Introduction to Electric Circuits

108 Three-phase a.c. circuits

Coil A has two ends labelled a and a" coil B has two ends b and b'; coil C has

ends c and c'. End b of coil B is displaced by 2~r/3 radians from end a of coil A,

and end c of coil C is displaced by 27r/3 radians from end b of coil B (in a

clockwise direction). This means that if the coils are rotated in an anticlockwise

direction at an angular frequency of to radians per second then coil A passes the

N-pole of the magnetic field 27r/3to seconds ahead of coil B which in turn passes

the N-pole 27r/3to seconds ahead of coil C. The phasor diagram of the voltages

is given in Fig. 5.2 and shows the symmetry of the system.

Ea

Ec Eb

Figure 5.2

5.3 PHASE SEQUENCE

The order in which the coils pass a given point in an anticlockwise direction is

called the positive phase sequence of the three-phase system, and in the case

shown, in which coil A generates phase A, coil B generates phase B and coil C

generates phase C, the phase sequence (the word positive is understood) is

ABC. The negative phase sequence of this system is ACB. The phase sequence

of any system can be reversed by reversing the connections to two of the coils

(say B and C) as shown in Fig. 5.3(a) and (b).

Reversing the connections to any one coil simply upsets the symmetrical

nature of the system as illustrated in Fig. 5.4(a) and (b).

o)

a

c' b'

a'

(a)

S

Eb Ec

(b)

Figure 5.3