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Answer Posted / partha podder
Starting current of a motor is high.The simplest way to
start a three-phase induction motor is to connect its
terminals to the line. In an induction motor, the magnitude
of the induced EMF in the rotor circuit is proportional to
the stator field and the slip speed (the difference between
synchronous and rotor speeds) of the motor, and the rotor
current depends on this EMF. When the motor is started, the
slip speed is equal to the synchronous speed, as the rotor
speed is zero (slip equal to 1), so the induced EMF in the
rotor is large. As a result, a very high current flows
through the rotor. This is similar to a transformer with the
secondary coil short circuited, which causes the primary
coil to draw a high current from the mains. When an
induction motor starts DOL, a very high current is drawn by
the stator, on the order of 5 to 9 times the full load
current. This high current can, in some motors, damage the
windings; in addition, because it causes heavy line voltage
drop, other appliances connected to the same line may be
affected by the voltage fluctuation. To avoid such effects,
several other strategies are employed for starting motors.
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