PHASE SEQUENCE

In a three-phase system, there are three voltages having same magnitude and frequency displaced by an angle of 120° electrical. They are attaining their positive maximum value in a particular order. The order in which the voltages (or emfs) in the three phases attain their maximum positive value is called the phase sequence.

The emfs induced in the three coils or phases attain their positive maximum value in the order of a₁a₂, b₁b₂, and c₁c₂; therefore, the phase sequence is a, b, and c. However, if the coils or phases are being named out as R, Y, and Bin place of a, b, and c,respectively, then the phase sequence will be RYB.

The sequence RYB (or YBR or BRY) is considered as positive phase sequence, while the RBY (or BYR or YRB) is considered as negative phase sequence. The sequence knowledge of phase sequence is essential in the following important applications:

1. The direction of rotation of three-phase induction motors depends upon the phase sequence of three-phase supply. To reverse the direction of rotation, the phase sequence of the supply given to the motor has to be changed.
2. The parallel operation of three-phase alternators and transformers is only possible if phase sequence is known.