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The main concept of this project is to maintain continuous AC supply to a single-phase load by an auto-changeover of load from the missing phase to any available phase in a three-phase system.

The proposed system uses a rectified-power supply. In this system, three phases areconnected to the control circuit through an Optoisolatorafterreducing the voltage level and rectifying it to DC using a step-down transformer and a half-wave rectifier, respectively. The load is selected from any of the availablethree phases, automatically, using relays,which are interfaced to the relay driver IC. The logic gates are used for detecting the availability of the phase.

The circuit is connected in such a way that the relays connected to the phase feeding the load are always energized. In case of the failure of this phase, the logic gates automatically generate an output to drive, so that the next relay is activated to connect the other available phases to feed the load.

This project in future can be enhanced by incorporating power semiconductor devices such as thyristors/IGBTs for instantaneous changeover to the next available phase. This overcomes the drawback of the change over time generally witnessed by the relay switching operations.