The star – delta starter requires an induction motor with six winding ends brought out of its terminal box. The starter can be operated manually using changeover switch or automatically which we will be explaining in this article.

The supply voltage per phase in star is only 58% of the value which would be applied if the windings were connected in delta and this reduces the starting torque and current to 1/3. This starting method is limited to light ans medium loads i.e. centrifugal pumps and fans having low inertia.

Picture 1 shows a wiring diagram of an automatic star – delta starter which involves three contactors K1, K2, K3 and the timer T1.

Star delta wiring diagram.

This is how it works :

When the start button is pressed, star contactor coil K2 is energized via normally closed contact K3 and timer contact T1. At the same time contactor coil K1 is energized. The motor windings are now connected in star. There is an interlock between the star and delta contactors but after o short period of time the T1 contacts open disconnecting star contactor K2, and energizing delta contactor K3.

To get some more information about star – delta starter please visit our Youtube channel