The project aims at controlling the AC power by using the concept of firing angle control of thyristors. One can enter the required percentage of power supply through a keypad. The input is provided to a microcontroller of 8051 family that initiate the firing angle to adjust the load power. For matching the power to the required one, a TRIAC is used in series with the AC load. A display unit is used to display the full power and one can go through the preferred percentage to decrease the power to the load. Here, to maintain the load power the firing angle would be changed automatically. The project employs a lamp such that the entered power equals the necessary one. The above process is carried out with the help of a TRIAC in series with the AC load. It uses 8051 family microcontroller. A keypad is used to give the input to the microcontroller and ZVS is given as reference. An LCD is used to display the information.

 

 

CHAPTER ONE
1.1                                                        INTRODUCTION
It is often desired to control the power fed to a load using electronic methods. Such methods permit a fine control of power with better efficiency than electrical methods. Semiconductor devices that are often used to control the flow of current in a circuit are diodes, diode ac (Diac) Thyristors or Silicon Controlled Rectifier (SCR) and Triode ac (Triac) [5]. Diodes only conduct when it is forward biased and have no external control of the start of conduction. Thyristors allow control of the start of conduction in the positive half-cycle of an ac voltage but rely on periodic reversal of current to turn them off. Triac on the other hand has the ability to conduct current in both half- cycles by using positive or negative gate pulse which provides control on the start of conduction; therefore it can be used to provide a control of power in ac circuits of lighting equipment, hot-air oven, electric incubator and electric heater and in universal single phase ac motor. In these systems, power is controlled by means of phase angle variation of the conduction period through the setting of different firing times corresponding to different firing angles.
The mains voltage is an alternating type that reverses its polarity many times per second and it is sinusoidal in nature. The instantaneous value of such a voltage is usually expressed as:
v(t) =Vsinωt                                                   (1)
Where V is the amplitude of the voltage in volts and ω is the angular frequency in radians per second and t is time in seconds, the period ‘T’ which is the time it takes the voltage to complete a whole cycle can be written in terms of angular frequency as:
T=2π/ω                                               (2)
The physical frequency f which is measured in hertz (Hz) is related to the angular frequency as:
ω = 2πf                                               (3)
Therefore,
T=1/f                                                              (4)
The root-mean-square (rms) value of an ac voltage described by (1) is proportional to the integral of the square of the instantaneous voltage over a period of the voltage, average power delivered to a load by such a voltage is the ratio of the square of rms voltage to the resistance of the load. An ac power controller is a unit that can vary the rms value of the voltage across a load while keeping the frequency constant. Methods that are often used in power control are ON/OFF control, Phase-angle control and pulse-width modulation ac chopper control [2].Power electronic components such as Triac and Thyristors are often used to delay the firing angle in a wave; this causes only part of the wave to be outputted to the load. The firing angle ‘α’ of these devices is the angle in a half-cycle of a sinusoidal signal at which they begin to conduct; is usually the object of control. The remaining angle of the cycle in which the device conducts is the conduction angle ‘ϕ’. These angles are complimentary.In analogue circuits the firing angle is usually controlled through an RC network in which a capacitor charges through a resistor and discharges through the gate of the Triac, such a network can only change the firing angle between 0° and 90°, but with operational amplifiers and more sophisticated circuits, the firing angle can be changed from 0° to 180° in both cycles of the ac voltage [4]. A digital circuit that can be used to provide control of ac power: precision ac power control was reported by [3]; apart from the complexity of the circuit, the system is not purely electronic. A microelectronics system has a microprocessor in itscircuitry. It stores the program on which it operates a memory unit and executes the program sequentially at the rate that depends on the clock signal applied to it. One of the numerous advantages of a microelectronic system is its ability to respond to internal or external stimuli called interrupts that must be addressed by the system. Interrupts are electrical signals that are either generated internally by timers or provided by a peripheral system to allow effective communication with the microprocessor or Central Processing Unit (CPU). The CPU checks for the presence of any interrupt at the end of execution of each line the program, the sequential order of execution of the program is broken if there is a interrupt and a particular segment of the program called the Interrupt Service Routine (ISR)which has been specifically written to take care of the interrupt is executed. The CPU resumes the execution of the main program after dealing with the interrupt [1].A microprocessor together with memory units that is fabricated on a chip is a microcontroller. Application of microcontrollers in electronics has led to high performance and reliability; low power consumption and space. Apart from these advantages, microcontrollers are able to meet the computing needs of many tasks efficiently. The availability of software development tools such as compilers, assemblers and debuggers and reliable sources of microcontrollers makes it possible to develop microelectronic systems easily. The use of a microelectronics system to control firing of the gate of a Triac as a means of increasing the reliability and reducing the complexity of a power control system investigated in this project. A microcontroller was used to generate a delay at the beginning of each half-cycle of an ac voltage and fires the gate of a Triac at the end of the delay, the period of the delay depends on the interrupt generated when a switch is pressed.

1.2                                                   AIM OF THE PROJECT
This work is aimed at controlling the AC power by using the concept of firing angle control of thyristors. With this device one can enter the required percentage of power supply through a keypad.
1.3                                             OBJECTIVE OF THE PROJECT
The main objective of this work is to vary the percentage of power supply by means of phase angle variation of the conduction period through the setting of different firing times corresponding to different firing angles. At the end of this work the student involved will be able to:

• Have the full understanding of firing angle.
• Understand the application of TRIAC and SCR in power electronics.

1.4                                                 SCOPE OF THE PROJECT
This work is on controlling the AC power by using the concept of firing angle control of thyristors. One can enter the required percentage of power supply through a keypad. The input is provided to a microcontroller of 8051 family that initiate the firing angle to adjust the load power. For matching the power to the required one, a TRIAC is used in series with the AC load. A LCD screen is used to display the power percentage that is provided by the user.

1.5                                          APPLICATION OF THE PROJECT

• Light dimming circuits for street lights
•  Industrial & domestic heating
• Induction heating
• transformer tap changing
• Speed control of Motors (variable torque)
• speed control of winding machines,fans
• AC magnet controls

1.6                                         SIGNIFICANCE OF THE PROJECT
AC power control with thyristor using microcontroller is designed to control AC power flow across load. This project is designed using  AT85s52 microcontroller and Zero crossing detection circuit. Phase angle control method is used for AC power control with thyristor. Thyristor is used as a switch to control flow of power. AT85s52 microcontroller is used to detect zero crossing and to generate pulse signals at a specific angle for thyristor. It is used in many power electronics project.

1.7                       PROJECT WORK ORGANISATION
The various stages involved in the development of this project have been properly put into five chapters to enhance comprehensive and concise reading. In this project thesis, the project is organized sequentially as follows:
Chapter one of this work is on the introduction to this study. In this chapter, the background, significance, objective limitation and problem of this study were discussed.
Chapter two is on literature review of the study. In this chapter, all the literature pertaining to this work was reviewed.
Chapter three is on design methodology. In this chapter all the method involved during the design and construction were discussed.
Chapter four is on testing analysis. All testing that result accurate functionality was analyzed.
Chapter five is on conclusion, recommendation and references.

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