CHAPTER ONEINTRODUCTION

1.0 INTRODUCTION

The inefficiency on the part of power supply authority to adequately ensure the

availability of power supply to consumers has led to a common practice where the

three phases are combined to a single phase to power their single phase loads. (See

figure 1.1). Most worrisome is method used in selecting these phases which is

manually done. The risk of electrocutions in this practice is very high as every

Tom, Dick and Harry always jumps at the cut-out fuse board as soon as a phase

fails. Though the idea of consumers changing phases on their own is not quite safe

for the power distribution equipment which could result in a situation where one

phase could be overloaded while the other is under loaded. This creates an unbal-

anced situation in the power distribution network. Since our people have wrong-

fully accepted this practice, to ensure safety, however, the idea of automating this

culture now forms the basis of this project. As soon as the power supply authori-

ties rises up to this challenge, the system to be developed in this project could still

be there but will not create any harm on the power distribution network.

Furthermore, an improvement on the manual method of phase change is the low

voltage detector that takes a low voltage below a preset value which could be

dangerous to our equipment as no power supply will be available and

consequently isolate the load.

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Figure 1.1 manual looping of three phases to one phase

1.1 AIM

The aim of this project is to design and construct an automatic phase selector for

safety of human life and equipment.

1.2 OBJECTIVES

To detect the availability of voltage supply in existing phase

To measure the value of voltage in each phases

Compare the different values of the phases

Automatically lock and connects the best phase to the load

1.3 APPLICATIONS

The work finds applications in:

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Power supply stability in cell sites, hospitals – during operation

Fire Prevention in homes

Security Efficiency in banks

Energy Efficiency

System Protection for file servers

Locations where Automatic selection of available phase for single phase

load required

As a safety equipment in industries and also where unmanned operations

are required

1.4 SCOPE OF THE PROJECT

The design of the project will involve the use of discrete components as well as

integrated circuits. It is will be a prototype which will be assembled on a board.

An average load of 25A will be considered in selecting main switching contactor,

however, with appropriate rating of the contactor the project can be used to control

heavier load switching without human involvement.

1.5 UNIQUENESS OF PROJECT

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Being an improvement to the manual method of phase change, the device can be

used to secure maximum safety of equipments and human life.

CHAPTER TWO

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LITERATURE REVIEW

2.0 HISTORICAL BACKGROUND

Older fuse boxes use rewirable fuses with no other protective device, and basic 4

ways boxes are very common. A lot of these boxes are made of brown-black

Bakelite, some times with a wooden base. Although their design is historic, these

were standard equipment for new installs as recently as the 1980s, so are very

common.

Users should be wary of these fuse boxes, as typically pulling a fuse carrier out

with the power on results in fingers grasping live connections, and these boxes are

wide spread even in modern installations.

The popular 4 way box usually usually takes heavy or sustained loads such as

immersion heaters and oven on a socket circuit. This arrangement is not a

recommended practice today, but it is common in existing installations.

Hence these fuse boxes do not have sufficient breaking capacity for safe reliable

operation in many premises.

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Voltage sensorBlue phase

Voltage sensorYellow phase

Scanning circuit

Load interfaceSwitching circuit

Comparators

Single phase Load

Low voltage detector

Low voltage detect - or

Low voltage detector

Voltage sensorRed phase

2.1 BLOCK DIAGRAM OF UNIT

Figure 2.1:The Block Diagram of the Unit

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Red phase

Blue Phase

Yellow

Neutral

2.2 METHODOLOGY

The approach to be adopted in the design of the automatic phase selector is to

develop a scanner and a full voltage presence sensor that is capable of detecting

the availability of supply in a phase automatically locking and connecting that

phase to the load. The block diagram of the unit is as shown in figure 1.2 above.

The scanner is made up of an astable multivibrator and a counter and the low

voltage detector is made up of a step down transformer, a rectifier, filter, a

regulator and a comparator. The switching will be done by electromechanical

relays and contactors. The power supply will be uninterrupted as the system must

run even when there is total power failure.

2.2.1 FUNCTIONAL PARTS OF THE BLOCK DIAGRAM

Voltage Sensor: The three phases usually available on a 220v supply will be

monitored by a voltage sensor on the device. The three phases are each having a

voltage sensor that is capable of determining the availability of supply in a phase.

The sole aim of this sensor is first to detect if there is voltage at all on the phases

and there by sending a signal to the scanner.

Low and Comparator Voltage Detector: The low voltage detector is made up of a

step down transformer, a rectifier, filter, a regulator and a comparator. The

regulated (Reference) and the unregulated voltages are compared. If the voltage is

low the unregulated voltage will be low since there is a linear relationship between

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the input and the output voltages of a transformer. Now the output of the

comparator will be low if the input voltage is low hence the counter gate can be

opened to allow the counter to advance to the next available phase.

Scanning Circuit: The scanner is made up of an astable multivibrator and a

decade counter. The astable multivibrator is used as a clock for the counter though

gated such that the counter receives the clock pulse only when the load is not

powered.

Switching Circuit: The switching circuit turns ON or OFF the interface

electromechanical relays and contactors which closes only on the availability of

correct voltage in that phase and only one is connected at a time so the short

between phases is completely avoided.

2.3 REVIEW OF COMPONENTS USED

Electronic systems are made from units which are in turn made from components

Basic circuit components have been used in this project. They include resistors,

capacitors, diodes, transistors and integrated circuit chips like the 555 timer,

operational amplifier, in his chapter; I will review the theory behind these

components as well as their applications to this project

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2.3.1 RESISTORS

Resistors are electronic devices designed to limit the amount of current flow in a

circuit. In other words they offer opposition to the flow of current in a circuit. This

ability to limit the flow of current in a circuit is called resistance of a resistor and

its unit is ohm. The symbols used to represent resistance is shown in figure 2.2

Fixed

[Variable]

Fig. 2.2 Resistor symbols

2.3.1.1 CLASSIFICATION OF RESISTORS`

Furthermore, the types of resistors that were used can be classified in terms of

their value. (i.e. fixed resistors and variable resistors), with each having different

maximum ohmic values, different tolerances, power or wattage ratings, as well as

stabilities.

2.3.1.2 FIXED RESISTORS

These are resistors whose ohmic values cannot be changed in a circuit except

when replaced. They are mainly of carbon wire wound types of resistors.

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Their resistance values are usually marked on their bodies. However, available

nowadays are resistors with their resistance values given on their bodies in the

form of a colour code consisting of four or five coloured bands

2.3.1.3 VARIABLE RESISTORS

These are the type of resistors used when it is required to vary resistance while the

circuit is in use (e.g. as in volume control in Radio and TV. Sets). They consist of

a circular carbon track on an insulating base upon which a metal contact wiper

moves; these are commonly referred to as rotary variable resistors or simply

potentiometers.

The maximum resistance values of this type of resistors are often indicated on the

casing, either in colour code or by letters. Some variable resistor may have

additional indications such as LOG or ‘LIN’. The LOG stands for logarithmic with

the effect that the log of the resistance of the resistor is proportional to the position

of the movable metal contact wiper. Similarly, LIN stands for Linear and the effect

is that, the resistance of the resistor is directly proportional to the position of the

movable contact wiper on the track.

These rotary variable resistors have power ratings ranging from 0.25W to

5 W and are available in ganged (resistance boxes or decades) and single forms.

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2.3.2 CAPACITORS

Capacitors are some of the widely used components in the electronic aspect of my

project. A capacitor is simply a device, which stores electrical energy or charges.

Essentially, a capacitor is made up of two sets of metal plates separated by an

insulator called dielectric. Capacitors derive their names from the kind of

dielectric employed in making them e.g.

(i) Electrolytic capacitor has electrolyte as dielectric

(ii) Air capacitor has air as dielectric

(iii) Mica capacitor has mica as dielectric

(iv) Paper capacitor has paper as dielectric

Capacitance is the measure of the capacitor ability to store charge.

The unit of capacitance is the Farad with a unit symbol ‘F’ and it is defined as the

capacitance of the capacitor when 1V p.d. gives it a charge of 1C.

There are however two major types of capacitors that were used which are:

2.3.2.1 THE FIXED CAPACITORS

The fixed capacitors have values that cannot be altered or varied. They are usually

classified according to the kind of insulator (dielectric) used, and they come in

various shapes and sizes ranging from the small button shaped or disc ceramic

capacitors (usually used in power stations and industries) and the electrolytic

capacitor. Electrolytic capacitors belong to a class of capacitor made through an

electrochemical process. It is made from rolling up two aluminum strips

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sandwiched with strapped gauze cloth soaked in borax. The borax serves as an

electrolyte for the electrochemical process involved in the manufacture. This

type of capacitors have polarity indicated on their bodies the positive being

connected to the more positive part of a circuit and the negative to the more

negative part of the circuit. The symbol is as shown in figure 2.3

This instruction should be observed otherwise the capacitor will be damaged. They

usually have large capacitance of the order of about 5μF to hundreds of μF

Figure 2.3: The circuit symbol of a capacitor

2.3.2.2 THE VARIABLE CAPACITORS

There variable capacitors used is:

(i) Variable Trimmer Capacitors

This is a much smaller type of variable capacitors. It is made up of two half-moon

shaped metal plates separated with a piece of mica. One of the plates is fixed while

the other can be rotated with a screwdriver in and out of mesh with the fixed one.

This varies the area of the contact.

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Typical values of these small variable capacitors are as from a minimum of about

2pF to maximum of about 30pF. Figure 2.4 shows the circuit symbol of a variable

capacitor

figure 2.4 circuit symbol of a variable capacitor

+ -

Fixed value capacitor Fixed value capacitor Variable capacitor

(Non – polarized) (Polarized)

Fig. 2.4 Capacitor Symbols

2.3.3 TRANSISTORS

Generally transistors are semiconductor devices. The transistors fall into the

category of bipolar transistor, either the more common NPN bipolar transistors or

the less common PNP transistor types. There is a further type known as a FET

transistor which is an inherently high input impedance transistor with behavior

somewhat comparable to valves. Modern field effect transistors or FET's including

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JFETS and MOSFETS now have some very rugged transistor devices. However,

the FET is not used in this project.

The transistor has three legs, the Collector (C), Base (B), and Emitter (E).

Sometimes they are labeled on the flat side of the transistor. Transistors always

have one round side and one flat side. If the round side is facing you, the Collector

leg is on the left, the Base leg is in the middle, and the Emitter leg is on the right

for the small type while the power types has a flat face with metal at the back.

Transistor Symbol

The symbol of figure 2.5 is used in circuit drawings (schematics) to represent a

transistor.

Figure 2.5: The circuit symbol of an NPN transistor

2.3.3.1 BASIC CIRCUIT

The Base (B) is the On/Off switch for the transistor. If a current is flowing to the

Base, there will be a path from the Collector (C) to the Emitter (E) where current

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output

Non-inverting input

Inverting input

+V

-V

can flow (The Switch is On.) If there is no current flowing to the Base, then no

current can flow from the Collector to the Emitter. (The Switch is Off.)

2.3.4 OPERATIONAL AMPLIFIER (OP-AMP)

An operational amplifier is 1C is a solid-state integrated circuit that uses external

feedback to control its functions. It is one of the most versatile device in

electronics. The term op-amp was originally used to describe a chain of high

performance dc amplifier that was used as basis for the analogue type computers

of long ago. The op-amp without any external devices is called ‘open loop’ mode,

referring actually to the so-called ideal operational amplifier with infinite open-

loop gain, input resistance, bandwidth and a zero output resistance. However, in

practice, no op-amp can meet these ideal characteristics. In fact, there is no such

thing as an ideal op-amp. Nowadays, the µA741 is a frequency compensated

device and although still widely used. The Bipolar are low-noise and replacing the

old style op-amps.

Shown in the figure 2.6 is an op-amp symbol as used today. It is common fashion

to omit the power supply connections as they are implied.

Figure 2.6 Symbol of an op-amp

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As indicated in the symbolic diagram, it has two inputs and a single output.

Additional terminals are also made available to which resistors may be connected

to correct any offset dc levels. It is usually operated using symmetrical positive

and negative dc power supplies. The integrated type op-amp such as the µA741

may be represented by its schematic and the equivalent circuit of the figure 2.7 (a)

and (b) below.

2.3.4.1 A 741 OP-AMP

The A741 is a high performance operational amplifier with high open loop gain,

internal compensation, high common mode range and exceptional temperature

stability. The A741 is short-circuit protected and allows for nulling of the off set

voltage. Fairchild semiconductor manufactures it and the data provided are as

shown in table 2.1.

Absolute Maximum Parameters

Maximum means that the op-amp safety tolerate the maximum rating given in the

data section of such am op-amp without the possibility of destroying it.

Table 2.1 : μA741 manufacturer data

Maximum Ratings

Supply Voltage 18 volts

Internal power dissipation 500mw

Differential input voltage 30volts

Input voltage 15 volts

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Voltage offset null/v 0.5 volts

Operating Temperature range 00 to + 700C

Storage temperature range -650C to + 1500C

Lead temperature, solder 60sec 3000C

Output short circuit Indefinite

2.3.4.2 DEFINITION OF TERMS

1. Supply voltage (+/-Vs) the maximum voltage (positive and negative)

that can be safely used to feed the op-amp.

2. Dissipation (pd): the maximum power the op-amp is able to dissipate by

specified ambient temperature (500mw at 800C)

3. Differential input voltage (Vid): This is the maximum voltage that can be

applied across the positive and negative inputs.

4. Input voltage: The maximum input voltage that can be simultaneous sly

applied between both input and ground also referred to as the common

mode voltage. In general, the maximum voltage is equal to the supply.

5. Operating temperature: This is the ambient temperature range for which

the op-amp will operate within the manufacturer’s specifications.

6. Output short-circuits Duration: This is the amount of time that an op-

amp’s output can be short circuited to either supply voltage.

Unlike the ideal op-amp, the op-amp that is used in more realistic circuits today

does not have infinite gain and bandwidth.

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One additional parameter is worth mentioning, the transient response or rise time

is the time it takes for the output signal to go from 10% to 90% of its final value

when a step-function pulse is used as an input signal and is specified under closed-

loop conditions. From electronics circuit theory, the rise time is related to the

bandwidth of the op-amp by the relation:

BW = 0.35/rise time.

The uses of op-amp are numerous. In this project, it has been used as a

comparator.

2.3.5 THE OPERATIONAL COMPARATOR

A comparator is a circuit which compares a time varying input signal Vs (t) with a

reference signal VR with a view to determining whether or not the analogue input

signal is greater than or less than the reference signal. The reference signal

determines the level at which comparison is made when the input signal exceeds

VR, the comparator output takes on a value which is different in magnitude from

when Vs (t) is less than VR

Assume the inverting input is grounded at zero volt. If a voltage signal V s is

applied to the non-inverting input, then the difference voltage V i across the input is

(Vs – O) = Vs. In the absence of feedback, the voltage gain Ao of the Op amp is

infinite. Thus, since the output voltage Vo is AoVs, the slightest departure of Vs

from zero volt produces a very large output voltage. The output voltage is

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however limited to the value of the power supply voltage (Vcc), unless it is

otherwise limited.

In the circuit of figure 2.9 (a), if Vs>O (ie Vs is positive), the output voltage Vo is

latched to the positive power supply +Vcc, and if Vs < O (ie negative), the output

will latch to the negative power supply – Vcc. This is illustrated in the associated

waveforms in figure 2.9 (b).

We should note that if the signal is applied to the inverting input, then the output

voltage will be inverted.

2.3.5.1 OFFSETS IN OP AMP

A re-occurring assumption in the analysis of op amp circuits is that the input

current is zero. Ideally, there should be no current flowing into the input of the

basic op-amp. In practice however, low-level input dc bias currents flow because

the transistors used in the internal amplifier draws bias currents needed for their

correct operation. If the currents at the input terminals are I1 and I2 when the

output voltage Vo is zero, then input bias current is given by

I bias = ½ (I1 + 12 ). It is of the order of 100nA for the µA741 op-amp.

Again I1 should equal 12. When I1 and 12 differ in value, the difference current

(11-12) = Ios is called the input offset current it is typically 10 to 20nA.

The flow of the bias current drops voltages across the internal resistors R1 and R2.

If the dc paths to ground are not identical for both input terminals due perhaps to

mismatch in the op-amp circuit elements, an offset voltage Vos is develop which is

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amplified at the output. The offset voltage is defined as the input voltage needed

to reduce the output voltage to zero. Ideally, the op-amp output voltage should be

zero for zero input voltage, in practice however, the offset voltage is of the order

of 1mv for the µA741 op amp, but is also known to drift as a result of temperature

and power supply variations,. Such fluctuation in dc condition is not

distinguishable from the signal to be amplified. Thus in measurement

applications, serious measurement error can occur. Offset voltages can be

minimized by making the dc paths to ground as nearly equal as possible, using

offset adjust bias circuits.

2.3.6 THE 555 TIMER IC

The 555 timer IC was first introduced around 1971 by the Signetics Corporation as

the SE555/NE555 and was called “The IC Time Machine” and was also the very

first and only commercial timer ic available. It provided circuit designers and

hobbyists with a relatively cheap, stable, and user-friendly integrated circuit for

both monostable and astable applications. The schematic diagram of the internal

structure is as shown below in figure 2.8.

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The 555 schematic is shown in figure 2.7

Definition of Pin Functions:

Refer to the internal 555 schematic of the Figure2.10 above.

Pin 1 (Ground): The ground (or common) pin is the most-negative supply

potential of the device, which is normally connected to circuit common (ground)

when operated from positive supply voltages.

Pin 2 (Trigger): This pin is the input to the lower comparator and is used to set

the latch, which in turn causes the output to go high. This is the beginning of the

timing sequence in monostable operation. Triggering is accomplished by taking

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Positive Vcc 4.5 -16V

Ground

the pin from above to below a voltage level of 1/3 V+ (or, in general, one-half the

voltage appearing at pin 5).

Pin 3 (Output): The output of the 555 comes from a high-current totem-pole

stage made up of transistors Q20 - Q24. Transistors Q21 and Q22 provide drive

for source-type loads, and their Darlington connection provides a high-state output

voltage about 1.7 volts less than the V+ supply level used.

Pin 4 (Reset): This pin is also used to reset the latch and return the output to a

low state. The reset voltage threshold level is 0.7 volt, and a sink current of 0.1mA

from this pin is required to reset the device.

Pin 5 (Control Voltage): This pin allows direct access to the 2/3 V+ voltage-

divider point, the reference level for the upper comparator. It also allows indirect

access to the lower comparator, as there is a 2:1 divider (R8 - R9) from this point to

the lower-comparator reference input, Q13. Use of this terminal is the option of the

user, but it does allow extreme flexibility by permitting modification of the timing

period, resetting of the comparator, etc.

Pin 6 (Threshold): Pin 6 is one input to the upper comparator (the other being

pin 5) and is used to reset the latch, which causes the output to go low. Resetting

via this terminal is accomplished by taking the terminal from below to above a

voltage level of 2/3 V+ (the normal voltage on pin 5). The action of the threshold

pin is level sensitive, allowing slow rate-of-change waveforms. The voltage range

that can safely be applied to the threshold pin is between V+ and ground. A dc

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current, termed the threshold current, must also flow into this terminal from the

external circuit.

Pin 7 (Discharge): This pin is connected to the open collector of an NPN

transistor (Q14), the emitter of which goes to ground, so that when the transistor is

turned "on", pin 7 is effectively shorted to ground. Usually the timing capacitor is

connected between pin 7 and ground and is discharged when the transistor turns

"on".

Pin 8 (V +): The V+ pin (also referred to as Vcc) is the positive supply voltage

terminal of the 555 timer IC. Supply-voltage operating range for the 555 is +4.5

volts (minimum) to +16 volts (maximum), and it is specified for operation

between +5 volts and + 15 volts.

2.3.6.1: MODES OF OPERATION OF 555 TIMER

The 555 timers have two basic operational modes: one shot and astable. In the

one-shot mode, the 555 acts like a monostable multivibrator (figure 2.9). A

monostable is said to have a single stable state--that is the off state. Whenever an

input pulse triggers it, the monostable switches to its temporary state. It remains in

that state for a period of time determined by an RC network. It then returns to its

stable state. In other words, the monostable circuit generates a single pulse of fixed

time duration each time it receives and input trigger pulse. Thus the name one-

shot. One-shot multivibrators are used for tuning some circuit or external

component on or off for a specific length of time. It is also used to generate delays.

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When multiple one-shots are cascaded, a variety of sequential timing pulses can be

generated. Those pulses will allow you to time and sequence a number of related

operations.

The other basic operational mode of the 555 is as and astable multivibrator (figure

2.10). An astable multivibrator is simply an oscillator. The astable multivibrator

generates a continuous stream of rectangular off-on pulses that switch between

two voltage levels. The frequency of the pulses and their duty cycle are dependent

upon the RC network values.

In the application of the 555 timers in the monostable mode, the duration of the

output pulse in seconds is approximately equal to:

T = 1.1 x R1 x C1 (in seconds)………………………………………………2.13

On the other hand, the frequency of operation of the astable circuit is dependent

upon the values of R1, R2, and C1. The frequency can be calculated with the

formula:

f=1/ (.693xC1x (R1+2xR2))………………………………………………..2.14

Figure 2.8: The 555 as a monostable multivibrator.

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Figure 2.9: The 555 as an astable multivibrator

2.3.7 DECADE COUNTER

The CD 4017 is called a counter or divider or decade counter. It is a very handy

chip for producing “Running LED effects” which is scanning.

It has 10 outputs. Output “0” goes HIGH on the rise of the first clock cycle.

On the rise of the second clock cycle, output “0” goes LOW and output “1” goes

HIGH. This process continues across the ten outputs and cycles to output “0” on

the eleventh cycle.

The “Carry Out” pin goes LOW when output “5” goes HIGH and goes HIGH

when output “0” goes HIGH.

When RESET (pin 15) is taken HIGH, the chip will make output “0” go HIGH

and remain HIGH.

When “Clock Inhibit” (pin 13) is taken HIGH, the counter will FREEZE on the

output that is currently HIGH

The CD 4017 is a divide-by-10 CMOS chip.

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Minimum supply voltage 6v

Maximum supply voltage 15v

Max current per output 15mA

Maximum speed of operation 5MHz

figure 2.10: the decade counter

2.3.8 INVERTER (NOT GATE)

In digital devices there are only two values, usually referred to as 0 and 1. 1 means

there is a voltage (usually 5 volts) and 0 means the voltage is 0 volts.

An inverter (also called a NOT gate) is a basic digital device found in all modern

electronics. So for an inverter, as the name suggests, it's output is the opposite of

the input (Output is NOT the Input). If the input is 0 then the output is 1 and if the

input is 1 then the output is 0. We can summarize the operation of this device in a

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truth table 2.2. Figure 2.12 shows the circuit symbol of the NOT gate

table2.2: the NOT truth table

Input Output

1 0

0 1

Figure 2.11: the circuit symbol of the NOT gate

2.3.9 THE OR GATE

The OR gate has an output of 1 when either A or B or both are 1. In other words,

there an output when any or all the inputs have a high logic. Figure 2.13 shows a

circuit analogy. the lamp will light up (logic 1) when either switch A or B or both

are closed. Obviously, the output would be 0 if and only if both its inputs are 0. In

terms of the switching conditions, it means that lamp would be OFF (logic 0) only

when both switches A and B are OFF. The above logic operation of the OR gate

can be summarized with the help of the truth table given in truth Table 2.3.The

electronic symbol for 2 two-input OR gate is shown in figure 2.13.

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Table 2.3.9.1: OR GATE TRUTH TABLE

A B C

0 0 0

0 1 1

1 0 1

1 1 1

Figure 2.12:.The electronic symbol for 2 two-input OR gate

2.3.10 THE AND GATE

The AND gate gives an output of a high logic only when all its inputs are high.

The electronic (or logic) symbol for 2- input AND gate is shown in figure 2.14.

The AND gate has a ‘1’ output when both A and B are 1. Therefore, in the AND

gate its output would be ‘1’ only if all its inputs are all ‘1’s true. Its output would

be ‘0’ if any of its inputs is ‘0’. Table 2.4 shows the truth table for a 2-Input AND

gate.

Table 2.3.10.1 AND GATE TRUTH TABLE

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A B C

0 0 0

0 1 0

1 0 0

1 1 1

Figure 2.13: The electronic (or logic) symbol for 2- input AND gate

2.3.11 CONTACTORS

Contactors are extremely useful when we have a need to control a large amount of

current and/or voltage with a small electrical signal. The contactor coil which

produces the magnetic field may only consume fractions of a watt of power, while

the contacts closed or opened by a magnetic field set up may be able to conduct

hundreds of times that amount of power to a load.

Contactors typically have multiple contacts, and those contacts are usually (but not

always) normally-open, so that power to the load is shut off when the coil is de-

energized. The contactor was used as a means of switching either of the phases to

the load.

CHAPTER THREE

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DESIGN OF THE AUTOMATIC PHASE SELECTOR

3.0 INTRODUCTION

This chapter is dedicated to the application of basic theories of electrical and

electronic Engineering to the development of a circuit diagram that is capable of

physically realizing the objectives of this project.

From the block diagram of figure 2.1, the following units can be identified:

(i). The scanning unit

(ii). Voltage presence sensor for each phase.

(iii). Low voltage detector for each phase.

(iv). Switching and load interface unit

3. 1 THE SCANNING UNIT

The unit is made up of a decade counter (4017) restricted to seven counts. The

eight - output is used as reset to the beginning of the counting process.

Count 1, 3 and 5 are used to scan for the red, yellow and blue phase respectively.

The clocking of the counter is done via an oscillator configured using a 555 timer

IC in a free running (astable multivibrator)

The output of the oscillator is gated to the counter with the output of the phase

voltage presence sensor.

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3.1.1 Design of 555 Astable multivibrator

The chosen astable NE555

The supply voltage 18V

The minimum current 20mA

Operating temperature 0 – 70 oC

The frequency of the oscillator in Hertz is given as f = 1

(R 1+2 R 2 ) 0∙ 693 ×C 1

Choosing a frequency of 1Hz, and setting capacitor C1 as 100µF, given

(R1 + 2R2) = 1100 x 0.6 93 x1 = 14430Ω

Taking R1 = 470Ω

2R2 = 14430 – 470

R2 = 13960 /2 =6980Ω

3.1.2 Design of the CD4017 Decade Counter

The count advances as the clock input becomes high (on the rising-edge). Each

output Q0-Q9 goes high in turn as counting advances. Counting to less than 9 is

achieved by connecting the relevant output (Q0-Q9) to reset, for example to count

0,1,2,3 connect Q4 to reset.

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Power Dissipation (PD)

Dual-In-Line 700 mW

Small Outline 500 mW

Lead Temperature (TL) (Soldering, 10 seconds) 260°C

The circuit diagram of the scanning unit is as shown in figure 3.1.

Figure 3.1:.The circuit diagram of the scanning unit

The clock pulse (output of the, oscillator) cannot get to the center except when the

phase detection given an output of zero which is then inverted to 1.

When a phase is encountered and thus in voltage, the output of the phase detector

produces a 1 which is then inverted to a zero and consequently disallows the clock

phase generated by the oscillator to getting through.

32

3.2 VOLTAGE PRESENCE SENSOR.

This unit is expected to produce an output of ‘1’ when there is power in the

encountered phase.

This unit is made up of a step down transformer selected as 12V, 300mA a bridge

rectifier, a filter capacitor and regulator.

The power supply unit is as shown in Fig 3.2.

3.2.1 Design of the step down transformer

The chosen step down transformer has the following ratings

Voltage rating 240V/12V

Current rating 500mA

3.2.2 Design of the bridge rectifier diode

The chosen diode is D3SBA10

Peak reverse voltage 800V

Forward voltage drop 0.7V

This diode satisfies the requirement because the maximum expected voltage is far

less than the peak reverse voltage rating.

3.2.3 Design of the filter capacitor

The maximum expected voltage (Vmax) = √2Vr.m.s

Vmax = √2×12 = 16.97V

Peak to peak ripple voltage (∆ Vp-p) = 10% Vmax / 2

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∆ Vp-p = (10 * 16.97) / (100 * 2) = 0.8485v

Id.c = 2 Imax / ∏ = 2√2Ir.m.s / ∏ = (2√2 * 0.5) / ∏ = 0.45A

Minimum required filter capacitor value is given by

C = Idc / (4√3f*∆Vp-p)

C = 0.45 / (4√3 * 50 *0.8485) = 1531.03ứF

A capacitor value of 4700ứF, 25V was used. It can serve the purpose since its

capacitance and voltage ratings are higher than the minimum values required

(1531.03ứF and 16.97v).

Output

Fig 3.2 – Block diagram of power supply.

3.2.4 Design of D.C Power supply

Two 9volts batteries was used as an alternative to provide the 12volt supply

needed by this project in the event of A.C power failure.

If there is voltage in any of the phases, there will be voltage at the output of the

regulator, when there is no power in that selected phase, there will be no voltage in

its regulated output.

The selection of the phase is done when the output of the counter closes and

interface relay.

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Mains220V

25V

12V

The output of the counter is approximately the supply voltage (Vcc) which is 9V.

The interface relay selected in 12V, 300Ω to be switched by an NPN transistor

(BC547) with amplification factor (β) of 60.

Therefore, the base resistor requirement

= RB = Vcc−VbeIB

But IB = ICβ

Then IC is the current that will flow when the transistor is saturated = Vcc300 = 9

300 =

30mA

So, IB = 30 x 10 ˉ ³60 = 0.54mA

RB = 9−0.70.5 x 10 ˉ ³

16600Ω

R3 = R4 = R5 is selected as for the three phases.

The circuit diagram of the voltage presence sensor is as shown in figure 3.3

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Figure 3.3: The circuit diagram of the voltage presence sensor

3.3 LOW VOLTAGE DECTECTOR.

This unit becomes necessary so that the power being supplied to the load is within

acceptable limits (and hence trigger the load interface contactor).

This unit is made up of a comparator and two preset resistance which is used to set

the reference voltage and sample of the output voltage to be checked.

Since the output of the step down transformer is proportional to the input supply

voltage the wider the value of the output voltage is a function of the unit voltage.

The capacitor output will be high if the voltage is within the acceptable and low

when the unit voltage is outside the range. The minimum voltage selected is 180V.

36

The outputs of the low voltage detectors are combined using an OR –gate which is

then inverted and then used as a gating pulse to the scanning clock low voltage

detector is as shown in figure 3.4

3.3.1 Design of the UA741 Comparator

VCC Supply Voltage ±22V

Vi Input Voltage ±15V

Vid Differential Input Voltage ±30V

Ptot Power Dissipation 500mW

Output Short-circuit Duration Infinite

Toper Operating Free Air Temperature Range 0 to +70°C

Tstg Storage Temperature Range -65 to +150°C

Since Transformer (T1-T3) = 220V/12V

Then setting reference voltage to (R10) to 180V

V2 = 12×180220 = 9.81V

V2 = V R 2R 1+R 2

Where R = R1+ R2 = 10kΩ

V2= 9.8 = 12 R 210 K

R2 =98 ×103

12= 8.166kΩ

R1= R – R2 =10 −¿ 8.166= 1.834kΩ

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Therefore using angular variation to set the potentiometer

R 1R

×100 %=8.16610

×100 %=81.66 %

R 2R

×100 %=1.83410

×100 %=18.34 %

3.3.2 Design of the Light Emitting Diodes

Parameter Bright Red Green Yellow Units

Power Dissipation 120 105 105 mW

DC Forward current 25 25 30 mA

Peak Forward Current 120 140 140 mA

Reverse Voltage 5 5 5 V

3.3.3 Design of Resistor for Light emitting Diode

Output of Comparator = 5V

Current Red and Green LED = 25mA

∴ V = IR

I = VR

= 925 mA

=360 Ω=0.36 KΩ

For Yellow LED

I = VR

= 930 mA = 300Ω = 0.3kΩ

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3.3.4 design of 4002(OR gate), 4049(INVERTER), 4081(AND gate)

Supply: 3 to 15V, small fluctuations are tolerated

Inputs have very high impedance (resistance)

Outputs can sink and source only about 1mA

Fan-out: one output can drive up to 50 inputs.

Gate propagation time: typically 30ns for a signal to travel through a gate

with a 9V supply

Frequency: up to 1MHz

Figure 3.4: low voltage sensor circuit.

39

3.4 SWITCHING AND LOAD INTERFACE UNIT

Relays RY1, RY2, and RY3 which switch power to the red, yellow and blue phase

step down transformers respectively are also used to switch power to the Red blue

and yellow phase contractors. There connections when energized supplies power

to the load.

Since these relays are energized only one at a time, then only one phase supply

gets to the load hence the load ‘sees’ only a single phase supply.

As soon as the phase fails, the scanning moves to the next available phase and

switch it to replace the failed phase.

The contactors selected are based on the load estimate. In this project, the

contractors used are D0910Q7 which has the capacity of supplying 25A. the unit is

energized by 220V AC

The current wiring of the switch and load interface is as shown in figure 3.5.

The combined circuit diagram of the automatic phase selector system is as shown

in figure 3.6

Figure 3.5:.The current wiring of the switch and load interface

40

Figure 3.6: The complete circuit diagram of the automatic phase selector system

41

3.5 LIST OF COMPONENTS

Component Description Value Unit

D1 –D21 IN4004 - Diode 800 V

C1 – C2 Capacitor 100uF F

C3 – C8 Filtering Capacitor 2200uF F

R2, R9 – R14 Variable Resistor 10k Ω

R1 Fixed Resistor 470 Ω

R2 Fixed Resistor 6980 Ω

R3 – R5 Fixed rsistor 16600 Ω

R6 – R8 Fixed Resistor 0.3k Ω

IC6 – IC8 uA 741- Comparator 500m W

IC1 NE 555 18 V

IC2 4017 3-15 V

IC3 4081 3 -15 V

IC4 4049A 3 - 15 V

IC5 4002 3 - 15 V

CCT1 – CCT3 D0910Q7 - Contactor 25 A

Q1 – Q3 BC 545 - Transistor 60 β

RY1 – RY3 Interface Relay 12, 300 V, Ω

T1 – T3 Transformer Step

Down

220/12 V

CHAPTER FOUR

42

CONSTRUCTION, TEST, RESULTS AND ANALYSIS

4.0 CHOICE OF COMPONENTS

The components used for this project are operational amplifiers, 555 timer,

counters, logic gates, regulators as well as discrete components like resistors,

capacitors, diodes, light emitting diodes and transistors.

The components were mainly CMOS types because of their low power

consumption as compared to TTL logics.

4.1 FABRICATION AND ASSEMBLY

The system was divided into blocks and each block was tested to ensure that the

desired result was obtained and it agrees with the characteristics desired for the

design.

The sequence followed was that the components got from the design analysis were

bought and connected on breadboard to see their response according to the circuit

diagram on a bread board. All units were tested to ensure proper functioning

before it was soldered them on the veroboard.

The Voltage presence sensors for each phase and low voltage detectors for the

three phases were first breadboarded. The transformers, rectifiers, diodes, filtering

capacitor, regulators, operational amplifier and resistor for phase power indicator

LED. After it was tested it was transferred to veroboard for soldering. The output

43

after construction was tested with a digital voltmeter and reading shows that the

performance is acceptable.

The next stage was the scanning unit. The oscillator and the counter as well as the

gate were assembled next. The components were connected on breadboard to see

their response according to the circuit diagram. The output was monitored using

and LED. The output frequency was fine tuned by adjusting R9. The components

were transferred to the veroboard for permanent soldering. .

The section that followed is switching and load interface unit. The relays and

contactors were connected. All connections were made according to the circuit

diagram. The unit was tested with the output of the counter connected to the base

of the switching transistor. The result shows a satisfactory performance.

In spite of the successes mentioned above, minor difficulties were encountered

after some blocks were transferred from breadboard to veroboard. However all

malfunctioning observed were corrected by proper trouble shooting of the faulty

section before proceeding to the next stage.

4.2 PACKAGING

The complete unit is housed in a metallic box with lagoon blue colour measuring

(410) mm x (110) mm x (250) mm (length, breath, height) having three LED

indicator attached to it which represents each of the phases with the scanning

power on toggle switch.

The output line and the three phase input are also attached to the casing.

44

The cover of the casing is a little bit gaped as to allow proper ventilation and

prevent external effects such as moisture.

Photograph of figure 4.1 shows the front and back view of the packaged project.

Diagram of Completed Project

Front View of Casing

Figure 4.1: The photograph of the packaged project.

Provision was made for connections terminals by proper drilling using drill bits

and other cutting tools.

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4.3 RESULTS AND ANALYSIS OF RESULTS.

When the construction was completed, series of test were carried out to ascertain

the performance of the project. The tests carried out include connecting the three

phases to power supply and switching off one after the other and systems response

noted. This was found to be working perfectly.

4.4 PRECAUTIONS

Many precautions were taken in the course of the construction of this project so as

to ensure a reliable result at the end of the project.

In the case of arranging the component onto a vero board, care was taken in

bending, aligning and inserting them.

During the soldering, the terminals to be soldered were scrapped off to remove

oxidation in order to ensure good connection. Excessive heat was avoided during

soldering because it could damage the components by soldering as quickly as

possible so as not to exceed the temperature each component could tolerate. Also,

the component terminals are held with pliers for this will absorb most of the heat

generated.

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4.5 BILL OF ENGINEERING MEASUREMENT AND

EVALUATION

The cost of the electrical and electronic components employed in this design is

shown in table 4.1 below.

Table 4. 1 Bill of Engineering Measurement and Evaluation

COMPONENT QUANTITY UNIT PRICE N AMOUNT NTransformer 220V/12V 3 280 840IN4004 21 10 210Resistors (fixed) 8 10 80Resistors (variable) 6 20 120Capacitors 8 60 480Ic sockets 8 20 160NE555 1 50 50Relay 3 100 300KA7809( Regulator ) 3 50 150Wire connectors 480Veroboard 2 150 300Toggle Switch 1 50 50LED 3 10 30Transistors 3 50 150Plug 1 60 604002 1 150 1504081 1 150 1504082 1 150 1504017 2 180 3604049 1 150 150Contactors 3 600 1800Battery contact 2 50 1009V battery 2 60 120Miscellaneous 1800Casing 1 1700Total 9,940

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CHAPTER FIVE

CONCLUSION AND RECOMMENDATION

5.0 CONCLUSION

The design and construction of the automatic phase selector system has been

successfully carried out. The system is one that will ease coordination, control and

management of phases in the power supply system. The system could also be

adapted to switch ON standby generator when the three phases fails.

The system has been designed using locally available materials and the cost has

been relatively minimized using cost effective circuits.

The system was designed using discrete electronic components and integrated

CMOS circuit devices. The system when tested fully met the specification

described in the preceding chapters and can be installed in real s(physical)

installation. Should higher load is required to be switched; the size of the

contactors can be increased.

Though, the system is relatively sensitive and reliable it is subject modification as

explained in the recommendation for further work.

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5.1 RECOMMENDATION

Some more work can be carried out on this project to make it more versatile and

efficient but will also make it costlier. These areas include:

1. Other circuit could be added to switch ON a standby generator when no

phase is available and switch OFF the standby generator when any phase

returns.

2. A complete phase failure alarm could be incorporated into the system.

3. The system could be interfaced with an automatic voltage regulator (AVR)

to make sure that only voltages within the acceptable limits are supplied to

the load.

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2. Theraja B.L., Theraja A.K.,2005 “A textbook of electrical technology” illustrative

multicolored edition, published by S.C Chand and company, pp 375 – 378

3. Madhuri J A., 1995 “Electronic Components and Materials”, Second Edition,

Wheeler Publishing & Co. Ltd. New Delhi, pp 17 – 45.

4. Comer D J.,”Modern Electronic Circuit”, Second Edition, 1975, Published by

McGraw-Hill Inc, pp 45-52.

5. Essien Mmekutmfon S. “Design and construction of a human body resistance

and temperature measurement device”.(B.Eng Project)unpublished 2007, pp 5 -30

6. Barry W ‘the essence of digital design’ (1998), prentice Hall, Europe.

7. Charles brooks ‘microelectronics: devices and applications ‘ (1985), national

education corporation ,England ,

8. Holdsworth B and R C Woods ‘digital logic design’ (2002), Newess press, Ox-

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9. Donald A Neamem ‘Electronic Circuit Analysis and Design’ (2001), 2nd edition,

McGraw-Hill press, New York, USA.

10. http://www.allaboutcircuits.com , 21/01/2008

11. http://www.discovercircuits.com, 13/03/2008

12. http://www.opamp-electronics.com/tutorials/index.htm, 22/07/2008

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