Page 425 - Basic Electrical Engineering

4

Three-phase System

TOPICS DISCUSSED

Advantages of a three-phase system
Three-phase windings and their connections
Active and reactive power
Measurement of power in three-phase circuits

4.1 INTRODUCTION

Generation, transmission, and distribution of electricity is done by three-

phase electrical networks consisting of generators, transformers, and
transmission and distribution lines forming the power system. In a three-
phase system we have three independent voltages induced in the three

windings of the generator. To understand the difference between a single-
phase voltage and a three-phase voltage, let us consider how these voltages

are generated in ac generators. We have known that EMF is induced in a coil
if it cuts lines of force. In Fig. 4.1 we have placed one coil in slots of a
hollow cylindrical stator core. A two-pole magnet is rotated at a particular

speed by some means. The flux lines will cut the conductors and EMF will be
induced in the coil. Since North and South poles’ flux will cut the conductors

alternately, an alternating single-phase voltage will be induced in the coil.
Here, we have seen that the coil is stationary and the magnetic field is

being rotated. However, we could have had the field stationary and the coil