Star Delta Starter | Working Principle, Power and Control Circuit Diagram

Introduction to Star Delta starter:

Star delta starter is widely used for the starting of a three phase induction motor. In star delta starting,the motor is first connected in star connections and after achieving the required speed the motor is connected in delta connection.

The small rating induction motor can be started on direct online starterl(DOL).The large rating motor if started on DOL draws a large starting current. The large starting current may cause voltage dip in the system voltage which may further cause power disturbance in the electrical network. To limit the starting current, Star Delta Starter is used. 

star delta starter

Why is the starter needed for induction motor?

When the induction motor is started it draws a large starting  current because the rotor impedance is more reactive at starting of the motor. The slip of the motor is unity when it is started. 

The voltage induced in 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 the rotor voltage and impedance respectively.

The rotor impedance Z2=√(Rr2 + s*Xr2)

The rotor reactance(Xr) is more than the rotor resistance.

Xr= 2πsfs XL

At start slip of the motor is unity and rotor induced voltage frequency is equal to 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 power factor of the motor is very low during starting. As a result, the starting torque delivery of the motor is very poor. As motor accelerates towards its base speed , the slips and the rotor voltage get reduced and the current decreases accordingly.

We use star delta starter to limit the starting current of the induction motor.

Methods of reducing of starting current

  1. Reducing the stator voltage- Star-delta starter
  2. Adding external resistance to the rotor circuit- For slip ring motor only

We will now discuss how the starting current can be limited with the use of star delta starter?

What is the star delta starter?

 star delta connection of motor at stator terminal

The winding of the stator of the motor is brought in the stator terminal box of the motor. The winding can be connected in the star or delta configuration.

When the motor is started by DOL starter the winding is configured in the Delta. The six pairs of the winding are brought in the starter panel to connect the winding in star configuration initially for certain time, and after the motor is connected in the delta configuration with star/delta contactor logic.

Principle of working of star delta starter

The motor is first started by connecting its stator winding in a star configuration. The phase voltage in 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 % of the line voltage. The starting stator current reduces to 1/3 of the full 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 of 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 % of the full torque capacity of the motor during the running of the motor in a star configuration. Therefore, the star delta starter can be used for starting loads that require less starting torque.

Circuit Diagram of Star Delta Starter

star delta starter circuit diagram

Star- Delta Starter Circuit Description:

When Start push button is pressed K1T timer gets energized and it operates the star contactor KM1 and then main contactor KM3. The motor starts in the star configuration. The motor operates in star for the time the timer K1T is programmed.

After the lapse of the timing of K1T timer, the delta contactor KM2 operates and motor now runs in delta mode. The overload relay can be mounted in mainline or in delta circuit. In the diagram, the overload relay is mounted in the delta circuit. In the delta circuit, the relay setting must be 1/3 of the motor full load current.

Applications of the star delta starter :

The star delta starter reduces the starting current, however, the starting torque is also get reduced. With star delta starter the starting torque is reduced up to 66 %. The motor can deliver only 33 % torque in star mode. The starter is best suited for the application where the mechanical load demands less starting torque.

Please follow and like us:

3 thoughts on “Star Delta Starter | Working Principle, Power and Control Circuit Diagram”

Leave a Comment