In star delta starting, the phase voltage is 58 % of the line voltage when the motor is first connected in the star connection. The reduced voltage leads to reduced motor current. The reduced current improves the motor starting characteristics, and the motor starts without any electrical and mechanical stress. That is why the star delta starter is used.
Why is a Starter used in induction Motors?
The induction motor takes 6 to 7 times its rated current at starting time. At starting, the rotor impedance is highly inductive( XL=2*pi*sfs*L) as the rotor current frequency equals the stator supply frequency(fr=fs), and the motor’s slip at the start is unity.
The rotor resistance of the induction motor is expressed with the following mathematical formula.
From the above equation, it is clear that the induction motor’s rotor resistance depends on the motor’s slip.
At the start, the slip of the motor is unity.
S=1
Putting the value of s=1 in the equation(1) , we get
At the start, the induction motor’s rotor resistance is zero and acts as a short circuit. Therefore, the induction motor draws a high current at the start. That is why a starter is used in induction motors to limit the starting current.
At the start, the induction motor draws a high current and delivers very little torque at the start.
To limit the starting current of an induction motor, the supply voltage needs to be reduced to reduce the voltage induced in the rotor circuit. The motor current will reduce with the decrease of the supply voltage. The small rating motors can be started with a DOL starter. However, as the rating of the motor increases, the starting of the motor must be done by a star delta starter.
Star Delta Starter
In the star delta starter, the winding of the stator is first connected in the star connection to reduce the stator phase voltage by 42 % of the supply voltage. The voltage across the winding will be 58 % of the supply voltage, and the starting current will be reduced accordingly. If the line voltage is 440 volts, the phase voltage across the stator winding is 0.58*440=255 Volts.
After running the motor in star connection for a certain time, the timer switches on the delta contactor, and now the motor winding gets connected in delta. The full supply voltage is impressed on the stator winding when connected in the delta.
Thus, the star delta starter limits the motor’s starting current and improves its speed-torque characteristics.
The large-rated squirrel cage motor can be started on DOL if the fluid coupling is used or if the VF Drive drives the motor. The VF drive starts the motor at reduced voltage and frequency and starts the motor in a soft starting mode.
Conclusion
The induction motor draws a large current at the start, and it is necessary to limit the current to enhance the motor’s life. That is why a starter is used in induction motors to limit the starting current.