Phase sequence

PHASE SEQUENCE

In fig 4.1(b), the peak value of voltage is the same in all the phases. But, the maximum values are attained at different times. The order in which the alternating quantities attain their maximum values is called the phase sequence. For the clockwise direction of rotation of the rotor field, the phase sequence is R Y B. It means that, the voltage in Y-phase reaches its maximum value 120° later than that in phase R. And, the voltage in B-phase reaches its maximum value 120° later than that in Y- phase.

If the direction of rotation of the field is reversed (ie., anti clockwise direction), the phase sequence becomes RBY.

RYB is called positive phase sequence whereas RBY is called negative phase sequence. Unless specified clearly, we take positive phase sequence.

The phasors can be named as R,Y, B, or A, B, C or 1, 2, 3. We prefer R,Y, B to other methods. It is because, we can give different colours to the conductors for easy identification (R-Red, Y-Yellow, and B-Blue or Black).