project 2: Automatic Phase Changer’

CHAPTER 1

INTRODUCTION

Our project ‘Automatic Phase Changer’ is a simple circuit. It is applicable in three phase circuits. If anyone wants their equipment works at rated voltage, this circuit will help him. The circuit provides correct voltage in the same power supply lines through relays from the other phase where correct voltage is available. Using it you can operate all your equipment even when correct voltage is available on a single phase in the building.

1.1. Objective:

In three-phase applications, if low voltage is available in any one or two phases, and you want your equipment to work on normal voltage, this circuit will solve your problem. However, a proper-rating fuse needs to be used in the input lines (R, Y and B) of each phase. The circuit provides correct voltage in the same power supply lines through relays from the other phase where correct voltage is available. Using it you can operate all your equipment even when correct voltage is available on a single phase in the building.

The circuit is built around a transformer, comparator, transistor and relay. Three identical sets of this circuit, one each for three phases, are used.

1.2. Components required:

The circuit consist the following - transformer, comparator, transistor and relay. Three identical sets of this circuit are used, one each for three phases. Here the IC 741 working as the comparator is used here is surrounded by all other components. Here we use transformer, a step down transformer. Transistor BC557 acting as a switch. Relay is electromagnetic type.

Transformer - 12 V, 300mA

Transistor – BC557 (PNP)

Diode - IN4007

Zener Diode - 5.1 V

Capacitor - 1000microF, 12 V

                 - 470microF, 35 V

Resistor - R1 & R2 – 3.3k

              - R3 – 10k

Potentiometer - 10k

CHAPTER 2

LITERATURE SURVEY

The aim behind the mini project is to improve the professional competency by selecting those areas which otherwise are not covered in the normal course. This is to enhance our knowledge into various fields, and thus to gain work experience, confidence, and logical thinking. Our aim was to select a topic which is simple enough to be done within the specified time. So we are planned to do a simple project using basic electrical and electronic concept that we have studied yet. We interested to apply and modify the basic concept than a new topic to be selected. While selecting a topic for our mini project, the first thing which came to our mind was that it should be a product that has got considerable importance in the modern era.

2.1. Selection:

Our concentration was to develop a system which can reduce the problems or difficulties in our life. Also one more thing was in mind that to develop a system which can be applied for several applications associated with modern science and developments in technology. So the concept of automatic phase changer was selected which can be used in 3-phase applications. In 3 phase applications, if low voltage is available in any one of two phases and want equipment to work in normal voltage this circuit will solve your problem. It is a simple circuit. The circuit consists a comporator,transistor,transformer and relays. We use 741 Op-Amp in ‘comporator’ mode. This allows it to compare two input voltages.

2.2 Design of the circuit:

The circuit is built around a transformer, comparator, transistor and relay. Three identical sets of this circuit, one each for three phases, are used. Here we used a step down transformer. Here the IC 741 working as the comparator is used here is surrounded by all other components. Transistor BC557 acting as a switch. Relay is electromagnetic type. In automatic phase changer the main processes can be divided into four.

• Step down the main supply
• Rectification
• Comparing
• Switching

Main supply R, Y, B is stepped down to desired voltage and current. Each transformer is individually connected to the phases R, Y, B respectively. In this case, only one phase work at a time. The diodes (IN4007) are used to rectify the ac to dc. The capacitors for removing the noises in the dc. The resistors and potentiometers of the circuit is gives the specified voltage input to the comparator. Based on the comparator output, the transistor (BC557) goes to on and off positions. Thus we can say that transistor work as a switch.

Transformer - 12 V, 300mA; Transistor – BC557 (PNP); Diode - IN4007; Zener Diode 5.1 V;  Capacitor - 1000microF, 12 V;   470microF, 35 V;     Resistor - R1 & R2 – 3.3k,  R3 – 10k;  Potentiometer - 10k.

2.3. Assembling the Project:

Main components needed for the project are resistors, capacitors, diodes, transformer, comparator and relays. The components were mounted on the bread board and were wired up. A 12V dc supply was generated. The main circuit consist comparator, transformer, transistor and relay. Three identical sets of this circuit connected on the breadboard. Each one corresponds three phases. Then the output is verified by connecting a load (bulb) at the output and got the desired output.

CHAPTER 3

PERIPHERAL DESCRIPTION

3.1. Op-Amp:

Operational amplifiers are important building blocks for a wide range of electronic circuits. They had their origins in analog computers where they were used in many linear, non-linear and frequency-dependent circuits. Their popularity in circuit design largely stems from the fact the characteristics of the final elements (such as their gain) are set by external components with little dependence on temperature changes and manufacturing variations in the op-amp itself.

An operational amplifier is a DC coupled high gain electronic voltage amplifier with a differential input and, usually a single ended output. An op-amp produces an output voltage that is typically hundreds of thousands times larger than the voltage difference between its input terminals.

Figure 3.1. 741IC

Op-amps are among the most widely used electronic devices today, being used in a vast array of consumer, industrial, and scientific devices. Many standard IC op-amps cost only a few cents in moderate production volume; however some integrated or hybrid operational amplifiers with special performance specifications may cost over $100 US in small quantities. Op-amps may be packaged as components, or used as elements of more complex integrated circuits.

The op-amp is one type of differential amplifier. Other types of differential amplifier include the fully differential amplifier (similar to the op-amp, but with two outputs), the instrumentation amplifier (usually built from three op-amps), the isolation amplifier (similar to the instrumentation amplifier, but with tolerance to common-mode voltages that would destroy an ordinary op-amp), and negative feedback amplifier (usually built from one or more op-amps and a resistive feedback network.

3.1.1. Circuit notation:

 

Figure.3.2. Symbol

The circuit symbol for an op-amp is shown, where:

·         : non-inverting input

·         : inverting input

·         : output

·         : positive power supply

·         : negative power supply

The power supply pins ( and) can be labeled in different ways. Despite different labeling, the function remains the same — to provide additional power for amplification of the signal. Often these pins are left out of the diagram for clarity, and the power configuration is described or assumed from the circuit.

The amplifier's differential inputs consist of a  input and a  input, and ideally the op-amp amplifies only the difference in voltage between the two, which is called the differential input voltage. The output voltage of the op-amp is given by the equation,

where  is the voltage at the non-inverting terminal,  is the voltage at the inverting terminal and A_OL is the open-loop gain of the amplifier. (The term "open-loop" refers to the absence of a feedback loop from the output to the input).

The magnitude of  A_OL is typically very large-10,000 or more for integrated circuit op-amps and therefore even a quite small difference between   and   drives the amplifier output nearly to the supply voltage. This is called saturation of the amplifier. The magnitude of A_OL is not well controlled by the manufacturing process, and so it is impractical to use an operational amplifier as a stand alone differential amplifier. If predictable operation is desired, negative feedback is used, by applying a portion of the output voltage to the inverting input. The closed loop feedback greatly reduces the gain of the amplifier. If negative feedback is used, the circuit's overall gain and other parameters become determined more by the feedback network than by the op-amp itself. If the feedback network is made of components with relatively constant, stable values, the unpredictability and inconstancy of the op-amp's parameters do not seriously affect the circuit's performance. If no negative feedback is used, the op-amp functions as a switch or comparator. Positive feedback may be used to introduce hysterisis or oscillation.

3.1.2. LM741 Operational Amplifier:

The LM741 series are general purpose operational amplifiers which feature improved performance over industry standards like the LM709. They are direct, plug-in replacements for the 709C, LM201, MC1439 and 748 in most applications. The amplifiers offer many features which make their application nearly foolproof: overload protection on the input and output, no latch-up when the common mode range is exceeded, as well as freedom from oscillations.

Figure.3.3. Schematic Diagram

3.1.3. Characteristics of Op-Amp (741):

The characteristics of an operational amplifier namely,

1.            Input Offset Voltage.                        

2.            Input Bias Current.

3.            Intrinsic input impedance.

4.            The Slew Rate.

5.            Common Mode Rejection Ratio (CMRR).

6.            The closed loop response by calculating the gain-bandwidth product (GBP).

Figure.3.4. Pinout Diagram

3.2. Comparator:

A comparator circuit compares two voltage signals and determines which one is greater. The result of this comparison is indicated by the output voltage: if the op-amp's output is saturated in the positive direction, the non inverting input (+) is a greater, or more positive, voltage than the inverting input (-), all voltages measured with respect to ground. If the op-amp's voltage is near the negative supply voltage (in this case, 0 volts, or ground potential), it means the inverting input (-) has a greater voltage applied to it than the no inverting input (+).

3.2.1. Comparator using Op-Amp:

Often two voltage signals are to be compared and to be distinguished which is stronger. For such situations, a comparator may be the perfect solution. It also forms the basic building block required for non-sinusoidal waveform generators or relaxation oscillators, so it deserves priority in discussion over relaxation oscillators.

We have studied that when the op-amp is used in open-loop configuration (or without feedback) any input signal (differential or single) which even slightly exceeds zero drives the output into saturation because of very high open-loop voltage gain (nearly infinity) of op-amp. It means that the application of a small differential input signal of appropriate polarity causes the output to switch to its either saturation. Thus op-amp comparator is a circuit with two inputs and one output. The two inputs can be compared with each other i.e. one of them can be considered a reference voltage, V_ref.

Figure shows an op-amp comparator circuit. A fixed reference voltage V_ref is applied to the inverting (-) input terminal and sinusoidal signal u_in is applied to the non-inverting (+) input terminal. When v_in exceeds V_ref the output voltage goes to positive saturation because the voltage at the (-) input is smaller than at the (+) input. On the other hand, when v_in is less than V_ref the output voltage goes to negative saturation. Thus output voltage u_out changes from one saturation level to another whenever v_in = V_ref ,.as illustrated in figure. In short, the comparator is a type of an analog-to-digital converter (ADC). At any given time the output voltage waveform shows whether v_in is greater or less than V_ref. The comparator is sometimes referred to as a volt-level detector because for a desired value of V_ref, the voltage level of the input voltage v_in can be detected.

Diodes D₁ and D₂ are provided in the circuit to protect the op-amp against damage due to excessive input voltage. Because of these diodes, the differential input voltage v_d is clamped to either + 0.7 V or -0.7 V, hence the diodes are called clamp diodes. There are some op-amps with built-in input protection. Such op-amps need not to be provided with protection diodes. The resistance R₁ in series with v_in is used to limit the current through protection diodes D₁ and D₂ while resistance R is connected between the inverting (-) input terminal and V_ref to reduce the offset problem.

3.2.2. Inverting comparator circuit:

When the reference voltage V_ref is negative with respect to ground, with a sinusoidal signal applied to the non-inverting input terminal, the output voltage will be as illustrated in figure. Obviously, the amplitude of v_in must be large enough to pass through V_ref for switching action to take place. Since the sinusoidal input signal is applied to the non-inverting terminal, this circuit is called the non-inverting op-amp comparator.

Similarly an inverting op-amp comparator can be had by applying the sinusoidal input to the inverting (-) input terminal to the op-amp.

Inverting comparator waveform

Non-inverting comparator waveform

Figure shows the circuit for an inverting comparator in which the sinusoidal input signal v_in is applied to the inverting (-) input terminal while the reference voltage V_ref is applied to the non-inverting (+) input terminal. In this circuit V_ref is obtained by the use of a potentiometer forming a potential divider arrangement with dc supply voltage + V_cc and – V_EE. As the wiper connected to (+) terminal is moved toward + V_cc, V_ref becomes more positive, while if it is moved toward – V_EE, V_ref becomes more negative.

The input and output waveforms are shown in figures. Comparators are used in circuits such as discriminators, voltage level detectors, oscillators, digital interfacing, Schmitt trigger etc.

3.3. PNP general purpose transistors:

FEATURES

· Low current (max. 100 mA)

· Low voltage (max. 65 V).

APPLICATIONS

· General purpose switching and amplification.

DESCRIPTION

PNP transistor in a TO-92;  SOT54 plastic package.

NPN complements: BC546 and BC547

Figure.3.5. Transistor

3.3.1. Limiting Values:

Table.3.1

3.4. Transformer:

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core, and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction.

If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (V_S) is in proportion to the primary voltage (V_P), and is given by the ratio of the number of turns in the secondary (N_S) to the number of turns in the primary (N_P) as follows:

By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be "stepped up" by making N_S greater than N_P, or "stepped down" by making N_S less than N_P. In the vast majority of transformers, the windings are coils wound around a ferromagnetic core, air-core transformers being a notable exception.

Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of power grids. All operate with the same basic principles, although the range of designs is wide. While new technologies have eliminated the need for transformers in some electronic circuits, transformers are still found in nearly all electronic devices designed for household ("mains") voltage. Transformers are essential for high voltage power transmission, which makes long distance transmission economically practical.

Figure.3.6. Transformer

The phenomenon of electromagnetic induction was discovered independently by Michael Faraday and Joseph Henry in 1831. However, Faraday was the first to publish the results of his experiments and thus receive credit for the discovery. The relationship between electromotive force (EMF) or "voltage" and magnetic flux was formalized in an equation now referred to as "Faraday's law of induction":

.

Where  the magnitude of the EMF in volts and ΦB is the magnetic flux through the circuit (in Webbers).

Faraday performed the first experiments on induction between coils of wire, including winding a pair of coils around an iron ring, thus creating the first toroidal closed-core transformer.

3.4.1. Induction coils:

The first type of transformer to see wide use was the induction coil, invented by Rev. Nicholas HYPERLINK "http://en.wikipedia.org/wiki/Nicholas_Callan"Callan of Maynooth College, Ireland in 1836. He was one of the first researchers to realize that the more turns the secondary winding has in relation to the primary winding, the larger is the increase in EMF. Induction coils evolved from scientists' and inventors' efforts to get higher voltages from batteries. Since batteries produce direct current (DC) rather than alternating current (AC), induction coils relied upon vibrating electrical contacts that regularly interrupted the current in the primary to create the flux changes necessary for induction. Between the 1830s and the 1870s, efforts to build better induction coils, mostly by trial and error, slowly revealed the basic principles of transformers.

3.4.2. Step down Transformers:

This is a very useful device, indeed. With it, we can easily multiply or divide voltage and current in AC circuits. Indeed, the transformer has made long-distance transmission of electric power a practical reality, as AC voltage can be “stepped up” and current “stepped down” for reduced wire resistance power losses along power lines connecting generating stations with loads. At either end (both the generator and at the loads), voltage levels are reduced by transformers for safer operation and less expensive equipment. A transformer that increases voltage from primary to secondary (more secondary winding turns than primary winding turns) is called a step-up transformer. Conversely, a transformer designed to do just the opposite is called a step-down transformer.

Figure.3.7. Windings

This is a step-down transformer, as evidenced by the high turn count of the primary winding and the low turn count of the secondary. As a step-down unit, this transformer converts high-voltage, low-current power into low-voltage, high-current power. The larger-gauge wire used in the secondary winding is necessary due to the increase in current. The primary winding, which doesn't have to conduct as much current, may be made of smaller-gauge wire.

The fact that voltage and current get “stepped” in opposite directions (one up, the other down) makes perfect sense when you recall that power is equal to voltage times current, and realize that transformers cannot produce power, only convert it. Any device that could output more power than it took in would violate the Law of Energy Conservation in physics, namely that energy cannot be created or destroyed, only converted. As with the first transformer example we looked at, power transfer efficiency is very good from the primary to the secondary sides of the device.

Figure.3.8. Step-down transformer: (many turns:few turns).

3.5. Diode:

A signal diode is one of many types of diodes, which are small components of electrical circuits, manufactured from semiconductors that force electricity to flow in only one direction. Signal diodes which are also sometimes known by their older name of the Point Contact or Glass Diode are physically very small in size compared to their larger Power Diode cousins and control small currents up to about 100mA. Generally, the PN-junction of a signal diode is encapsulated in glass to protect it and they generally have a red or black band at one end of their body to help identify which end is its Cathode terminal.

Figure.3.9. Diode

Signal diodes are designed to pass very small currents, and have several applications in the signal processing field. The arrow in the symbol of diode points in the direction of conventional current flow through the diode meaning that the diode will only conduct if a positive supply is connected to the Anode (A) terminal and a negative supply is connected to the Cathode (K) terminal thus only allowing current to flow through it in one direction only, acting more like a one way electrical valve, (Forward Biased Condition).  However, we know that if we connect the external energy source in the other direction the diode will block any current flowing through it and instead will act like an open switch, in reverse biased mode as shown in Figure.3.10.

Figure.3.10. Diode in Forward and Reverse Biased Condition

The characteristics of a signal point contact diode are different for both germanium and silicon types and are given as: Germanium Signal Diodes - These have a low reverse resistance value giving a lower forward volt drop across the junction, typically only about 0.2-0.3v, but have a higher forward resistance value because of their small junction area. Silicon Signal Diodes - These have a very high value of reverse resistance and give a forward volt drop of about 0.6-0.7v across the junction. They have fairly low values of forward resistance giving them high peak values of forward current and reverse voltage. Signal Diodes are manufactured in a wide range of voltage and current ratings. There are bewildering arrays of static characteristics associated with the humble signal diode but the important ones are as follows; maximum forward current, peak inverse voltage and maximum operating temperature. The diode characteristics are shown in Figure.3.11.

 

Figure.8. Diode Characteristics

Figure.3.11. Diode Characteristics

3.5.1. Zener Diode:

Figure.3.12. Symbol

3.5.2. Zener characteristics:

Figure.3.13

3.6. Resistors:

A resistor is a two-terminal electronic component that produces a voltage across its terminals that is proportional to the electric current passing through it in accordance with Ohm's law, V = IR.

Resistors are elements of electrical networks and electronic circuits and are ubiquitous in most electronic equipment. Practical resistors can be made of various compounds and films, as well as resistance wire. The primary characteristics of a resistor are the resistance, the tolerance, maximum working voltage and the power rating. Other characteristics include temperature coefficient, noise, and inductance. Less well-known is critical resistance, the value below which power dissipation limits the maximum permitted current flow, and above which the limit is applied voltage. Critical resistance is determined by the design, materials and dimensions of the resistor.

 

3.6.1. Ohm's Law:

The behavior of an ideal resistor is dictated by the relationship specified in Ohm's law: V=IR. Ohm's law states that the voltage (V) across a resistor is proportional to the current (I) through it where the constant of proportionality is the resistance (R). Equivalently, Ohm's law can be stated: V/R = I. This formulation of Ohm's law states that, when a voltage (V) is maintained across a resistance (R), a current (I) will flow through the resistance. For example, if V is 12 volts and R is 400 ohms, a current of 12 / 400 = 0.03 amperes will flow through the resistance R.

 

3.6.2. Power Dissipation:

The power dissipated by a resistor (or the equivalent resistance of a resistor network) is calculated using the following:  All three equations are equivalent. The first is derived from Joule's first law. Ohm’s Law derives the other two from that. The total amount of heat energy released is the integral of the power over time.

If the average power dissipated is more than the resistor can safely dissipate, the resistor may depart from its nominal resistance and may become damaged by overheating. Excessive power dissipation may raise the temperature of the resistor to a point where it burns out, which could cause a fire in adjacent components and materials. There are flameproof resistors that fail (open circuit) before they overheat dangerously. Note that the nominal power rating of a resistor is not the same as the power that it can safely dissipate in practical use.

3.6.3. Color Code:

Four-band identification is the most commonly used color-coding scheme on resistors. It consists of four colored bands that are painted around the body of the resistor. The first two bands encode the first two significant digits of the resistance value, the third is a power-of-ten multiplier or number-of-zeroes, and the fourth is the tolerance accuracy, or acceptable error, of the value. The first three bands are equally spaced along the resistor; the spacing to the fourth band is wider. Sometimes a fifth band identifies the thermal coefficient, but this must be distinguished from the true 5-color system, with 3 significant digits. For example, green-blue-yellow-red is 56×104 Ω = 560 kΩ ± 2%. An easier description can be as followed: the first band, green, has a value of 5 and the second band, blue, has a value of 6, and is counted as 56. The third band, yellow, has a value of 104, which adds four 0's to the end, creating 560,000 Ω at ±2% tolerance accuracy. 560,000 Ω changes to 560 kΩ ±2% (as a kilo- is 103). The color code of resistor is shown in Table.2.

                                                                         

Table.3.2. Colour Code of Resistors

3.7. Capacitors:

A capacitor (formerly known as condenser) is a passive electronic component consisting of a pair of conductors separated by a dielectric (insulator). When a potential difference (voltage) exists across the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force between the conductors. The effect is greatest when there is a narrow separation between large areas of conductor, hence capacitor conductors are often called plates. An ideal capacitor is characterized by a single constant value, capacitance, which is measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them. In practice, the dielectric between the plates passes a small amount of leakage current. The conductors and leads introduce an equivalent series resistance and the dielectric has an electric field strength limit resulting in a breakdown voltage.

Figure.3.14. Capacitor

A capacitor consists of two conductors separated by a non-conductive region. The non-conductive substance is called the dielectric medium, although this may also mean a vacuum or a semiconductor depletion region chemically identical to the conductors. A capacitor is assumed to be self-contained and isolated, with no net electric charge and no influence from an external electric field. The conductors thus contain equal and opposite charges on their facing surfaces and the dielectric contains an electric field.  The capacitor is a reasonably general model for electric fields within electric circuits. An ideal capacitor is wholly characterized by a constant capacitance C, defined as the ratio of charge ±Q on each conductor to the voltage V between them: Sometimes charge buildup affects the mechanics of the capacitor, causing the capacitance to vary. In this case, capacitance is defined in terms of incremental changes. The simplest capacitor consists of two parallel conductive plates separated by a dielectric with permittivity ε (such as air). The model may also be used to make qualitative predictions for other device geometries. The plates are considered to extend uniformly over an area A and a charge density ±ρ = ±Q/A exists on their surface. Assuming that the width of the plates is much greater than their separation d, the electric field near the centre of the device will be uniform with the magnitude E = ρ/ε. Capacitor is the fundamental component in ant circuit.

3.8. Relays:

Here we use electromagnetic attraction type relays. Electromagnetic attraction relays operate by virtue of an armature being attracted to the poles of an electromagnet or a plunger being drawn into a solenoid. Such relays may be actuated by dc or ac quantities. Fig 3.15 shows the schematic arrangement of an attracted armature type relay. It consists of a laminated electromagnet M carrying a coil C and a pivoted laminated armature. The armature is balanced by a counter weight and carries a pair of spring at its free end. Under normal operating conditions, the current through the relay coil C is such that counter weight holds the armature in the position shown. However, when a short-circuit occurs, the current through relay coil increases sufficiently and the relay armature is attracted upwards. The contacts on the relay armature bridge a pair of stationary contacts attached to the relay frame. This completes the trip which results in the opening of the circuit breaker and disconnection of the faulty circuit. The minimum current at which the relay armature is attracted to close the trip circuit is called pick up current. It is a usual practice to provide a number of tapping’s, on the relay coil so that the number of turns in use and the setting value can be varied.

Figure.3.15. Relay

Figure.3.16. Internals

                                                CHAPTER 4

PRINCIPLE OF OPERATION

Automatic phase changer works depends on the output of comparator. It compares two voltage signals and determines which one is greater. The result of this comparison is indicated by the output voltage: if the op-amp's output is saturated in the positive direction, the non inverting input (+) is a greater, or more positive, voltage than the inverting input (-), all voltages measured with respect to ground. If the op-amp's voltage is near the negative supply voltage (in this case, 0 volts, or ground potential), it means the inverting input (-) has a greater voltage applied to it than the no inverting input (+).

The mains power supply phase R is stepped down by transformer, which is rectified by diode and filtered by capacitor to produce the operating voltage for the operational amplifier. The voltage at inverting pin 2 of operational amplifier is taken from the voltage divider circuit of resistor and preset resistor. Preset resistor is used to set the reference voltage according to the requirement. The reference voltage at non-inverting pin 3 is fixed to a particular voltage through zener diode. Till the supply voltage available in phase R is in a normal range, the voltage at inverting pin 2 of IC remains high, i.e., more than reference voltage and its output pin 6 also remains high. As a result, transistor does not conduct, relay remains de-energised and phase ‘R’ supplies power to load via normally closed contact of relay. As soon as phase-R voltage goes below normal range of supply voltage, the voltage at inverting pin 2 of IC goes below reference voltage of and its output goes low.

As a result, transistor conducts and relay energises and load is disconnected from phase ‘R’ and connected to phase ‘Y’ through the second relay. Similarly, the auto phase-change of the remaining two phases, viz, phase ‘Y’ and phase ‘B,’ can be explained. Switch is mains power ‘on’/’off’ switch.

Use relay contacts of proper rating and fuses should be able to take-on the load when transferred from other phases. While wiring, assembly and installation of the circuit, make sure that you:

1. Use good-quality, multi-strand insulated copper wire suitable for your current requirement.

2. Use good-quality relays with proper contact and current rating.

3. Mount the transformer(s) and relays on a suitable cabinet. Use a Tag Block (TB) for incoming/outgoing connections from mains.

CHAPTER 5

CIRCUIT WORKING

The circuit is built around a transformer, comparator, transistor and relay. Three identical sets of this circuit, one each for three phases, are used. Let us now consider the working of the circuit connecting red cable (call it ‘R’ phase). The mains power supply phase R is stepped down by transformer X1 to deliver 12V, 300 mA, which is rectified by diode D1 and filtered by capacitor C1 to produce the operating voltage for the operational amplifier (IC1). The voltage at inverting pin 2 of operational amplifier IC1 is taken from the voltage divider circuit of resistor R1 and preset resistor VR1. VR1 is used to set the reference voltage according to the requirement. The reference voltage at non-inverting pin 3 is fixed to 5.1V through Zener diode ZD1. Till the supply voltage available in phase R is in the range of 200V-230V, the voltage at inverting pin 2 of IC1 remains high, i.e., more than reference voltage of 5.1V, and its output pin 6 also remains high. As a result, transistor T1 does not conduct, relay RL1 remains de-energised and phase ‘R’ supplies power to load L1 via normally closed (N/C) contact of relay RL1. As soon as phase-R voltage goes below 200V, the voltage at inverting pin 2 of IC1 goes below reference voltage of 5.1V, and its output goes low. As a result, transistor T1 conducts and relay RL1 energizes and load L1 is disconnected from phase ‘R’ and connected to phase ‘Y’ through relay RL2. Similarly, the auto phase-change of the remaining two phases, viz, phase ‘Y’ and phase ‘B,’ can be explained. Switch S1 is mains power ‘on’/’off’ switch.

During testing in the lab, we used a 12V, 200-ohm, single phase change over relay with 6A current rating. Similarly, ampere-rated fuses were used. If the input voltage is low in two phases, loads L1 and L2 may also be connected to the third phase. In that situation, a high-rating fuse will be required at the input of the third phase which is taking the total load.

Figure.5.1. Circuit diagram

Figure.5.2.PCB layout

CHAPTER 6

CONCLUSION

          By using this circuit we can solve the problem of low voltage in three phase systems. We can use this circuit in lower cut and upper cut voltage ranges by adjusting the potentiometer. By adjusting the relay connection and using PNP or NPN transistor, can vary upper and lower cut. Relay operation depends on comparator. If we get a positive signal at the output of comparator and there use a PNP transistor, then it works on lower cut.

REFERENCES

1. Electron Devices and Circuits by P. Ramesh Babu, T.R. Ganesh Babu,

   Scitech Publications(INDIA) Pvt. Ltd. Chennai. Pages: 9.19.

2. http://www.circuitstoday.com/wp-content/uploads/2009/09/741ic-inverting- comparator-circuit.jpg

3. http://www.circuitstoday.com/wp-content/uploads/2009/09/741ic-inverting- comparator-waveform.jpg

4. http://www.circuitstoday.com/op-amp-comparator#ixzz0mPevClUJ

5. http://www.creativecommons.org/licenses/by/3.0

6. http://www.Angelfire.com/electronic/beausalector/intermediatecircuits.html

7. http://www.datasheetarchive.com

8. http://www.wp-content/upload/2008/10/armature-relay.jpg

9. http://www.en.wikipedia.org/wiki/tansformer#mw-head