Design And Construction Of A 3-Phase Automatic Power Change Over With Generator Set Switching Facility
In this research, a “3-Phase Automatic Power Change Over with generator set switching facility” has been designed and implemented using three voltage Comparators (LM741 AH1883), 3-input-AND gate (4073), two BC 108 transistors and 12V, 30mA relay as well as some biasing resistors. The voltage Comparators (LM741 AH1883) were biased to sense the unregulated voltage – one for each of the three phases (Rø, Yø, Bø) and then couple the analogue outputs to the 3-input-AND gate (4073). The AND gate produces an output of „0‟ (OFF) when all the three phase input voltages are all within the normal (preset) range, else it produces an output of „1‟ (ON) implying a voltage drop or phase failure in at least one of the compared phases. The output of the gate when coupled to the base of switching transistors (BC 108) determines their states (OFF or ON). Since the transistors are configured in a Darlington pair arrangement, the second is ON only when the first is OFF. This then triggers the public power supply ON due to normal phase voltage. On the contrary, when the first transistor is ON, the 12V battery produces a potential which triggers ON the alternative power source (Generator) via the 12V, 30mA relays hence breaking contact from the public power supply to the Generator side. The switch is tested to have function optimally within ±5% nominal voltage of 220 or 415V supply at the point of changing over to an alternative power source. Hence this device can be of Industrial or domestic use where 3-phase power supply is available with a stand-by power source.
The evolutionary trend in industrial electronics and automation has engulfed all parts of the world and proved more challenging to as third world like Nigeria. Regarding the milestone, it was decided to make the choice of project research on the design and implementation of an automatic 3 – phase change – over with Generator switching which will eliminate the stress and lapses involved in the manual process. This electronics system is realized using 3 – phase regulated Dc power supply unit whose output is compared and fed to a 3 – input AND gate that encodes control logic to the driver stage realized using NPN Bipolar junction transistors which energize three electromagnetic relays. These relays in turn energize single coil three contacts contactors that switch the phases. (Manaha, 2008) A control/switching circuit is also integrated that takes care of the Gen set switching whenever at least one phase from mains is low/faulty.
1.1. General Description of the Project
The project design and implementation of automatic 3 – phase change – over with Gen set switching is built in several functional blocks all combined to form one functional unit. The first is made up of regulated Dc voltage power supply unit whose output is compared and fed into 74 LS11 that generates the control logic. This logic is used to bias the driver stage which energizes electromagnetic relay. These relays serve as auxiliary to a single coil 3 – contact contactor of suitable rating. The Gen set switching module incorporates delay timer and solid state relay that takes care of the process.
The project, so constructed is packaged in a plastic casing for compatibility. On the surface of the package, there are three 100k vol/pot which are used to demonstrate low voltage in a given phase. Three amber LEDs represents the mains while the three blue LEDs are for the Gen set. And these make up of the panel indicator.
1.2. Objectives of the Project
One of the objectives of this project is to design and construct an automatic 3 – phase change – over with Gen set switching facility which will automatically switch ON/OFF Gen set and change over between PHCN and Gen set. It is also aimed at exposing the graduating student to electronics system designing using random logic – especially in designing in industries and power electronic. Another objective of this study is to ground the graduating students in Engineering, scientific and technical writing which is indispensable in their professional practice.
1.3. Significance of the Project
This project work signifies a lot in industrial automation and power electronics. It is very useful in wood processing industries, oil rig equipments and other manufacturing industries that used 3 – phase machines. A project of this nature can be modified to function as automatic phase selector which can be used anywhere.
1.4. Scope of the Project
This project work covers the following; transistor drivers, electromagnetic relay, contractors, 3 – input AND gate encoder, shunt regulation and comparator using Zener diode, delay/un-delay timer and other system
This automatic change-over switch is limited to only 200A alone.
- It is also limited to auto starter generator.
- It is limited to a generator of three (3) phase supply.
1.6 Definition of Terms
In electrical engineering, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another. In the design, I used the switches S1 S2, S3. Switch S1 is to prevent the generator from starting in case PHCN restores power, Switch S2 is to prevent the auxiliary contact from energizing the relay that connects the starter; Switch S3 is to start the generator manually.
A cable is defined as an insulated conductor, single or stranded which makes an overall mechanical protection. Cables have two major parts, the conductor and the insulator. The conductor is that part of the cable which allows current to flow through it, while the insulator is the outer covering which serves as a protection to the conductor and does not allow current to flow through it. Cables are channels through which current are being transferred in various instrument/equipment.
Resistors are devices that can resist the flow of electrical current. They are bi-directional components. Resistors are made of metal, carbon or ceramic. They are of two types namely fixed type and variable types.
In resistors, colour coding is used to identify their values which are measured in a unit Ohms with the symbol (Ω). The schematic diagram of resistors is shown below.
Capacitors are electronics components that store electrical changes/energy by electrostatic stress in a dialectric. Capacitors basically consist of two conducting surface separated by a layer of an insulating medium lied dielectric. The dielectric may be air, vacuum, per, mica, ceramic plastic electrolyte. Each of these materials has the values called dielectric constant.
Diodes are two terminal semi-conductors that allow current to flow in one direction. The terminals are anode and cathode. When connecting positive terminal of the source (EMF) to the positive terminal of the diode and negative of the source to the negative of the diode, it is said to be in forward biased mode and it will conduct. But when connecting positive terminal of the source to the negative terminal of the diode and negative of the source to the positive terminal of the diode, it is said to be in reverse biased mode and it will not conduct.
This is a static electrical machine through which power from one part of the circuit (primary side) is transferred to the other part (secondary side) of the circuit by means of electromagnetic induction. It input and output current/ voltage differs depending on the windings.
Transistors are made up of P-type and N-type semi-conductor materials. It is being formed into two junctions. PN junction and NP junction separated by either N or P semi—conductor in between the junctions. This results in a three terminal electrode of emitter, collector and base.
This is a five layer bi-directional device which can be triggered into conduction by both positive and negative triggering pulse at it gate. It is used to control AC power to a load by switching ON and OFF during positive and negative half cycles of the input AC power.
1.7. Project Report Organization
This project report is presented in five chapters to appropriately illustrate the steps involved in its implementation.
- Chapter One introduces the project overview, objectives and its significance. Chapter Two is basically the literature review and some theories relevant to the design.
- Chapter Three x–rays the methodology and analysis while
- Chapter Four elaborates on the implementation, testing and result.
- Finally, Chapter Five is about the summary of the entire theories and recommendation.
5.0 Conclusion and Recommendation
After the implementation of the design, various tests were carried out and the results obtained demonstrated that the 3-phase automatic change over switch achieved its design and construction purpose. The system worked according to specification by monitoring phase failure and under-voltage thereby changing over to the alternative power supply. This automatic electronic system operates without human intervention hence the sluggishness of manual operation is eliminated and production downtime reduced to the barest minimum thereby reducing production losses and costs.
The automatic changeover system with feedback performs optimally if the recommended operating conditions are observed by users. The following are the recommended conditions for optimum performance.
- Power supply 12-18V dc
- Maximum operating temperature +500C
- Minimum operating temperatures 9oC
- Maximum load 120watts
- Minimum DTMF signal +1Db
- (each tone of composite signal) 869mvrms Exceeding any of these values may cause a permanent damage to either the chips in particular or the entire system as a whole. Because of the problems encountered during the project design and construction, the following recommendation should also be put in place in order to achieve a result oriented design.
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