16.3 Three-Phase Systems & Phase Sequence

Three-Phase Systems & Phase Sequence

Quick Answer: Three windings spaced 120° apart produce three EMFs displaced by 120 electrical degrees. In a star-connected Australian supply, Vₗ = √3 × Vₚ gives 230 V phase and 400 V line. Balanced three-phase power is P = √3 × Vₗ × Iₗ × cos φ. Always confirm phase rotation with an indicator before energising rotating plant.

Why Three Phase

Three phases deliver constant total power to a balanced load, allow smaller conductors for the same power transferred, and produce the rotating magnetic field that makes induction motors start without additional apparatus. Every commercial and industrial installation you will meet on the capstone practical is three phase at the switchboard even if the final subcircuits are single phase.

Generation and the 120° Displacement

An alternator carries three windings physically spaced 120° apart in the stator. As the rotor field sweeps past them, each winding reaches its peak EMF one third of a cycle after the previous one. At 50 Hz, one third of a 20 ms cycle is 6.67 ms.

Because the three sinusoids sum to zero at every instant, a perfectly balanced star-connected load draws no neutral current. Real installations are never perfectly balanced, which is why the neutral is still installed and still sized properly, and why harmonic currents from electronic loads can make neutral current exceed line current in some circuits.

Star (Wye) Connection

The three winding ends are joined at a common star point, which becomes the neutral and, in a MEN installation, is earthed at the distributor’s transformer.

• Line voltage Vₗ = √3 × phase voltage Vₚ → 230 × 1.732 = 398 V, nominally 400 V.
• Line current Iₗ = phase current Iₚ.

That single relationship explains the Australian 400/230 V description: 400 V between any two actives, 230 V between any active and the neutral.

Delta Connection

The three windings are connected end to end in a closed loop, with the lines taken from the junctions. There is no neutral.

• Line voltage Vₗ = phase voltage Vₚ.
• Line current Iₗ = √3 × phase current Iₚ.

Delta is common for motor windings and for the primary of distribution transformers. A three-phase motor nameplate showing 400 V delta / 690 V star tells you which connection suits which supply.

Power and Energy in Three-Phase Circuits

For a balanced load:

• Apparent power S = √3 × Vₗ × Iₗ (VA)
• True power P = √3 × Vₗ × Iₗ × cos φ (W)
• Energy W = P × t (Wh, or kWh when P is in kW and t in hours)

Worked example. A balanced three-phase load draws 25 A per line at 400 V with a power factor of 0.85.

1. S = 1.732 × 400 × 25 = 17,320 VA ≈ 17.3 kVA
2. P = 17,320 × 0.85 = 14,722 W ≈ 14.7 kW
3. Running 8 hours a day: W = 14.7 × 8 = 117.8 kWh per day

Reverse-worked example — the version that appears on maximum demand papers. A 30 kW balanced load at 400 V and 0.9 power factor draws:

Iₗ = P / (√3 × Vₗ × cos φ) = 30,000 / (1.732 × 400 × 0.9) = 30,000 / 623.5 = 48.1 A

That current — not the kilowatt figure — is what you take into cable selection and protective device coordination.

Phase Sequence and Rotation

Phase sequence is the order in which the three phases reach their positive peaks: L1–L2–L3 (often called ABC or clockwise) or the reversed L1–L3–L2.

Why it matters:

• A three-phase induction motor rotates in the direction set by the sequence. Reverse any two lines and the motor runs backwards — which can destroy a pump, a conveyor, a compressor or a lift.
• Parallel sources and generator changeover schemes require matching sequence.
• Multi-board installations must keep sequence consistent so downstream plant behaves predictably.

Methods of determining rotation:

1. Rotating-disc or LED phase rotation indicator connected to the three lines — the standard site instrument, giving a clear clockwise/anticlockwise or L1-L2-L3 indication.
2. Two-wattmeter or phase-angle instruments in engineering contexts.
3. Oscilloscope comparison of waveforms where instrumentation permits.

Trial-and-error — energising the motor and watching which way it spins — is not an acceptable capstone answer where the driven machine can be damaged or people can be hurt.

Correcting rotation: swap any two line conductors at the isolator or motor terminals. Never swap the neutral or the protective earthing conductor to change rotation. Re-label conductors correctly after the change, retest polarity, and record the result.

Practical Discipline

• Confirm rotation before connecting rotating plant, and after any board or supply alteration.
• Check that phase identification is consistent from origin to final board — crossed phases between boards is a defect even when everything still runs.
• Measure and record all three line-to-line and line-to-neutral voltages during commissioning; unequal readings point to a loose connection, a lost phase or an unbalanced load.
• Remember that a lost neutral on a star-connected installation lets connected single-phase loads see up to line voltage — one of the most destructive faults in a domestic or commercial board.