This is very simple;If r1,r2,r3 are the arms of the delta then the formula converting them into star;

R12=R1*R2/R1+R2+R3

R13=R1*R3/R1+R2+R3

R23=R2*R3/R1+R2+R3

Rab = Ra + Rb + [Ra.Rb]/Rc.

Rbc = Rb + Rc + [Rb.Rc]/Ra.

Rca = Rc + Ra + [Rc.Ra]/Rb.

this is the case when you have to convert star into delta...where Ra Rb Rc is the resistance conected through the arms in star and Rab ,,,,are that connected in DELTA.

Ra = [Rab].[Rbc]/K.....{ where K is Rab + Rbc + Rca}.

similarly Rb and Rc,,,,,,,,,

 you can easily convert a complicated network consisting of stars and deltas using star delta transformations.

a star connection can easily be converted into a delta connecton and and vice versa.

for conversion of star to delta-

                                                       Ra=(R1R2+R2R3+R3R1)/R2

                                                       Rb=(R1R2+R2R3+R3R1)/R3

                                                       Rc=(R1R2+R2R3+R3R1)/R1

for conversion of delta to star-

                                                       R1=(RaRb)/(Ra+Rb+Rc)

                                                       R2=(RbRc)/(Ra+Rb+Rc)

                                                       R3=(RcRa)/(Ra+Rb+Rc)

hence,with the help of above formulae you can convert a complicated network into a simple one.if you need i can provide u with the proof for the above formulae.

Biki, a goiitian, wrote an article on this :

In many circuit applications, we encounter components connected together in one of two ways to form a three-terminal network: the ?Delta,? or ? (also known as the ?Pi,? or ?) configuration, and the ?Y? (also known as the ?T?) configuration.

It is possible to calculate the proper values of resistors necessary to form one kind of network (? or Y) that behaves identically to the other kind, as analyzed from the terminal connections alone. That is, if we had two separate resistor networks, one ? and one Y, each with its resistors hidden from view, with nothing but the three terminals (A, B, and C) exposed for testing, the resistors could be sized for the two networks so that there would be no way to electrically determine one network apart from the other. In other words, equivalent ? and Y networks behave identically.

There are several equations used to convert one network to the other:

? and Y networks are seen frequently in 3-phase AC power systems (a topic covered in volume II of this book series), but even then they're usually balanced networks (all resistors equal in value) and conversion from one to the other need not involve such complex calculations. When would the average technician ever need to use these equations?

A prime application for ?-Y conversion is in the solution of unbalanced bridge circuits, such as the one below:

Solution of this circuit with Branch Current or Mesh Current analysis is fairly involved, and neither the Millman nor Superposition Theorems are of any help, since there's only one source of power. We could use Thevenin's or Norton's Theorem, treating R₃ as our load, but what fun would that be?

If we were to treat resistors R₁, R₂, and R₃ as being connected in a ? configuration (R_ab, R_ac, and R_bc, respectively) and generate an equivalent Y network to replace them, we could turn this bridge circuit into a (simpler) series/parallel combination circuit:

After the ?-Y conversion . . .

If we perform our calculations correctly, the voltages between points A, B, and C will be the same in the converted circuit as in the original circuit, and we can transfer those values back to the original bridge configuration.

Resistors R₄ and R₅, of course, remain the same at 18 ? and 12 ?, respectively. Analyzing the circuit now as a series/parallel combination

the equivalent resistance of the circuit canbe calculated very very easily....

 

 

Unbalanced bridges can be cracked very easily by this method instead of using the long and clumsy Kirchoff's Voltage Law....

 

 

http://www.goiit.com/posts/list/community-shelf-making-star-out-of-delta-is-of-great-use-in-19381.htm

 

p.s " ?" r kept as it was

Ramyani has given detailed explanation about delta, star, pi..... connections of resistors. Congrats

thanxxx a lot.................may god bless you all and hope all of you get in top 100 of jee.