What is Star Delta Starter?
The star-delta starter is the most commonly used starter for starting a 3 phase induction motor and the star-delta starter is a reduced voltage starter. During starting period of 3 phase induction motor. induction motor is connected in star connection and when the motor reaches the required speed, then the motor is connected as delta connection.
In the star delta starter, 3 phase induction motor stator connection configuration changed from star to delta. The 3 phase induction motor start in a star stator connection and when the motor reaches 80 % of its full speed, the stator connected changes from star to delta.
The star-delta starter is used for reducing the starting current of the 3 phase induction motor without using any external device. The star-delta starter has less inrush current( 1/3 of D.O.L starter) than the direct online starter.
The main problem of starting of induction motor is starting inrush current which is two to five times of full load current. For reducing the starting heavy inrush current star-delta starter is used.
Working of Star delta starter
How is the current reduced to 1/3 in 3 phase induction motor in star delta starter than the direct online starter?
VL =
Supply line voltage
IL =
Supply line current
Iw =
Supply winding current per phase
Z = Impedance of
winding per phase
When stator winding is connected in star configuration the winding current is equal to the phase current.
Supply line Current ILS = Winding current per phase IwS
ILS = IwS
When stator winding is star connected current in each phase of the stator winding is given by
The voltage across each phase of winding = VL/ √3
Winding current per phase is given by
IwS = VL/ √3 Z
In star connection line current is equal to the winding current in each phase
ILS =VL/ √3 Z = Iw
Now motor reached 80 % of full speed and now motor in a run in a delta configuration
In the delta connection line is current is equal to the √3 times the winding current of each phase.
ILD = √3 IwD
When winding is delta connected Line voltage is equal to the phase voltage or Winding voltage per phase.
VLD = VwD
Winding current per phase when winding is connected in Delta
IwD = VL/ Z
Now
ILD = √3 IwD
ILD = √3 VL/ Z
Now we compare the line current when the motor is connected to the delta connection to the star connection
Line current in Delta ILD / Line current in Star ILs
= √3 VL/ Z / VL/ √3 Z = 3
Line current when stator winding is connected star to delta
ILS = 1/3 ILD
From the above equation, we know that starting current of 3 phase induction motor in the case of the start delta is one-third of direct switching in the delta.
The star-delta starter also reduces the starting torque of the induction motor to one-third of that produced by the Direct online starter.
The star-delta starter is one type of autotransformer with 57.7 % of tapping.
Advantages of star delta starter:
1. Star delta starter has a simple design and operation.
2. Star delta starter has less cost than other voltage controlling starters.
3. In the star delta starter, heat is not produced and no tap changing device is required so the efficiency of the starter is increased.
4. In the star delta starter, current and torque performance is very good.
5. In the star delta starter starting torque is reduced to 1/3 then the direct online starting.
6. In the star delta starter high starting torque is produced.
7. In the star-delta starting motor draws two times starting current of the full load current.
8. The star-delta starter is used as a power-saving device in various industries.
Disadvantages of Star Delta Starter:
1. In star-delta starting of induction motor starting torque is reduced to 1/3 of full load torque.
2. In the star delta starter particular sets of induction motor is required. In the star delta starter, six lead induction motor is delta connected.
3. When induction motor delta connected supply voltage is equal to the rated voltage of induction motor.
4. When the motor switches from star to delta and the speed of the induction motor are not more than 80 %, the current peak is very high which is damage contactor contacts and it is not reliable.
5. The star-delta starter has require more maintenance as compared to the direct online starter.
6. In star delta starter. induction motor transition takes place from the star to delta, so two separate cables are required in the terminal box.
Application of star delta starter:
1. Star delta starter is suitable where starting current is low and also starting torque is very low. So this starter is used in centrifugal Compressors.
2. Star-Delta starter is used in low to medium voltage and low starting torque induction motors. Star delta starters are also used in grinding mills.
Features of star delta starter:
1. In the star delta starter starting current is 33 % of the full load current.
2. In the star delta starter starting torque is 33 % of full load torque.
3. In the star delta starter, there are a reduced starting torque and starting current.
4. In the star delta starter 6 terminal cables are needed for the motor terminal box.
5. In the star delta starter's peak starting current is 1.3 to 2.6 of full load current.