what is the reason behind using a star-delta and DOL starter for A.C motors??

Answer 1

When an induction motor is started it takes a starting current several times more than the running current as there is no back emf. The starting current of a typical 3phase induction motor is 6 times its running current. Therefore as the motor is started high starting current flows and voltage drop occurs in the cable connecting the motor with the source. If the voltage across the motor drops below 80% of the motor rated voltage the motor will not start. At the same time if a higher capacity motor is started with other connected load in the system the resultant voltage drop in the system may stop other loads in the system due to heavy voltage drop ( during running the voltage drop across the motor should not be less than 10% of rated voltage). Direct On Line or DOL starter is used where such drop is not detrimental to the system ( other loads). In a Star-Delta starter the motor is started as star connection and when the motor starts the connection is changed to delta. With star connection the motor takes 1.732 times less current thus the starting voltage drop reduces. However as the torque is proportional to square of the current the starting torque also reduces. This is actually a system design and many factors are related. The figures given above are as per IS standards.
Hope this will satisfy you.

Answer 2

With DOL commencing the situation is the excessive line voltage drop as a results of excessive commencing contemporary which could influence the neighbouring electrical energy shopper. yet,below 5 HP, the commencing contemporary and hence the voltage drop are no longer very important. In a start up delta starter you ought to incur fee on an further 2 contactors and a timer too. to shrink fee, DOL starters are favorite as right here you ought to use basically one contactor and no timer is needed.

Answer 3

star and delta configurations have sifferent phaqse to phase voltages. In the delta configuratrion the phase to phase voltage is equal to the same in the star configuration multiplied by a factor of 1.732. While starting a motor there is no back emf generated by the motor as bac k emf is dependent on speed of the motor. So if the full force the high voltage available in the delta configuration is applied to the motor while starting it, tje resultant heavy rush of current to the motor would damage the motor windings and burn up the motor. Hence as aprotection to the motor, the motor is started in star comfiguration and then it is transferred to the delta configuration, allowing the full voltage to be applied to the motor during its running so as to get the full torque output.

Answer 4

commonly starters are used to avoid damage of motor windng due to the passage of heavy current during the starting of motor. during the starting, it requires large amount of current(heavy load).