MICROCONTROLLER BASED DELTA MODULATED PWM FOR 3-PHASE INVERTER AYU ANNUR FETRAH BT YUSOFF

Transcription

1 i MICROCONTROLLER BASED DELTA MODULATED PWM FOR 3-PHASE INVERTER AYU ANNUR FETRAH BT YUSOFF This thesis is submitted as partial fulfilment of the requirement for the award of the Bachelor Electrical Engineering (Power System) Faculty of Electrical & Electronic Engineering Universiti Malaysia Pahang JUNE, 2012

2 vi ABSTRACT One of the PWM schemes proposed for the control of inverters supplying ac drive is Delta Modulation. Presently, microcomputer has been used as an alternative to the standard Delta Modulation PWM technique. Even though the microcomputer is considered as power computing machine, their weakness is that to communicate with peripheral equipments, they must use specialized circuit added as an external chips. By utilizing the features of microcontroller, they can just fit inside other device like the inverter or other appliance and also they does not need any other specialize external component for its application. The microcontroller based delta modulated PWM for 3- phase inverter has been proposed in this project in order to get better solution for the control of inverter. The simulation of Delta Modulation techniques is carried out using PROTEUS ISIS 7.0 in order to get six modes of delta modulated PWM switching scheme. Then, the microcontroller PIC18F4550 circuitry to generate Delta PWM is assembled. The circuit from the simulation can produce the necessary Delta PWM signal to control the inverter.

3 vii ABSTRAK Salah satu skim PWM yang dicadangkan untuk kawalan penyongsang membekalkan AC pemacu adalah Delta Pemodulatan. Pada masa ini, mikrokomputer telah digunakan sebagai alternatif kepada teknik PWM Delta standard Pemodulatan. Walaupun mikrokomputer dianggap sebagai mesin kuasa pengkomputeran, kelemahan ia untuk berkomunikasi dengan peralatan persisian, mereka mesti menggunakan litar khas yang ditambah sebagai cip luar. Dengan menggunakan ciri-ciri mikropengawal, ianya hanya boleh dimuatkan di dalam peranti lain seperti penyongsang atau perkakas lain dan juga ia tidak perlu mana-mana orang lain yang mempunyai kepakaran komponen luaran untuk aplikasi tersebut. Mikropengawal berdasarkan delta termodulat PWM untuk penyongsang 3-fasa telah dicadangkan dalam projek ini untuk mendapatkan penyelesaian yang lebih baik bagi kawalan penyongsang. Simulasi Delta Pemodulatan teknik dijalankan menggunakan Proteus ISIS 7.0 untuk mendapatkan enam mod delta termodulat PWM menukar skim. Kemudian, mikropengawal PIC18F4550 litar untuk menjana PWM Delta dipasang. Litar daripada penyelakuan boleh menghasilkan isyarat PWM Delta perlu untuk mengawal penyongsang.

4 viii TABLE OF CONTENTS TITLE DECLARATION DEDICATION ACKNOWLEDGEMENT ABSTRACT ABSTRAK TABLE OF CONTENTS LIST OF FIGURES LIST OF SYMBOLS LIST OF ABBREVIATION i ii iv v vi vii viii xi xii xiii CHAPTER 1 : INTRODUCTION Background Project Objective Scope of Project Project Contribution Report Organisation 3 CHAPTER 2 : LITERATURE REVIEW Introduction Review of Pulse Width Modulation Techniques (PWM) Schemes Single PWM Multiple PWM Sinusoidal PWM Dual Slope Delta Modulated Pulse Width Modulation (DPWM) Methods To Generate Pulse Width Modulation Via Analogue Circuits 11

5 ix Via Digital IC Via Microcomputer/Microprocessor Via Microcontroller Architecture of Microcontroller 14 CHAPTER 3 : METHODOLOGY Introduction Dual Slope Delta Modulated PWM (DPWM) Implementation Analysis Hardware Development Microcontroller PIC18F Power Supply +5V LED Indicator Software Implementation Algorithm and Programming in PIC C Compiler Explanation on Main Programming 27 CHAPTER 4 : RESULT & DISCUSSION Introduction Analysis on Determine The Effect Of Changing The Parameters in DPWM Algorithm Overall Result LED Indicator LCD Display 35 CHAPTER 5 : CONCLUSION & RECOMMENDATION Conclusion Problems Recommendation Costing and Commercialization 38 REFERENCES 40 APPENDICES 39

6 x LIST OF FIGURES Figure 2.2(a) Single PWM with 180 pulse. 6 Figure 2.2(b) Multiple PWM with 6 pulses. 6 Figure 2.2(c) Sinusoidal PWM. 6 Figure 2.3(a) Switching waveforms for PWM Inverter Obtained By Delta Modulation Technique. 7 Figure 2.3(b) Waveform of Delta Modulator For Rising Slope. 9 Figure 2.3(c) Waveform of Delta Modulator For Falling Slope. 9 Figure Generate PWM Signals Via Analogue Circuits. 11 Figure Block Diagram of 555 Timer. 12 Figure 2.5 Microcontroller Versus Microptocessor. 15 Figure 3.1 Work Flow of The Project. 16 Figure 3.2 Calculation for Rising Slope. 17 Figure 3.3(a) Block Diagram for PIC Microcontroller. 19 Figure 3.3(b) PWM Switching Control Circuit. 19 Figure 3.3(c) +5V Regulated Power Supply. 20 Figure 3.4 PIC18F4550 Pin Connection Circuit. 21 Figure 3.5 IC LM Figure 3.6 Schematic Circuit of +5V Power Supply. 23 Figure 3.7 LED Schematic. 24 Figure 3.8 Program Flow Chart. 26 Figure 4.1 Pulses Generated at Dual Slope Operation when modulating frequency is changed 31 Figure 4.2 Figure 4.4 Pulses Generated at Dual Slope Operation when rising slope is changed 32 PWM signals with 60 degree pahse shift between the signals 33 Figure 4.4 LEDs Indicator 34

7 xi List of Symbols dv = Window width in delta modulator in volts. f m = Modulation frequency. S = Slope of the triangular estimated wave. S F = Falling slope of carrier/estimated wave. S R = Rising slope of carrier/estimated wave. t i = Terminating position of i-th pulse. t i 1 = Starting position of i-th pulse. V F = Estimated wave (Carrier wave). V I = Modulated signal. V L = Lower boundary of estimated wave of DM. V m = Magnitude of sine wave. V R = Reference sine wave (Modulating sinewave). V S = Magnitude of DC supply to electronic circuit. V U = Upper boundary of estimated wave of DM. = Fundamental frequency in rad/sec. R = frequency of input sine reference wave in rad/sec. V = Window width in delta modulation.

8 xii List of Abbreviation CPU dc DM I/O PWM RAM ROM RWDM SPWM Central Processing Unit. Direct Current. Delta Modulation. Input Output. Pulse Width Modulation. Random Access Memory. Read Only Memory. Rectangular Wave Delta Modulation. Sinusoidal Pulse Width Modulation.

9 CHAPTER 1 INTRODUCTION 1.1 Background This project is focusing on how to generate the signal of Pulse Width Modulation (PWM) as the input signal for 3-phase inverter. The 3-phase inverters are an electronic device used to convert a DC input voltage to symmetrical AC input voltage. Generally, the inverter output voltage is supplied by DC input voltage and controlled by the PWM voltage signal. The PWM signals may be generated either form a sinusoidal Pulse Width Modulation (SPWM), Dual Slope Pulse Width Modulation (DSPWM) or Sigma Pulse Width Modulation. DSPWM generated using a Personal Computer (PC) has been reported in the past however, generating the signal based on microprocessor or microcontroller has not yet reported. Therefore, in this project, the DSPWM generated by a microcontroller is proposed. 1.2 Project Objective The aim of this project is obviously to develop a controlling system which can be used to drive the 3-phase inverter. The main objectives are as follow: i. To implement Delta Modulation PWM switching scheme for the 3-pahse inverter using microcontroller. ii. To analyze the generated Delta Modulation PWM of its feasibility in controlling the 3-phase inverter.

10 2 1.3 Scope Of Project There are several scopes that need to be accomplished for this project i. This project will design on how to generate delta modulated PWM signals using microcontroller PIC18F4431. ii. This project will generate the six signals of delta modulated PWM with 60 degree phase shift between the pulses sequence. 1.4 Project Contribution The PWM signals have a wide range in electrical and electronics applications. It can be seen in industrial application, the PWM signals has been widely used in controlling the speed of the motor and also the output of the inverter. As we know, PWM is digitally controlling, hence, system cost and power consumption can be reduced. The same thing goes in communication system because the digital signals are more robust and less vulnerable to noise. Furthermore, most of today s microcontroller is built with PWM capability. Thus, by using this capability, the implementation for controller will be easier.

11 3 1.5 Report Organization This project report is organized into five chapters. The content of each chapter is outlined as follows: Chapter 1 explains the basic theory and advantages of PWM signal. The overview of project objectives, scope of project and the contributions has been discussed in this chapter. Chapter 2 is discussing the literature review that related to this project. This chapter is review the conventional method to generate the signals and also the types of PWM that have been generated. Lastly, the overview of the microcontroller characteristics. Chapter 3 is focusing the work flow of the project from beginning. This chapter also present all the circuit related to this project and method for the development of PWM signals including software and hardware. Chapter 4 discusses about the type of PWM scheme that has been generated as the input signal for 3-phase inverter. This chapter also described the PWM signals when the value parameters of DPWM algorithm are changed. Chapter 5 outlines the conclusions and the future work. This chapter concludes all the chapters covered in this report.

12 4 CHAPTER 2 LITERATURE REVIEW 2.1 Introduction Pulse width Modulation has been widely used in controlling the output of the inverter because its provide a way to reduce the harmonics problems. Instead of the inverter, the PWM also is widely used in other electrical and electronic devices. Thus, a suitable switching method is needed to generate the input signal for the device. Hence, in this chapter, the conventional method to generate the PWM signals will be reviewed. Furthermore, the architecture of the microcontroller also will be presented. 2.2 Review Pulse Width Modulation (PWM) Schemes PWM is the technique used to generate digital signals from analogue signals. Digital signals are used to create a square wave, a signal switched between on and off. Today s there are various methods and theories has been developed to generate advanced pulse width modulation (PWM) signals easier than previous methods. There are several PWM techniques that has been developed (Norkharziana, M.N, 2009) : i. Single PWM ii. Multiple PWM iii. Sinusoidal PWM iv. Modified Sinusoidal PWM

13 5 v. Sinusoidal PWM with unipolar voltage switching vi. Sinusoidal PWM with bipolar voltage switching vii. Phase displacement control viii. Harmonic Elimination PWM ix. Vector PWM x. Hysterisys PWM However, there are four types of PWM switching schemes that recently be applied in industrial applications and home appliances for power converter. The schemes are Single PWM, Multiple PWM and Sinusoidal PWM. The Sinusoidal PWM is the most common used in power converter applications Single PWM In Single PWM control, there is only one pulse per half-cycle and the width of the pulse is varying to control the output voltage. The undesired lower order harmonics can be eliminated using this switching scheme (Norkharziana, M.N, 2009). Figure 2.2 (a) shows the PWM switching scheme Multiple PWM For this PWM, the harmonic content can be reduced by using several pulses in each half-cycle of output voltage (Mohammad K. Abuzalata (2012)). The generation of gating signals is slightly different with single PWM which is the duty cycle is becoming smaller. The signal waveforms consist of several pulses on each half cycle at specified frequency. The signals for multiple PWM are illustrated on Figure 2.2 (b).

14 Sinusoidal PWM The most common PWM approach is sinusoidal PWM and it is a very poplar switching scheme in industrial applications. The generation of gating signals is by comparing a sinusoidal reference signal with a triangular carrier wave of frequency. The signal produced as shown in Figure 2.2 (c). Figure 2.2(a) Single PWM with 180 pulse. Figure 2.2 (b) Multiple PWM with 6 pulses Figure 2.2 (c) Sinusoidal PWM

15 7 2.3 Dual Slope Delta Modulated Pulse Width Modulation (DPWM) The Dual Slope Delta Modulated PWM switching scheme will be applied in this project. Delta modulation is the technique to convert the analog signal to digital signal. The delta modulation technique can be illustrated from the figure 2.3 (a). The triangular wave Vf is the estimation of a sine modulating wave and it is allowed to oscillate within a defined window V above and below the reference voltage VR (Rahman, M.A., Quaicoe, J.E., and Choudhury, M.A. (1981)). Thus, the PWM is determined by the change of slope in carrier signals. Figure 2.3(a) Switching Waveforms for PWM Inverter Obtained by DM Technique.

16 8 Thus, the switching points for each pulses are determined by the following equations (Rahman, M.A., Quaicoe, J.E., and Choudhury, M.A. (1981)): ( ) (2.3.1) Where, V = window widths in volts S = slope of the carrier wave V F V R = maximum voltage of modulating sine wave t i = ith pulse termination time R = ripple frequency of modulated wave The concept from the previous theory that can be seen above is slightly different with the concept of dual slope delta modulated. Dual slope means that the slope for carrier frequency consists of rising slope and falling slope. The illustration of this slope can be seen on Figure 2.3 (b) and (c) (Ramdan Razali, Zulkeflee Khalidan, M.Badaruddin, M.S Najib, 2009).

17 9 (b) (c) Figure 2.3 Waveform of Delta Modulator For Rising Slope (b) and Falling Slope (c) of the carrier signal. From the research (Ramdan Razali, Zulkeflee Khalidan, M.Badaruddin, M.S Najib, 2009), the algorithm for rising edge slope S R can be related as follows: = ( ) (2.3.2) After replacing (sin ) = ( )[ ], obtains = + (2.3.3)

18 10 Meanwhile for the falling slope, the expressions are as follows: = + (2.3.4) Hence, the switching point t i can be expressed in terms t i-1 as = ( ) + (2.3.5) Where, V = window widths in volts S = S R or S F depending on i = odd or even (in normal modulation S R = S F ) V m = maximum voltage of modulating signals R = 2 f m, where f m is the frequency of modulated signal. When the value for t i-1 is known, the value of t i can be calculated. 2.4 Methods To Generate Pulse Width Modulation Pulse Width Modulation (PWM) is a very efficient way in controlling the output of the inverter and also another 3-phase induction motor. Thus, there are various of methods that have been introduced to generate the PWM signals. The methods to generate the PWM signals are : i. Develop Analogue Circuit ii. Develop Digital IC iii. Develop microcomputer or microprocessor

19 11 iv. Develop microcontroller The most commonly used method to generate PWM nowadays via microcontroller. In this project also, the dual slop delta modulated PWM (DPWM) will be generated suing microcontroller Via Analogue Circuits Analogue circuits is the conventional method of generating PWM signal which is by using a high ratio of carrier frequency signal and fundamental frequency signal (Norkharziana, M.N, 2009).This conventional method is shown graphically in Figure 2.4 (a). This method uses an analog device i.e a comparator to generate the signal. Pulse width Modulation Figure Generate PWM signals via analogue circuits. Comparator is used to compare the desired modulating signal with the carrier signal. The resulting signals is depending on modulating signal and carrier signal whether the modulating signals is larger or smaller than the carrier signal. There are some consideration need to be taken in this method which is frequency modulation ratio and amplitude modulation ratio.

20 Via Digital IC Digital IC also known as the PWM generator chips which convert a DC level into a PWM output[5]. This method is very useful in any applications where the PWM setting need only be changed occasionally. The example of Digital IC that can be used is 555 Timer. The 555 Timer in the PWM circuit is configured as a stable oscillator. This means that once power is supplied, the 555 Timer will oscillate without any external trigger[6]. The figure 2.4.2(a) and (b) illustrate the diagram for 555 Timer. Figure Block diagram of 555 Timer Via Microcomputer / Microprocessor As can see nowadays, instead of 3-phase inverters, there are many other power electronic devices have been developed rapidly. Thus, there is a need to get an improvement in controlling the switching technique. Instead of analogue circuits and digital IC, the conventional method to produce the signal of Delta Modulated PWM was by applying the microcomputer has been introduced as an alternative. In earlier system,

21 13 a microcomputer was used to generate the reference signal, while the timing signal and actual PWM signals were produced by electronic hardware (Marlen Varnovitsky, 1983). In a speed control system with a wide range of generated frequencies (V. V. Athani and S. W. Deshpande, 1980), a microprocessor was used to generate a PWM or a six-step unmodulated signal and to maintain a constant voltage/frequency ratio. Because there is computation speed limitations found in many of the available microcomputers, it was concluded that on-line computation of switching patterns was not practical (G. S. Buja and P. Fiorini, 1980). However, the latest advances in microelectronics produced new microcomputers with faster speeds which in turn made it practical to compute the switching pattern on-line Via Microcontroller However, the development of digital technology today has enabled us to apply another method in order to produce the signal of PWM such as digital controller(i.e. microcontroller). Microcontroller is differs with microcomputer in many ways. For instance, microcontroller just can easily be fitted inside other devices and also they don t need any other specialized external components for its application. Therefore, microcontroller is chosen due to the its characteristics which are small in size, less hardware requirement, low cost implementation and consume less power.

22 Architecture of Microcontroller Most microcontrollers also contain read-only memory (ROM), programmable read-only memory (PROM), or erasable programmable read-only memory (EPROM). Al1 of these memories are permanent: they retain what is programmed into them even during loss of power. They are used to store the firmware that tells the microcontroller how to operate. They are also used to store permanent lookup tables. Often these memories do not reside in the microcontroller; instead, they are contained in external ICs, and the instructions are fetched as the microcontroller runs. This enables quick and low-cost updates to the firmware by replacing the ROM. Where would a microcontroller be without some way of communicating with the outside world? This job is left to input/output (I/O) port pins. The number of I/O pins per controllers varies greatly, plus each I/O pin can be programmed as an input or output (or even switch during the running of a program). The load (current draw) 18 that each pin can drive is usually low. If the output is expected to be a heavy load, then it is essential to use a driver chip or transistor buffer. Most microcontrollers contain circuitry to generate the system clock. This square wave is the heartbeat of the microcontroller and all operations are synchronized to it. Obviously, it controls the speed at which the microcontroller functions. All that needed to complete the clock circuit would be the crystal or RC components. We can, therefore precisely select the operating speed critical to many applications. To summarize, a microcontroller contains (in one chip) two or more of the following elements in order of importance [8]: i. Instruction set ii. RAM iii. ROM,PROM or EPROM

23 15 iv. I/O ports v. Clock generator vi. Reset function vii. Watchdog timer viii. Serial port ix. Interrupts x. Timers xi. Analog-to-digital converters xii. Digital-to-analog converter Figure 2.5 Microcontroller versus Microprocessor

24 16 CHAPTER 3 METHODOLOGY 3.1 Introduction This chapter explains about hardware development such as equipments, procedures and method design for microcontroller circuit in order to generate the signals. However, before looking at the details subtopics, it is good to begin with the reviewed of the work flow of this project. The work flow diagram is shown in Figure 3.1 : Figure 3.1 Work Flow of the Project.