Single and Three-Phase Supplies

Most domestic supplies are single-phase AC with a phase and neutral wire. However, most electricity is generated and transmitted as three-phase AC. Rather than having a single coil rotating in a magnetic field, three-phase generators have three coils fixed at 120° to each other; thus three voltages, that are 120° out of phase with each other, are produced in three separate circuits (figure 4.1).

The phases are normally called red (R), yellow (Y) and blue (B) or line 1 (L1), line 2 (L2) and line 3 (L3), or in the USA they are called A-phase, B-phase and C-phase.

       Figure 4.1: A three-phase AC supply with 240V RMS generated in each coil:
                   (a) Generation; (b) Wave diagram; (c) Phasor diagram.


The advantages of using three-phase AC for transmission and distribution of electricity are:

(a) Less copper (or aluminium) is needed for the conductors of a three-phase system which transmits    a given power at a given voltage over a given distance than for a simple single-phase system.

(b) Three-phase motors have many advantages over single phase motors, including smaller size,steady torque output, and the ability to self-start.

(c)when connected in parallel, single-phase generators present difficulties which do not occur with three phase generators.