CHAPTER ONE

INTRODUCTION

This report focuses on DC to AC power inverter, which aim is to efficiently transform a DC power source to a high voltage

AC source, similar to power that would be available at an electrical wall outlet.

Inverters are used for many applications, as in situations where low voltage DC sources such as batteries, solar panels or fuel

cells must be converted so that devices can run off of AC power. One example of such a situation would be converting

electrical power from a car battery to run a laptop, TV or cell phone.

The method, in which the low voltage DC power is inverted, is completed in two steps. The first being the conversion of the

low voltage DC power to a high voltage DC source, and the second step being the conversion of the high DC source to an

AC waveform using pulse width modulation. Another method of completing the desired outcome would be to first convert

the low voltage DC power to AC, and then using a transformer to boost the voltage to 220 volts. This project focuses on the

first method described and specifically the transformation of a high voltage DC source into an AC output.

Again, in each and every field you will find some electrical and electronic device, be it in general household use or in some

specialized industrial use. These electrical or electronics devices require electrical power for their operation and most of

these devices are very particular about the quality of the power given to them. If the power given to these

electrical/electronic devices can get damaged.

Also, these devices will not be of any use if you do not provide them with proper power supply.

In Nigeria a standard electrical/electronics equipment works on 230v/50Hz A.C power supply. This power supply should not

contain any problem such as spike, noise etc, otherwise it could damage the equipment. In a developed country are good

qualities without any of the above problems, but in a developing country like Nigeria, the power provided by PHCN might

contain some of the problems. Also power cuts and line problems are very frequent which gets worse in some seasons.

In a situation like this, an efficient inverter such as the one described on this work could be adopted to bridge the gap

between power failures. Inverter produces a good quality A.C power supply which could be used to power most electrical

electronics devices.

1.1 OBJECTIVE

The primary aim of this project is to produce a very efficient, reliable and cost effective device that can convert DC

Voltage (battery) to AC power supply with the following features:

i. Automatic change over

ii. Automatic charging system

iii. Battery full cut off

iv. Automatic mosfet cut off

v. Feedback mechanism

1.2 SCOPE AND LIMITATION

The scope of this work is to represent theory, design and analysis, and construction of an inverter that convert a 24 volts DC

to 220 volts AC at a frequently of 50 hertz and produces a power of 5000 watts. However, a research of this nature was never

without limitation. Some factor that poses difficulties to the construction on this project includes inconsistency in power

supply. In addition, the finance at disposal was inadequate. Furthermore, insufficient research material poses a problem.

In order to achieve the objective of the project, we studied some areas that are related to the project such operations of a

transistors, oscillators, transformers, relays, diodes to mention but a few and had interview with the people currently making

use of the project to understand their difficulties.

1.3 EXPECTED RESULTS

The project is expected to produce a 220V ac supply from a 24V Battery having the same frequency as that of the PHCN

supply and with the aforementioned features.

CHAPTER TWO

LITERATURE REVIEW

2.1 BACKGROUND

In the world today there are currently two forms of electrical transmission, Direct Current (DC) and Alternating Current

(AC), each with its own advantages and disadvantages. DC power is simply the application of a steady constant voltage

across a circuit resulting in a constant current. A battery is the most common source of DC transmission as current flows

from one end of a circuit to the other. Most digital circuitry today is run off of DC power as it carries the ability to provide

either a constant high or constant low voltage, enabling digital logic to process code executions. Historically, electricity was

first commercially transmitted by Thomas Edison, and was a DC power line. However, this electricity was low voltage, due

to the inability to step up DC voltage at the time, and thus it was not capable of transmitting power over long distances4.

V =IR

P=IV=I 2R (1)

As can be seen in the equations above, power loss can be derived from the electrical current squared and the resistance of a

transmission line. When the voltage is increased, the current decreases and concurrently the power loss decreases

exponentially; therefore high voltage transmission reduces

Power loss. For this reasoning electricity was generated at power stations and delivered to homes and businesses through AC

power. Alternating current, unlike DC, oscillates between two voltage values at a specified frequency, and its ever changing

current and voltage makes it easy to step up or down the voltage. For high voltage and long distance transmission situations

all that is needed to step up or down the voltage is a transformer. Developed in 1886 by William Stanley Jr., the transformer

made long distance electrical transmission using AC power possible. Electrical transmission has therefore been mainly based

upon AC power, supplying most American homes with a 120 volt AC source. It should be noted that since 1954 there have

been many high voltage DC transmission systems implemented around the globe with the advent of DC/DC booster,

allowing the easy stepping up and down of DC voltages6.

Like DC power, there exist many devices such as power tools, radios and TV’s that run off of AC power. It is therefore

crucial that both forms of electricity transmission exist; the world cannot be powered with one simple form. It then becomes

a vital matter for there to exist easy ways to transform DC to AC power and vice versa in an efficient manner. Without this

ability people will be restricted to what electronic devices they use depending on the electricity source available. Electrical

AC/DC converters and DC/AC inverters allow people this freedom in transferring electrical power between the two.

In the past, pulse width modulation techniques were employed in voltage source and current source inverter only.

Availability of self-commuted devices, such as power transistor, Mosfets and GTOs, have made pulse width modulation AC

to DC converter also popular in many applications. The steady state and dynamic performance of inverters and AC to DC

converters and DC and AC drives are significantly dependent on the pulse width modulation techniques.

Pulse width modulation (PWM) of a signal or power source involves the modulation of its duty cycle, to either convey

information over a communication channel or control the amount of power sent to a load.

Battery Oscillator Mosfet drivers Buffers Transformers

Change overOutlet socket

Currently whatever work you do, in each and every field you will find some electrical/electronic devices in general

household use or in some specialized Industrial use. The electrical/electronic device require electrical power for their

operation, the devices will not be of any use if you do not drive them with proper power supply. In Nigeria the standard

electrical/electronic equipment works on 220V/50HZ AC power supply. This power supply should not contain spikes, noise

etc which could lead to damage of the equipment.

With this direct current (DC), to alternating current (AC) power inverter people now have an alternative to power supply

when there is an outage thereby reducing irregularities of power supply by the National power body (PHCN) and since this

device can also be used in remote areas were there is no electricity/power, the commutation problems associated with

generator are eliminated.

CHAPTER THREE

METHODOLOGY AND CIRCUIT DESCRIPTION

3.1 METHODOLOGY

The construction of the PWM inverter can be complex when thought as a whole but when broken up into smaller projects

and divisions it becomes much easier to manage.The divisions comprises of the Inverter DC-AC, Battery charger, Battery

full cut off, feedback system, Mosfet cut off, transformer, automatic change over and delay system. The above divisions will

be design separately and assembly to achieve the desired aim. The above divisions were critically searched through several

ways which includes internet, resource text book, and consultation. The specific operation and construction of each block

will be discuss in the subsequent sections.

Fig. 3.1 Simple inverter block diagram

3.2 MATERIALS

In this project design, tools and components are carefully and properly selected. Materials for design should be done in a

proper and careful manner because of the major role it plays in performance of any designed product and its marketability.

If a product is designed at a very high cost, no matter how perfectly it was designed, it might meet consumers’ resistance in

terms of product purchasing.

For a proper material selection, we have to put these into consideration: Availability of materials or components in the area

where units is to be constructed. Reliability of the units components parts, this leads to the choice of components to be

selected for a particular project design or construction. Another of our consideration should be ensuring that materials best

meet the requirements.

This section explains what materials, equipments and instruments that will be used in executing the project in particular.

Our main objective here is not only to obtain an output but to achieve the desired output and qualities which would be

affordable for everybody. To achieve this, we have to put caution in our choice of components to make use of a few

components and so make it cheaper.

The materials and tools employed includes soldering iron, casing, drilling machine, saw, to mention but a few. Most of the

mechanical works such as drilling, cutting painting, etc were taken to a welders shop.

3.3 CIRCUIT COMPONENT DESCRIPTION

a. TRANSFORMER

Transformer is a device which steps up or down a.c signal to any required magnitude. The transformer operation is based on

the principal of electromagnet induction, which states that e.m.f is induced whenever there is a change in the flux flowing in

the electromagnetic circuit. The transformer has two coils the primary and the secondary. The coil which makes

electromagnet is called the primary coil while the outer coil into which the current is induced can be called secondary coil.

The process of inducing current in the in the secondary coil because of change of current in the primary is called induction.

The induced current in the secondary coil changes at the same rate which the current in the primary coil is changing.

The current in the secondary is generated by induced voltage across the secondary winding. The magnitude of this induced

E.M.F. is proportional to the rate of change of the flux i.e. the line of magnetic force field, of the magnetic field generated by

the primary. With the induce emf of the secondary coil, a self induced EMF is generated in the primary coil as well. This

EMF is induced by the primary coil to oppose the applied EMF. This generation of self induced EMF in the primary coil is

known as “self induction” when current is induced in a coil due to change of current in another nearly coil, it is known as

“Mutual Induction”.

Hence, a transformer is a device which works on the principles of mutual induction. A transformer needs two coils, which

wound on a laminated core. These coils are call:

- Primary coil and

- Secondary coil.

Fig. 3.2Transformer basic diagram

24V

24V

0V

240V

220V

0V

The coil to which the AC supply is provided is called primary coil/winding while the coil in which EMF is induced, and

from which the o/p is taking is called secondary coil.

In the transformer electric energy is transferred from one circuit to another circuit. During this transfer the current and the

voltage can be changed i.e., they can be increased or reduced. There is no direct electrical connection between the primary

and secondary coil in the transformer.

The voltage generated in the secondary coil depends on the ratio between the number of turns in the primary and number of

turns in the secondary. Below, is the relationship between voltage, current and number of turns in the coil:

E1 = N1 = I2 = R1

E2 N2 I1 R2

Where;

E1 = Input voltage to primary coil

E2 = Output voltage to secondary coil

N1 = Number of turns in primary coil

N2 = Number of turns in secondary coil

I1 = Current in primary

I2 = Current in secondary

R1 = Resistance presented by primary winding to the source

R2 = Actual load resistance.

The kind of transformer employ in inverter design and construction is bifilar winding transformer.

Fig. 3.3 Inverter transformer (bifilar)

As seen in the diagram above, it is designated 12v – 0v – 12v, 24v – 0v – 24v, etc on the primary side. 0v centre tapping is

connected to the positive terminal of the battery while each channel of the MOSFETs, in the o/p section is connected to the

24v – 0v – 24v.

b. BATTERY AND ITS MAINTENANCE

A battery is something that supplies DC power through chemical reaction. The kind of battery employ in inverter

design are those capable of relaying after exhausting the stored charge in them. In this work, 24volts transformer is adopted

hence two 12v battery are connected in series to obtain 24v to improve the duration of an inverter over a load, the battery can

be used to build what we refer to as POWER BANK.

Which are several battery connected in series to increase the voltage and in parallel to increase the power.

The life of battery gets reduced with time owing to some factors ranging from undercharging, overcharging, rapid high

current discharge to mention but few.

The under listed point should be adopted to increase the life span of the battery:

i. Overcharging: when a battery is overcharged, active material lead peroxide on the positive plate of the

battery starts to fall. This damages the plate grids and the life of the battery is shortened. Overcharging

also generates heat. This heat evaporates the water (if a wet cell is used) in the electrolyte taste, if the

water is not restored regularly, the damages the battery plates and battery performance suffers.

ii. Undercharging: Protect the battery from undercharging; if the battery is

undercharged regularly, sulphate starts to accumulate on the positive plates of battery.

iii. Leaving the battery unused: During the discharge cycle lead sulphate is formed on the battery plates. If

the battery is allowed to remains in a discharged condition for a long time, these deposits harden. This

battery then no longer responds to the recharging process, it cannot be restored to its full capacity. If the

sulphation extends for a long period, it makes the battery useless.

iv. Damage to the battery container: If the battery is not handled properly, or if the battery sustains a fall,

or if the nuts and bolts are too tight or too loose then the battery plates and separators can get damaged.

Also, the electrolyte could leak from the container making the battery unrepairable.

v. Forget to add water: When the water content of the electrolyte reduces, its acid content increase, if the

acid concentration increases beyond a limit, it would damage the battery plates. To save the battery from

this type of condition, timely check electrolyte level and the specific gravity of the electrolyte. Add water

into the electrolyte, if the required ratio of water is not found. One should regularly check the water the

water level in a battery.

vi. Use only distilled/pure water: If the water used to restore the loss of water from the electrolyte is not

pure then this could increase the impurities in the electrolyte. Main impurities due to impure are iron and

chlorine, these impurities attack the battery plates and the battery life gets reduced. The chlorine in the

tap water can damage the battery separator. Use only pure/distilled water to restore the electrolyte. If

distilled water is not available use cooled boiled water.

c. AUTOMATIC MOSFET CUT OFF AND CHARGING SYSTEM

The 24v and center tap sides of the transformer which suppose to join to the drain of the mosfet is connected to the common

of a relay while the drain of the mosfet is connected to the normally close of the same relay .The a.c input of the bridge

rectifiers is connected to the normally open of the relay. When inverter is operating, the signal from the mosfet is extended to

the transformer via the normally close of the relay but when ac mains is available the relay switches to the normally open

while the step down ac output is extended to the bridge rectifier via the normally open.

Fig. 3.4 Automatic Mosfet cut off with charging system

d. BATTERY CHARGING:

Charging is the process of restoring the exhausted charges from a rechargeable battery.

The charging section in this our inverter system comprises of a step down transformer (which is the same inverter

transformer), Relay, rectifier & capacitor. The relay is connected to the secondary side of the transformer as shown in fig

(3.4). The drain of the MOSFET is connected to the normally connected of the relay while the secondary side of the

transformer is connected to the common of the relay. The connection is such that when the AC power is available, the relay

will be energizing to cut off the MOSFET and switch to the normally open. Hence, the transformer secondary output appears

at the normally open of the relay.

Fig. 3.5Full Bridge rectification

Fig.3.6Action of a Filter

-VE

+VE

The step down A.C. power on the N/O of the relay is now extended to the rectifier, which converts the A.C. supply into a

D.C. supply:

The three most common types of rectifier circuits are

half wave rectifier

full wave rectifier type

full wave bridge rectifier type

The one employed in the designed project is full wave rectifier. In this rectifier, four diodes are connected across

the secondary winding in a special arrangement of diode called “bridge arrangement”, as shown in fig. 5.1.

The so called D.C. output from the rectifier is containing ac ripple which may damage/reduce the life span of the battery.

Hence, it is passed through a capacitor. A capacitor is connected between the output terminals of rectifier to act as a filter.

This give a D.C. output with reduced ripple. The output of the filter is what charges the battery.

e. BATTERY FULL CUT OFF

Fig. 3.7 Battery Full cut off circuit

For its operations, the inverter gets voltage and current from battery. When the battery becomes fully changed, my further

voltage added to it damages the battery. Thus, when the battery voltage goes up to a set limit, the charger should be cut off.

A fully charged battery voltage will show voltage of around 13.5V (for 12V battery) or 26.5V (for 24V battery).

LM 324 was used to implement the design.

Fig. 3.8 LM324 internal circuitry

LM 324N is an operational Amplifier (op-Amp) 1C. it has four different Op-Amp units, which can be used individually.

Output

+

-Inverting

Input

Non-InvertingInput

Fig. 3.9OP AMP

+

_

R4

R2

R1

V-

V+

R3

+ Vb

Vcc

OUTPUT

OP Amp

Operational Amplifier plays a big role in the PWM based inverter.

The OP-AMP is used as a comparator in the inverter circuit. Each OP Amp has 3 pins, two for input and one for output. Out

of the two input pins, one is called inverting (-) input and other is called non- Inverting (+) input.

Non- Inverting Input (+)

If the input given to this pin is less than the input given to the inverting input pin, then the op-Amp output will be low. If not,

the output will be high.

Inverting input (-):

If the input given to this pin is less than the input given to the non-inverting input pin, then the OP Amp output will be high.

If not, it is LOW.

So, we can see that the OP-Amp output depends on the values of the signal not its input pins. This property of the OP-AMP

makes it useful in comparator in Inverting circuit.

As can be seen from the figure above, a fixed reference voltage, 6v, is at the non- inverting input. A variable voltage is

placed at the inverting input which is dependent on the battery voltage and variable (10k) resistor.

Mathematically

but (4.1)

(4.2)

In the work, the values of the resistors were choosen to fixed a 6v on the non – inventing input of the op-Amp and to fixed a

voltage greater than 6v wherever the battery is fully charged.

Provided the circuit is arranged as shown in the figure above when ever there is AC power supply and the battery is not fully

charged, V+ > V-. Thus, a HIGH is at the output which activates the LED, D5 to show that the battery is charging. The

HIGH cannot bias the PNP transistor hence no voltage is at the collector emitter junction. Immediately, the battery is fully

charged, V- becomes greater than V+. Thus, a low biases the PNP transistor, hence collector emitter voltage energized the

relay to switch to N/O and cut off the charging supply.

f. THE OSCILLATOR

Fig. 3.10 Oscillator circuit with fan control

The oscillation section of this inverter uses a very common PWM controller IC SG3524. This IC is used to generate the

50HZ frequency required to generate A.C supply by the inverting DC supply from battery to AC.

To start this process, battery supply is given to the pin 15 of SG3524 via the control relay, RLA, and inverting ON/OFF

switch.

Pin 8 is connected to the negative terminal of the battery. Pin 6 and 7 of the IC are oscillation section pins. Frequency

produced by the IC depends on the value of the capacitor and resistance at these pins. A variable resister is employ to serve

as a preset to adjust the frequency output to a constant 50 Hz.

The signal generated by oscillator section within the SG3524, reach the frequency section of the SG. This section converts

the in coming signal into signal with changing polarity. The output is a two signal with changing polarity, when the first

signal is positive, the 2nd is negative. This process is repeated 50 times per second, i.e. an alternating signal with 50 Hz freq

is generated inside the flip flop. This 50Hz frequency alternating signal is output at pin 11 and 14 of the SG. This alternating

signal is known as “MOS drive signal “. The signal voltage is between 3-4v, any difference could change the MOSFET.

The MOS drive signals are given to the base of MOS drive T1 and T2. This result in the MOS drive signal getting separated

into two different channels. Transistor, T1 and T2 amplify the 50HZ mos drive signal at their base to a sufficient level output

then form emitter, the 50hz signal from the emitter of T1 is given to the gate of each MOSFET in the first MOSTET channel

and the same 50hz signal from T2 is applied to the gate of the second mosfet channel.

Fig. 3.11 Complete Inverter circuit without mosfet driver, cut off but with automatic changeover

g. OUTPUT SECTION

The 50Hz alternating MOS drive signal reach each MOSFET channel separately the result in the MOSFET channels being

alternatively ON/OFF. When first channel is ON, the 2nd will be OFF, and when the 2nd is ON the 1st will be OFF. This

ON/OFF switching process is repeated50 times per second. DRAIN.D, of all the MOSFET of one end are connected

together. The 24 volts wire of the bifilar winding is connected to the drain. The Drain of the MOSFET of the second channel

are also connected together and the other end of the inverter transformer’s bifilar winding is connected to this connection.

Positive terminal of the battery is connected to the centre tapping of the winding. The source terminals of the two MOSFET

channels are connected together to the negative terminal of the battery.

When the first MOSFET channel is ON, the current flows through first half of the inverter transformer bifilar winding.

When second MOSFET channel turns ON, the current flows through second half of the inverter transformer winding. This

switching ON/OFF of the MOSFET channels will start an alternating current in the bifilar winding of the inverter

transformer. This AC current in the bifilar winding will induce an AC current of 50Hz, in the 270v tapping of the

transformer. The AC voltage input from 270v tapping of secondary winding is regulated to 220v by feeding it to an upto

coupler IC, 4N35.

To make the PWM work, the PWM, SG3524 should receive a feedback of AC supply generated by the inverter circuit. If the

SG3524 does not receive feedback, then as the value of the load connected to the inverter output socket changes, the PW

output from PIN 11 and 14 will also change. This results in fluctuation of the inverter output supply at its output socket.

3.4 FEEDBACK

To provide feedback to the PWM controller, the AC voltage generated at the 270v winding of the inverter transformer is

given to pin 3 of the connector CN14. this AC voltage at pin 3 of CN4 is given to dropping resistance, R and then connected

into DC voltage by bridge rectifier D1 – D4. DC voltage from the bridge rectifier is sent to pin 1 & 2 of the opto coupler,

4N35. where pin 1 & 2 receive supply, a LED inside the IC starts to glow, the light from the LED falls in the base of the

photo – transistor inside the 4N35. this will conduct the photo- transistor.

Collector of photo-transistor is connected to pin 5 of 4N35 & the emitter is connected to pin 4. Pin 5 of 4N35 receives 12v

supply from the battery when the photo transistor conducts; the supply at the collector of the photo-transistor is output as

feedback at its emitter that is pin 4. feedback signal at pin 4 is given to Pin 1 of SG, through a potential divide circuit made

of 100k and PWM adjustment present, VR. PIN 1, 2 and 9 of the SG are pins 7 or OP Amp, pin 1 and 2 are input pins & pin

9 is output. As explained earlier, pin 1 of SG is given feedback signal from the supply, pin 2 is given 5v regulated supply as

reference voltage.

When the value of the load connected at the inverter output changes, the voltage at pin 4 of 4N35 will also change. This will

result in variations in the feedback voltage reaching pin 1 of SG will result in change in output from pin 9. Pin 9 of IC2 is

internally connected to the section, which controls the width of the oscillatory frequency. A 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 50Hz freq output at pin 11 and 1.

This change in the width of 50Hz frequency will bring back the inverter output to its original 220V. Preset VR connected to

the pin 4 of 4N35 is used to set the inverter output by changing the width of the 50Hz frequency signal.