*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 120^{0} 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. *

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