Q. What do you mean by Phase Sequence?

It is standard practice in the United States to designate the phase A-B-C such that under balanced conditions the voltage and current in the A-phase lead in time the voltage and current in the B-phase by 120° and in the C-phase by 240°. This is known as positive phase sequence A-B-C. The phase sequence should be observed either from the waveforms in the time domain shown in Figure (a) or from the phasor diagrams shown in Figure (b), and not from space or schematic diagrams, such as Figures (a). If the rotation of the generator of Figure is reversed, or if any two of the three leads from the armature (not counting the neutral) to the generator terminals are reversed, the phase sequence becomes A-C-B (or C-B-A or B-A-C), which is known as negative phase sequence.

Only the balanced three-phase sources are considered in this chapter. Selection of one voltage as the reference with a phase angle of zero determines the phase angle of all the other voltages in the system for a given phase sequence. As indicated before, the reference phasor is chosen arbitrarily for convenience. In Figure(b), ¯V_BC is the reference phasor, and with the counterclockwise rotation (assumed positive) of all the phasors at the same frequency, the

sequence can be seen to be A-B-C. Unless otherwise mentioned, the positive phase sequence is to be assumed.