Page 426 - Basic Electrical Engineering

rotating by placing the field magnets on the stator and the coil on a
cylindrical rotor. What is required is to produce a relative motion between the
magnetic field and the conductor. The magnitude of the induced EMF will

depend upon the number of coils, the strength of the magnetic field, and the
speed of rotation of the magnet. The frequency of the induced EMF will

depend upon the number of magnetic poles confronted by the coils per
revolution. Normally, the number of magnetic poles and the speed of rotation

of the magnetic poles by a drive, usually a turbine, is kept constant. The
number of coils and number of turns used in each coil are kept as per design

and are constant once the machine is constructed. That is why the magnitude
of the induced EMF and its frequency is constant. Thus, we get a single-
phase voltage from the single-phase winding which can be used to supply an

electric circuit comprising resistance, inductance, and capacitance elements.

Figure 4.1 Generation of single-phase voltage

In a three-phase system we will have three-phase voltages induced in the
three-phase windings of the generator. In all generating stations three-phase

generators are installed.

4.2 ADVANTAGES OF THREE-PHASE SYSTEMS

A three-phase system has a number of advantages over a single-phase

system. Some of these are mentioned as follows:

i. the output of a three-phase machine generating electricity is more than the output of a single-
phase machine of the same size;
ii. the most commonly used three-phase induction motors are self starting. For single-phase