Design and Construction of Control System for Automatic Voltage Stabilizer

In Myanmar, voltage fluctuation always occurs in electrical supply system. Due to voltage fluctuation, life of electrical equipment consumed electricity is shorted really. To solve this problem, automatic voltage stabilizer is needed for domestic and industries. Both single phase and three phases are available. In this paper intend to known that automatic voltage stabilizer plays efficient role in all type of load i.e. resistive, inductive and capacitive loads. This paper present control circuit for automatic voltage stabilizer provides voltage comparator, relays and servo-controlled motor that compare instantaneous input and output voltage. Automatic voltage stabilizer consists of two unit; measuring unit and regulating unit. In this stabilizer, toroidal type variable autotransformer is used for regulating unit and electronic control circuit is used for sensing unit. Electromechanical or servo control system is used for measuring unit to sense the supply voltage. This electronic control circuit will operate within the fluctuation range from 120V to 250V.The rating of this automatic voltage stabilizer is 7kVA (single phase) and its frequency range is 50Hz. The output sensitivity is ±1%. If input voltage is lower than 120V or higher than 250V, the system will be automatic shutdown.


INTRODUCTION:
Servo controlled voltage stabilizer is a useful and effective device used to maintain a constant power supply. Voltage fluctuation is a common problem in Myanmar, which can cause damage in electronic devices used in home and in industries. To solve these appliances safe is to use voltage stabilizers. The automatic voltage stabilizers are widely used in industrial application to obtain the stability and good regulation for the sophisticated electrical and electronic equipments such as communication equipments and system, process controller, computer equipment etc [1].
Servo controlled is a closed loop systems for electric motors. The motor used in servo control is usually DC motor used in servo is also possible. The servo system uses a sensor to sense motor position/speed. Servo control has a feedback circuit which changes the drive power going to motor according the control input signals and signal from sensors [2].
There are various types of stabilizers available in market. Both single phase and three phases are available. The rating of this type of stabilizer is quite high and is more economical for high power rating .Based on the change in main voltage, the automatic voltage stabilizers increase or decreases the power supply to rectify the deviation and brings the power supply to normal level. Automatic voltage stabilizer provides a continuous monitoring of output voltage by means of an electronic control circuit that compares the instantaneous output voltage with the set value. Servo voltage stabilizer uses an advance electronic controlled servo controlled motor concept to govern a motorized variable autotransformer. Because of the motor involved, there is a small delay in voltage correction. However, output voltage accuracy is usually ±1% with input voltage change of ±50%. This stabilization method does not create interference or harmonics in the supply system.  The regulating unit consists of toroidal type variable autotransformer. The purpose of the regulating unit is that of acting under the signal from the measuring unit in such a manner as to correct the output voltage of the stabilizer, as near as possible, a constant or predetermined value. Measuring unit includes control circuit. The function of the measuring unit is that of detection a change in the input or output voltage of automatic voltage stabilizer and producing a signal to operate the regulating unit. Overall circuit diagram of control circuit for AVS is shown in fig: 2. Transformer steps down the AC source voltage to 12V. Then, the transformer output is rectified by bridge rectifier. The rectifier output voltage is filtered by capacitor. In this circuit, LM324 is used as comparator and indicator. The noninverting input (-Vin) the op-amp is greater than the inverting input (-Vin) the op-amp is ON-state. At normal condition, positive voltage sensing only. If VR1 is adjusted so that when output AC voltage is 220V, the op-amp output is zero. Op-amp (3) is used for over-voltage condition. It is connected as voltage comparator. When transistor Q2 base receives forward bias and Q2 goes 'ON' motor runs in the direction to raise voltage. When transistor Q1 base is forward biased and Q1 goes 'ON' motor runs in the other direction to lower voltage. After the present time, opamp output swings positive and release the relay. The control system is automatically shut down when the voltage fluctuation is lower than 120V and higher than 250V.

Design Consideration
Power supply system is an essential part of each electronic system from simplest to the most complex. Input voltage supply is 220V AC supply. Automatic voltage stabilizer control system is based on mainly control circuit. In figure 3, LM324 is a 14 pin IC consisting of four independent operational amplifiers (op-amp) compensated in a single package. Op-amp is high gain electronic voltage amplifier with differential input and usually, a single-ended output. The output voltage is many times higher than the voltage difference between input terminals of an op-amp.

RESULTS AND DISCUSSION
In this section the results obtained from the design calculation of stabilizer controller. There are three conditions by the unstable input supply.

Normal Condition
The parameter of Op-amp 2,

Under-Voltage Condition
The parameter of Op-amp 4, Reference voltage at non-inverting input is 6.03V Assume, fixed voltage V=6.03V and I=1mA Since 6.02 < 6.03, op-amp output goes positive and drives the relay.
A variable resistor VR2 is used so that the user can adjust the voltages settings at the desired position. In this case, over-voltage setting  240V and undervoltage setting 180V. If the supply voltage is lower than 120V and more than 250V, the supply of the motor will be cut out. After that the relay will cut out the supply. So, automatic voltage stabilizer will not produce the power supply without being 220 stable voltage.   10 shows that under voltage condition, the input voltage of the stabilizer is lower than output voltage, the sensing circuit is unbalanced that and the op-amp output is positive. The motor rotates clockwise direction as to increase the stabilizer output voltage to 220V.By testing digital meter (yellow for input and red for output).  11 shows that over voltage condition, the input voltage of the stabilizer is higher than output voltage, the sensing circuit is unbalanced that and the op-amp output is positive. The motor rotates anti-clockwise direction as to decrease the stabilizer output voltage to 220V. By testing digital meter (yellow for input and red for output) Figure 11.Photo showing 7kVA, Automatic Voltage Stabilizer Control System at over-voltage Condition

CONCLUSION
In this circuit when the input voltage is lower than 220V, the relay starts energized and the motor rotates the clockwise direction. And then the input voltage is higher than 220V, the motor rotates the anticlockwise direction. In this research, the input voltage fluctuation can be with stand 120V and 250V for single-phase 7kVA. The control circuit components are available in local market. So the circuit component can be replaced easily when they are damaged. This automatic voltage stabilizer is very suitable and economical for all electrical equipments. This automatic voltage stabilizer is very convenience and economic for domestics and industries. So, automatic voltage stabilizer having with these conditions will offer the stable output voltage or stable output voltage for all electrical equipments and will improve productivities and reduce downtime.

ACKNOWLEDGMENTS
The author is deeply grateful to her dissertation superior and co-supervisor. The author also thanks to all teachers at Technological University (Thanlyin) and all who provided her with necessary assistance for this paper. The author wishes to express her guidance to all persons who helped directly or indirectly towards the successful completion of paper. Finally, the author wishes to express her special thanks to her parents for their support and encouragement to attain her destination without any trouble.