why do we use star-delta starter. what is its need?
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Answer Posted / james allan
Star Delta starters make use of the way current flows in
Star and Delta. In Star Line current = phase current and in
Delta line current =1.73 phase current. During starting it
is also accepted the the current drawn can me as high as 6
times the motor rated current for a short period. When
starting a motor the load needs to be accelarted from rest
to speed and thus requires massive torque. Torque is
directly proportional to the current (which is also
determines the stregth of the magnetic field in the poles
that grab hold of the rotor to drag it along). When
starting in star your line current will equal the phase
current. This means that line current at start will be 1.73
times less than startig the motor in Delta. Power of the
motor stays the same as the voltage in star reduces by a
factor of 1.73. However current through windings generates
heat so you do not want the high current constantly in the
windings as this equates to heat losses by the square of
the current multiplied by the resisitance (Actually it
should be impededance Z which takes induction and
capacitance in the motor into account. Once the load is
accelarated, it does not require as much torque to keep
rotating at its speed so requires less current. At this
point the motor switches over to delta. The phase voltage
ups by a factor of 1.73 and the current reduces by a factor
of 1.73 thus the power consumed is constant. less current
in the windings means less heat losses. Star delta does not
affect the spead of an induction motor. A motor sunning in
star runs at the same speed as a motor running in delta.
Speed of an induction motor is detemined by the frequence
from te supply 50Hz or 60Hz, and the number of poles in the
motor. This is fixed. Only frequency drives can control the
speed iin an induction motor.
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