CERTAIN INVESTIGATIONS ON VSI FED INDUCTION MOTOR DRIVES

Synopsis of the Thesis CERTAIN INVESTIGATIONS ON VSI FED INDUCTION MOTOR DRIVES Submitted to ANDHRA UNIVERSITY for the fulfillment of the requirement for the award of the degree of DOCTOR OF PHILOSOPHY in ELECTRICAL ENGINEERING By K. VIJAYA BHASKAR REDDY M.TECH Under the guidance of Dr. G.V. SIVA KRISHNA RAO M.TECH., Ph.D. PROFESSOR DEPARTMENT OF ELECTRICAL ENGINEERING AU COLLEGE OF ENGINEERING (A) ANDHRA UNIVERSITY VISHAKAPATNAM- 530003 ANDHRA PRADESH, INDIA FEBRUARY-2013

1.1 INDUCTION MOTOR DRIVES INTRODUCTION Many research works are focusing in the development of the efficient control algorithms for high performance variable speed induction motor (IM) drives. Induction motor has been operated as a work horse in the industry due to its easy build, high robustness and generally satisfactory efficiency. Recent development of high speed power semi conductor devices, three phase inverters take part in the key role for variable speed AC motor drives. Traditionally, Three Phase inverters with six switches (SSTP) have been commonly utilized for variable speed IM drives; this involves the losses of the six switches as well as the complexity of the control algorithms and interface circuits to generate six PWM logic signals. So far researchers mainly concentrated on the development of new control algorithms. However, the cost, simplicity and flexibility of the overall drive system which are some of the most important factors did not get that much attention from the researchers. That is why, despite tremendous research in this area, most of the developed control system failed to attract the industry. Thus, the main objective of this work is to develop a cost effective, simple and efficient high performance IM drive. Recently, some efforts have been made on the application of four switch Three Phase (FSTP) inverter for variable speed drives. Some advantages of the FSTP inverter over the conventional SSTP inverter such as, reduced price due to reduction in number of switches, reduced switching losses, reduced number of interface circuits to supply logic signals for the switches, simpler control algorithms to generate logic signals, less chances of destroying the switches due to lesser contact among switches and less real time computational burden. The traditional inverters are Voltage Source Inverter (VSI) and Current source Inverter (CSI), which consists of diode rectifier front end, DC link and Inverter Bridge. In order to improve power factor, either an AC inductor or DC inductor is normally used. The DC link voltage is roughly equal to 1.35 times the line voltage and the Voltage source inverter is a buck converter that can only produce an AC voltage limited by the dc link voltage. Because of this nature, the Voltage source inverter based PWM VSI and CSI are characterized by relatively low efficiency because of switching losses and considerable Electromagnetic Interference (EMI) generation. 1

Currently, there are two existing inverter topologies used for induction motor drives, the conventional three phase Pulse Width Modulation (PWM) inverter and a three phase PWM inverter with a DC-DC boost converter, which is also very popular in other applications like hybrid electric and fuel cell vehicles. Because of the wide voltage range and limited voltage level of fuel cell stack, the conventional PWM inverter topology imposes high stresses to the switching devices and motor. The DC-DC boosted PWM inverter topology can alleviate the stresses and limitations, but suffers from problems such as high cost and complexity associated with the two stage power conversion. 1.2 A BRIEF REVIEW OF THE LITERATURE Many interesting investigations in the area of four switch three phase inverter and Z- source inverters fed induction motor drives have been reported in the literature. Notable among them are presented in the following. Van der Broeck HW et al. [1], suggested the four switch inverter employing four switches and four diodes as a practical alternative to the six switch inverter with six switches and six diodes. Meanwhile in four switch inverters, since one of the motor phase windings is connected directly to the neutral point of the DC- link capacitors, the single phase current flowing into the DC-bus will directly charge one of the capacitors and discharge the other. This can cause significant fluctuations of the DC-bus voltage and subsequently unbalance three phase currents at the inverter output, thus, requires much larger DC-link capacitors. Van der Broeck HW et al. [2], have reported the harmonic effects of a two phase PWM inverter reducing the number of switching devices at the output stage from six to four and the method of generating three phase waveforms with two DC-link voltages. The modulation strategy suggested can produce three phase balanced sinusoidal wave forms at a reduced output voltage of 0.866 compared with the conventional six switch inverter. The analysis of harmonic losses and torque pulsation produced by the four switch inverter was carried out assuming that the switching frequency is much larger than the fundamental frequency. In that case, one can provide the fundamental voltage and current values which are approximately constant during a switching period. High switching frequency, however, may actually cause a reduction of efficiency, as switching losses in such a system account for a considerable amount of the total losses. 2

Peters GL et al. [3], proposed a new strategy that utilizes front end rectifier, which enables unity input power factor and an improved DC-link voltage fluctuation that provides three balanced currents in the output of the inverter. Nasir Uddin M et al. [4], proposed Hysteresis current controller technique for four switch inverter. Inherently, this scheme has fast dynamic response and it is suitable for IM drives with SSTPI, but in FSTPI structure, the hysteresis band limitation increases the output current distortion. Meanwhile, in this scheme the switching frequency is variable that makes the output filter design difficult. Experimental results demonstrate high distortion and unbalanced three phase output currents with high THD. The current controlled PWM method, instead of the voltage controlled PWM, which is called direct current controlled PWM has been applied for four switch three phase brushless DC motor (BLDC) drive. Miaosen Shen et al. [5], have investigated and compared three different inverters, traditional PWM inverters, DC-DC boosted PWM inverter and Z-source inverter. Hossein Madadi Kojabadi et al. [6], have made a comparative analysis of different pulse width modulation methods for low cost induction motor drives, they reported the unbalance three phase output currents due to it the dynamic unbalance between the two capacitors by different pulse width modulation methods. Ciro Attaianese et al. [7], developed space vector modulation Algorithm for Torque control of Inverter fed Induction Motor Drive. The Advent of vectorial control strategies in conjunction with space vector modulation techniques for voltage source inverters have made possible to achieve high quality current and torque responses in induction motor drives. T. Brahmananda Reddy et al. [8], work on A Variable Structure Controller Approach for Sensor less Direct Torque Control of Induction Motor based on Hybrid Space Vector Pulse width Modulation to Reduce Ripples and Switching Losses. This scheme is for direct torque control of induction machines using hybrid space vector PWM to reduce switching losses and the study state ripple in torque and flux. S. Chakrabarti et. al.[9], used Space Vector Modulation Based Pulse Width Modulation for reduction of Torque Ripple in Direct Torque Control of Induction Motor Drives. This method 3

approaches the switching instants of different space vectors which are determined to reduce torque ripple compared to commonly used space vector concept based on sinusoidal PWM technique. Ebenezer V. et. al. [10], developed a sensor less Vector Control Scheme for Induction Motors using a Space Phasor based Current Hysteresis Controller. This scheme measures the stator current hysteresis error direction to determine the rotor flux position, during inverter zero vector states. This measurement of the rotor flux position is done indirectly by sensing the motor back emf. D. Hadiouche et. al. [11], conducted simulation studies of Space Vector PWM Control of Double-Star Induction Motors. The modeling of DSIM is made using an arbitrary shift angle between the two-three phase windings. Joao Pinto et. al.[12], discussed a Stator-flux-oriented Vector-Controlled Induction Motor Drive with Space-Vector PWM and Flux-Vector Synthesis by Neural Networks. This method provide the ANN s, when implemented by dedicated hardware application-specific integrated circuit chips, which extreme simplification and fast execution for control and feedback signal processing functions in high performance AC drives. Pedro Ponce et. al.[13], This method adopts a new direct torque control scheme which reduces the stator flux ripple and keeps the switching frequency constant using space vector modulation based on fuzzy logic controller and developed Fuzzy Logic Controller Based on Space Vector Modulation for Induction Motor Control. Kuo-Kai Shyu et. al.[14], described Variable Structure Current Control for Induction Motor Drives by Space Voltage Vector PWM. This method proposed current controller, which is based on space voltage vector PWM drive, which exhibits several advantages in terms of reduced switching frequency, robustness to parameter variations, elimination of current/torque ripple, and improved performance in induction motor drive. S.Williamson et al. [15], developed space Vector representation of advanced motor models for vector controlled induction motors. This method presents a transient induction motor model that accounts for skin effect and saturation in both the main and rotor leakage flux paths 4

which is described and also shown that magnetizing flux oriented control is a more suitable form of vector control. M. Zelechowski et al.[16], designed Controller for Direct Torque Controlled Space Vector Modulated (DTC-SVM) Induction Motor Drives. In a DTC-SVM scheme the switching harmonics are neglected in the control algorithm. S.Ekram et al. [17], works on Effects of harmonics on PWM inverter fed induction machines. This method presents the effects of harmonics upon current, torque & speed of PWM inverter fed induction machine. Heinz Willi Van Der Broeck et al. [18], work on Analysis and Realization of a Pulse width Modulator Based on Voltage Space Vectors. This method of using a space vector concept for PWM VSI is compared with the commonly used established sinusoidal concept. Armando Bellini et al. [19], works on Comparison between Sinusoidal PWM and Space Vector Modulation Techniques for NPC Inverters. The method proposes analyse and compares two different kinds of modulation techniques, specially designed for NPC inverters. Mehmet Yumurtaci et al. [20], work on Comparison of output Current Harmonics of Voltage Source Inverter used Different PWM Control Techniques. This paper finds output current harmonics of open loop voltage source inverter for different PWM techniques which are measured by harmonic analyzer. The above literature does not deal with comparison of modified sine PWM and SVM methods. This work deals with the comparison between sine PWM and SVM methods. The above mentioned paper does not use Photovoltaic cell for the control of induction motor. This work proposes PV array for the control of induction motor. 1.3 OBJECTIVES AND ORGANISATION OF THE THESIS The objectives of the present work are as follows: To model, simulate and implement VSI fed induction motor drive. To simulate modified sine PWM VSI fed induction motor drive and compare it with conventional VSI fed drive. 5

To model and simulate SVM based VSI fed induction motor drive. To compare Sine PWM based VSI fed induction motor drive with SVM based VSI fed induction motor drive. To implement PV powered induction motor drive. ORGANISATION OF THE THESIS Chapter 1 deals with Introduction, literature review, concepts of Matlab and theory of Induction motor and problem formulation. Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 deals with VSI fed induction motor drive with the modeling, design & simulation and experimental results are presented. deals with simulink modeling, simulation and hardware implementation of modified Sine PWM inverter fed induction motor drive. deals with SVM inverter fed induction motor drive. Also, basic sine PWM inverter system compared with SVM inverter fed induction motor drive. deals with PV powered inverter fed induction motor drive. The PV power is used to drive the induction motor system. The simulation and experimental results are presented. deals with conclusion and scope for further work 1.4 VSI FED INDUCTION MOTOR DRIVE DC motors have been used during the last century in industries for variable speed applications, because its flux and torque can be controlled easily by means of changing the field and armature currents respectively. Furthermore, operation in the four quadrants of the torque speed plane including temporary standstill was achieved. Almost for a century, induction motor has been the workhorse of industry due to its robustness, low cost high efficiency and less maintenance. The induction motors were mainly used for essentially constant speed applications because of the unavailability of the variable- frequency voltage supply. The advancement of power electronics has made it possible to vary the frequency of the voltage supplies relatively easy, thus extending the use of induction motor in variable speed drive applications. But due to 6

the inherent coupling of flux and torque components in induction motor, it could not provide the torque performance as good as the motor. In AC grid connected motor drives, a rectifier, usually a common diode bridge providing a pulsed DC voltage from the mains is required. Although the basic circuit for an inverter may seem simple, accurately switching these devices provides a number of challenges. The most common switching technique is called Pulse Width Modulation (PWM). PWM is a powerful technique for controlling analog circuits with a processor s digital outputs. PWM is employed in a wide variety of applications like UPS, electric drives, HVDC reactive power compensators in power systems, ranging from measurement and communications to power control and conversion. In AC motor drives, PWM inverters make it possible to control both frequency and magnitude of the voltage and current applied to a motor. As a result, PWM inverter-powered motor drives are more variable and offer a wide range better efficiency and higher performance when compared to fixed frequency motor drives. The energy, which is delivered by the PWM inverter to the ac motor, is controlled by PWM signals applied to the gates of the power switches at different times for varying durations to produce the desired output waveform. To improve the quality of the product, variable speed is required, for that step less speed control is required. Depending on the type of load and type of speed different methods are adopted for speed control of motors. For step less speed control below and above the rated speed with high torque and to avoid the harmonics, the PWM inverter fed induction motor control is best suitable one. 1.5 THREE PHASE PWM INVERTER The recent advancement in power electronics has initiated to improve the level of the inverter instead of increasing the size of the filter. In multilevel inverter, design involves parallel connection of the inverter. For these redundant switching a space vector modulation is needed which is based on vector selection in dq stationary reference frame. For a multi level system either space vector modulation or sinusoidal triangle modulation may be taken. However space vector modulation is having more advantages due to low harmonic production. The performance of the multi level inverter is better than the classical inverter. The total harmonic distortion of the classical inverter is very high. The diode clamped inverter provides multiple voltage levels from a series capacitor bank. The voltage across the switches is only half of the DC bus voltage. These features effectively double the power rating of voltage source 7

inverter to the given semiconductor device. The total harmonic distortion is analyzed between multilevel inverter and other classical inverters. Field oriented control (FOC) of induction motor was introduced which has opened a new horizon to the induction motor applications. The method, which uses frame, has transformed the performance of induction motor similar to that of the DC motor. The implementation of this system however is complicated and furthermore FOC, in particular which is an indirect method and widely used, is known to be highly sensitive to parameter variations due to the feed-forward structure of its control system. In the DTC drive, flux linkage and electromagnetic torque are controlled directly and independently by the selection of optimum inverter switching modes. The required optimal switching voltage vectors can be selected by using a so called optimum switching voltage vector look up table. In the present work an attempt is made to simulate DTC system. 1.6 MODIFIED SINE PWM VSI FED INDUCTION MOTOR DRIVE Three phase induction motors are the most widely used motors for industrial control and automation. Hence they are often called the workhorse of the motion industries. They are robust, reliable, less maintenance and have high durability. When power is supplied to an induction motor with recommended specified voltage and frequency, it runs at its rated speed. However many applications need variable speed variations for improving the quality of the product. The development of power electronic devices and control systems has to mature to allow these components to be used for speed control of AC and DC motors control in place of conventional methods. This type of control not only controls the speed of AC and DC motors, but can improve the motor s dynamic and steady state characteristics. 1.7 MODIFIED SINE PULSE WIDTH MODULATION For wide variation in drive speed, the frequency of the applied AC voltage needs to be varied over a wide range. The applied voltage also needs to vary almost linearly with the frequency. The switches of the PWM inverters are turned on and off at significant higher frequencies than the fundamental frequency of the output voltage waveform. The harmonic content in the output of the inverter can be reduced by employing pulse-width modulation (PWM). The PWM techniques and strategies have been the subject of intensive research since 8

1970 s were to fabricate a sinusoidal AC output voltage. The classical Sinusoidal pulse-width modulation (SPWM) is the basis of state-of-the-art PWM techniques. Sinusoidal PWM (SPWM) is effective in reducing lower order harmonics while varying the output voltage and have gone through many revisions and it has a history of three decades. In Sine PWM Inverter, the width of the voltage pulses over the output cycle, vary in a sinusoidal manner. The scheme, in its simplified form, involves comparison of a high frequency triangular carrier voltage with a sinusoidal modulating signal that represents the desired fundamental component of the pole voltage waveform. The peak magnitude of the modulating signal should remain limited to the peak magnitude of the carrier signal. The comparator output is then used to control the high side and low side switches of the particular pole. Some of the following constraints for slow varying sinusoidal voltage be considered as the modulating signal are 1) the peak magnitude of the sinusoidal signal is less than or equal to the peak magnitude of the carrier signal. This ensures that the instantaneous magnitude of the modulating signal never exceeds the peak magnitude of the carrier signal. 2) The frequency of the modulating signal is several orders lower than the frequency of the carrier signal. A typical figure will be 50 Hz for the modulating signal and 20 KHZ for the carrier signal. Under such high frequency ratios the magnitude of the modulating signal will be virtually constant over any particular carrier signal time period. 3) A three phase Sine-PWM inverter would require a balanced set of three sinusoidal modulating signals along with a triangular carrier signal of high frequency. For a variable voltage- variable frequency (VVVF) type inverter, a typical requirement for adjustable speed drives of AC motor, the magnitude as well as frequency of the fundamental component of the inverters output voltage needs to be controlled. This calls for generation of three phase balanced modulating signals of variable magnitude voltage and frequency which it may be emphasized, need to have identical magnitudes and phase difference of 120 degrees between them at all operation frequencies. Generating a balanced three phase sinusoidal wave forms of controllable magnitude and frequency is a pretty difficult task for an analog circuit and hence a mixed analog and digital circuits is often preferred. The widths of the pulses near peak of the sine wave do not change much when modulation index is changed. According to M.H. Rashid in this method carrier triangular wave is suppressed at 30 0 in the neighborhood of peak of sine wave. Hence triangular wave is present for 9

the period of first 60 0 and last 60 0 of the half cycle of sine wave. The middle 60 0 of the sine wave does not have triangular wave. Hence the generated PWM has less number of pulses as compared to sinusoidal wave. This type of modulation is known as Modified SPWM. Its RMS value can be changed by changing the amplitude of sinusoidal wave. This modulation scheme reduces harmonic content and switching losses but implementation of this scheme is tougher than Sinusoidal PWM technique. 1.8 COMPARISION OF SINE PWM & SVM CONTROLLED INDUCTION MOTOR DRIVE The three phase induction motor, which is most widely used AC motor type in the industry, has been favored because of its self starting capability, simple & rugged structure, low cost, less weight per watt, high reliability and high efficiency. Along with variable frequency AC inverters induction motors are used in many adjustable speed applications, which do not require fast dynamic response. There are many possible PWM techniques like sinusoidal PWM, selected harmonic elimination PWM, space vector PWM etc used for speed control of induction motor. The concept of space vector PWM control has opened a new possibility that induction motors can be controlled to achieve dynamic performance as good as that of DC motors. In sine PWM Inverter the widths of the pole voltage pulses over the output cycle, vary in a sinusoidal manner. By comparing the output, SVPWM is superior as compared to sinusoidal PWM in many aspects like; 1) the modulation index is higher for SVPWM as compared to sinusoidal PWM. 2) The output voltage is about 15% more in case of SVPWM as compared to sinusoidal PWM. 3) The current & torque harmonics produced are much less in the case of SVPWM. SINE PULSE WIDTH MODULATION The PWM inverters are very commonly used in adjustable AC motor drive loads, where one needs to feed the motor with variable voltage variable supply frequency. For wide variation in drive speed, the frequency of the applied AC voltage needs to be varied over a wide range. The applied voltage also needs to vary almost linearly with the frequency. The switches of the PWM inverters are turned on and off at significant higher frequencies than the fundamental frequency of the output voltage waveform. 10

In sine PWM Inverter, the width of the pole voltage pulses over the output cycle, vary in a sinusoidal manner. The scheme, in its simplified form, involves comparison of a high frequency triangular carrier voltage with a sinusoidal modulating signal that represents the desired fundamental component of the pole voltage waveform. The peak magnitude of the modulating signal should remain limited to the peak magnitude of the carrier signal. The comparator output is then used to control the high side and low side switches of the particular pole. Generating a balanced three phase sinusoidal wave forms of controllable magnitude and frequency is a pretty difficult task for an analog circuit and hence a mixed analog and digital circuits is often preferred. SPACE VECTOR MODULATION SVM is a digital modulating technique, where the objective is to generate PWM load line voltages that are in average equal to a given load line voltage. This is done in each sampling period by properly selecting the switching states of the inverter and the calculation of the appropriate time period for each state. The SVM for a three-leg voltage source inverter is obtained by sampling the reference vector at the fixed clock frequency 2fs. All the eight possible switching combinations of the switching network are mapped into an orthogonal plane. The results are six non-zero vectors and two zero vectors. 1.9 PHOTOVOLTAIC POWERED INDUCTION MOTOR DRIVE Solar energy is the most low cost, competition free, universal source of energy as sunshine's throughout. This energy can be converted into useful electrical energy using photovoltaic technology. The steady state reduction of price per peak watt and simplicity with which the installed power can be increased by adding panels are attractive features of PV technology. Among the many applications of PV energy, pumping is the most promising. In a PV pump storage system, solar energy is stored, when sunlight is available as potential energy in water reservoir and consumed according to demand. There are advantages in avoiding the use of large banks of lead acid batteries, which are heavy and expensive and have one fifth of the lifetime of a PV panel. A number of experimental DC motor driven PV pumps are already in use in several parts of the world, but they suffer from maintenance problems due to the presence of the commutator and brushes. Hence a pumping system based on an induction motor can be an attractive proposal where reliability and maintenance-free operations with less cost are 11

important. The effective operation of Induction motor is based on the choice of suitable converter-inverter system that is fed to Induction Motor. Converters like Buck, Boost and Buck- Boost converters are popularly used for photovoltaic systems. But these converters are limited to low power applications. For PV applications like pumping these converters could do a good job as pumping is carried out at high power. Thus a new push pull converter which is two switch topology can do justice by giving a high power throughput. The Induction Motors are the AC motors and hence from converter, an inverter system is also required to obtain an AC voltage. This inverter is chosen based on its advantages and it is fed to induction motor. Among the many applications of photovoltaic energy, pumping is one of the most promising. In PV pump storage system, solar energy is stored, when sunlight is available as potential energy in a water reservoir and consumed according to the demand. There are advantages in avoiding the use of large banks of lead acid batteries, which are heavy and expensive and have one fifth of lifetime of PV panel. A typical configuration of a battery less photovoltaic pumping system comprises of 1) PV panels 2) DC/DC converter 3) DC/AC converter 4) an induction motor and 5) centrifugal pump The design of an effective PV pumping system without the use of a battery bank represents a significant challenge. It is necessary to deal with the effect of the stochastic nature of solar installation on the entire energy conversion chain, including the nonlinear characteristics of PV pumping, the voltage boost converter, and the electromechanical power conversion device. In general terms, it is necessary to obtain the best performance from each system component over a wide input power range.currently, solar water pumps are used in the western United States as well as in many other countries or regions with abundant sunlight. Solar pumps have proven to be a cost effective and dependable method for providing water in situations where water resources are spread over long distances, power lines are few or non-existent, and fuel and maintenance costs are considerable. PUSH PULL CONVERTER Push-Pull type DC - DC converter is suitable to boost up the voltage from low to high voltage. This converter may be used in conjunction with a high frequency transformer to boost the output voltage with the advantage of providing isolation between the input and output stage. In this project a simple two switch push pull converter topology is used, which will step up a 12

12V DC voltage supply to the required output voltage. A 12Vsupply is used as the input supply. The high frequency transformer is known as the push pull transformer. This push pull transformer is usually the preferred choice in high power switching transformer applications exceeding one kilowatt. Power ratings for push pull transformer can vary from a fraction of a Watt to Kilowatt 1.10 CONCLUSION VSI fed three-phase induction motor drive is simulated, fabricated and tested. The reliability is increased by using microcontroller as the on chip intelligent controller. The experimental results closely agree with the simulation results. Modified Sine PWM is used to reduce switching losses. The contribution of this work is the development of modified Sine PWM model using the blocks of simulink. Modified Sine PWM inverter fed induction motor drive is a viable alternative to the VSI fed induction motor drive due to the reduced switching losses. The present work indicates that SVM inverter fed induction motor drive is an economical drive with reduced harmonics Sine PWM and SVM inverter fed induction motor drives are compared. The Photo Voltaic powered three phase induction motor drive system is successfully designed, modeled and simulated. The experimental results are similar to the simulation results. 13

1.11 PAPERS PUBLISHED INTERNATIONAL JOURNALS 1. K Vijaya Bhaskar Reddy, G V Siva Krishna Rao and T S Surendra Digital Simulation of Space Vector Modulation Based Induction Motor Drive, Published in Research Journal of Applied Sciences, Engineering and Technology, ISSN 2040-7467, April 2011, pp 329-336. 2. K Vijaya Bhaskar Reddy and G V Siva Krishna Rao, Comparison of Sine PWM and SVM Controlled Induction Motor Drives, published in CIIT international journal of artificial intelligent systems & machine learning, ISSN 0974-9667, VOLUME 3, NO13, pp 826-830 and December 2011. 3. Vijaya Bhaskar Reddy, G V Siva Krishna Rao and T S Surendra, On chip implementation of VSI fed induction motor drive, published in ARPN Journal of Engineering and Applied Sciences, ISSN 1819-6608, volume 7, April 2012 and pp492-498. 4. K.Vijaya Bhaskar Reddy, G V Siva Krishna Rao, Modelling and Simulation of Modified Sine PWM VSI fed Induction Motor Drive, published in IJEET journal of engineering, ISSN 0976-6545, volume 3, September 2012. CONFERENCES 1. Transient Effect of PWM VSI Inverter on 3 phases Induction Motor, in Vignan s Institute of Information Technology at Visakhapatnam on March 2007. 2. Winding failures in Induction Motor Due to Static VSI Drives, in SAURASHTRA UNIVERSITY, RAJKOT on October, 2006. 14

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