PLC Tutorial: Star delta starter for induction motors

This plc tutorial will enlighten the user about the programming of star delta motor starter in PLC.  A brief explanation of star delta starter diagram is included followed by programming and demonstration of star delta starter in LOGO SOFT software.

Introduction to star delta motor starter

When electric motor is started, it draws a high current typical 5-6 times greater than normal current. In DC motors there is no back emf at starting therefore initial current is very high as compared to the normal current. This concept is explained in detail in my article

Three point starter for DC motor.
In induction motor, the winding or rotor are normally shorted. Also there is no opposing flux against the actual flux. As a result motor draws high current in starting. High starting current of Induction motor is explained in.


Starting of induction motors

Working principle of star delta starter

The following figure shows the star delta configuration



Figure A shows the star configuration. If the motor windings are connected in configuration, the total voltage applied to each winding will be



So the total applied voltage will be reduced to 58% of total line voltage. For instance of total or line voltage is 400V, than in star configuration the total voltage applied to each winding is 220V.
On the other hand, the total voltage applied to each winding in delta configuration will be full line voltage.

Due to the reduced applied voltage in star configuration, the total starting current will be 1/sqrt(3) or 58% of the total current in delta configuration.

Star Delta starter diagram

The following figure shows the winding connections in star and delta configuration one by one.

It can be seen that in star connection, one end of all three windings are shorted to make star point while other end of each winding is connected to power supply. In delta configuration, the windings are connected such that to make a close loop. The connection of each winding is shown in above figure. In actual motor the three phase connections are provided in the following order as shown

So in order to make winding connection in star and delta style in practical motor, the connection is shown below.

To change the motor connections from star to delta, contactors are employed. To practically shift connection of motor from star to delta in running posito, three contactors are required. It is shown in the following star delta starter connection diagram

Main contractor is used to supply power to the windings. It must be turned on all the time. Initially the star contactor is closed while delta contactor is open It makes the motor windings in star configuration. When the motor gains speed, the star contactor is opened while delta contactor is closed turning the motor windings into delta configuration.

The contactors are controlled by using PLC. The following section of PLC tutorial will explain the ladder programming for star delta motor starter.

PLC Tutorial program for star delta motor starter

The ladder program for the star delta starter is written in LOGOSOFT PLC software as shown in following figure



Rung 1 Main contactor : The main contactor depends upon the normally open input start push button (I1), normally closed stop button (I2) and normally closed overload relay. It means that Main contactor will only be energized if start button is pressed, while stop is not pressed and overload relay is not activated.  A normally open input named (Q1) is added in parallel to the start button I1.  By doing so, a push button is created which means that once motor is started, it will be kept started even if start button is released

Programing of push button and other requirements for simple motor starter is explained in PLC Tutorial: Motor starter


Rung 2 Star contactor: Star contactor depends upon main contactor, normally close contacts of timer (T1), and normally close contacts of output delta contactor (Q3). So star contactor will only be energized if main contactor is ON, time output is not activated and delta contactor is not energized.


Timer T1: Timer T1 measures the time after which the winding connection of star delta starter is to be changed. It will start counting time after main contactor is energized.


Rung 3 Delta contactor: Delta contactor will be energized when main contactor (Q1) is energized, timer T1 is activated and star contactor (Q3) is de-energized.





Following video will show the plc tutorial of star delta motor starter in LOGO SOFT PLC software.

If you have any questions please ask in comments.