5 Three-phase a.c. circuits
5.1 INTRODUCTION
Three-phase has a number of advantages over single-phase:
9 A three-phase machine of a given physical size gives more output than a
single-phase machine of the same size and most electrical power generation
is carried out by means of three-phase synchronous generators.
9 There is a considerable amount of saving in conductor material to be gained
by using three-phase rather than single-phase for the purposes of power
transmission by overhead lines or underground cables.
* The three-phase induction motor is the cheapest and most robust of
machines and accounts for the vast majority of the world's industrial
machines.5.2 GENERATION OF THREE-PHASE VOLTAGES
We saw in Chapter 4 that a single-phase a.c. voltage is generated by rotating a
single coil in a magnetic field. A three-phase a.c. system is generated by rotating
three coils in a magnetic field, the coils being mutually displaced in space by
2~r/3 radians (120 ~ as shown in Fig. 5.1(a). The waveforms of the three-phase
system of voltages thus produced are shown in Fig. 5.1(b). The coils in which
the voltages are generated constitute the armature, while the system producing
the magnetic field is called the field system. In the large generators found in
power stations the armature system is stationary and it is the field system which
is made to rotate.Figure 5.1
O)
a
b' c'a'
(a) (b)