Model no.13:
Star/Delta starters are probably the most common reduced voltage starters in the 50Hz industrial motor world (Known as Wye / Delta starters in the 60Hz USA world). They are used in an attempt to reduce the start current applied to the motor during start as a means of reducing the disturbances and interference on the electrical supply. The Star/Delta starter is generally manufactured from three contactors; an electromechanical timer and a thermal overload for operating a 3 phase motor at 440 volt at ac mains supply 50 Hz. The interlocking arrangement of all the contactor coils is traditionally wired in 440 volt ac. However in our project we have taken up the same to operate a 3 phase motor at 440 volt ac mains supply 50 Hz with a set of 12 volt DC relays, an electronically adjustable timer and a set of miniature circuit breakers. The interlocking arrangement of the relay coils and the electronic timer are all wired in low voltage dc of 12 volt fed from an inbuilt dc power supply for safe handling of the starter during the study, still retaining its is application for a 3 phase motor starting with single phasing prevention .as the project would be expensive in using a 3 phase induction motor the project shall be tested with a set of lamps for each phase. During star operation the lamps would glow dim indicating the supply voltage across the coils are 440/sqrt (3) and while in delta condition after the timer operates the lamps would glow with full intensity indicating full supply voltage of 440volts. The timer comprises of a 555 in mono-stable mode the output of which is fed to a relay for changing the mains supply from 3 phase star to delta. The project also has the provision of single phasing protection since 3 phase motors get burnt if any one phase goes missing during running. The output to the lamps shall be completely cut-off in the event of any phase failure.
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