Design And Construction Of A Thyristor Based Cycloconverter

ABSTRACT

The project aims at controlling the speed of a single phase induction motor making use of cyclo convertor mechanism through thyristors. It becomes difficult to control the speed of induction motors that usually operates at constant speed. The process is carried out in three steps. The project uses a microcontroller of 8051 family interfaced to a pair of slide switches for selecting the required speed range as F, F/2 and F/3. Thyristor is used for the purpose of controlling speed as the Ac supply frequency to the induction motors cannot be changed. As the switches are set the microcontroller generates the pulses to trigger the SCR’s in a dual bridge.

TABLE OF CONTENTS

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWELDGEMENT

ABSTRACT

CHAPTER ONE

INTRODUCTION

1.1      BACKGROUND OF THE PROJECT

• AIM/OBJECTIVE OF THE PROJECT
• SCOPE OF THE PROJECT
• ADVANTAGES OF THE PROJECT
• APPLICATION OF THE PROJECT
• SIGNIFICANCE OF THE PROJECT
• ADVANTAGES OF THE PROJECT

CHAPTER TWO

LITERATURE REVIEW

• OVERVIEW OF THE STUDY
• TYPES OF CYCLOCONVERTERS
• METHODS FOR SPEED CONTROL OF INDUCTION MOTOR
• TYPES OF INDUCTION MOTORS
• DESCRIPTION OF AN SCR
• SCR IN FORWARD AND REVERSE CHARACTERISTICS

CHAPTER THREE

METHODOLOGY

• SYSTEM BLOCK DIAGRAM
• COMPONENTS LIST THYRISTOR CONTROLLED CYCLOCONVERTER
• WORKING PRINCIPLE
• SYSTEM OPERATION
• CIRCUIT DIAGRAM
• SYSTEM INSTALLATION

CHAPTER FOUR

TESTING AND RESULTS

• CONSTRUCTION PROCEDURE AND TESTING
• ASSEMBLING OF SECTIONS
• PACKAGING
• MOUNTING PROCEDURE
• TESTING OF SYSTEM OPERATION
• RESULT

CHAPTER FIVE

• CONCLUSION
• REFERENCES

CHAPTER ONE

1.1                                                        INTRODUCTION

In industrial applications Speed control of induction motor is necessary. There are several methods for the speed control of induction motor. Cycloconverter are used in very wide variable frequency drives with ratings from few megawatts up to many tens of megawatts. A cycloconverter is controlled through the timing of its firing pulses, so that it produces an alternating output voltage. It can also be considered as a constant frequency changer and typically contains silicon-controlled rectifiers. The development of the semiconductor devices has made it possible to control the frequency of the cycloconverter according to the requirement and deliver a large amount of controlled power with the help of semiconductor switching devices like thyristors, in order to get alternating output of variable frequency. The quality of the output waveform improves if more switching devices are used. Split- phase induction motors are widely used in many applications due to their energy efficient characteristics. Improvements in its performance mean a great saving in electrical energy consumption. Thus, a cycloconverter has the facility for continuous and independent control over both its output voltage and frequency.

This paper uses a thyristor controlled cycloconverter which enables the control of speed in steps for an induction motor. A pair of slide switches is provided to select the desired speed range (F, F/2 and F/3) of operation of the induction motor. These switches are interfaced to the microcontroller. The status of the switches enables the microcontroller to deliver the pulses to trigger the SCR‟s in a dual bridge. Thus, the speed of the induction motor can be achieved in three steps i.e. (F, F/2 and F/3).

Traditionally we use the converter and inverter to vary the AC supply frequency (i.e. it converts AC to DC by using converter and then inverter for DC to AC) to change the frequency which is very costly and complicated. Due to this switching of AC to DC and DC to AC the noise produces also the harmonics creates so the sensitive electronic devices may get damaged ,if the input and output waveforms is small then sub harmonics also get produced and this limitation is overcome by using the cycloconverter i.e. Intermediate DC stage is not used in this conversion.

Cycloconverter is used to convert the AC supply frequency from one input frequency to another output frequency. Cycloconverter is used for high power applications for driving induction and synchronous motor. So, cycloconverter is used for providing a variable frequency due to its 4-quadrant operation .Intermediate DC stage is not used in this conversion. In cycloconverter power flow is bidirectional.

1.2                                        AIM/OBJECTIVE OF THE PROJECT

The main aim of this work is to write on a device that changes the speed of an ac motor using the variable frequency drive. This device is a frequency converter from one level to another that can change AC power from one frequency to AC power at another frequency. Thyristor used enables the control of speed in steps for an induction motor

1.3                                                 SCOPE OF THE PROJECT

The single phase to single phase cycloconverter connects with split phase induction motor. Which is operating without circulating current; the non-conducting thyristors should always be kept off otherwise the input power supply could be shorted. When the load current is positive, the output voltage is only controlled by phase control of thyristors T1 and T3 at the same time, the other two negative thyristors T2 and T4 are kept Off and vice-versa when the load current is negative. When the load current changes its direction at the same time ensuring that the two thyristor half bridges do not conduct at the same time.

1.4                                          ADVANTAGES OF THE PROJECT

1. This thyristor controlled cyclo converter drive can be used in industrial and domestic application such as the machine which have single-phase motor can be drive through this drive.
2. This thyristor controlled cyclo converter drive can be used in vacuum cleaner, washing machine and water pumps.
3. This drive can be used for controlling the speed of drill machine because some time we have required low speed and high torque for this purpose this is best drive.
4. This drive can also be used for the speed control of cement mill drives, Ship propulsion drives, rolling mill drives, Scherbius drives, Ore grinding mills and mine winders.
5. This thyristor controlled drive is less costly and efficient and compared to the other motor drives.

1.6                                   APPLICATIONS OF CYCLOCONVERTER

Applications include controlling the speed of AC machines like It is mainly used in electric traction, AC motors having variable speed and induction heating.

• Synchronous Motors
• Mill Drives
• Ship propulsions
• Grinding Mills

1.7                                         SIGNIFICANCE OF THE PROJECT

Cycloconveters are extensively used for driving large motors like the one used in Rolling mills, Ball mills Cement kils etc. The out frequency of a Cycloconverters can be reduced upto to zero which helps us to start very large motors with full load at minimum speed and then gradually increase the speed of the motor by increasing the output Frequency. Before the invention of Cycloconverters, these large motors has to be unloaded completely and then after starting the motor it has to be loaded gradually which results in time and man power consumption.

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