DESIGN AND CONSTRUCTION OF A 60AMP THREE PHASE AUTOMATIC CHANGE OVER SWITCH
The design and construction of a 60A, 3-Ф automatic power changeover from public utility supply to generator and vice versa was aimed at providing a useful system for use in our homes or offices. The system was achieved with a 66A 3-Ф contactor, 100mA transformers and relays. The system turns on a generator when the public utility (PHCN) is off by cranking the starter and a provision was made to stop cranking when the generator is fully started. A delay was provided to stop the generator five minutes after the PHCN is restored to avoid unnecessary off and on. A display that shows when PHCN or GEN is on was provided.
This project work deals on the design and construction of an automatic power changeover that will change/transfer control from MAINS to Gen set and can switch ON/OFF the Gen automatically. The system integrates phase selector in MAINS and changeover to Gen. It is designed in several blocks all combined to from one functional unit. A +5v regulated DC power supply unit forms the first block whose output is fed to priority encoder and decoder block. The output of the BCD – Decimal decoder is used to select the phase in priority through a transistor driver stage and electromagnetic relay that activates contactors.
Industrial automation and control has contributed immensely to the high rate of the world’s technological growth. Power control and automation is a part of this field which is applied in virtually anywhere electrical power is used, generated or transmitted. Manual power change-over switches have been in existence and in different forms and shapes. 1959, the advent of microelectronics has paved way to modify the manually operated change-over switches. This project deals with the design and construction of automatic change-over switch with indicator. It eliminates the time and energy spent in operating the manual change – over and starting of Gen. Set. It is fully automated. It can be used in residential houses, offices and most importantly, in factories that required constant power supply. It has so many advantages over the existing ones/types.
1.1. BACKGROUND OF THE STUDY
This project work is embarked upon to eliminate nearly every shortcomings associated with the manually operated type to ensuring that it is fully automated. This system is realized in several functional blocks all combined to form one functional system. The first block is a 3-phase MAINS regulated dc power supply unit that powers the overall circuit and acts as the input. The output of thin block is fed into a priority encoder (74147) which selects the most prioritized phase at a time. The third block is a BCD – to – DECIMAL Decoder (7442) used to decode the prioritized phase.
The fourth is the driver stage which comprises of NPN transistor that energizes the electromagnetic relay which switches the phase in priority. The Gen set switching section is achieved using the least prioritized phase. This section switches ON the Gen set when all the NEPA phases are low, OFF or faulty. Once the Gen set phase is selected, a switching transistor fires an SCR which conducts large battering current to the ignition coil of the Gen set. Panel indicators are used to show the status of the system.
1.2. OBJECTIVES OF THE PROJECT
The major objectives of this project is to design and construct an automatic change-over switch with Gen set switching facility capable of transferring control from MAINS to Gen set.
It is also aimed at equipping the graduating student with Hands – on experiences which is invaluable in their professional practice. It is aimed at grounding the graduate of EEE in electronic system designs involving power control and automation using random/Hard wired logic. Another objective of this project is to acquaint the graduating students on the ethics of Engineering, technical and scientific writing which is a nucleus of engineering professional practice.