In 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 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 as clockwise sequence and similarly the Negative phase sequence will be RBY, BYR, YRB respectively and known as an anti-clockwise sequence.
- The parallel operation of three phase transformer or alternator is only possible when its phase sequence is known.
- The rotational direction of 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.