23-08-2014, 04:59 PM
SERVO CONTROLLED VOLTAGE STABILIZER SEMINAR REPORT
SERVO CONTROLLED VOLTAGE.docx (Size: 409.22 KB / Downloads: 23)
ABSTRACT
The project titled “SERVO CONTROLLED VOLTAGE STABILIZER” aims in maintaining a constant output voltage. The conventional voltage stabilizers are used for feeding controlled voltages to appliances, where the voltage predominantly varies during peak loads. These voltage stabilizers normally use one or two relays and voltages stabilized in two steps. The other type of stabilizers available in the market is manually operated. In such stabilizers the output is changed manually with switch to maintain the output voltage constant. During the peak period this manual operation of stabilizer has to be done frequently. This problem is solved by servo controlled voltage stabilizer efficiently. With this circuit it is possible to keep output voltage constant even if the mains voltage goes to a maximum or minimum value.
The project also incorporates both manual and automatic methods of voltage stabilization. A protection circuit is also provided with this circuit to isolate the circuit from mains supply from reaching the appliances, the voltage exceeds or falls below a certain predetermined value
INTRODUCTION
While the provision of Indian electricity Act required that power supply voltages should not drop or rise more than four percent, we find voltage fluctuations taking the 230 volts mains supply ,apply voltages to as low as 170 volts or as high as 270 occasionally. With enormous increase in loads connected to a distribution transformer, the electricity suppliers now find it exceedingly difficult to maintain the voltage with in the updated values. And that has necessity and use of automatic voltage stabilizer for almost every instrument. Even domestic appliances like refrigerator and T.V. sets need a stabilizer before connecting power to them, not to speak of computers and other expensive equipment.
With the wide spread use of switched mode power supplies, color T.V. sets today have eliminated the need for a voltage stabilizer. But it is preferable to employ one even for them, to safeguard against momentary voltages over 250v and below 150v on the mains.
Fig. 1 is a block diagram of a voltage stabilizer connected to an appliance or load. The stabilizers size increase generally with its rating, which is given in KVA
IMPORTANCE OF VOLTAGE CONTROL
When the load on the supply system changes, the voltage at the consumer terminal also changes. The variations of voltage at the consumer terminal are undesirable and must kept within prescribed limits for the following reasons.
1) Incase of lighting load, the lamp characteristics are very sensitive to changes of voltage. For instance if the supply voltage to an incandescent lamp decreases by 6 percent of rated value, the illuminating power may decrease by 20 percent.
2)Incase of power load consisting of induction motors, the voltage variation may cause erratic operation. If the supply voltage is above the normal, the motor may operate with saturated magnetic circuit with consequent large magnetizing current,heating and low power factor.On the other hand, if the voltage is too low it will reduce the starting torque of the motor considerably.
3) Too wide variations of voltage cause excessive heating of distribution transformer. This may reduce their rating to a considerable extent.
It is clear from the above discussion that voltage variation in a power system must be kept to minimum level in order to deliver good services to the consumer. With the trend towards larger and larger interconnected system, it has become necessary to employ appropriate methods of voltage control
OVER VOLTAGES
There are several instances when the elements of a power system are subjected to over voltages i.e., voltage greater than normal voltages. These over voltages on the power system may be caused due to many reasons such as lightning, the opening of circuit breaker, the grounding of a conductor. Most of the over voltages are not of large magnitude but many still be important because of their effect on performance of circuit interrupting equipment and protecting devices. An appreciable number of these over voltages are of sufficient magnitude to cause insulation break down of equipment in power system therefore, power system engineers always desire ways and means to limit the magnitude of the over voltages produced and to control their effects on the operating equipment
INTERNAL CAUSES OF OVER VOLTAGES
Internal causes of over voltages on the power system are primarily due to oscillations set up by the sudden changes in the circuit condition. These sudden changes may be a normal switching operation such as opening of circuit breaker or it may be fault condition such as a grounding of aline conductor. In practice, the normal system insulation is suitably designed to withstand such surges
INSULATION FAILURE
The most case of insulation failure in power system is the grounding of conductor, which may cause over voltages in the system. Suppose a line at a potential of ‘E earthed at a point x. The earthing of the line causes two equal voltages of -e to travel along ‘xq’ and ‘xp’ containing current –E/Zn and +E/Zn respectively. Both these currents pass through x to earth so that current to earth is 2E/Zn.
THEORY AND OPERATION OF SERVO CONTROLLED VOLTAGE STABILIZER
These stabilizers employ a toroidal auto transformer and a servomotor driven by a circuit, which senses the voltage.
The toroidal auto transformer has a toroidal core. It has a contact arm housing a carbon brush, which makes a sliding contact with the coil wound over the toroid, just as in a potentiometer. The toroidal core is circular with a diameter of about 35cm.
Enameled copper wire, which is wound around the toroid uniformly, is exposed (uncovered by enamel) at the top side where the contact is made by the carbon brush of the moving assembly.
The output voltage is varied automatically on varying the position of this contact. For this purpose, a servo motor fitted with gears is coupled to the contact arm.
The sensing circuit senses the voltage difference between the output and the nominal voltage. It drives the servomotor, after suitable power amplification, in clockwise or anticlockwise direction. As the servomotor moves the contact on the winding of the autotransformer, it reduces the voltage difference which becomes zero when output voltage reaches the nominal value. As there is no error signal now, the servomotor stops
STABILIZER WORKING PRINCIPLE
The output voltage of which has to be maintained constant is sensed by the sensing transformer i.e., sensing transformer provides a signal proportional to the output voltage. The voltage being AC is rectified by the rectifier and filter circuit and applied to the precision grade adjustable voltage regulator (LM723 IC). With this it is possible to get the voltage regulation of about + or – one volt. The output of lm723 being the reference voltage which is about 12v. The output stage of LM723 consists of two 741ICs out of which one act as inverting comparator and the other acts as Non-inverting comparator. The output of LM723 is applied to Inverting terminal of one Op-amp and Non-inverting terminal of other op-amp.
The ICs used here acts as comparators which compares the two voltage levels applied at two input terminals. One input to each comparator being at 12v, other inputs are greater than 12v for one comparator and less than 12v for other comparator. The output stages of the comparator consists of two triacs one to increase the voltage and other to decrease the voltage. These triacs used here acts like switches, which when closed gives the supply to the Ac servomotor, thus running the motor, which in turn runs the knob of the autotransformer through gear mechanism
OVER/UNDER VOLTAGE PROTECTION
The power line fluctuations and cut-offs cause damages to electrical appliance connected to the line. It is more serious in the case of domestic appliances like fridge, air conditioners.
The under/over voltage protection circuit presented here is a low cost and reliable circuit for protecting such equipment from damages. Whenever the power line is switched on it gets connected to the appliance. If there is a high/low fluctuation beyond set limits the appliance gets disconnected. The system tries to connect the power back after the main circuit is working with in the set limits
WORKING PRINCIPLE
The basic principle behind the operation of the circuit is described here. Two DC values are derived from the line voltage, which represents the higher and lower permitted values of the line output. The values are compared a fixed reference using comparators, one as a positive comparator and other as a negative comparator. The comparator outputs are used to control a switching circuit directly for switch off and to switch on.
In the circuit shown here resistive network of R1, R2, VR1 and VR2 along with the diode D5 and the capacitor C2 derives the upper and lower input voltages for the comparators corresponding to the line voltage. Two of the comparators of IC2 LM324, quad comparators are used to compare these values with respect to a single reference voltage. This reference voltage is obtained by the six volts zener diode D6 and the resistor R3. One of the comparator as used as a positive comparator and the as a negative comparator sothat the former gives HIGH output for over voltage and the later gives HIGH output under voltage. By adjusting the presets VR1 and VR2 the comparator inputs can be set to desired values of the upper and lower limits of the line voltage. The outputs of the operators are connected together through diodes D7 and D8 to derive a signal. A low value of signal (Vs.) activates the main circuit
SERVOMOTOR
The motors that are used in automatic control systems are called servomotors. When the objective of the system is to control the position of an object then the system is called servomechanism. The servo motor are used to convert an electrical signal (control voltage) applied to them into an angular displacement of the shaft. They can either operate in continuous duty or step duty depending on construction.
There is variety of servomotors available for control system applications. The stability of a motor for a particular application depends on the characteristics of the system, the purpose of the system and its operating conditions. In general a servomotor should have the following feature.
1) Linear relationship between the speed and electric control signal
2) Steady state stability
3) Wide range of speed control
4) Linearity of mechanical characteristics throughout the entire speed range
5) Low mechanical and electrical inertia
AC SERVOMOTOR
An AC servomotor is basically atwo phase induction motor except for certain special design features.
Two phase servomotors differ in the following two ways from normal induction motor.
1) The rotor of the servomotor is built with high resistance, so that its X/R ratio is small which results in linear speed torque characteristics(but conventional induction motors will have high value of X/R ratio which results in high efficiency and non-linear speed torque characteristics) the speed torque characteristics of normal induction motor (curve-a) and ac servomotor(curve-b) are shown in figure
2) The excitation voltage applied to two stator windings should have aphase difference of 90 degrees.
WORKING PRINCIPLE OF SERVOMOTOR
The stator windings are excited by voltages of equal rms magnitude and 90 degrees phase difference. These results in exciting currents i1 and i2 that are phase displaced by 90 degrees and have equal rms values. These currents give rise to a rmf of a constant magnitude. The direction of rotation depends on the phase relationship of the two currents (or voltages). The exciting currents produces a clock wise rmf and phase shift of 180 degrees in i1 will produce an anti-clockwise rmf.
The RMF sweeps over the rotor conductors. The rotor conductors experience a change in flux and so voltages are induced rotor conductors. These voltage circuit currents in the short circuited rotor conductors and the currents create rotor flux
AUTOTRANSFORMER
A transformer, in which a part of the winding is common to both the primary and secondary circuits, is called autotransformer. In an auto transformer the two windings are not electrically isolated.
In this project the transformer used is toroidal autotransformer. It has a toroidal core. It has a contact arm housing a carbon brush, which makes a sliding contact with the coil wound over the toroid just as a potentiometer. The toroidal core is circular in shape with a diameter of 35cm.
Enameled copper wire, which is wound around the toroid uniformly, is exposed at the top side where the contact is made by the carbon brush of the moving assembly.
The output voltage is varied automatically on varying the position of this contact. For this purpose, a servomotor fitted with gears is coupled to the contact arm
BUCK BOOST TRANSFORMER
Isolation transformers have separate primary and
Secondary windings, electrically insulated and isolated
from one another. With a relatively high voltage primary
(Typically 120, 240 or 480 Volts) and a relatively low voltage
Secondary (typically 12, 16, 24, 32 or 48 Volts), buck-boost
transformers are designed to be field connected as
Auto-transformers. These are transformers with one
Continuous winding, a portion of which is jointly shared
between the input and the output. No electrical isolation is
present in an autotransformer.
Buck-Boost transformers have two major uses:
1. When field connected as an autotransformer, they
Can be used to Buck (lower) or Boost (raise)
available line voltage in the range of 5 to 27% andat a KVA rating many times that listed on the transformer nameplate.
2. When left as an isolation transformer, they can be
Used to supply power to low voltage circuits at the name plate rating listed
CONCLUSION
The project servo controlled voltage stabilizer employs a adjustable precision grade voltage regulator LM723.With this circuit it is possible to get a voltage regulation of +1 or -1 volt .This circuit works for a voltage range of 170 to 250 (180 to 250)volts. If the voltage is beyond these limits a protection circuit called under/over voltage protection is provided to protect the main circuit from damages. In this circuit ac servo motor is used to drive knob of auto transformer, but for higher ratings of servo motor a buck boost transformer can be used and accordingly the output voltage is either bucked or boosted. Here with the implementation of circuit using buck boost transformer, the rating of the stabilizer is increased to 5/1 kva