Updated: Aug 20
Star Delta starter is used to start the squirrel cage induction motor. Initially at start the motor winding remains in the star configuration and after lapse of some time which is set time of timer the motor stator winding gets connected in the delta configuration.
The induction motor takes about 5 -7 times current of its FLC when it is started. The star delta starter is used to limit the starting current of the induction motor. In this post, we will confine our discussion on power and control circuit of the star delta starter.
How Does Star Delta starter work?
The power and control circuit diagram of the star delta starter is as given below.
In above circuit,
KM3 is The Main Contactor
KM2 is Delta Contactor
KM1 is Star Contactor
Operation Of Main Contactor KM3
The main contactor KM1 remains energized all the time when start button is pressed. This is because in star mode KM1 is energized, KM1 operated main contactor KM3 and KM3 remains latched through NO contact of KM3.
Operation Of Star Contactor KM1
The conditions of operation of star contactor are as given below.
Delta Contactor should be in OFF state
Main Contactor should be in OFF state
When start push button is pressed, the star contactor KM1 first gets energized through NC contact of timer and NC contact of delta contactor KM2.
The NO contact of star contactor energize the main contactor KM3 and motor starts in star.
Therefore, the sequence of opeartion is;
First, the one end of coils is shorted through the star contactor KM1
KM1 operates the main contactor
Motor starts in star configuration
In star mode, voltage across the stator coil is 58 % of the line voltage. If 415 volt motor is started with star delta starter then in star mode the voltage across motor terminal is equal to;
= 0.58 x 415
= 240 Volt
This reduction in voltage reduces the starting current of the motor.
Operation Of Delta Contactor KM2
Sequence of opeartion is as under;
The timer K1T starts operating with pressing of start push button.
After lapse of star operation timer, the NC contact of timer drops the star contactor and NO contact of timer energize the delta contactor.
Now keep on running in delta mode
Choice of Placement of Overload Relay in star delta contactor & Its setting
Overload Relay in Delta Circuit
In the given above circuit of star delta starter, the overload relay is placed in the delta circuit. The phase current will flow through the overload relay. We know phase current is 58 % of the line current. If motor is drawing 100 ampere line current then the current through the overload relay will be about 58 amperes.
Therefore, the overload relay settings needs to be done according to connection of the overload relay in the circuit.
For example, 30 KW motor is started with star delta starter. The overload relay is in the delta circuit. The full load current of the motor is 52 amperes. Then, the overload realy setting must be,
= 0.587 x 52
= 30.52 ampere
Overload Relay in line Circuit
If the overload realy is mounted in line cicuit then;
Overload setting of the relay = Motor Full load Current