Previous Year Question

Correct answer: A — Interchanging any pair of lines.

Meaning of phase sequence

Phase sequence is the order in which the three phase voltages reach corresponding points, such as their positive peaks. For a balanced R–Y–B system, the phase voltages are 120 electrical degrees apart, and their order repeats cyclically.

Interchange any two line connections while leaving the third unchanged. For example, exchanging Y and B changes R–Y–B into R–B–Y. The cyclic order is now reversed. The same reversal occurs when either of the other two pairs is interchanged.

In a three-phase motor, the time sequence of stator currents determines the direction of the rotating magnetic field. Reversing the phase sequence reverses this field direction and consequently changes the normal motor rotation direction.

A cyclic relabelling such as R–Y–B to Y–B–R does not reverse sequence; it merely changes the phase chosen as the starting reference. Similarly, shifting all three phase voltages by the same angle leaves their relative sequence unchanged. The required change is the interchange of two lines.