Electrical Engineering ⇒ Topic : Phase sequence

Phase sequence. The order in which the voltages in the three phases (or coils) of an alternator reach their *maximum positive values is called phase sequence or phase order.The phase sequence is determined by the direction of rotation of the alternator. Thus in Fig. a (i), the three coils A, B and C are producing voltages that are displaced 120 electrical degrees from one another. Referring to the wave diagram or phasor diagram in Fig. a, it is easy to see that voltage in coil A attains maximum positive value first, next coil B and then coil C. Hence the phase sequence is ABC. If the *direction of rotation of the alternator is reversed, then the order in which the three phases attain their maximum positive values would be ACB. Hence, the phase sequence is now ACB i.e. voltage in coil A attains maximum positive value first, next coil C and then coil B. Since the alternator can be rotated in either clockwise or anticlockwise direction, there can be only two possible phase sequences.

Phase sequence is important in certain applications. For example, in 3-phase induction motors,the phase sequence of the supply determines whether the motor turns clockwise or anticlockwise.

                                                                        figure (a)