**1.Introduction**

utilisation of A.C. electric energy almost invariably involve a type of

system or circuit called a polyphase system or polyphase circuit. In

such a system, each voltage source consists of a group of voltages

having relative magnitudes and phase angles. Thus, am-phase system will

employ voltage sources which, conventionally, consist of m voltages

substantially equal in magnitude and successively displaced by a phase

angle of 360° I m.

consist of three voltages substantially equal in magnitude and

displaced by phase angles of 120°. Because it possesses definite

economic and operating advantages, the 3-phase system is by far the most

common, and consequently emphasis is placed on 3-phase circuits.

**2.Advantages of Poly phase Systems**

1. A polyphase transmission line requires less conductor material

than a single-phase line for transmitting the same amount power at the

same voltage.

2. For a given frame size a polyphase machine gives a higher output

than a single-phase machine For example, output of a 3-phase motor is

1.5 times the output of single-phase motor of same size

3. Polyphase motors have a uniform torque where most of the single-phase motors have a pulsating torque.

4. Polyphase induction motors are self-starting and are more

efficient. On the other hand single phase induction motors are not

self-starting and are less efficient.

5. Per unit of output, the polyphase machine is very much cheaper.

6. Power factor of a single-phase motor is lower than that of polyphase motor of the same

7. Rotating field can be set up by passing polyphase current through stationary coils.

8. Parallel operation of polyphase alternators is simple as compared

to that of single-phase alternators because of pulsating reaction in

single-phase alternator.

It has been found that the above advantages are best realized in the

case of three-phase systems. Consequently, the electric power is

generated and transmitted in the form of three-phase system

**3.Generation of Three-phase Voltages**

Fig: 1. On the armature are three coils, ll’, mm’, and nn’ whose axes

are displaced 120° in space from each other.

When the field is excited and rotated, voltages will be generated in

the three phases in accordance with Faraday’s law. If the field

structure is so designed that the flux is distributed sinusoidally over

the poles, the flux linking any phase will vary sinusoidally with time

and sinusoidal voltages will be induced in three-phases. These three

waves will be displaced 120 electrical degrees (Fig. 2) in time as a

result of the phases being displaced

120° in space. The corresponding phasor diagram is shown in Fig. 3.

The equations of the instantaneous values of the three voltages (given

by Fig. 2) are:

e l’ l = E max .. sin wt

e m ‘m = E max. sin (wt- 120°)

e n ‘n = E max. sin (wt – 240°)

The sum of the above three e.m.fs. is always zero as shown below :

Resultant instantaneous e.m.f.

= e l’ l + e m’ m + e n’ n

= E max. sin wt+ E max. sin (wt- 120°) + E max. sin ( wt – 240°)

= E max. [sin rot + (sin wt cos 120° – cos wt sin 120° + sin wt cos 240° – cos wt sin 240°)]

= E max. [sin rot+ (- sin wt cos 60° – cos rot sin 60° – sin wt cos 60° + cos wt sin 60°)]

= E max. (sin rot – 2 sin wt cos 60°)

= E max. (sin rot- sin wt) = 0.

**4.PHASE SEQUENCE AND NUMBERING OF PHASES **

*By phase sequence is meant the order in which the three phases attain their peak or maximum, *

of the field system in Fig. 1 was assumed. This assumption made the

e.m.f. of phase ‘

**m**‘ lag behind that of ‘

**l**‘ by 1200 and in a similar way, made that of ‘

**n**‘ lag behind that of ‘

**m**‘ by 120° (or that of

**l**by 240°). Hence, the order in which the e.m.fs. of phases

**l, m**and

**n**attain their maximum value is

**Imn.**It is called the

*phase order*or phase sequence

**l → m → n**. If now the rotation offield structure of Fig. 1 is reversed

*i.e.*made

counter-clockwise, then the order in which three phases would attain

their corresponding maximum voltages would also be reversed. The phase

sequencewould become

**l → n → m**. This means that e.m.f. of phase

**‘n’**would now lag behind that of phase ‘

**l**‘ by 120° instead of 240° as in the previous case.

The phase sequence of the voltages applied to a load, in general, is

determined by the order in which the 3-phase lines are connected. The *phase sequence can be reversed *by *interchanging any pair of lines *. (In the case of an induction motor, reversal of sequence results in the reversed direction of motor rotation).

The three-phases may be *numbered ***l, m, n **or 1, 2, 3 or they may be given three colours(as is customary).

The colours used commercially are *red, yellow *(or sometimes white) and *blue. *In this case sequence is RYE.

Evidently in any three-phase system, there two possible sequences, in

which three coils or phase voltages may pass through their maximum

value *i.e., *red → yellow → blue (RYE) or red → blue → yellow (RBY).

By convention:

RYE ….. taken as *positive. *

RBY ….. taken as *negative. *

**5. INTER-CONNECTION OF THREE PHASES **

**[**one ‘start’ (S) and another ‘finish’

*(F)*]and if

individual phase is connected to a separate load circuit, as shown in Fig. 4, we get a non-interlinked

3-phasesystem. In such a system each circuit will require two conductors, therefore, 6 conductors

all. This makes the whole system

*complicated*and

*expensive.*Hence the

*three phases are gene*

interconnected which results in substantial saving of copper.

interconnected which results in substantial saving of copper.

The general method of inter-connections are:

1. Star or Wye (Y) connection.

2. Mesh or delta (∆) connection.

**TO BE CONTINUOUS……….**

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