Page 273 - Basic Electrical Engineering

load when connected across the battery terminals. Since the magnitude is
assumed constant, it is represented through a graph as shown in Fig. 3.1 (b).
Fig. 3.1 (c) shows the nature of the current when the switch is turned ‘ON’

and ‘OFF’ at regular constant intervals. By using a reverse switching
arrangement, as shown in Fig. 3.1 (d), we can have an ac flowing through the

load. Here, through a reversing switch, supply terminal 1 is connected to load
terminal 2′ and supply terminal 2 is connected to load terminal 1′. It can be

noted that if the period of switching ON in both the directions in kept
constant, the load current will be alternating in nature and its wave shape will

be square or rectangular type as shown in Fig. 3.1 (e) and 3.1 (f).
To generate ac from an available dc source, we need an automatic
switching arrangement. This is achieved by using electronic circuitry as in the

case of inverters used as emergency lighting arrangement.
However, alternating current on a large scale is made available by using ac

generators installed in power houses. AC generators are driven by turbines
(gas, steam, water). Turbines are used to create a relative motion between a

set of magnets and a set of coils. The rate of change of magnetic flux linkages
or the rate of cutting of flux by the conductors of the coils causes EMF to be

induced in the coil windings. The relative motion between the coils and the
magnets producing a magnetic field can be created by making one system
rotating with respect to the other. For example, we may have a stationary

magnetic field system and inside the magnetic field we can place the coils
which will be rotated by a prime mover (i.e., a turbine).

Alternately, the coils could be kept stationary and a set of magnets could
be made rotating, thus causing EMF to be induced in the coils. We shall see
the nature of EMF induced when we create a relative motion between a set of

coils and a magnetic field. For simplicity we will consider only one coil
rotating in a magnetic field created by a North and a South Pole.

3.1.2 Generation of Alternating Voltage in an Elementary Generator

In Fig. 3.2 (a) is shown a coil having a few turns rotated in a magnetic field.
If ϕ is the flux produced in Webers in the magnetic field and N is the number