Star Delta Stater
The star delta starter is widely used for the starting of a 3 phase induction motor. In star delta starting,the motor is first connected in star connections and after achieving the specified speed the motor is connected in delta connection. The small rating induction motor are often started on direct online starterl(DOL).The large rating motor if started on DOL draws an outsized starting current. the massive starting current may cause voltage dip within the system voltage which might further cause power disturbance within the electrical network. To limit the starting current, Star Delta Starter is utilized .
Why is that the starter needed for induction motor?
When the induction motor is started it draws an outsized starting current because the rotor impedance is more reactive at starting of the motor. The slip of the motor is unity when it’s started.
The voltage induced within the Rotor is;
Er = s*Es* stator rotor turns ratio
Er = Rotor induced voltage
s = Slip
At starting, slip=1
Rotor Current(Ir) = Er/Zr
Er and Zr is that the rotor voltage and impedance respectively.
The rotor impedance Z2=√(Rr2 + s*Xr2)
The rotor reactance(Xr) is kind of the rotor resistance.
Xr= 2πsfs XL
At start slip of the motor is unity and rotor induced voltage frequency is capable the stator supply frequency. Therefore, the reactance of the rotor is more during starting of the motor.At starting,the rotor draws large inductive current.
Moreover, the ability factor of the motor is extremely low during starting. As a result, the starting torque delivery of the motor is incredibly poor. As motor accelerates towards its base speed , the slips and so the refore the rotor voltage get reduced and therefore the current decreases accordingly.
To limit the starting current of the induction motor, the starter is employed.
The starting current are often limited by;
- Reducing the stator voltage- Star-delta starter.
- Adding external resistance to the rotor circuit- For connection motor only.
We will discuss how the starting current are often limited with the use of star delta starter?
What is the star delta starter?
The winding of the stator of the motor is brought within the stator terminal box of the motor. The winding are often connected within the star or delta configuration.
When the motor is started by DOL starter the winding is configured within the Delta. The six pairs of the winding are brought within the starter panel to attach the winding in star configuration initially sure as shooting time, and after the motor is connected within the delta configuration with star/delta contactor logic.
Working principle of star delta starterThe motor is first started by connecting its stator winding during a star configuration. The phase voltage within the star configuration is 1/√3 of the phase to phase or line voltage. Thus the voltage applied to the stator winding is about 58 you take care of the road voltage. The starting stator current reduces to 1/√3 of the whole load current when the motor is started on star winding configuration.
However, with a reduction of the starting current, the starting torque delivering capacity of the motor get also adversely affected. The torque within the induction motor is proportional to the square of the applied voltage.
T= Slip* V2 ———-(1)
At starting, slip=1
Tst= K* V2 —————(2)
Tst= K* (1/√3)2
Tst= K*1/3 —————(3)
The torque delivering capacity of the motor becomes 33 you take care of the whole torque capacity of the motor during the running of the motor during a star configuration. Therefore, the star delta starter are often used for starting loads that require less starting torque.
Circuit Description:When S1 electric switch is pressed K4 timer gets energized and it operates the star contactor K2 so main contactor K1. The motor starts within the star configuration. The motor operates in star for the time the timer K4 is programmed.
After the lapse of the timing of K4 timer, the delta contactor K3 operates and motor now runs in delta mode. The overload relay are often mounted in mainline or in delta circuit. We see that the overload relay is mounted during a delta circuit. within the delta circuit, the relay setting must be 1/√3 of the motor full load current.
Applications of the star delta starter :
Star Delta Starter reduces starting current. Although the starting torque, the star delta starter reduces about 66%. Motor can give only 33% torque in Star mode. The starter is most fitted for that application where the mechanical load demands less early torque.