Benefits of three-phase over single-phase delivery

Three-phase AC delivers power with three voltages displaced by 120°. It is preferred over single-phase for generation, transmission and distribution because:

1. Constant power: Total instantaneous power in a balanced three-phase system is constant ($p = 3V_{ph}I_{ph}\cos\phi$), while single-phase power pulsates at twice the supply frequency. This gives smooth torque and less vibration in machines.
2. Less conductor material: For the same power, voltage, distance and loss, a 3-phase 3-wire line needs only 75 % of the conductor of a single-phase 2-wire line.
3. Higher machine output: A 3-phase generator or motor of the same frame size gives about 1.5 times the output of a single-phase machine, and has higher efficiency and p.f.
4. Self-starting motors: 3-phase induction motors produce a rotating magnetic field and start by themselves; single-phase motors need auxiliary windings or capacitors.
5. Cheaper transformers and switchgear per kVA.
6. Parallel operation of 3-phase alternators is easier and steadier.
7. Flexibility: A 3-phase 4-wire system gives both 400 V (3-phase loads) and 230 V (single-phase loads) from the same supply.
8. Better voltage regulation and lower losses for the same power transfer.