01-03-2013, 10:21 AM
THREE PHASE VOLTAGE REGULATION USING SCR & MICRO - CONTROLLER
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EXECUTIVE SUMMERY
In our day to day life we use a lot many devices to satisfy our needs or to make our life comfortable and luxurious. Every device needs a power supply, to work on. And for the optimum functioning of the device it is necessary that the supply should be reliable. That is, it should provide a constant voltage.
But this is not possible always. There are many reasons due to which there is a fluctuation in the supply voltage. This change in the supply voltage may cause the device to damage or make it work in an undesired way, which no one would desire.
Hence the best alternative is to regulate the supply voltage. This is what we have tried to achieve here. Our project is supply voltage regulation, using controller and SCR.
In our project we provide the load with a constant voltage of 240 V ac., in spite of any variation in the input voltage. The voltage regulation is achieved by controlling the firing angle of the SCR so precisely that the load receives a constant supply. The voltage across the load is stepped down and provided to ADC. ADC will produce a digital signal corresponding to the input analog signal. This digital signal from ADC is then processed by the controller and generates a firing pulse for SCR, hence controlling the load current.
INTRODUCTION
In our day to day life we use a lot many devices to satisfy our needs or to make our life comfortable and luxurious. Every device needs a power supply, to work on. And for the optimum functioning of the device it is necessary that the supply should be reliable. That is, it should provide a constant voltage.
But this is not possible always. There are many reasons due to which there is a fluctuation in the supply voltage. This change in the supply voltage may cause the device to damage or make it work in an undesired way, which no one will desire.
Hence the best alternative is to regulate the supply voltage. This is what we have tried to achieve here. Our project is supply voltage regulation, using controller and SCR.
Silicon Controlled Rectifiers also called Thyristors controller, employing novel technology, which is designed to provide a price effective solution for applications that require power, current or voltage regulation with some power factor correction and a smother process control. Traditional phase-angle control causes lots of harmonic current distortion on the main power supply. This in turn creates voltage distortion which affects power quality. There is no simple accessory available for reducing this problem.
However, when simple voltage or current regulation is required often phase-angle control is the most cost effective solution.
AIM & OBJECTIVE
AIM:-
To develop a system for controlling fluctuation in the three phase Voltage supply using SCR and Controller.
OBJECTIVE:-
To upgrade the existing three phase analog regulatory system, to a three phase, microcontroller based SCR drive system. So that if any fluctuation comes in three phase voltage supply, controller will Sense that fluctuation and accordingly give triggering pulses to the SCR to get controlled regulated output at the load.
• POWER SUPPLY
This is the first block of our system. We have used a step-down centre tap transformer, with the voltage rating of 240V ac as primary voltage and 24-0-24V ac as the secondary voltage. The current rating of the transformer is 500mA.
The stepped-down ac signal is supplied to the rectifier & regulator. It consists of a simple rectifier diode bridge network along with some filtering circuit, for smoothing out the input signal. This filtered and rectified signal is then regulated using a positive voltage regulator, to the desired value (say 5 V dc & 15 V dc) and also negative voltage regulator to the desired value (say -15 V dc).For these purpose; we are using three regulator chips.
ZERO CROSSING DETECTOR
This circuit is containing of OP-AMP UA 741.This is mainly used to detect the zero crossing of the input sine wave so that we can get Synchronization.
The output of ZCD is given to the PORT pin 2.5 of the Microcontroller. Here ZCD is used so that we can give trigger angle to the SCR at Correct time.
The op-amp in the ZCD is just a sine to square wave generator. It converts in the input 24V ac signal to the square wave of 5 V and of the same frequency as that of the sine wave. Op-amp UA 741 is provided with a dual supply, obtained from the positive and negative regulators (+15V & -15V dc).
The output pin of the ZCD is provided with a rectifying diode which restricts the negative signal from reaching the controller pin to avoid any damage to it.
• SCR BRIDGE NETWORK
This block consists of a pair of SCR & diodes. Input to the SCR Bridge circuit is fluctuated Single phase voltage supply, which is given to anode of both the SCRs and cathode of both the diodes. Cathode of both the SCRs and Anode of both the diodes are provided to the load. We have assumed a resistive load of 10K ohm. From this load resistor one voltage signal will go to the Potential divider for feedback purpose. This will act as the input signal to the ADC.
The gate of the SCR is connected to the PORT2.0 and PORT2.1. A specific triggering pulse is provided to the gate of the SCR of sufficient time delay so as to keep the load voltage constant.
• ANALOG TO DIGITAL CONVERTER (ADC 0808)
Here we get input from potential divider network which is around +5V. Then this analog value is converted to digital data and is given to Microcontroller. ADC 0808 has four channels but we need only one, hence we have selected channel 0 for input. The 8 bit digital output of ADC is provided to the port 1 of controller.
• MICROCONTROLLER 89C51RD2
This block is the only decision making block, which decides whether any fluctuation in the supply line has occurred or not. It continuously compares the signal with the reference described in the software. If there is no change then SCR will be fired by it at phase angle 0 deg. But if it finds some fluctuation, then it will generate the pulse at a measured time delay to provide the firing angle of the SCR (through gate) such that the fluctuations will be nullified, and the supply to the load remains unaffected, in-spite the fluctuations.
Description information
This series of fixed-voltage integrated-circuit voltage regulators is designed for a wide range of applications. These applications include on-card regulation for elimination of noise and distribution problems associated with single-point regulation. Each of these regulators can deliver up to 1.5 A of output current. The internal current-limiting and thermal-shutdown features of these regulators essentially make them immune to overload. In addition to use as fixed-voltage regulators, these devices can be used with external components to obtain adjustable output voltages and currents.
SPECIFICATION OF MICROCONTROLLER 89C51RD2
• The 89C51RB2/RC2/RD2 device contains a non-volatile 16kB/32kB/64kB Flash
• Program memory that is both parallel programmable and serial In-System and In-Application Programmable. In-System Programming (ISP) allows the user to download new code while the microcontroller sits in the application. In-Application Programming (IAP) means that the microcontroller fetches new program code and reprograms itself while in the system. This allows for remote programming over a modem link. A default serial loader (boot loader) program in ROM allows serial In-System programming of the Flash memory via the UART without the need for a loader in the Flash code. For In-Application Programming, the user program erases and reprograms the Flash memory by use of standard routines contained in ROM. This device executes one machine cycle in 6 clock cycles, hence providing twice the speed of a conventional 80C51. An OTPconfiguration bit lets the user select conventional 12 clock timing if desired. This device is a Single-Chip 8-Bit Microcontroller manufactured in advanced CMOS process and is a derivative of the 80C51 microcontroller family. The instruction set is 100% compatible with the 80C51 instruction set. The device also has four 8-bit I/O ports, three 16-bit timer/event counters, a multi-source, four-priority-level, nested interrupt structure, an enhanced UART and on-chip oscillator and timing circuits. The added features of the P89C51RB2/RC2/RD2 make it a powerful microcontroller for applications that require pulse width modulation, high-speed I/O and up/down counting capabilities such as motor control.
FUNCTIONALITY
• Here, in our project we are controlling the single phase supply voltage 240V ac by triggering the SCR from Microcontroller.
• Main purpose of our project is to get constant 240 V dc at load. To fulfill this task we have to control the firing angle of SCR trigger pulse. And for control purpose we have used Philips 89C52RD2 Microcontroller.
• Main parts of our circuits are SCR bridge circuit, Power supply, Zero crossing detector, Potential divider.
• The input to the SCR bridge circuit is 240V ac. From this circuit we get output which will initially be fluctuating so for controlling purpose we will take a feedback signal from output.
Now, we have to give this feedback signal to Analog to Digital converter but here the feedback signal is of around 240V dc. When ADC 0808 can operates up to +5 V dc. It will be damaged if we apply 240V dc to it. So for that we must have to use some kind of step down circuitry. Here we have used Potential divider circuitry. By the use of Potential divider we will step it down to around +5V dc signal. Now it is safe to apply that signal to ADC 0808.