Design And Construction Of An Oscillating Stage Of A 3.5KVA Inverter

ABSTRACT

An oscillator is an electronic device that converts dc power supply to ac power having the desired characteristics.

Power inverters come in a wide variety of power capacities and they are distinguished primarily by the wave shape (sine or cosine, square and others as shown in the figure 2) of the alternating current they produced from the oscillator employed for the design. The major types are: 1. Pure sine-wave inverter, 2. Square wave inverter, and 3. Modified sine-wave inverter. The basis of the oscillation in an electric circuit is the positive feedback.

Oscillator falls into the following classes; Positive feedback (RC and LC networks), Negative resistance, and Relaxation type. The discussion of the oscillators in this paper will be focused on the low frequency oscillators that can be employed for the design of power inverters operating at 50/60Hz.

 CHAPTER ONE

1.0                                                        INTRODUCTION

DC to AC inverter is an electronic device which is able to convert a DC potential normally derived from a lead-acid battery into a stepped-up AC potential which may be quite comparable to the voltage that is found in our domestic AC Mains outlets. It’s a device which converts or inverts a low voltage, high DC potential into a low current high alternating voltage such as from a 12V automotive battery source to 220V AC output.

The basic principle behind converting a low voltage, DC to high voltage AC is to use up the stored high current inside a DC source and step it up to a high voltage. This is basically achieved by using an inductor, which is primarily a transformer having two sets of winding namely primary (input) and secondary (output). The primary winding is meant for receiving the direct high current input while the secondary is for inverting this input into the corresponding high voltage low current alternating output.

By alternating voltage we mean a voltage which switches its polarity from positive to negative and vice versa many times a second depending upon the set frequency at the input of the transformer.

Generally this frequency is a 50Hz or 60 Hz depending upon the particular country’s utility specs. An artificially generated frequency is used at the above rates for feeding the output stages which may consist of power transistors or MOSFETS or GBTs integrated with the power transformer. An inverter normally may be divided into three important stages viz. oscillator, amplifier and the transformer output stage. The frequency which an inverter used to operates always been generated by oscillating stage of the inverter.

This stage is basically responsible for the generation of oscillating pulses either through an IC circuit or a transistorized circuit.

These oscillations are basically the productions of alternate battery positive and negative (ground) voltage peaks with a particular specified frequency (number of positive peaks per second.) Such oscillations are generally in the form of square pillars and are termed as square waves, and the inverters operating with such oscillators are called square wave inverters. The above generated square wave pulses though are too weak and can never be utilized to drive high current output transformers. Therefore these pulses are fed to the next amplifier stage for the required task.

1.2                                             OBJECTIVE OF THE PROJECT

Oscillating stage of an inverter is a stage basically responsible for the generation of oscillating pulses either through an IC circuit or a transistorized circuit. This work focused on the low frequency oscillators that can be employed for the design of 3.5KVA power inverters operating at 50/60Hz.

1.3                                         SIGNIFICANCE OF THE PROJECT

Oscillating stage of an inverter is basically responsible for the generation of oscillating pulses either through an IC circuit or a transistorized circuit. It is cheap and simple to design because it contains few electronics components with some Resistors and Capacitors as the major generating components. It provides good Frequency stability. The Circuit is simpler than other OSCILLATORs.

1.4                                                 SCOPE OF THE PROJECT

An inverter uses the power from a huge battery to create 50Hz or 60Hz at the same voltage as the electricity in homes but the waveform can be a square-wave for a very cheap circuit and many electronic products won’t work from it, or the waveform can be a modified sinewave that is a little more complicated but nearly all products work from it, or the waveform can be a pure sine-wave which is made with a complicated circuit. The oscillator in the inverter creates the 50Hz or 60Hz.

If you use a sine-wave oscillator driving a linear power amplifier then the amplifier wastes a lot of power as heat. The battery current is double what a modern circuit would be. A modern sine-wave inverter uses a high frequency voltage step up circuit that has a tiny transformer and a rectifier to make high voltage DC. Then another high frequency high voltage circuit uses pulse-width-modulation to create a stepped sine-wave then a small high frequency filter smoothes the steps into a pure sine-wave. The parts that operate at high frequency barely get warm because they switch on and off.

1.5                                          APPLICATION OF THE PROJECT

Apart from using oscillator in an inverter, it can also be used:

• as clock as it generates periodic waveform.
• In uninterruptible power supply(UPS)
• In multivibrators
• the sounds produced by electronic beepers
• signals broadcast by radio and television transmitters,
• clock signals that regulate computers and quartz clocks,
• video games.

1.6                                          LIMITATION OF THE PROJECT

• operates on low power
• Inverter oscillator can only be produced by configuration of CMOS integrated circuits or an arrangement of power transistors
• The output depends on the feedback.
• Any little change in the circuit (that is capacitor or resistor) will affect the output frequency.