Introduction to Electric Circuits

72 Single-phase a.c. circuits

Figure 4.5

N

a

• tf ,.,A A
  oB
  b

Single-phase a.c. quantities

Fig. 4.5 shows an elementary generator. It consists of a single coil having sides a

and b rotating in a magnetic field produced by permanent magnets. As the coil

sides are cutting magnetic flux, they will have emfs (voltages) induced in them.

These emfs could be measured at terminals A and B and would be alternating

in nature as the coils come under the influence of first a north pole and then a

south pole alternately. Because there are only two terminals and only one emf

can be generated in the coil, the a.c. generated is said to be single phase. The

emf produced by this simple machine would be alternating but not sinusoidal.

By careful machine design, however, generators can be made to generate

sinusoidal emfs.

4.2 SINGLE-PHASE A.C. CIRCUITS IN THE STEADY

STATE

Steady state operation means that any transient effects following the switching

on of a circuit have died away and that the waveforms of voltages and currents

are continuous sine waves.

Purely resistive circuits

Fig. 4.6 shows a single-phase voltage source V supplying a pure resistor R. The

arrowheads indicate that if the voltage is going positive in the direction shown

by its arrowhead then the current will be going positive in the direction shown

vtC DR

Figure 4.6