Difference between DOL, Star Delta, VFD and Soft starter

Difference between DOL, star delta, vfd and soft starter:

The difference between DOL- direct online starter ,star delta, vfd, and soft starter is explained below with operation diagram and their uses in different applications.

DOL-Direct on line Starter:

here the starting current will
be 6 times of full load current. A bimetallic thermal
overload relay provides protection against the over load.
There will be no reduction of starting torque during

Difference between DOL and star delta starter

Star Delta Starter:

In order to limit the starting current of motors this method is followed. Initially motor windings are connected in star configuration. The applied voltage across the winding is reduced 1/1.73 times of rated voltage due to reduced the voltage current also reduced. when the motor obtain rated speed then motor connected into delta connection.

Difference between DOL and star delta starter

Auto Transformer Starters:

Auto transformer connection always  the motor connected in delta connection. so the delta connection starting voltage is reduced by 1.732 times so starting current also reduced. the auto transformer there are different tapping point is there 60%, 70% and 80%. when the motor start the auto transformer automatically adjust the taping points to limit he starting current. once the motor obtain full speed the tapping windings are short circuited and line voltage is directly applied to the motor windings.

Difference between DOL and star delta starter

Soft Starter :

Soft starters are Thyristor Drives The rms value of
the applied voltage is reduced by controlling the firing
angle of power Thyristor. The achieving the full speed
Thyristor conducts in full. Here the starting voltage can
be varied from Zero to maximum value and the motor starting
is vary smooth. Similarly soft-stop facility is also
available thereby the motor can be stopped by gradually
reducing the speed.

Difference between DOL and star delta starter


In VFDs the ac voltage with fixed frequency  is converted into DC voltage and further its converted into AC voltage with variable frequency range. the variable frequency can be produced thyristor controller. the frequency ranges from 0- 200%. this type of starter is used in squirrel cage induction motor.


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