phase sequence

phase sequence

1. What Phase Sequence Is

Phase sequence (phase order) = order in which the three phase voltages/currents reach their peak as time progresses.

• Positive (ABC) sequence: $v_a$ peaks first, then $v_b$ (120° later), then $v_c$ (240°) — i.e., $v_a =V_m sin ωt$, $v_b =V_m sin(ωt-120°)$, $v_c =V_m sin(ωt+120°)$. Also written RYB (Red-Yellow-Blue).
• Negative (ACB) sequence: $v_a$, then $v_c$, then $v_b$ — opposite rotation: $v_a =V_m sin ωt$, $v_b =V_m sin(ωt+120°)$, $v_c =V_m sin(ωt-120°)$.

Swapping any two phases reverses sequence (e.g., swap $b$ and $c$ ⇒ sequence flips).

2. Why Phase Sequence Matters

• Direction of rotating magnetic field in 3-phase motor depends on sequence: ABC ⇒ clockwise, ACB ⇒ counter-clockwise (or vice versa depending on winding). So swapping two supply wires reverses motor direction — used to reverse induction motor.
• Phase-sequence indicator checks before connecting large motors or paralleling generators (wrong sequence causes large circulating current).

Oscilloscope: three traces 120° apart; observe which peaks first.

Phase-sequence meter: neon lamps or electronic detector.

In phasor diagram: ABC goes counter-clockwise (a at 0°, b at -120°, c at +120°).

5. Exam Points

• Define phase sequence as order peaks occur (ABC vs ACB) — 3-5 marks, very frequent.
• Show $v_a=V_m sinωt$, $v_b=V_m sin(ωt-120°)$, $v_c=V_m sin(ωt+120°)$ for ABC — 5 marks, frequent.
• Explain swapping two phases reverses motor direction — 5 marks, very frequent (common practical question).
• Mention rotating field direction depends on sequence — 3-5 marks, frequent.