Three-phase power
Three AC supplies offset in time, giving smooth power and a rotating field for motors.
What it is
Three-phase power is three separate AC supplies of the same voltage, offset from each other by a third of a cycle. Together they deliver power more smoothly and efficiently than a single phase.
How it works
Because the three phases peak at different moments, the total power delivered is far steadier than the pulsing of a single phase. The same offset lets three windings create a smoothly rotating magnetic field — which is exactly what an induction motor needs to turn.
Loads can connect phase-to-phase or phase-to-neutral. Balancing single-phase loads across the three phases keeps the system efficient and the neutral current low.
Why it matters
Three-phase is the backbone of commercial and industrial supply and almost all larger motors. Imbalance between phases, or loss of a phase, causes overheating and nuisance tripping (see single-phasing).
Related faults
Three-phase distribution board badly unbalanced
One phase of a three-phase board runs much hotter / higher current than the others — nuisance tripping on that phase, a hot neutral, or a warm phase conductor at the board.
Three-phase equipment single-phasing (lost a phase)
Three-phase equipment is misbehaving — motors humming, struggling, overheating, or tripping — because one phase has been lost somewhere between the supply and the load.
Three-phase supply voltage high, low, or unbalanced
Measured supply voltages are out of the expected range or unbalanced between phases — causing equipment trips, poor performance, or protection operating across the board.
Related definitions
Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.
Single-phasing
What happens when a three-phase load loses one phase — and why it's so damaging to motors.
Neutral vs earth
The neutral carries return current; earth is for safety. Confusing them causes real, dangerous faults.
Distribution & sub-mains
How power is split from the main board into final circuits and sub-boards, with protection at each level.