elta
conversion. Star/Delta starters are probably the most common reduced voltage
starters in the 50Hz industrial motor world. Star delta is used in an attempt
to reduce the start current applied to the motor. Thereafter, full load current
is applied to the motor.
The
Star/Delta starter is generally manufactured from three contactors; and
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.
This
project uses a system to start a 3 phase motor at 440 volt AC mains supply 50
Hz with a set of 12 volt DC relays in star mode first and then to delta mode by
an electronically adjustable timer. A set of relays are used to shift the motor
connections from star to delta with a time delay.
The
project is supplied with six lamps instead of a 3 phase motor i.e., two lamps
representing each phase winding of the motor. 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. It still retains its application for a 3 phase motor starting
with single phasing prevention also.
During
star operation the lamps would glow dim indicating the supply voltage across
the coils are 440/root of (3). 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. Further
the project can be enhanced by using a thyristors in firing angle control
principle for soft start of the induction motor that would overcome all the
drawbacks of star delta starter.
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