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to fulfil the requirement of high starting current and initial torque the motor is run in star first which provides more current for duration of starting time prescribed by motor manufacturer and then when motor develops required torque and speed then motor is run in delta whic h fulfils the running current of motor
A typical 3 phase induction motors have very high current during starting, hence power is supplied initially via star connection in order to supply very low current, The voltage across the winding will be( 1/Sqrt(3))*Line Voltage.
Once the motor attains constant speed of rotation, then the power supplied via Delta connection by which time the current in the motor will be equal to the supplied line current.
Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes less losses. Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused.
All 3 Phase Motors are connected through Self starters in which Automatically switching is done from Star to Delta Connections of the winding.
As all of you know any Electrical equipment is to be connected to +ve and -Ve supply to complete the circuit but in 3 phase its not possible to connect neutral to the 3 phase motor. so the motor supply first is connected to Star connection where the winding connecting acts as Neutral and motor starts to pick up the Speed or Torque, it needs to draw high current for starting from idle load to movementum, Once the Speed is reached as per the requirement of the roatar pump of motor then the load is Automatically switched to Delta connection through timmer fixed inside the starter and connetced to 3 Phase contactors where neutral is not required for smooth running of the motor.
so the sequence of Star and delats is used to run the motors.
Star Delta Motors are preferred as a method of starting in Low starting Torque Applications. The starting current of Induction motors are six to seven times the full load current. Starters are employed to reduce the starting current.
A star Delta Starter reduces the starting current by connecting the motor winding in Star at the time of starting. This reduces the voltage across the winding.The voltage across the winding will be( 1/Sqrt(3))*Line Voltage.
After the starting time set in the timer the winding will be connected in Delta. The voltage across the winding will be equal to the line voltage.
The switching of winding configuration is done by devices called contactors. Typical application of Star Delta motors are compressors
Note: Since the Torque is Proportional to Square of voltage. A high starting torque is not possible.
At star connection starting voltage applied against motor winding is V/root3, hence lesser voltage and lesser current keeps the winding protected from higher current jerk results in efficient starting of motor adding more life to windings.
Keeping the force of inertia in mind.
In star connection the line voltages are less and the motor starts with low current and slow but smooth speed. After getting specific speed and torque then transfer to delta with full running.
Motor start in start to limit the starting current,afert 6-10 S motor switch to delta for mas torque/power
to reduce the starting current and voltage for smooth running and prevent other equipments.
to reduce the starting current and torque of the motor
