Three Phase Systems in Electrician And Wireman : Practice MCQ Questions & Answers with Explanations

The correct answer is B: Voltages.

Phase sequence in a three-phase system refers to the order in which the voltages of the three phases attain their maximum positive value.

Working Principle:

• In a three-phase system, the three phases are designated as R, Y, and B (or A, B, C).
• The voltages are sinusoidal and mutually displaced by 120 degrees.
• In a positive (standard) sequence, voltages peak in the order R → Y → B.
• Reversing the sequence (e.g., R → B → Y) changes the direction of motor rotation.

Importance of Phase Sequence:

• Motor Rotation: Phase sequence directly determines the direction of rotation of three-phase motors. An incorrect sequence causes reverse rotation, potentially damaging equipment.
• Power Distribution: Correct phase sequence is essential for proper operation of all equipment connected to a three-phase supply.
• Synchronization: Critical when synchronizing multiple generators or motors in a system.