Star Delta Starter Principle Circuit Diagram and Explanation

Star Delta Starter Principle Circuit Diagram and Explanation

Introduction:- A three phase motor has three windings with terminal numbers A1-A2, B1-B2 and C1,C2. In some motor terminal numbers are U1-U2, V1-V2 and W1-W2. These three winding can be connected in two way known as star connection or delta connection. In motor name plate we can get motor type as three phase ac induction motor, kw as 5kw 10 kw, voltage 230 or 400, star or delta conection full load current power factor efficiency duty cycle etc.

Star connection:- In star connection the second point of each winding ie A2, B2, C2 are conected (shorted) to each other making a point known as star point. And remaining three points A1, B1, C1 are connected to 3 phase supply points R phase Y phase and B phase respectively. The connection is as shown in diagram. In star connection the voltage given to each winding known as phase voltage is equal to line voltage/1.732 (root 3). And phase current is equal to line current.

Delta connection:- In Delta connection the second point A2 of first winding is connected to first point B1 of second winding. Again the second point B2 of second winding is connected to first point C1 of third winding. And second point C2 of third winding is connected to first point A1 of first winding. It forms a tringle shape closed circuit by three  winding as arm of tringle thats why it is known as Delta connection. In delta connection winding phase voltage is equal to line voltage of supply. But the phase current is line current/root 3.

So as per name plate voltage and connection a motor can be connected in star or delta connection.
If a motor name plate shows delta connection then  it can be run first  in star  and then in delta.This motor can be used with star delta starter.
If a motor is 400 volt star connection it can not be used with star delta starter because motor can not connected in delta connection.

Principle of star delta starter:- while starting a motor 3 phase supply is given to motor by contactor. While giving direct full voltage to motor causes very high current which causes line voltage drop. To reduce starting current a method used is known as reduce voltage starting and full voltage runnung scheme is used. This is achieved by two way ie

l. by using star delta starter
2. by using auto transformer srarter

Here we will discuss star delta strater.

In old time manual star delta starter was used. but now automatic star delta starters are used.

Construction;- It consist of three starter and one time relay and two push button overload relay. all parts are connected as shown in circuit diagram.

When start button is pressed contactor c1 and c3 pick up and motor start in star conection with reduced voltage taking low current. after few seconds when motor picks up speed time relay oprates and contactor c3 drops and c2 pickup which changes motor connection to delta. now motor runs at full voltage. The overload relay provides protection against overload condition by trippning the motor. When stop button pressed both the contactors drop and motor stops.time relay coil is connected in parallen with c1 contactor coil. c2 and c3 nc contact is used in interlocking so that c2 can pick up only when c3 gets drop.similarly c3 can only pick up when c2 is drop.
I hope this will help full to you. please comment for any more information.


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