Phase Sequence

In a three-phase system, the order in which the voltages attain their maximum positive value is called Phase Sequence. There are three voltages or EMFs in the three-phase system with the same magnitude, but the frequency is displaced by an angle of 120 deg electrically.

Taking an example, if the phases of any coil are named as R, Y, B then the Positive phase sequence will be RYB, YBR, BRY also called clockwise sequence and similarly the Negative phase sequence will be RBY, BYR, YRB respectively and known as an anti-clockwise sequence.

It is essential because of the following reasons:

1. The parallel operation of the three-phase transformer or alternator is only possible when its phase sequence is known.
2. The rotational direction of the three-phase induction motor depends upon its sequence of phase on three-phase supply. And thus to reverse its direction the phase sequence of the supply given to the motor has to be changed.