4. PHASE SEQUENCE AND NUMBERING OF PHASES

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

In the generation of three-phase e.m.fs. in Fig. 2 clockwise rotation 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.