What is the difference between direct on line DOL starter an

What is the difference between direct on line (DOL) starter and star delta starter?

Solution

Direct On-Line Starter (DOL):

The Direct On-Line (DOL) starter is the simplest and the most inexpensive of all starting methods and is usually used for squirrel cage induction motors. It directly connects the contacts of the motor to the full supply voltage. The starting current is very large, normally 6 to 8 times the rated current. The starting torque is likely to be 0.75 to 2 times the full load torque. In order to avoid excessive voltage drops in the supply line due to high starting currents, the DOL starter is used only for motors with a rating of less than 5KW There are safety mechanisms inside the DOL starter which provides protection to the motor as well as the operator of the motor.

Star-Delta Starter:

The star delta starting is a very common type of starter and extensively used, compared to the other types of the starters. This method used reduced supply voltage in starting. The method achieved low starting current by first connecting the stator winding in star configuration, and then after the motor reaches a certain speed, throw switch changes the winding arrangements from star to delta configuration. By connecting the stator windings, first in star and then in delta, the line current drawn by the motor at starting is reduced to one-third as compared to starting current with the windings connected in delta. At the time of starting when the stator windings are star connected, each stator phase gets voltage V_L/(3)^(1/2) , where V_L is the line voltage. Since the torque developed is proportional to the square of the applied voltage, star- delta starting reduced the starting torque to one – third that obtainable by direct on line starting.