Question

Please answer the bleow question (typing but not handwriting):

In the industries, Y-∆ starter is usually used to start the induction
motors. What is the significance of using Y-∆ starter?

Answer 1

First of all we have to understand why we need a starter?

In case of induction motor the rotor winding is short circuited ,so when a rotating flux produced by stator 3 phase winding cuts the rotor conductor an emf induces in rotor winding. Since the coil is short circuited a current will flow in rotor conductors. The magnitude of this rotor current will depend upon the emf induced.

Initially during start, the rotor is at rest so the slip will be 1 and due to this during start the large emf will induce,which will result in large current. This large current may damage rotor winding. So we need a starter to have less current during starting of induction motor.

Star Delta starter is mainly used because it's cheap and easy in maintenance. What actually happens in star Delta starter is ,it's set up is such that during start the stator winding is connected in star so the phase voltage will get reduced by a factor of 1/^{3. Hence the rotor emf will also reduced and current too(transformer action). In running condition the stator winding gets connected into Delta to have rated voltage across stator winding .}

^{​​ In running condition Current will not be high because emf induced in rotor winding will be less due to less slip less than 1. You might be thinking how this connection change is possible in the mid way operation. This is done by a special triple throw switch and some relay arrangements. It happens automatically.}