A star-delta starter is a very common type of starter and is widely used compared to other types of induction motor starters.
The star-delta starter is used for the squirrel cage induction motor that is designed to run on delta-connected stator winding.
The connection diagram of a start-delta starter for a three-phase induction motor is shown in the figure below.
The stator winding is connected in star connection at the time of starting (switch s is connected at START position).
When the motor reaches 80% of rated speed, the changeover switch s thrown quickly to the RUN position. And the stator winding is connected in a delta connection.
During starting condition, the stator winding is connected in a star connection. Hence, the starting current is reduced to one-third as compared to the delta connection.
At the time of starting, stator winding is connected in star connection. Therefore, each stator phase gets a voltage of VL/√3 (VL is line voltage).
Since the torque developed in the motor is directly proportional to the square of the voltage, the start-delta starter reduces the starting torque to one-third of that obtained by direct delta starting.
Calculation of Star-delta Starter
At the time of starting, the stator winding is connected in a star connection. Therefore, the voltage across each phase winding is equal to 1/√3 times the line voltage.
VL = Line voltage
ISYP = Starting current per phase with stator winding connected in star connection
ISYL = Starting line current with stator winding in star connection
For Star connection
V1 = Phase voltage
ISDP = Starting current per phase by direct switching with stator winding connection in delta connection
ISDL = Starting line current by direct switching with stator winding connection in delta connection
Ze10 = Standstill equivalent impedance per phase of motor referred to stator
For delta connection;
Line current = √3 x Phase current
Therefore, in star-delta starter, the starting current from the main supply is one-third of that with direct switching in delta.
Also, the starting torque is reduced to one-third of the starting torque obtained with direct switching in the delta.
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