Phase Sequence
By phase sequence is meant the order in which the three phases attain their peak or maximum values. In the development of the three-phase e.m.fs. in Figure (A) & (B), clockwise rotation of the field system in Figure (C) was assumed. This assumption made the e.m.fs. of phase ‘b’ lag behind that
of ‘a’ by 120° and in a similar way, made that of ‘c’ lag behind that of ‘b’ by 120° (or that of ‘a’ by 240°). Hence, the order in which the e.m.fs. of phases a, b and c attain their maximum values is a b c. It is called the phase order or phase sequence a→b→c as illustrated in Figure (A).
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If, now, the rotation of the field structure of Figure (A) & (B) is reversed i.e. made anticlockwise, then the order in which the three phases would attain their corresponding maximum voltages would also be reversed. The phase sequence would become a→b→c. This means that e.m.f. of phase ‘c’ would now lag behind that of phase ‘a’ by 120° instead of 240° as in the previous case as shown in Figure (B). By repeating the letters, this phase sequence can be written as acbacba which is the same thing as cba. Obviously, a three-phase system has only two possible sequences : abc and cba (i.e. abc read in the reverse direction).
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