at a constant speed of ω radian per second. The above discussed theories of SPWM & SVM techniques are frequently used by engineers and researchers to control and to make, the output of conventional six switch three phase voltage inverter, smoother. In this paper author apply these two above discussed techniques on four switch three phase inverter (FSTPI) and then analysis the result. But before apply SPWM and SVM techniques on FSTPI, author first discuss about FSTPI in brief. 18

6 From the different literature reviews author find that there are many techniques for speed control of three phase induction motor, among those techniques, constant V/Hz technique of speed control of three phase induction motor is one of the easy implemented method. In this method the input voltage and frequency both are vary in such a way that their ratio kept constant to control the speed of three phase induction motor. To vary the voltage and frequency in such a way, researchers use six switch three phase voltage source inverter (SSTPI) and apply different methods to control its switching and get smooth output voltage form it and apply that output on induction motor to achieve its smooth control. But SSTPI has some drawbacks such as: Complex Control Algorithm High Electromagnetic Interference High Switching Losses Complex Interface Circuit To overcome these drawbacks some researchers suggest the concept of four switch three phase inverter in which two DC link capacitors will be used in place of two switches due to which all the four above mentioned difficulties will reduces. The circuit diagram of FSTPI is as follows: Fig 3: Four Switch Three Phase Inverter Fed Induction Motor Drive System Now author applies SPWM & SVM techniques on FSTPI and analyzed the result. The simulation circuit and its results are discussed by the author as follows. V. SIMULATION RESULTS OF FSTPI USING SVM AND SPWM TECHNIQUES According to the above research, researchers first apply SVM technique on four switch three phase inverter (FSTPI) and investigate the result. According to that result the THD of FSTPI reduce to 3.96%. The simulation diagram and the THD analysis display of FSTPI using SVM technique are shown below: 19

7 In the above SIMULINK model, researcher create a block of inverter subsystem which is actually represent a four switch three phase inverter containing two DC link capacitors. The output of inverter has given to the three phase induction motor and according to the change of output of inverter the input voltage and frequency of induction motor also change and control the speed of three phase induction motor. The output of inverter changes due to the controller design by the researcher which is represented as pulse generator in the above SIMULINK model. The pulse generator is based on the mathematical equations discussed above for space vector technique. The output waveform of the SVM controlled four switch three phase inverter is as follows: Fig 5: Output Voltage and Current Waveforms of all the Three Phases of SVM Controlled FSTPI The THD analysis of SVM controlled FSTPI is as follows: FFT window: 1 of 250 cycles of selected signal Time (s) Fig 6: THD Analysis of Output of SVM Controlled FSTPI From the above analysis, author found that the output voltage and current of SVM controlled FSTPI are almost sinusoidal in nature and the THD of output voltage has reduced significantly, up to 3.96% which can be considered as very good result from the THD point of view. But SVM technique has a disadvantage, application of SVM technique is complicated means researchers have to use complex control algorithm during the use of this technique. So SVM technique can be considered as useful technique for higher rating motors or where high degree of accuracy require, but for small rating motors or for the applications where high degree of accuracy is not required for those applications SVM technique is not considered as better technique. To overcome this difficulty researchers have to apply a different techniques which can able to reduce THD just like SVM technique and easier to implement in comparison to SVM technique. 20

8 There are many other modulation techniques which can be applied on FSTPI with easier control algorithm. From the above discussion author get that in case of SPWM technique the control algorithm can be simpler, so here author apply SPWM technique on FSTPI and analyzed the result that whether it can able to reduce THD in output of FSTPI like SVM technique or not. The simulation model of SPWM controlled FSTPI and its results are shown as follows: The output waveform of SPWM controlled four switch three phase voltage source inverter fed induction motor drive is as follows: Fig 8: Three Phase Output Voltage of SPWM Controlled FSTPI Fed Induction Motor Drive System The THD analysis of output of four switch three phase voltage source inverter is as follows: 21

9 FFT window: 1 of 250 cycles of selected signal Time (s) Fig 9: THD Analysis of Three Phase Output Voltage of SPWM Controlled FSTPI VI. CONCLUSION AND RECOMMENDATIONS As per the investigation, researchers concluded that four switch three phase voltage source inverter has some advantages over six switch inverter, but there is one disadvantage of FSTPI, its output contain more THD as compare to six switch inverter. Researchers use many techniques to reduce the THD in six switch inverter. As per the review author got that the most commonly use techniques to reduce the THD are modulation techniques. There are many types of modulation techniques and among those techniques pulse width modulation techniques are mainly used by researchers. After the investigation, author found that these techniques are applicable and give significant result in case of FSTPI also. Here author chose two most popular techniques that is SVM technique and SPWM technique and apply them on FSTPI. After the application author found that the SVM technique can reduce the THD in significant amount, the THD in output of FSTPI after applying SVM technique is about 3.96%, but the main problem with SVM technique is that to apply SVM technique one has to use complex algorithm, so author suggest that SVM technique is appropriate for high rating motors and where high accuracy required, but in case of small rating motors and the applications where high degree of accuracy is not required the use of SVM technique is not so much feasible. After applying SVM technique researcher apply SPWM technique on FSTPI and analyzed the result, from the result author found that THD of output of FSTPI reduces to about 2.58%, which is better than SVM technique and also the application of SPWM technique is easier as compared to SVM technique because application of SPWM technique require easier algorithm, but in case of SPWM technique the three phase output voltage is not as sinusoidal as the output that author get in case of SVM technique, so researcher suggested that SPWM technique is more appropriate for low rating motors and applications where high degree of accuracy is not required. 22

10 Circuit Method Used THD Result Four Switch Three Phase Inverter SVM Technique THD reduced to 3.96% Four Switch Three Phase Inverter SPWM Technique THD reduced to 2.58% REFERENCES [1] Bose B. K., Modern Power Electronics and AC Drives, Pearson Eduction, 4 th edition, 2011 [2] Dr. S. K. Bhattacharya, Dr. S. Chatterjee, Industrial Electronics and Control, Tata McGraw Hill Publishing Company Limited. [3] M. Nasir Uddin, Fuzzy-Logic-Controller-Based Cost Effective Four-Switch Three-Phase Inverter- Fed IPM Synchronous Motor Drive System, Proceding of IEEE. February [4] Mohanty N. K., Microcontroller based PWM Controlled Four Switch Three Phase Inverter Fed Induction Motor Drives. Serbian Journal Of Electrical Engineering, vol.7, No. 2, pp , November [5] Rashid Muhammad H., Power Electronics, Circuits, Devices and applications, Pearson Education, Third Edition [6] Kothari D. P., Nagrath I. J., Electric Machine, Tata Magraw-Hill Publishing Company Limited, 3 rd Edition, [7] G. Arunkumar et al, A Novel Topology for Seven Level Inverter in Industrial Drives Applications, International Journal of Research in Electrical & Electronics Engineering, Vol.1, Issue 1, July-Sept.2013, pp [8] Krishna C. V., Shyam H.N., Rishi S., Moorthi S. FPGA Based Implementation of variable voltage variable frequency controller for a three phase induction motor, IEEE transactions On Electronics Engineering, Vol. 34, no.4, pp , June [9] [10] MATLAB/SIMULINK User Guide, Math Works Inc., [11] Sunil Panda Control of Voltage Source Inverters using PWM/SVPWM for Adjustable Speed Drive Applications, National Institute of Technology, Rourkela. [12] Banerjee S., Reduction of Total Harmonics Distortion in Four Switch Three Phase Inverter Fed Induction Motor Drive. International Journal of Researcher in Electrical & Electronics Engineering, Vol. 1, Issue 2, pp.82-91, Oct.-Dec.,