Three Phase Supply

A three-phase supply is generated when three coils are placed 120° apart and the whole rotated in a uniform magnetic field as shown in Fig (a). The result is three independent supplies of equal voltages which are each displaced by 120° from each other as shown in Fig (b).

The convention adopted to identify each of the phase voltages is: R-red, Y-yellow, and B-blue, as shown in Fig.

The phase-sequence is given by the sequence in which the conductors pass the point initially taken by the red conductor. The national standard phase sequence is R, Y, B.

A three phase a.c supply is carried by three conductors, called ‘lines’ which are coloured red, yellow and blue. The currents in these conductors are known as line currents (I_L) and the p.d.’s between them are known as line voltages (V_L). A fourth conductor, called the neutral (coloured black, and connected through protective devices to earth) is often used with a three phase supply.

If the three-phase windings shown in Fig are kept independent then six wires are needed to connect a supply source (such as a generator) to a load (such as motor). To reduce the number of wires it is usual to interconnect the three phases. There are two ways in which this can be done, these being:

(a) a star connection, and (b) a delta, or mesh, connection. Sources of three-phase supplies, i.e. alternators, are usually connected in star, whereas three-phase transformer windings, motors and other loads may be connected either in star or delta.