inverter systems 2010

CHAPTER 1

INTRODUCTION

1.1 BACKGROUND HISTORY OF INVERTER SYSTEMS

Since the first day of creation when God said “Let there be light” and light was created and

separated from darkness, light has been a major issue in the planet earth.

From the pre-historic times to the medieval periods to the Renaissance period, the period of

enlightenment, different forms of lights were developed and used. In the pre-historic times,

smooth stones were hit together, and the resultant friction results in a spark which after many

trials may result in fire. Around 300 years ago, different highly combustible chemicals were

developed which when mixed together can explode to serve as a source of light. But these light

sources were often uncontrollable thus it cannot necessarily be used in a controlled environment.

[13]

But with the evolving of civilization came the lanterns and the light sticks which could be lighted

and used safely. Since the invention of electricity by Michael Faraday, various means of

electricity generation has been invented, developed and used with both their positive and

negative effects.

The most popular being the hydro-electric power generated using turbines, a mechanical device.

Its major strength lies in consistent generation and supply of power but its major limitation is in

the transmission and distribution of the generated power to the end users/final consumer. This is

because the amount of electricity generated is proportional to the water level such that it goes

500VA Inverter System 1

down in dry seasons. Also, during rainy seasons electrical transmission and distribution lines get

damaged due to thunderstorms, thereby increasing power problems.

Due to these power problems, alternatives to the hydro-electric power are the electricity

generating plants containing internal combustion engines which uses petroleum products for

power generation. But its weakness inherently lies in the noise generation and usage of

inflammable products for power generation. To overcome these problems, the inverter systems

were invented and developed.

Early inverters

From the late nineteenth century through the middle of the twentieth century, DC-to-AC power

conversion was accomplished using rotary converters or motor-generator sets (M-G sets). In the

early twentieth century, vacuum tubes and gas filled tubes began to be used as switches in

inverter circuits. The most widely used type of tube was the thyratron.

The origins of electromechanical inverters explain the source of the term inverter. Early AC-to-

DC converters used an induction or synchronous AC motor direct-connected to a generator

(dynamo) so that the generator's commutator reversed its connections at exactly the right

moments to produce DC. A later development is the synchronous converter, in which the motor

and generator windings are combined into one armature, with slip rings at one end and a

commutator at the other and only one field frame. The result with either is AC-in, DC-out. With

an M-G set, the DC can be considered to be separately generated from the AC; with a

synchronous converter, in a certain sense it can be considered to be "mechanically rectified AC".

Given the right auxiliary and control equipment, an M-G set or rotary converter can be "run

backwards", converting DC to AC. Hence an inverter is an inverted converter. [2]


http://en.wikipedia.org/wiki/Electric_power_conversion

http://en.wikipedia.org/wiki/Electric_power_conversion

http://en.wikipedia.org/wiki/Thyratron

http://en.wikipedia.org/wiki/Gas_filled_tube

http://en.wikipedia.org/wiki/Vacuum_tubes

http://en.wikipedia.org/wiki/Motor-generator

http://en.wikipedia.org/wiki/Rotary_converter

In 1957, solid state devices such as thyristors or silicon controlled rectifiers were introduced in

the design of inverter systems. However, Metal Oxide Semiconductor Field Effect Transistors

(MOSFETS) are currently used in most inverter systems designs.

1.2 OBJECTIVES OF THE SYSTEM

Basically, an inverter system is a DC-AC converter. It uses the potential energy stored in the

battery for its various processes. The major objectives of developing the modernized version of

the inverter system the micro-controller based inverter system are:

A.) To develop and demonstrate how an electronic system can be used as an alternative to

hydro-electricity at a domestic and commercially competitive cost.

B.) To verify the belief that an electronic system can be developed which can complete

favourably with electricity generators

C.) In terms of environmental and human factors, to show that the inverter system is more

convenient, environmental and human friendly than generation of electricity by

mechanical generators or power plants.

D.) To show the advancements that has been made in the quest to develop highly efficient

inverter systems.

E.) To establish the environmental acceptability and operational safety of inverter systems. [3]

1.3 STATEMENT OF THE PROBLEM

The effective utilization of the inverter system has many serious problems that must be

overcome if it is to be generally accepted as an alternative means of power supply in the absence


of mains power supply when compared to the electricity generators. The major challenge

encountered is the source of energy which in this case is the battery.

The battery most times is not as durable as the petroleum products required to operate the

generators. The number of hours that the inverter systems operate is solely dependent on the

voltage and current ratings of the battery as well as the output load. The charging of the battery

depends on the presence of the mains power supply.

If the battery discharges before the mains power is restored, the inverter must be shut

down hence the inverter system application is limited to regions where power cut lasts for short

periods of time.

Secondly, for 100% efficiency, a pure sine wave alternating current voltage must be

obtainable. But presently, modified sine wave AC voltage is gotten this introduces humming in

inductive load like the fans and sound systems. Steps are currently being taken to obtain pure

sine wave AC V.

1.40 SCOPE

The project – A Micro controller based inverter system is expected to have the following circuit

specifications

Output capacity 1000VA

Input (AC Mains) Voltage 220V, 50Hz frequency

Output (inverter) voltage 220V AC, 50Hz frequency


The battery rating is expected to be 12V/60AH. If all conditions are met the expected efficiency

is 80%. The operating time of the inverter system will depend on the load.

1.50 DESIGN SCOPE (PULSE WIDTH MODULATION WITH MODIFIED

SINE WAVE OUTPUT)

PWM is mainly used to keep the AC supply output by the inverter constant at 220V. The PWM

based inverter maintains constant output voltage despite variation in load. This is achieved by

changing the width of the switching frequency generated by oscillator.

Thus in the PWM based inverter, the AC supply at the inverter output depends on the width of

the oscillator frequency generated by the oscillator section.

A modified sine wave is the type of AC generated by pulse width modified sine wave generated

by inverter systems. It consists of a number of very small on/off steps rather than a fully smooth

sine wave. While the sine wave smoothly rises steadily to its peak and decreases, modified sine

waves and square waves sharply rise and ‘level off’ at the peak voltage and drop straight down.

The main difference between the modified sine waves and the square wave is that it sits at zero

for a short period.

Modified sine wave inverter systems can be used to power most electric devices such as

computer systems, light bulbs, television sets and a host of others. However, some devices such

as electric drills require smooth sine waves for their operation. Such devices should not be

connected to a modified sine waves inverter system. [3]


1.60 ECONOMIC IMPORTANCE

The PWM inverter system with modified sine wave output is of great economic importance.

This is discussed in the following four points.

1.) Inverter systems can be used to provide power for homes and offices in the absence

of mains supply. The modified sine wave output can be used to power a broad range of

electronic devices.

Presently, power supply is erratic and highly unreliable. This poses a serious

challenge to industries as they are highly dependent on power for their production and

profitability. Thus, inverter systems provide power to run a range of home and office

equipment in the absence of AC mains supply. Its batteries can be charged when mains is

available and can be automatically switched from battery to mains and vice versa as

required.

Inverter systems are the perfect alternative to generators. They provide power

noiselessly and require little effort to operate. The table below summarizes the

advantages the inverter system has over the generator.

Generator Inverter

Generator generates a lot of noise during its

operation.

Inverter works noiselessly.

Normally generators do not have automatic

start/stop function. When mains AC fails one

has to manually start the generator and switch

Inverters provide completely automatic switch

over function. When the mains supply fails the

inverter immediately switches the output to its


the power supply back from generator to

mains.

internal battery and when the mains supply

returns the inverter shuts down its operation

and provides the mains AC supply at its output.

Starting the generator is a process which

requires some force. An old or sick person may

not be able to start the generator.

Currently some generators are coming with

starting switch. By pressing this switch one can

start the generator.

But, this additions makes the generator cost

very high and after some time this starting

arrangement starts giving problems.

Also, after starting the generator, switching of

output supply from mains to generator to mains

needs to be done manually.

Inverter does not require any special starting

process and the switching of output from mains

to inverter and inverter to main is done

automatically.

Generator requires petrol, diesel, etc for its

operation. These are highly inflammable

products and they generate bad smell in the

area. Also during its operation it emits smoke

(that contains carbon mono-oxide) which can

be harmful to the people around.

Inverter works on battery which works

noiselessly without producing any smell or

other harmful emissions etc.

As a generator has many mechanical parts, it

requires constant maintenance. Parts such as

Since the inverter is a purely electrical device,

it requires no special maintenance. Only the


carburetor, etc. require regular maintenance. battery used with the inverter will require some

routine service such as topping it with distill

water once in 15-20 days.

2.) Clean source of energy. Since the inverter system emits no dangerous substances to its

environment, it provides clean energy. Currently, industries utilize energy sources that

emit carbon for their operation. Carbon is a pollutant that contributes to the depletion of

the ozone layer, global warming, destruction of certain ecosystems and other

environmental threats.

With the use of inverter systems as energy sources for home and offices, carbon

emissions will be lowered and its environment will suffer no negative impact.

3.) Employment Generation. As inverter systems are increasingly being used in homes and

offices as alternative to the unreliable power supply plaguing the economy, the inverter

systems manufacturing industry is expected to grow.

This growth will spur employment generation in the economy as

electrical/electronics technicians and engineers will be employed to build, repair and

install systems to meet the rising demand.

4.) Inverters reduce running costs for businesses. Inverter systems are renewable sources

of electrical energy and can reduce the operating costs for businesses. The inverter

batteries are charged when mains power supply is available and can provide power from

stored power when mains power supply is unavailable. Thus, businesses will not have to

purchase fuel or diesel on a regular basis. Therefore, running costs will be reduced by the

use of inverter systems.


1.50 BLOCK DIAGRAM

Fig 1.0: Block diagram of inverter system

1.60 DESIGN METHODOLOGY

The inverter system is basically a DC-AC converter circuitry. It serves as an alternative to both

hydro-electric power and petroleum power generators. Basically a micro-controller based

inverter system consist of the following integrated circuits (ICs): the SG3524, the pulse width

modulator controller IC, the 4N35 which is an opto-coupler IC, the 4066 IC which serves as the

data selector, the analogue to digital converter IC, (ADC), as well as the 89C52 which is the

micro-controller (i.e. a system on a chip).

For the discreet electronic devices: the MOSFET (IRFP150N), 12V/220V 50Hz step-up

transformer, the relays, the liquid crystal display (LCD), as well as the capacitors (electrolytic

and non-electrolytic), the resistors, the light-emitting diodes, (LEDs), the variable resistors and

the crystal oscillator. The SG3524, a key IC in the inverter system is responsible for the 50Hz

frequency generation, shutting down of the inverter system in case of system overload or low DC

battery voltage and also maintaining a constant output voltage despite variation of the load.


The 50Hz frequency alternating signal from the output pins of the SG3524 known as the MOS

drive signal is sent to two different MOSFET channels. Because of the alternating signals at the

output of the oscillator IC, the two MOSFET channels are alternately ON/OFF (i.e. when one

channel is on, the other is off). This ON/OFF switching of the MOSFET channels starts an

alternating current in the primary winding of the step-up transformer. The AC current induces a

220v/50Hz at the secondary winding which is then sent to the output socket.

Some of the output current is sent to the SG3524 through the opto-coupler which maintains an

optical contact between them in order to maintain output voltage stability in case of variation in

the load, for easy monitoring of the conditions (i.e. the MOSFET temperature, the battery voltage

and the output voltage) of the inverter system, the micro-controller is also responsible for

sending the signal that shuts down the inverter system if the above conditions are not within

specified limits.

The micro-controller is a digital IC that is programmable thus it cannot make use of the variation

oriented analogue signals from the output section for its normal operation thus the analogue to

digital converter (ADC) is added to undertake the function of converting the analogue signal to

discreet digital signals of 8 bits.

Three signal conditions circuitry incorporate transducers; one for sensing the output voltage, and

rectifies it and moderates it to serves as an input to the data selector. Another one makes use of

the temperature sensor (LM35) in converting the MOSFET temperature to voltage signal before

being sent to the data selector IC and an RC network that moderates the battery voltage before

being sent. [4]


The micro controller generates an address bit which the data selector IC interprets and reads the

analogue values sent by the respective conditioners and sent it individually for conversion by the

ADC. The liquid crystal display (LCD) reads the digital signals (8 bits) sent to it by the micro-

controller and displays the ASCII equivalent alphanumeric characters.


CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

In the course of this project, two project reports related to our project’s topic: The design and

Construction of 1kva Micro-Controller Based Inverter System, previously done by the graduated

students of this department were analyzed.

One was the design and construction of the 625VA inverter system done by Okonta N. N. and

Co. [2002]. [5] From the analysis of the project report, it was discovered that while following the

basic principles utilized in the designing of an inverter circuitry, they used the NE555 timer

integrated circuit, a timer IC for the generation of a 100Hz frequency when configured to operate

as an astable multivibrator. The SN74LS112, a J-K flip-flop IC, was used to break up the 100

Hz signal generated by the timer into two 50 Hz pulse switching frequencies of opposite polarity.

And for the amplification of the switching frequency generated by the timer, they made use of

transistors.

The usage of the NE555 timer, the SN74LS 112 IC at the oscillator section and transistors at the

driver section will result in some drawbacks such as:


i) Absence of a mechanism for shutting down the inverter system in situations of system

overload. When excess load is connected, high current will flow to the transformer

and output sections of the inverter system. This could damage the transformer and the

transistors.

ii) Lack of a mechanism for disconnecting the charging section from the battery when

the battery is fully charged as the battery is considered fully charged at 13.5V.

iii) Also absence of facility for shut-down of the inverter system when it is fully

discharged as the battery is considered fully discharged at 10V.

iv) There was no mechanism for keeping the output voltage constant thus the variation of

the load connected to the output voltage will result in the changing of the output

voltage.

v) To ascertain the conditions of the system (ie the battery voltage and the output

voltage) the LEDs were used. It has the disadvantage of not knowing the exact value

of the conditions of just the range. Also, an indicator for the systems temperature was

not incorporated.

The above stated drawbacks were addressed and eliminated in our project design. The second

project report reviewed was: The design and construction of 1KVA Inverter System done by

Etuk K. & Co. (2005). [2] From the analysis of the circuit design and explanation, we actually

discovered that their project was closely related to our project circuit design because the

Oscillator and the Driver sections made use of the SG3524 IC and MOSFET respectively. But

the major differences between the two were also clearly defined.


In contrast, our own project is micro-controller based and incorporates the liquid crystal display

to display the actual value of the system conditions. Also, the microcontroller is programmed to

monitor when these conditions fall outside the design specifications. With the exception of the

MOSFET temperature, the other conditions were monitored by comparators and other discreet

components and their value displayed as LB- low battery, CH- for charging, OL- overload, and

when the inverter is working on battery (DC), inverter is not working on battery (AC).

2.2 THE INVERTER SYSTEM [3]

An inverter system basically is a device which converts the DC Voltage supply to AC supply.

The basic idea of the inverter system can be explained from the circuit below:

Fig 2.1: Basic Circuit of an Inverter System

When the switch is closed, current starts to rise in the circuit. The rising current makes the

transformer to generate an EMF (electromotive force) opposing the EMF of battery. The rise of

the current will depend on the inductance of the transformer. The higher the inductance, the more

time needed to generate the EMF that will balance that of the battery.

500VA Inverter System

BAT11.5V

BAT21.5V

TR1

TRAN-2P2S

14

If the switch is opened and closed at a constant interval, he current changes in the primary

winding of the transformer will induce an output at the secondary winding of the transformer.

The output will be a square wave of the frequency at which the switch is opened and closed. The

idea applied in real inverter system can be understood from the circuit diagram below

I 2

2 4 V

I 1

S 1

S 2

T R 1

O / P

Fig 2.2: Circuit of a simple Inverter System using two switches

These switches S1 and S2 are turned on/off alternatively to generate current in the primary coil

when the switch S1 is closed and S2 is open, the current flows in the first part of the primary

winding and the EMF is induced in the secondary winding. When S1 is opened and S2 is closed,

the reverse is the case.

If the switches S1 and S2 are alternatively opened and closed at constant rate, then the output

from the secondary winding is a square wave of the frequency at which the switches S1 and S2

are opened and closed. This is the basic principle behind the operation of the inverter system. But

for a modern digital inverter system with efficiency of about 85%, a lot of both discreet

electronic consumables and integrated circuit devices are used.

The rest of this chapter will look at the internal configuration of these electronic devices that

makes them relevant in the development of the modern digital inverter systems.


2.3 BATTERY

For an inverter to function, it requires a source of power supply. Because if is being used as an

alternative to generating sets, it requires a device that can store electrical energy when mains

power supply is available which can then be used when mains is unavailable.

The lead accumulator also known as storage battery is used. A battery is a device that supplies

DC power through a series of chemical reactions. Batteries in different forms have existed for

over 150 years.

Alessandro Volta developed battery around year 1800 when he found that electrical current can

be generated from the chemical reaction between different metal of different properties.

Batteries can be divided into two categories:

The primary cell

The secondary cell

The primary cell is a single used battery. The chemical reaction that produces electrical current is

irreversible. Thus once the useful life expires, it is discarded.

The secondary batteries are rechargeable batteries i.e. they can be used multiple times, after

using the charge stored in them, they can be recharged and reused.

The lead accumulator is based on the principle of electrolysis.


Lead(IV)oxide lead electrode

+

88

.8

Volts

C A T H O D E A N O D E

Dilute tetraoxosulphate (VI) acid lead(II)

tetraoxosulphate(VI)

solution deposited both plates

Fig 2.3 Diagram of a lead acid battery

A battery consist of two electrodes; the cathode Lead (IV) Oxide, PbO2 and the anode, metallic

lead with Dilute Hydrogen Tetraoxosulphate (VI) Acid solution (H2SO4) as the electrolyte.


When a battery is in operation i.e. discharging, two processes occur, oxidation and reduction. At

the anode, the metallic lead releases two electrons to become oxidized to lead (II) ion. The lead

ions combine with tetraoxosulphate (VI) ions in the electrolyte to become deposited on the anode

as lead (II) tetraoxosulphate (VI).

I.e. Pb(S) PB2+(aq) + 2e-

Pb2+(aq) + SO4

2-(aq ) PbSO4(s)

The electron released pass round external circuit as an electric current which is used by the

inverter system for operation before arriving at the cathode.

At the cathode, the electrons from the anode are accepted at the cathode.

At the cathode, the electrons from the anode are accepted. The lead II oxide combines with the

electrons and hydrogen ions to form lead II ions and water.

The lead ions combine with sulphate ions to become deposited at the cathode as PbSO4(s)

I.e.

PbSO2(s) + 4H+(aq) + 2e - Pb2+(aq) + 2H2O(l)

Pb2+ (aq) + SO42-

(aq) PbSO4(s)

During the discharging process, the density of the acid decreases to 1.15gCm -2 due to the

absorption of hydrogen& H2SO4 from the electrolyte, also, the e.m.f of the cell drops to 1.8V.


When both electrodes are completely covered with lead (II) tetraoxosulphate (VI) deposits, the

lead accumulator will stop discharging current and needs to be recharged. To recharge is to pass

electric current through the electrode.

The process is as shown:

Pb2+(aq)

+2e Pb(s) At The Cathode

PbSO4 SO42-

(aq) (REDUCTION PROCESS)

Pb2+(aq)

2e- PbO2(s) + 4H+(aq) At The Anode

PbSO4 SO42-

(aq) (OXIDATION PROCESS)

After recharging, the acid density returns to initial value of 1.25gCm-3 and the e.m.f returns to

2.2V.

2.3 THE SG3524 (OSCILLATOR IC)

In the traditional method of developing inverter system, the output of the inverter changes with

any change in the load connected to the output of the inverter. To eliminate this problem, the

modern digital inverter system incorporates the SG3524, a pulse width modulation IC. The

PWM IC keeps the output constant by increasing or decreasing the width of the oscillation pulse

generated by the IC with respect to the reference voltage which is a small part of the inverter

output that is fed into it.

For the PWM IC to function effectively both as a frequency generator and

pulse width modulator, the internal configuration is as shown.


3

21

41

1

1

R 2

R 1

3

21

41

1

4 A M P

P A C K A G E = D I L 1 4

3

21

41

1

5

P W M2

1 0

9N O R G A T E

V I3 V O 1

GN

D2

1 5

1 6

7 8 0 5

12

1 31 2

7

F L O PF L I P

12

1 31 2

6

O S C

N O R G A T E

1 2

N P N

1 3

1 4

N P N

8

1 1

3

Fig 2.4: Internal block diagram of IC SG3524

For the effective operation of the IC, a normal 12V from the dc battery is fed to pin 15 of the IC

which serves as the positive supply of the battery. An internal voltage regulator breaks it down

to 5V which is fed to the different components internally connected namely the comparators, the

flip-flops, the gates as will and the crystal oscillator. The inputs to the error amplifier are

connected to pins 1&2 whereas pin 1 is given feedback signal for output regulation and pin 2 is

given a reference voltage from pin 16 which outputs a constant 5V at its output pin. For effective

amplification, pins 4 and 5 are connected to an amplifier whose output is tied to the output of the

error amplifier also the shutdown input pin which helps to shutdown the inverter once voltage

greater than 0.6V is sent to pin 10.

The inverting input of a comparator which serves as a PWM is connected to pin 9 also known as

the compensation input while the non inverting input is connected to the crystal oscillator to

which both pin 6 and pin 7 are connected .


An externally connected RC network to pins 6 and7 serves to set the frequency of oscillation.

The inputs of the NOR gates are connected the outputs of the PWM amplifier, the flip-flop and

the oscillator amplifier. The outputs of the NOR gates connected to two NPN transistors while

the collector and emitter serves as the positive supply for oscillator section and output for the

MOS drive signals respectively. [3]

2.4 MOSFET

The MOSFET is called a voltage controlled device because the gate voltage controls the drain

current.

Basic Construction

A narrow bar of n-type semiconductor material is taken and then two p types junctions are

diffused on opposite sides of its middle part. The junction forms two p-n diodes of gates and the

area between these gates is called channel. The p-region is internally connected and single lead is

brought out which is called gate terminal. Ohmic contact (direct electrical connections) are made

at the two ends of the bar one load is called source terminal S and other drain terminal D.

Fig 2.5: Construction of n-channel MOSFET showing source, drain and gate


The gate is insulated from its conducting channel by an ultra thin metal oxide insulating film

(usually SiO2).

Operation

a. Depletion mode of N-channel MOSFET.

When Vgs = 0, Id = 1

Vgs <0, Id decreases

Vgs<<<0, Id = 0

b. Enhancement mode

Vgs = 0, Id = 1

Vgs > 0, Id increases

Vgs>>0, Id ++++

For a MOSFET to be used as an amplifier,

Id = Idss(1 – Vgs/Vp)2

= Idss(1 – Vgs/Vgs(off))2

NB – The SS in Idss, also known as zero gate voltage drain current, indicates that the gate is shorted

to source to make sure that Vgs= 0. [7]

General Specifications of MOSFET

1. The forward transconductance is about 1-10 mA/V.


2. The input resistance is very high, about 10-12 watts.

3. The output resistance is about 10-50 KΩ.

4. Other specifications that must be considered before selecting a MOSFET are

a. Breakdown voltage (250-1000V)

b. Switching characteristics

c. Zero gate voltage

d. Drain current

e. Input capacitance, etc.

These depend on the type of application for which a MOSFET would be used. MOSFETs are

specifically used in application with high operating frequency (> 200 KHz), wide load variation

and lower output power. They can also provide large output current with a small input.

ADVANTAGES OF MOSFET’s OVER BJT’s

1. It has faster switching time than BJT (10 times faster).

2. It makes use of every small switching current.

3. Temperature has least effects on it.

4. It has lower losses than BJT’s.

But it can get damaged at higher voltages especially static electricity.

N/B: The MOSFET has very small gate leakage current.

2.5 TRANSFORMERS


The design of transformers is based on the principle of electromagnet induction postulated by

Michael Faraday. The basic idea behind transformers is the passage of an electromagnet (a

combination of a soft iron bar inside a current carrying coil) In and out of a coil to which a meter

is connected.

The meter shows null deflection when the electromagnet is within or out of the coil but deflect

on/off when the electromagnet is in motion. The electromagnet can be kept permanently inside

the coil while an on/off switch is connected as shown below:

The switch can further be replaced with an AC voltage supply to give a constant supply of

alternating current. This alternating current flowing in the electromagnet induced an EMF in the

coil in which the electromagnet is embedded.

Thus a transformer results when an alternating current flowing in a coil wound round a soft iron

coil induces an EMF in another coil (sec coil) which is also wound round the metallic soft core.

The frequency of the current in the secondary coil depends on the input frequency of the

alternating current in the primary winding. [6]

A transformer is a general term for a pair of mutual inductors, made of two windings -

primary and secondary. [8] The winding to which a signal is applied is the primary winding while

the winding from which the signal is taken is the secondary winding. There is no direct

connection between the primary and secondary windings in the transformer.


Fig 2.6: A Transformer

The transformer is responsible for converting the 12V battery supply to 220V supply. The

primary winding of this transformer is generally 12V - 0V - 12V, and the secondary winding is

of 220V. This is a step up transformer.

Thick wire is used in the primary winding while the secondary is made of thin wire.

When AC mains is not available, the center tapping of the primary winding is given positive

supply from the battery. Each end of the primary winding is given alternate supply from a

switching circuit (driver section), resulting in an AC current in the primary winding of the

transformer. AC current in the primary is stepped up 220V AC current in the secondary winding

of the transformer. This stepped up current is then sent to the inverter output socket.

Transformations in a transformer

Types of transformations in a transformer are:

1) Voltage Transformation (V1/V2 = N1/N2)

2) Current Transformation (I2/I1 = N1/N2)


3) Impedance Transformation (R1/R2 = N1/N2)

Types of Transformers

Transformers can be differentiated according to

i) Output voltage

ii) Core type

iii) Usage

i) Output Voltage

a) Step - Up Transformer

This transformer is used to increase the secondary (output) voltage in proportion to primary

(input) voltage. In this transformer, the number of turns in the primary winding is less than the

number of turns in the secondary winding and the secondary voltage is given by:

Vs = (Vp/Np) * Ns

Where

Vp is the primary voltage

Vs is the secondary voltage

Np is the number of primary coil turns

Ns is the number of secondary turns

b) Step - down Transformer

This transformer is used to decrease the secondary (output) voltage in proportion to primary


(input) voltage. In this transformer, the number of turns in the primary winding is more than the

number of turns in the secondary winding and the secondary voltage is given by

Vs = (Vp/Np) * Ns

Where

Vp is the primary voltage

Vs is the secondary voltage

Np is the number of primary coil turns

Ns is the number of secondary turns

ii) Core

The core is made from materials of different properties. The properties of a good core

material are:

-Permeability

-Electrical Resistivity

a) Permeability

Permeability is defined as the ability to conduct flux and is expressed as the ratio of flux

density (B) to the magnetizing force/field strength (H) that causes it.

This is given by = B/H

Where

B is flux density

H is the magnetizing force

A good core should have high permeability.


b) Electrical Resistivity

The lines of flux that link the winding of transformer also pass through the core and induce

current in it. This induced current is known as eddy current. The eddy current heats up the core

thus wasting power. If the resistance of the core is high, less current flows through it and the

power loss is reduced. Thus, the core with high resistance is preferred in transformer to reduce

the losses.

Different types of cores used in transformers are:

Laminated Iron Core

This type of core is constructed by cutting the metal into sheets of the required shape and

size (usually of the English letter E and I or U and T) and insulated by iron oxide and varnish.

These are used to reduce the iron loss (or eddy current loss). The transformers with this type of

core are used in low frequency circuits.

Ferrite Core

It is a ferromagnetic material and had high permeability and give high flux density. The

transformer with this core material is used in high frequency circuits. The core losses in this type

of transformer are very low.

Air core

It is constructed by a simple insulated former/bobbin on which the coil is wound. Here


the medium of flux is air. These types of transformers are used in high frequency circuits.

Usage

Different kinds of transformers based on their usage are:

- Voltage or Potential Transformer

-Current Power Transformers

- Series Transformer

- Impedance or Coupling Transformer

- Isolation Transformer

- Auto Transformer

Different Types of Winding

Types of winding used in making a transformer is as given:

- Layer Winding

- Random Winding

- Bifilar Winding

Layer Winding

In this winding the wire is rotated from one end to another forming a layer. This layer is

insulated by a paper or tape and the second layer wound on it, this process is continued till the

required number of turns is completed. The transformer with this kind of winding has higher load

bearing capability and is used to construct the power transformer.


Random Winding

In this winding, enameled copper wire is used and hence insulation is not required. After

completing the primary winding, insulating paper is used and after completing the secondary

another insulation paper is used.

Bifilar Winding

In this type of winding two wires are rotated along till the required turns are completed,

then the two wires having continuity are connected together. The point where they are joined

together is known as center tap. If the resistance of winding is 100E thin in the bifilar winding

the resistance of center tap to each end will be equal to 100E.

Transformer Ratings (Watts/ Volt-Ampere)

The watt is calculated by multiplying volt ampere, but in case of the transformer both are

not the same thing. The transformer is rated in Volt-Ampere rather than in Watts. If the load

connected to the transformer is purely resistive, then the load current is in phase with voltage,

and the voltage dropped multiplied by the load is in watts. This power is referred to as true

power.

But if the load is inductive or capacitive then the voltage across it is not in phase with the

load current (current is lagging in case of inductive load and leading in case of capacitive load)

and multiplication of voltage and ampere is known as apparent power (volt-ampere).

This causes power to be consumed in the windings, which in turn creates heat. Thus watts

and volt-amperes are interchangeably used but they should be differentiated according to the


load connected.

The ratio of watts and volt-ampere (true and apparent power) for a load is known as

Power Factor (pf). The power factor is never greater than 1.0. In case of pure resistive load, the

power factor is 1 and if the power factor is less than 1 then both true power (watts) and apparent

power (volt-ampere) will have different values.

Applying DC to Transformer

When AC supply is given to the transformer the current in the primary winding of

transformer changes and the flux produced by the winding also changes. This changing flux

induces EMF in the secondary winding. But if DC is applied which has zero frequency and is

constant, no EMF is induced in the secondary winding. In this case, if the resistance of primary

winding is very less then high current flows through it, which may damage the transformer,

hence DC should not be applied to the transformer.

Advantages of Transformer

Some of the advantages of a transformer are:

1) The output can be varied proportional to the input

2) Being a static device, it requires low maintenance

3) Noiseless transfer of power from primary to secondary as there is no mechanical parts

involved.

4) As there is no electrical contact between the primary and secondary of the transformer, circuit

connected to the secondary is safe from electric shock.


Parts of a Transformer

Fig 2.7: Diagram showing primary winding, secondary winding and core of a transformer.

Coil

The coil is a requirement for the core. Coils made from inferior materials can melt when

hot, or short circuit when slightly higher load is connected to the transformers output. Higher

quality wires such as the green/blue colored super enameled wire are recommended. These wires

are rated in Standard Wire Gauge (SWG) standard. In SWG system, the higher the SWG

number, the thinner the wire gets. So a 5SWG wire will have diameter of 5.4mm, but a 50SWG

will have a diameter of only 0.025mm.


Former/Bobbin

Fig 2.8: Bobbin

In the transformer, the coil is wrapped around a former or a bobbin. Bobbin is made of

some insulation material such as plastic, paper, fiber etc. In power transformers the bobbin is

usually made of plastic or fiber. This keeps the bobbin safe from heat/cold and humidity. A

paper/cardboard bobbin is used in cheap transformers, but this type of bobbin generates many

problems.

When a plastic bobbin is used, it is recommended that is must be of good quality core,

otherwise the heat of the core will distort the shape of bobbin. Bobbin made using fiber may also

burn if the core gets too hot. Normally, standard companies use good quality plastic bobbin in

their transformers.

Core

Core is the heart of a transformer. It must be of high resistance to reduce the flow of eddy

current in the core and further limit power losses. To increase the resistance of the core, thin

laminated iron sheets are joined together to form the core instead of using a single iron piece.

This action increases the resistance and narrows down the eddy current path. Which in turn


improves the productivity of the transformer and makes it less hot? These sheets are normally

available in the shape of English letter E and I to use in transformer. One can get the E & I shape

sheets from a rectangular shaped iron sheets without any wastage.

The core can be made from a range of materials such as low-Carbon steel, silicon steel,

nickel-iron, cobalt-nickel-iron and cobalt iron. Magnetic ceramic materials known as soft-ferrite

ceramics or simply ferrite are also being used to make core of unusual shapes.

The core is available in various sizes and numbers. Other than E & I shapes, the shape of

English letter U & T is also very common for the core. Green/Gray color Japanese core is of the

best quality. These have very smooth surface which can be joined together without leaving any

air gaps. Common thickness for the core material is 0.35mm to 0.5mm.

Core used in power transformers can be divided into the following types:

- Dynamo grade Lamination: These cores are also known as ordinary or Tata core

- CRGC (cold rolled grain oriented) & CRNGO (cold rolled Nickel Grain Oriented): These cores

are also known as special cores. When good quality core is used, a transformer gives long trouble

free service. [3]

2.8 OPTOCOUPLER

An optocoupler is a device consisting of a semiconductor photo transistor in close contact

with a light-emitting diode (LED). Signals into the LED are optically transferred to the photo

transistor, but there is a considerable degree of isolation, depending on the packaging used, so

that the input and output can be at very different DC potentials.


Fig 2.12: The IC 4N35

Pin Configuration

The IC 4N35 is a 6 pin optocoupler integrated circuit (IC). It contains a LED and a photo

transistor. Pins 1 and 2 are connected to the LED. Pin 1 is given positive supply while pin 2 is

given negative supply to light the LED. The light from the LED falls on the base of the transistor

inside the IC. Pins 4, 5 and 6 are connected to the emitter, collector and base points of the photo

transistor respectively. Light falling on the base conducts the photo transistor and output from

the transistor is made available at the emitter.

Fig 2.13: The Block Diagram of IC 4N35

Pin description of IC 4N35 is given below:

- Pin - 1- This pin is connected to the anode of the LED.

- Pin - 2 - This pin is connected to the cathode of the LED.

- Pin - 3 - This pin is not used.


- Pin - 4 - this pin is connected to the emitter of the photo-transistor.

- Pin - 5 - This pin is connected to the collector of the photo transistor.

- Pin - 6 - This pin is connected to the base of the photo transistor. [1]


CHAPTER 3

DESIGN AND ANALYSIS

The inverter system is divided into several sections that perform specific functions. These

sections are:

i) Oscillator Section

ii) Driver Section

iii) Feedback Section

iv) System’s Monitoring Main Section

v) Data Selection and Conversion Section

vi) AT80C52 Microcontroller Section

vii) Changeover Section

viii) Charging Section

The rest of this chapter discusses the functionality, design and analysis of the inverter system on

a section-by-section basis.


3.1 OSCILLATOR SECTION


Fig 3.1 Internal configuration of SG3524 oscillator[3]

This section describes the operation of the oscillation section, with emphasis on the generation of

the 50Hz by the section. The battery supply (lead acid battery rated at 12V, 60A) is given to pin

15 of IC2 (SG3524) through the inverter power switch. Pin 8 of the IC is connected to the

negative terminal of the battery (ie. it is grounded). Pin 6 and 7 of the IC are oscillation section

pins. Frequency produced depends on the value and resistance at these pins. A 0.1uF mica

capacitor is connected to pin 7. The capacitance of the capacitor decides the 50Hz frequency

output by the IC. Pin 6 is timing resistance pin. A resistance at this pin keeps the oscillator

frequency constant. Preset (50K) is connected to the ground from pin 6 of the SG3524 IC. A

preset is used at this pin to ensure frequency can be constantly adjusted to 50Hz[3].


The frequency of the oscillator is determined according to the equation [7]:

f0 = (L = inductance, C = Capacitance)

But based on the phase shift principle, tuned circuits( LC circuits) are not an essential

requirement for oscillation. For devices that makes use of RC network circuits for frequency

generation,

fo =

i.e. crystal replaces the inductor for tuned circuits.

In SG3524, the crystal oscillator used is the Wien Bridge oscillator that is capable of generating

frequencies of 5Hz – 500Hz, low distortion, tunable, high purity sine wave generator.

Signal generated by oscillator section reaches the flip flop section of the IC. This section

converts the incoming signal into signal with changing polarity. This follows that, when the first

signal is positive, the secondary signal will be negative. This process is repeated 50 times per

second, i.e. an alternating signal with 50Hz frequency is generated inside the flip flop section of

the IC. This 50Hz frequency alternating signal generated by the oscillator IC is sent to pins 11

&14 [3]. This alternating signal is called “MOS drive signal. Voltage at these pins must be the

same so as not to damage the MOSFET at the output.


3.2 DRIVER SECTION

14 IRFP150N

IRFP150N

11

Q4 Q5

12V TO 230V 50HZ

TRAN-2P3S

12V 60 A / HR BATTERY

Fig 3.2: The Driver Section, showing the connection of the SG3524 to the MOSFET channels

which is further connected to the transformer.


MOS drive signals from pin 11 and 14 of the oscillator IC are fed to the gates of the MOSFET

via a biasing resistor. The resistors are needed for faithful amplification of the MOS drive

signals. The 50Hz alternating MOS drive signal reach each MOSFET channel separately,

resulting in the MOSFET channels being alternatively ON and OFF. When the first channel is

on, the second will be off, and when the second is on, the first will be off. This on/off switching

process is repeated 50 times per second.

The drain (D) of the two MOSFET channels are both connected together and tied to each end of

the transformer’s bifilar winding. The positive terminal of the battery is connected to the center

tapping of the bifilar winding. This results in the positive supply reaching the drain of each

MOSFET transistor, through each end of the bifilar winding.

Source terminal of each MOSFET is connected to the negative terminal of the batter through a

shunt (low value resistance). When the first MOSFET channel is on, the current flows through

the first half of the inverter transformer bifilar winding. When second MOSFET channel is on,

current flows through the second half of the inverter transformer winding.

This switching on/off of MOSFET channels will start an alternating current in the bifilar winding

of inverter transformer. This AC current in the bifilar winding will induce an AC current of

50Hz, in the 240V winding of the transformer.

From the SG3524,

VS = 2.7V

VGS = ?

IG = ?


Each MOSFET operates with a current rating of 15A.

Transformer – 1KVA (Apparent power)

Real power = Apparent power × Power Factor = 1000 × 0.8 = 800W

Battery rating = 60Ah / 12V

For the primary winding

P = IV

800 = Ip × 12V

Ip = 66.66A

Secondary winding,

800 = Is × 240

Is = 3.33A

A transformer of 800W/ 12V can sink up to 66.66A depending on the thickness of the coil.

Each MOSFET operates at 15A

If each channel has 2 MOSFETs

i.e. 15A × 2 = 30A

For 2 channels,

30 × 2 = 60A


60A < 66.66A

If MOSFETs used per channel,

15 × 3 = 45A

45 × 2 = 90A

Thus, MOSFET can comfortably accommodate 66.66A which is below 90A. The transformer

must have a thickness that must accommodate 66.66A without heating up at primary winding

and also accommodate 3.33A at secondary winding.

Thus, thickness of coil at primary winding must be higher that of the secondary.

From the transformer equation,

E.M.F. of primary coil, E1 = 4.44fN1 m

E.M.F. of secondary coil, E2 = 4.44fN2 m

f = frequency,

N1 = no. of primary turns

N2 = no. of secondary turns

m = maximum flux in core webers

Thus =


For idea transformer, input power = output power

Thus V1I1 = V212

= =

Based on the principle of transformer, the primary turns must be lesser than the secondary turns.

i.e. the higher the number of turns, the higher the induced voltage.

N1 = 27 turns

Thus, = = N2 = = 540 turns

Also =

N2 = = 540.486 turns

Thus power remains constant.

The waveform generated at the inverter output is termed modified sine wave. A sample of this

waveform is shown below:


Fig 3.3: Modified Sine Waveform

The modified sine wave inverter can be used to operate various electronic gadgets including

computers, motor-driven appliances, toasters, coffee makers, most stereos, ink jet printers,

refrigerators, TVs, VCRs, many microwave ovens, etc. [9]


3.3 FEEDBACK SECTION

6

5

4

1

2

SG3524

4N35 OPTOCOUPLER-NPN

1

1000uF

12V BATTERY 230V/12V 50HZ500mA

BRIGE RECTIFIER

PC802

FROM INVERTER OUTPUT

0.1uF

5K VR

RES-VAR

5KR

470R

Fig 3.4: The Feedback Section

Current, IR = 30mA

Voltage, VLED = 3V (max)

Vs = 15V

ILED =

30 X 10-3 =


R = = 400

Due to availability and tolerance, 470R was used.

Pulse width modulation is used to keep the inverter output at a constant 240V AC. The SG3524

receives feedback, as the value of the load connected to the inverter output socket change, the

width of the output from pin 11 and 14 will also change. This results in fluctuation of the inverter

output socket. Thus the inverter output is first converted by bridge rectifier to DC voltage. DC

voltage from the bridge rectifier is sent to pin 1 and 2 of opto coupler IC (4N35). When pin 1 and

2 receive supply, a LED inside the IC starts to glow and the light from this LED falls on the base

of a photo transistor inside the optocoupler. This causes the conduction of the photo transistor.

The collector of photo transistor is connected to pin 5 of the optocoupler and emitter is connected

to pin 4. Pin 5 of 4N35 recieves 12V supply from the battery. When the photo transistor

conducts, the supply at collector of photo transistor is output as feedback at its emitter ie. pin 4 of

the optocoupler.

Feedback at pin 4 of the optocoupler is given to pin 1 of SG3524 via a potential divider resistor

network. As explained earlier, pin 1 of oscillator IC is given feedback signal from the output

supply, pin 2 is given 5V regulated supply as reference voltage, through 4.7k resistors connected

in potential divider technique. This 5V reference voltage is taken from pin 16 of the osc. IC.

When the value of the load connected at the inverter output change, voltage at pin 4 of IC5 will

also change. This will result in variations in the feedback voltage reaching pin 1 of the osc IC.


Any change in the feedback signal reaching pin 1 of osc. IC will result in change in output from

pin 9.

Pin 9 of osc. IC is internally connected to the section, which controls the width of the oscillation

frequency. Change in the signal at pin 9 will result in change in the width of the output

frequency. This will in turn result in change in the 50Hz frequency output of pin 11 and 14.

This change in the width of 50Hz frequency will bring back the inverter output to its original

240V.

4

RV1 3

C2

5

Q4IRFP250

Q5IRFP250

Q7IRFP250

Q8

15

Q6

16

Q3

IRFP250

1

2

TR1

TRAN-2P3S

6

5

4

1

2

U2

OPTOCOUPLER-NPN

C41nF

BAT29V

TR2

TRAN-2P2S

C51nF

R5

7

R6

13

R712

R14

10

R15

11

C11nF

R9

6

RV2

RES-VAR

RV3RES-VAR R4

9

R8

8

Fig 3.5: Complete Circuitry for 12V DC/ 240V AC Voltage Generation


3.4 SYSTEM’S MONITORING MAIN SECTION

The 4066 Data Selector is essentially a quad bilateral switch. It can be likened to 4 completely

independent on-off switches, through which current flows in either direction. It constantly

monitors:

- Battery Voltage

- Inverter Output Voltage

- MOSFET Temperatures

It selects one of three data channels (output voltage, input voltage and temperature sensor)

sequentially depending on which enable pin is activated by the micro controller. The data

selector is connected to the Analogue to Digital Converter (ADC).

As stated earlier, the temperature is an atmospheric condition which left by itself is useless for

electrical applications. Transducers which convert physical data such as temperature, light

intensity, flow and speed to electrical signal become necessary in the circuit for this project; the

linear temperature sensor LM 35 was used [4]. The LM35 outputs 10mV per degree rise in

centigrade temperature i.e. 10mV/°C.

The maximum battery output voltage is 13.5V while the maximum inverted output voltage is

240V.


3.5 SIGNAL CONDITIONING AND DATA SELECTION SECTION


12V BAT

50K VR

LS4066

240V TO 12V 50 HZ

TRAN-2P2S

FROM INVERTER O/P

TO ADC

1000UF

GND3 +VS 1VO

UT

2

LM 35 FOR MOSFET CHANNEL 1

LM35

GND 3+VS1 VO

UT

2LM 35 FOR MOSFET CHANNEL 2

LM35

7805

50k vr

VI

1V

O3

GND 2

DATA SELECTOR

ENABLE SIGNALS frm CONTROLLER

1K

10k

1K1k

Fig 3.6: The Data Conditioning and Selection Section

The 220V/50Hz output from the inverter system is stepped down by a 240V/12V transformer.

This AC signal is rectified by a bridge rectifier and filtered by the 3000uf capacitor to produce

pure DC signal. Input V1 of 7805 voltage regulator is connected to the output of the rectifier,

while V0 of the regulator is connected to Vs of the LM35 temperature sensor. The 7805 ensures

that the voltage is kept at constant 5V while the LM35 temperature sensor is mounted at the

MOSFET channels to sense variations in temperature. These temperature variations are

converted to electrical signals. The signals are relayed to the ADC, the microcontroller and

finally to the LCD for display.

The output voltage of the LM35 is proportional to the Celsius temperature, with a scale factor of

10mV/°C. It requires no external calibration or timing and maintains an accuracy of +/-0.4°C at


room temperature and +/-0.8°C over a range of 0°C to +100°C. Another important characteristic

of the LM35 is that it draws only 60 micro amps from its supply and possesses a low self-heating

capability. The sensor self-heating causes less than 0.1°C temperature rise in still air. The output

voltage of the LM35 is converted to temperature by a simple conversion factor [ ]. The general

equation used to convert output voltage to temperature is:

Temperature (°C) = Vout * (100°C/V)

So if Vout is 1V, then Temperature = 100°C.

LM35 Vout is connected to an input pin of the 4066 data selector.

3.5.1 TEMPERATURE CONDITIONING

R1

0R1

R20R1

1K

1K

Fig 3.7: Potential divider network for temperature conditioning

=

=


1000(12 – V0) = 1000V0

12 – V0 = V0

12 = 2V0

V0 = 6V

Thus to get half Vcc, equal values of resistors are used in potential divider network.

But Vmax =2.56 is required as ADC output is 8bits thus 28 ie 256 steps.

N.B. – for the LM35 conditioning, 10mV/°C

Digital output =

350 C gives 350mV

360C gives 360mV

Thus step size = (360 – 350) mV = 10mV

Digital Output = = 35 = 001000112

3.5.2 BATTERY VOLTAGE CONDITIONING

The 12V input voltage from the battery is also fed through the potential divider network to the

input of the data selector.


12V BATTERY

50K VR

RES-VAR

1KR

Fig 3.8: The Battery Voltage Conditioning Circuit

Maximum voltage from fully charged battery = 14V (for a new battery of 12V)

With respect to the ADC, with a step size of 10mV,

Vmax or 14V must be conditioned to give an output of 140mV

i.e. 14V => 140mV

13V => = 130mV

Thus 13V => 130mV

Step size => 140mV – 130mV = 10mV

For 140mV i.e. 0.14V

A variable resistor is used to fix the output value at 0.14V. But what value of variable resistor

must be used?

If VR = 50K


=

=

14 – V0 = 50V0

V0 = = 0.275V

If 50K varies from 0 – 50K, just as voltage varies from 14V -> 0V

i.e. at 0 -> 14V

at 50K -> 0.257V

But 0.14V lies between 0V and 0.257V

Thus 50K can be used.

3.5.3 OUTPUT VOLTAGE CONDITIONING


The inverter output voltage from the bridge rectifier is filtered and connected through the

potential divider network to input of the 4066 data selector as shown in the following circuit:

TR1

TRAN-2P2S

BR1

PW01 C11nF

R11k

RV1

RES-VAR

240/12V

100uf 50K

1K

0.15V

Fig 3.9: The Output Voltage Conditioning Circuit

=

15V – V0 = = 0.294

50K (VR) can also be used.

15V -> 150mV


14V ->

= 140mV

Step size = 10mV

Digital output =

Digital output = 150mV / 10mV = 15

Three values are required to be converted by the analogue to digital converter but the ADC can

convert a single value at a time. Thus a quad (i.e. 4 input – 4 output) data selector 74LS4066 is

incorporated. It selects the analogue signals individually and sequential and sends it to the ADC.

The outputs of the data selector are tied together to give a single output which is then fed into the

Vin of the ADC.

Circuit Analysis

2R5

68R

3.3V10K VR

RES-VAR

+88.8Volts

Generally, zener diode operates at on and off state. The zener diode used in the circuit ensures

high stability of the output voltage at exactly 1.28V. For a zener diode of 3.3V, it becomes


5V

Fig 3.11: The zener diode connection

necessary to get a resistor values that will ensure that the zener diode is at ‘ON” state. For the

circuit, a 2.5Kohm and 10Kohm resistors are use in potential divider network.

Eo = I RL and I =

Eo = .Ei

3.3V – the breakdown voltage

Eo = = = 4V, with zener diode unconnected.

Since 4V > 3.3V, the zener diode is operating the ‘on’ state.

Voltage drop across 2.5K = 5V – 3.3V = 1.7V

Current through R = = 0.68mA

Load current IL = = = 0.33mA

I = IL + IZ


IZ = 0.69mA – 0.33mA = 0.35mA

R = = 1.65K

But because of availability, 2.5K was used. Basically, a 10K resistor was used to adjust the

output voltage to 2.8V.

i.e. Eo = Vz = = = 4V

If RL= 0, IL = 0

EO = = = 2V

And 2.8 lies between 4V and 2V.

For free running mode, the chip select, the read, the write and the interrupt pin (i.e. end of

conversion) are tied low to ensure automatic analogue signal conversion and data transfer.


CHAPTER 4

RESULTS AND DISCUSSION

After the design and construction of the inverter system, there arises a need to conduct various

tests to verify its functionality, reliability and efficiency. This chapter discusses the testing of the

inverter system and the implications of the test results.

4.1 TESTING AND RESULTS

Certain requirements are needed for testing a new inverter section. These are:

Connect a fully charged battery to the inverter.

A current meter or ampere meter is required to check the load current and charging

current. For this, connect a 50A meter in series between the positive terminal of battery

and the positive terminal of inverter.


Connect a 0 – 300V AC voltmeter parallel to the inverter output socket to test the output

voltage.

To check the AC supply frequency of inverter output, connect a oscilloscope parallel the

output socket.

The following steps were taken to test the operation of the inverter system.

The battery was connected and inverter was switched on .The 12V supply from battery

reached pin-15 of the oscillator SG3524.

The MOS drive voltage at pin 11 and 14 of the SG3524 were tested using a multimeter

set in the 10V AC range. The voltage reading for each of the pins was 3.5V. Any

difference in or absence of voltage at these pins would indicate a fault in the circuitry.

The gates of each MOSFET were tested for the availability of MOS drive signal.

To test for the functionality of the entire system, it was loaded with a 60W bulb and

switched on. The bulb glowed, indicating the presence of output at the secondary winding

of the transformer and the functionality of the inverter system.

The data conditioning and selection section were tested by activating each input pin of

the data select IC to ensure data transfer of conditioned signals.

The frequency of the inverter output supply was set and tested by connecting an

oscilloscope at the output socket. The frequency adjustment preset (50K connected to

pin-6 of the SG3524) is set to 50Hz. Any frequency outside to 50Hz ±1% range will

cause the inverter to operate inefficiently.

After setting the output frequency, the PWM voltage at the output socket was set. An AC

voltmeter was connected to the output socket and the PWM adjustment preset (4.7K) at


pin – 4 of the optocoupler 4N35 was adjusted. The preset was adjusted such that the

output voltage was 220V.

It is very necessary for the user of the inverter system to monitor the conditions of the

systems as any variation outside the pre-determined range can head to partial or complete

damage of the system’s circuitry. Because of the drastic nature of the system’s conditions, an

alarm was incorporated to alert the user when the conditions exceed the desired value.

4.2 PROBLEMS ENCOUNTERED

The oscillator section was first tested and it was discovered that the SG3524 was not

giving any output at its various output pins. Therefore, it was subsequently replaced.

The frequency generated by the oscillator was initially 72Hz, thus the 50K variable

resistor connected to pin 7 was adjusted while the output from pins 11 and 14 were

monitored using a cathode ray oscilloscope until it gave output of 3.5V/50Hz.

During the operation of the MOSFET section, one of the MOSFETs in the channel heated

up exponentially and burnt. Upon trouble shooting, it was discovered that there was a

short circuit between the gate and the drain due because excess lead was fitted off the

vero board. Therefore, the burnt MOSFET was the replaced.

Due to the manual winding of the transformer, there were copper losses (I2R). The

transformer was designed to give an output of 240V/3.3A with 527 turns but the output

was 205V as a result of losses. The transformer was consequently re-winded to produce

an output of 285V to compensate for the 45V not generated due to losses.

When the feedback section was isolated from the rest of the circuit and tested, an output

signal was detected at pin 4 of 4N35 IC. However, when the section was integrated with


the rest of the circuit, the 4N35 generated no output. Upon troubleshooting, it was

discovered that the optocoupler was damaged due to excess voltage (15V) from the

bridge rectifier. The excess voltage burnt the LED which is internally connected to pin 4

of the 4N35. The internal LED operates with a voltage of 3 – 4V and current of 30mA.

Thus a 470ohm current limiting resistor is connected in series with the 4N35 optocoupler

to keep the voltage and current in the proper range.

Short-circuiting of any component leads which often resulted in heating up and damage

of the component. This usually resulted from excess lead or internal short-circuiting of

the component due to excessive heat from the soldering iron heat during soldering.

Some electronic components were inferior and were not able to sink the amount of

voltage and current that is within the component specification. This resulted from

inadequate doping materials used during the manufacture of the components.

4.3 INSTALLATION

After the entire circuitry was tested and verified as fully operational. The circuit was packaged in

a metallic casing fabricated especially for it to form a complete system. Before loading the

system, a fully charged 12V/60AH battery was connected to the inverter DC V inputs using a 12

auto wire. The positive terminal of the battery was connected to the positive terminal of the

inverter. The same was done for negative terminal of the connection.

The 12mm auto wire is most suitable for 1KVA inverter system as lesser wire guage will heat up

and melt the wire and at a higher value, the current drawn from the battery and sent to the

inverter will exceed the required specification.


Also, the battery is placed close to the inverter system to minimize the current loss due to the

resistance of the wire. The mains AC supply was also fed to the system to charge the battery

when main power is restored.

4.5 LOADING

Before system was loaded, some factors were considered:

1) The power rating of the various appliances to be connected.

2) The optimum operational time of the inverter system for a 1KVA inverter system, the

total power rating of the system must not exceed 80% of the real power i.e. 800W

because of the two factors.

i) Each electronic appliance has two essential power ratings, the surge power

rating and the continuous power rating.

ii) The continuous power rating is the power required by the system for

normal operation. The surge power rating is power is required for the

initial start up of the appliance.

The continuous power rating is the typical power rating an electronic

appliance while the surge power rating varies depending on the appliance.

Television sets, refridgerators, air conditioning systems have a higher

surge, almost twice its continuous power rating, basically 60 – 80% while

iron fans, electric bulb has surge power rating close to the continuous

power rating basically 25% - 50% of continuous power rating.

For a 1KVA inverter system,


Real Power = 1000VA × 0.8 = 800 W

Loading must not exceed 80%

i.e. 800 × 0.8 = 640W

Actual loading:

Computer System = 300 W

3 Light Bulbs at 60W each = 60 × 3 = 180W

Fan = 60W

A 100W fluorescent lamp = 100W

Total = 640W

Operational Time

The operational time is the duration in hours that the battery can adequately supply the inverter

system on load without damaging the battery circuit. It depends on the total current consumed by

the individual loads.

Loaded Appliances current rating,

Computer system, P = IV

300W = I × 240 = 1.25A

Light bulbs, P = IV

180 = I × 240 = 0.75


Standing fan, 60W = I × 240

I = 0.25A

Fluorescent lamp, P = IV

100W = I × 240

I = 0.42A

Total current consumed = (1.25 + 0.75 + 0.25 + 0.42) A = 2.67A

The total current rating did not exceed the inverter output current rating of 3.33A.

Thus,

Operational time = = = 22hrs 28mins.

The efficiency of the system basically depends on the load at the output. Inductive loads

generally have a lower power factor. Thus excessive current is drawn from the system. This

reduces the efficiency of the system. Capacitive load has a higher power factor thus better

performance of the system.

Generally,

Efficiency = =


For capacitive load,

Real Power= IVcos = 1000 × 0.8 = 80%

For inductive load,

Real power = IVcosØ = 1000 × 0.6 = 60%

Average = % = 70 %

The efficiency of the system is as low as 70%. Thus, it generally affects the performance of the

system.

4.4 APPLICATIONS

The 1KVA PWM microcontroller-based inverter system generates a modified sine wave output.

Thus, equipment that senses voltage peaks or zero voltage crossings such as medical equipments,

motor speed controllers, laser printers and some battery chargers are not compatible with this

inverter system.


However, the modified sine wave inverter system can be used to operate a range of electrical

gadgets including bulbs, computers, motor-driven appliances, toasters, most stereos, ink jet

printers, refrigerators, VCD/DVD players, TVs, many micro wave ovens. The LCD display

feature enables output voltage, battery voltage and MOSFET temperature to be monitored,

making the system ideal for home and office use.


CHAPTER 5

CONCLUSION AND RECOMMENDATIONS

5.1 CONCLUSION

The inverter system is the ideal alternative source of electricity for our homes and offices. They

are renewable sources of energy which do not produce carbon emissions. Inverter systems also

operate noiselessly and require minimum effort on the part of the operator.

The inverter system designed for this project is rated 1KVA and is powered by a 60Ah 12V lead

acid battery. It features an LCD which displays battery voltage, output voltage and MOSFET

temperature. The system can initiate a shutdown in case of overloading or overheating.

This inverter system generates a modified sine wave output, not a pure sine wave output. Pure

sine wave inverters are expensive since they require large transformers to design. Thus, the

modified sine wave system cannot be used with equipment that senses voltage peaks or zero

voltage crossings such as medical equipments, motor speed controllers, laser printers and some

battery chargers. Pure sine wave inverters enable appliances to operate more efficiently and have

a long useful life. However, modified sine wave inverter systems are cheaper and can be used to

operate a variety of electric gadgets including computers, motor-driven appliances, toasters, toast

makers, coffee makers, most stereos, ink jet printers, refrigerators, TVs, VCRs, many microwave

ovens, etc.


The price of inverter systems in developing countries like Nigeria is highly unaffordable by most

electricity consumers. This factor has discouraged many from buying inverters and resort to

alternatives such as fuel/ diesel generating sets. These generating sets emit pollutants, such as

carbon monoxide, which are highly hazardous to the health and the environment.

5.2 RECOMMENDATION

Despite the numerous advantages of inverter systems in terms of portability, ease of use and

environmental friendliness, we are yet to realize the full potential of its application in our homes

and offices. Most electricity consumers are not well informed of the benefits that accrue from

the use of inverter systems. In this regard, awareness campaigns must be carried out to keep the

consumers conversant with the technology.

Cost is another factor hindering the use of inverter systems in the country. The price of inverter

systems is simply unaffordable for the majority. Therefore, it is recommended that more

investments into research and development should be carried out to produce cheaper and more

efficient inverter systems. The government must also support the science and technology sector

with funding to enable inverter systems to be manufactured locally, and at a lower price to the

consumer.

Finally, it is recommended that students should be encouraged to design and construct pure sine

wave inverter systems as projects. This will enable students to produce inverters with limitless

applications, without the drawbacks of the modified sine wave types.


REFERENCES

1. Anyakoha J.O “Senior Secondary School Physics”, Africana feb Publishers, Onitsha,

Nigeria, 2nd edition, 2005.

2. Etuk, K & Co. “Design and Construction of 1KVA Inverter System”, 2005.

3. Manahar, L “Modern Digital Inverter – Introduction, Servicing and Troubleshooting”,

BPB Publications, New Delhi, India, 1st edition, 2001.

4. Mazidi, A and Mazidi, C “The 8051 Microcontrollers and Its Applications”, Prentice Hall

Inc, 1st edition, 2000.

5. Okunta, N. & Co. “Design and Construction of 625VA DC/AC Inverter System”, 2002


6. Theraja, B.C. and Theraja, A.K. “A Textbook On Electrical Technology”, S. Chand and

Company Ltd, New Delhi, India, 23rd edition, 2003.

7. Theraja, B.C. “Basic Electronics (Solid State)”, S. Chand and Company, New Delhi,

India, 17th edition, 2007.

8. Sinclair, I “Dictionary of Electronics”, HarperCollins Publishers, Glasgow, UK, 2nd

edition, 2004.

9. What is the difference between pure sine wave and modified sine waves?

<http://wiki.answers.com> Retrieved on 28.12.2009

10. Kleitz, W “Digital Electronics, A Practical Approach”, Pearson Education Inc, Upper

Side River, New York, 7th edition, 2005.

12. Wikipedia, “Inverter (Electrical)”, on the Wikipedia Website at

<http://www.wikipedia.org/inverter_(electrical).htm> Accessed 3.06.2009

13. Snyder, E “Man and the Physical Universe”, Bell & Howell Publishing Company, 1st

edition, 1976.


http://www.wikipedia.org/inverter_(electrical).htm

http://wiki.answers.com/

inverter systems 2010

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