Design And Construction Of 25 Amps Single Phase Automatic Change Over

1.0 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 change over without fluctuations.
The first type of change over switch were 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:
1) Switching of the contactors (i.e C/O)
2) Switching of the generator started if power fails
3) 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.

1.2 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.
BLOCK DIAGRAM