automatic phase selector

AUTOMATIC PHASE SELECTOR


A project report Submitted in partial fulfilment of the requirement

For the award of degree of

BACHELOR OF TECHNOLOGY

IN

ELECTRICAL ENGINEERING

Department of Electrical Engineering

Delhi Technological University

(Formerly Delhi College of Engineering) New Delhi, India

Submitted By:

Saurabh 2K12/EE/120

Shubham Singh 2K12/EE/126

Supriya Azad 2K12/EE/134

Sushil Singh 2K12/EE/137

Under the esteemed guidance of

Mr. D.C. Meena Assistant Professor

EE Department DTU


CERTIFICATE

This is to certify that the project report entitled


which is submitted by

Saurabh (2K12/EE/120)

Shubham Singh (2K12/EE/126)

Supriya Azad (2K12/EE/134)

Sushil Singh (2K12/EE/137)

For the partial of the requirement for the award of degree Bachelor of

Technology in Electrical Engineering from DTU is a record of the candidates

own work carried out by him under my supervision. To the best of my knowledge,

the matter embodied in thesis has not been submitted to any other

university/institute for the award of any degree or diploma.

..

Date: 6th December 2014 Project Guide

Mr. D.C Meena

(Assistant Professor)



(Formerly Delhi College of Engineering)

New Delhi, India


DECLARATION

We hereby declare that this submission is our own work and that to the best of our knowledge &

belief. It contains no material published previously or written by other person. Also, it does not

contain any material that to a substantial extent has been accepted for the award of any other degree

or diploma of the University or other institute of higher learning except where due

acknowledgement has been made in the text.

Sign:

Name: Saurabh (2K12/EE/120)

Sign:

Name: Shubham Singh (2K12/EE/126)

Sign:

Name: Supriya Azad (2K12/EE/134)

Sign:

Name: Sushil Singh (2K12/EE/137)

APPROVED BY

Mr. D.C Meena

(Assistant Professor)



(Formerly Delhi College of Engineering)

New Delhi, India


ACKNOWLEDGEMENT

This project by far the most significant accomplishment in our life and it would have been

impossible without people who supported us and believe in us.

We would like to extend our gratitude and our sincere thanks to our honorable, esteemed

supervisor Mr. D C Meena, Assistant professor. He is not only a great teacher professor

who guided and encouraged us towards the successful completion of this project. His trust

and support inspired us to extend our horizons of knowledge and we are really glad to work

with him. Our special thanks go to Prof. Madhusudan Singh, Head of Department

Electrical Engineering, and Delhi Technological University for providing us with best

facilities in the department and his timely suggestion.

We would like to thank all our friends and especially our classmates for all the thoughtful

and mind simulating discussion we had, which prompted us to think beyond the obvious.

Last but not the least we would like to thank our parents, who taught us the value of hard

work by setting an example themselves. They rendered us enormous support during the

whole tenure of our studies at the college.

Saurabh

Shubham Singh

Supriya Azad

Sushil Singh


CONTENTS

1. INTRODUCTION..1

2. OBJECTIVE...1

3. REQUIRED COMPONENTS...1

4. BLOCK DIAGRAM...2

5. CIRCUIT DIAGRAM3

6. PICTORIAL VIEW4

7. LITERATURE SURVEY..5

8. CIRCUIT DESIGN.5

9. COMPONENTS DESCRIPTION.6

9.1 OP AMP....6

9.2 Comparator9

9.3 PNP General Purpose Transistor.11

9.4 Transformer.13

9.5 Step down Transformer..14

9.6 Windings.....14

9.7 Diode..15

9.8 Resistors..17

9.9 Ohms Law.....18

9.10 Power Dissipation......18

9.11 Color Code............18

9.12 Capacitors......19

9.13 Relays...21

10. PRINCIPLE OF OPERATION.....................................................................22

11. CIRCUIT WORKING....23

12. CONCLUSION....23


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.

2. 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.

3. REQUIRED COMPONENTS

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 - 1N4007

Zener Diode - 5.1V

Capacitor - 1000mF, 12V & 470mF, 35 V

Resist or - R1 & R2 - 3.3k & R3 10K

Potentiometer - 10k


5. CIRCUIT DIAGRAM


6. PICTORIAL VIEW


7. LITERATURE SURVEY The aim behind the 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 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.

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 selector 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 consist a

comparator, transistor, transformer and relays. We use 741 op-Amp in 'comparator'

mode. This allows it to compare two input voltages.

8. CIRCUIT DESIGN

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 selector the main processes can be divided

into four.

Stepping 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 (IN40O7) 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 (8C557) goes to on and off

positions. Thus we can say that transistor work as a switch.

Transformer - 12 V, 300mA; Transistor - 8C557 (PNP); Diode - 1N4007; Zener

Diode 5.1 V; Capacitor - 1.000microF, 12 V; 47OmicroF, 35 V; Resistor R1 & R2

- 3.3k, R3 - 10k; Potentiometer - 1Ok.

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 consists of 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.

9. COMPONENT DESCRIPTION

9.1 OP AMP

Operational amplifiers are important building blocks for a Wide range of electronic circuits.

They had their origin 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.

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.


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

Input t1 : Inverting Input (2)

Input t2 : Non-Inverting Input (3)

+V Supply : No. 4 Pin

-V Supply : No. 7 Pin

The power supply pins (and) can be labelled in different ways. Despite different labelling,

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 the voltage at the non-inverting terminal is, is the voltage at the inverting terminal

and AOL 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 AOL 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 AOL is

not well controlled by the manufacturing process, and so it is impractical to use an

operational amplifier as a standalone 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. lf 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. lf no negative feedback is used, the op-amp functions as a switch or

comparator. Positive feedback may be used to introduce hysteresis or oscillation.

LM741 Operational Amplifier:

The LM741 series are general purpose operational amplifiers which feature improved

performance over industry standards like the LIVI709. They are direct, plugin 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.

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 by the gain bandwidth

produce (GBP).


9.2 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 opamps 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

opamps 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 non-

inverting input ().

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. lt 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 one

of them can be considered a reference voltage, Vref.

Figure shows an op-amp comparator circuit. A fixed reference voltage Vref is applied to the

inverting (-) input terminal and sinusoidal signal Um is applied to the non-inverting (+)

input terminal. When Vm exceeds Vref the output voltage goes to positive saturation

because the voltage at the input is smaller than at the (+) input. On the other hand, when

Vin is less than Vref the output voltage goes to negative saturation. Thus output voltage

Unit changes from one saturation level to another whenever Vin Vref ,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 Vin is greater or less than Vref.

The comparator is Sometimes referred to as a volt-level detector because for a desired value

of Vref, the voltage level of the input voltage Vin can be detected.

Diodes Dl and DZ 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 Vm is

clamped to either 0.7 V or -O.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 Vin is used to limit the current through


protection diodes D1 and D2 While resistance R is connected between the inverting (-)

input terminal and Vref to reduce the offset

Problem. Inverting comparator circuit:

When the reference voltage Vref 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 Vin must be large enough to pass through Vref 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

Figure shows the circuit for an inverting comparator in which the sinusoidal input signal

Vin is applied to the inverting () input terminal while the reference voltage Vref is applied

to the non-inverting () input terminal. In this circuit Vref is obtained by the use of a

potentiometer forming a potential divider arrangement with dc supply voltage Vcc and VEE. As the wiper connected to () terminal is moved toward Vcc, Vref becomes more

positive; while if it is moved toward VEE, Vref 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.


9.3 PNP General Purpose Transistors:

FEATURES

Low Current (max 100 mA)

Low Voltage (max 65V)

APPLICATIONS

. General Purpose Switching and Amplification.

DESCRIPTION

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

. NPN Complements: BC546 and BC 547


9.4 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. !n an ideal transformer, the induced voltage in the secondary

winding (VS) is in proportion to the primary voltage (VP), and is given by the ratio of the

number of turns in the secondary (NS) to the number of turns in the primary (NP) 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 NS greater than NP, or "stepped down"

by making NS less than NP. ln 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.


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 OB is the magnetic flux throu$h 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.

Induction coils:

The first type of transformer to see wide use was the induction coil, invented by Rev'

Nicholas 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 1g70s, efforts to build better induction coils, mostly by trial and error, slowly

revealed the basic principles of transformers.

9.5 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.

9.6 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.

9.7 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.


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 of low

Through it none 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.l0.

Diode Characteristics

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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

.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 6-0 across the junction 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 be

wildering 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.


9.8 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.

9.9 Ohm's Law:

The behaviour 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 (l) will flow through the resistance. For

example, if V is L2 volts and R is 400 ohms, a current of L2 / 400 = 0.03 amperes will flow

through the resistance R.

9.10 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.

9.11. Colour Code:

Four-band identification is the most commonly used color-coding scheme on Resistors. It

consists of four colour 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 S colour system, with 3 significant digits. For example

green-blue-yellow-red is 56x104 = 560 k+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 55. The third band, yellow, has a value of 104 which adds four 0's to the end,

creating 560,000 2% tolerance accuracy' 560'000 changes to 560 k 2% (as a kilo- is 10

3).


9.12 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.

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 build up 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 e (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 = 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 = /e. Capacitor is the fundamental component in ant circuit.


9.13 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

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' lt 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.


10. 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-energized

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 energizes 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.

11. 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 XL to deliver 12V, 3OO 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

5.1 V, and its output pin 6 also remains high. As a result, transistor T1 does not

conduct relay RL1 remains de-energized 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

2ooV, 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. lf 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.

12. 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.

automatic phase selector

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