Analyzing of three phase circuits

INTRODUCTION :

A Single phase a.c voltage can be generated by rotating turn made up of conductors,in a magnetic field.But voltage produced by such a single turn is very less and not enough to supply practical loads.so here no of turns are connected in series to form one winding .such a winding is called is called armature winding.

Generation,transmission and heavy-power utilization A.C electric energy almost invariably involve a type of system or circuit called poly phase or 3 phase circuit . 

A three phase system will employ voltage sources which,conveniently consist of three voltages substantially equal in magnitude and displaced by 120 degree.

Three phase circuits:

There are two types of circuits available in our electrical power. They are single phase circuits and 3 ph circuits.

1 phase circuits :Here 1 ph circuits having only two conductors.They are phase wire and neutral wire.Then 3 ph circuits having 3 wire circuits such as (R,Y,B).

Each phase are having a phase difference of 120^o. So from the total of 360^o (R,Y,B),three phases are equally divided into 120^o each. The power in three phase system is continuous as all the three phases are involved in generating the total power.
3 ph star connected system:

from above fig ,having three ph star connected system such as R,Y,B and its phase shift is 120 deg.three impedance having such as Z1,Z2,Z3.
The star connection formed by connecting starting or terminating end of all three winding's together.This common point is neutral point(N).These ends are generally referred as R-Y-B to which load is to be connected.
concept of Line voltage and Line current for star connections:

Line voltage :The potential difference between any two lines of supply is called line voltage.Line voltage denoted as VL.

VL = √3 Vph for star connection

Thus line voltage is √3 times phase voltage in star connections.

Line current :current passing through any line of three lines is called Line current.Line current is denoted as IL.

IL = Iph for star connection

 3 ph delta connections :

from above fig having delta connected system.If three impedance Z1,Z2,Z3 are connected such that starting end one of one is connected to terminating end of other,to form a closed loop it is called delta connection.

IL = √3 Iph for delta connection

Thus line current is √3 times the phase currents in delta connection

VL = Vph for delta connection

Important definitions to Related to Three phase system:

1.Symmetrical system :

It is possible in poly phase system that magnitudes of different alternating voltages are different.But a three phase system in which the three voltages are same magnitude and frequency and displaced from each other by 120 deg phase angle is defined as symmetrical system.

2.Phase sequence :

The sequence in which the voltages in three phases reach their maximum positive values is called Phase sequence.Generally the phase sequence is R-Y-B.

The phase sequence is important in determining direction of rotation of ac motors,parallel operations of alternators etc.

Advantages of 3 ph system :

1.For transmission and distribution,three phase system needs less copper or less conducting material than single phase system for given volt amperes and voltage rating so transmission becomes very much economical. 

2.It is possible to produce rotating magnetic field with stationary coils by using 3 ph system.

3.Three ph system gives steady output.

4.single phase supply can be obtained from three phase but three can not be obtained from single phase.

5.power factor of single phase motors is poor than three phase motors of same rating.

6.For converting machines like rectifiers,the d.c output voltage becomes smoother if number of phases are increased.