Design And Construction Of 25 Amps Single Phase Automatic Change Over

ABSTRACT

The Changeover Switch is a device used to switch off a power supply and subsequently switch on another power supply

An Automatic Changeover Switch switches off the generator and automatically changes over to NEPA. This Automatic Changeover Switch will automatically switch off your generator and change over to MAIN (Public power supply/NEPA) on resumption. You will save fuel and no risk of electrocution. It also serves as power surge protector for appliances.

The main aim of any electric power supply in the world is to provide uninterrupted power supply at all times to all its consumers. Although, in developing countries, the electric power generated to meet the demands of the growing consumers of electricity is insufficient, hence power instability and outage.

TABLE OF CONTENT

 Title page

Approval page

Dedication

Acknowledgement

Abstract

Table of content

CHAPTER ONE

• introduction
  • general description and application
  • aim/objective of the project
  • significance of the project
  • purpose of the project
  • statement of problem
  • limitation of the project
  • application of the project
  • definition of terms
  • project organisation

• block diagram representation

CHAPTER TWO

• Theory Of Operation
  • The General Starter
  • The Change Over Switch
  • The General Turn Off Switch
  • The Power Supply

CHAPTER THREE

COMPONENT DESCRIPTION AND RATING

THE  COMPONENT USED FOR THE PROJECT ARE LISTED

• Definition And Analysis Of Component And Ratings
  • The Component Used For The Project
  • Power Transformers
  • Relays
  • Contactors
  • Battery
  • Switch
  • Capacitors
  • Capacitor Ics
  • Logics Ics
  • Timer
  • Diodes
  • Indicator lamps
  • Circuit breaker
  • Wires jumpers and connectors
  • Transistors
  • Resistors

CHAPTER FOUR

• Circuit Analysis
  • The Voltage Converters
  • The Comparators
  • The Logic Gates
  • The Generator Starter
  • The Change Over Switch

CHAPTER FIVE

CONSTRUCTION AND ECONOMIC OF THE PROJECT

• Construction Of The Casing
  • Economic Of The Project
  • Reliability
  • Maintainability
  • Project Evaluation

chapter six

• Testing
  • Trouble Shooting And Remedy

CHAPTER SEVEN

CONCLUSION

REFERENCES

Chapter one

• INTRODUCTION

as the quest for change over on public power supply and stand by generation increase. Engineering have researches deep into the different methods of achieving to both manual and automatic types.

It may not be taken seriously in some places where power supply is up to 90 constant or where intermittent outage causes no great damage. However, place where they have business centers, industrial centers and even cold rooms need to almost 100% supply. So in this case there is need to provide a standby power supply in the event of power outage. The most important task is just to provide a standby power supply by great attention is paid to the reliability of the system. That is immediately and changes over without fluctuations.

The first type of change over switch was purely manual operated. They usually have three switch able position, main, off and generator. An arm is provided from where the operator can have access to the switching. Here, the effectiveness of the switching (i.e how fast it is effected each time power needs to be changed) largely depends on the operator. The change over system will be ineffective when the operator is not around.

The second generation of change over switches were made semi-automatic in which the mains power supply directly powers the contactors. This arrangement is wired is such a way that if the mains power supply fails, the contractors will all drop leaving the supply to the generator position. The effectiveness of the system will now depend on the operator top start up the generator on time.

The main problem suffered by the above mentioned two versions of the change over switch is that the operator must have to be continuously on duty for it to be very effective. They lent use full closed loop system to operate.

The present day technology has made it possible for the change over switch to come in a number of designs switch using the closed loop system at feed back control. Some of them use comparators, logic circuits and electronic timers to achieve the task. Some even used simple micro processor chips to function.

The design of this project is such that a combination of comparators digitals switches (logic IC), electronics timers and electromagnetic switches are used to achieve a very fast and effective change over in this case. This  project was entirely constructed and the thesis written by Nwalua Chukwukereze under the supervision of Engr. Ihedioha in partial fulfillment of the requirements for the award of Higher National Diploma in Electrical Electronics Engineering.

The automatic change over switch makes use of protective relays and contractors among other things which constantly monitor the power system to ensure maximum continuity of the electrical service with minimum damage to life and property. They are found mostly in domestic installations, hospital threatres, conference halls, computer centres, data and research centres, laboratories and industries that require three phase constant power supply. This equipment however is used to protect some house holds appliances from electrical faults which may arise due to voltage drop. It was designed and constructed purposely to alleviate the suffering encountered by the consumers following supply authority’s consistent failure to meet its obligations of constant and uninterrupted power supply. A circuit breaker is in corporate as a back up protection to improve the reliability of the system and to meet it’s specification, although more back-up protection could have been incorporated monitoring the supply but due to financial constraints, it has to be limited to this operation. The schematic diagram of the automatic change over switch was clearly explained as far as possible to be apprehended by any person within or outside the engineering field. It is my intention however to present the work to Institute of Management and Technology (IMT) students in Engineering and to the general public for consumption.

 1.1             GENERAL DESCRIPTION AND APPLICATION

BRIEF DESCRIPTION: The change over switch is said to be automatic because it employs full closed loop system. It has comparators that test both the input and output conditions and then uses the information to switch the relays to the appropriate positions.

There are three important switching position which are:

• Switching of the contactors (i.e C/O)
• Switching of the generator started if power fails
• Switching off the generator as soon as power is restored

The system uses 3 comparators, one for each phase to test the voltage conditions to a certain full voltage, low voltage or even power failure. When the 3 phase voltage are okay between 190v and 230v not neutral, the voltage from the converters to the input of comparators is higher than voltage reference at the non-inverting input. Hence, the output of the comparators will be below (zero volts).

If there is low voltage (below 190v) in one or more of the phase voltages, the voltage at the inverting input will be less than reference voltage at the non-inverting input this will produce a high output (+9v) from the comparator. One or more of this art the input of the OR gate will cause the output of the OR gate to be high and after a delay time of about 30 seconds this voltage appears at the inputs of the AND gate.

Now the inputs of the AND gate will depend on the position of the enable switch. If it is switched off, the information from the comparators will not pass the AND gate. But if it is switched on (i.e both inputs will now  be high) the information will be reflected at the output of the AND gate and it will be used to achieve the 3 switches generator on/off and then change over switch.

Also if there is complete power failure in one or more phase, the operation of the system will be the same as described above for low voltage. For the switching actions, the information at the output of the AND gate operates the 3 switches at the same time. It supplies voltage to the coil of contactor “A” leaving the contacts on the normally close (NC) position ready for generator supply. Also the generator starter and turn off receive information, the turn off will switch on d.c supply to the high voltage coil while the starter supplies starting pulses (through the multivibrator) to the kick starter. When the generator starts running, a signal is sent to the starter to disable it.

Also the generator sends signals to the delay circuit and after some delay time. It will switch on contactor “B” and the generator will start supplying. But if the power is restored and the comparators okay the supply (i.e if it is above 190v) the OR gate output goes low and time delay, the C/O will be effected  again and the generator will be turned off by cutting off  d.c supply to the high voltage coil. The aim of the delay is to stoop any fluctuation in power which is less than 15 seconds from reaching the control relays the power supply uses a 9v back.up battery with a 9v regulator. It is interesting to note that when supply is restored, the system automatically returns the consumers to his original power supply as the case may be.

• AIMS AND OBJECTIVES

The main aim of any electric power supply in the world is to provide uninterrupted power supply at all times to all its consumers, a device which switches on the head from power Holding Company (PHC) to a generator when power fails and from generator to PHC when power comes back, using contactor as the switching device.

1.2                          SIGNIFICANCE OF THE PROJECT

The automatic change over switch, the switch aimed at achieving the following automatic actions;

• To change power over to generator
• To change back to PHC
• To change the generator.

The automatic change over unit can be operated in single or three phase system. The automatic change over switch has the following advantages;

It minimizes damages to lives/equipment since it has its own monitoring system and its switching requires no human contact with the switch, thus eliminating human error.

It reduces its change over timing to the minimum due to its fast response to power outage.

It maintains high quality of service through its fast and prompt response.

Moreover, the size and captivity of the unit will depend upon the load for which it will be used. The unit is also portable, easy, convenient and safe to install.

1.4                           PROBLEMS OF THE PROJECT

In every home, office or industries, automatic power changeover  plays a vital role, that is, It provides a means of switching from utility AC mains to generator in the case of power failure; This project has been improved on the existing types of electromechanical device that has being in use over the years.

In the course of designing this project, different kinds of problem was notice such as:

• Difficulty in troubleshooting with circuit without the circuit diagram
• Difficulty in connecting the output without short-circuiting, until a multiplexing circuit was gotten.
• Difficult in wiring because of the strong wiring the project required.

1.5                SCOPE AND LIMITATION OF THE STUDY

This work covers only a one phase automatic changeover which can only be used for providing a means of switching from one phase of AC mains to another in the case of failure in public utility.

1.6                            APPLICATIONS OF THE PROJECT

1. This circuit can be used as a home lighting system with few modifications.
2. It can be used to drive other DC loads like a DC motor of any electronic appliance or other toy applications.

1.8                               DEFINITION OF TERMS

CHANGEOVER: Generally, a changeover switch is a system which could be operated manually or automatically , that changes one source of power supply to another source in case of power failure from either of the two sources.

POWER FLUCTUATIONS: is a periodic dip or spikes in the electrical current of any given circuit.

POWER FAILURE: is a short- or long-term loss of the electric power to an area.    

1.9                   BLOCK DIAGRAM REPRESENTATION

The block diagram representation of the automatic change over switch is the one shown in figure 2.1 below, though for the previous years the details of the Automatic C/O switch have never been shown in block form. Therefore, there is need to show it in block form for proper understanding of the major component parts of the system.