Electrical Engineering ⇒ Topic : Polyphase Circuits

Introduction

The a.c. circuits discussed so far in the book are termed as single phase circuits because they contain a single alternating current and voltage wave. The generator producing a single phase supply (called single-phase generator) has only one armature winding. But if the generator is arranged to have two or more separate windings displaced from each other by equal electrical angles, it is called a *polyphase generator and will produce as many independent voltages as the number of windings or phases. The electrical displacement between the windings depends upon the number of windings or phases. For example, a 2-phase generator has two separate but identical windings that are 90° electrical apart and rotate in a common uniform magnetic field. Obviously, such a generator will produce two alternating voltages of the same magnitude and frequency having a phase difference of 90°. Similarly, a 3-phase generator has three separate but identical windings that are 120° electrical apart and rotate in a common uniform magnetic field. A 3-phase generator will, therefore, produce three voltages of the same magnitude and frequency but displaced 120° electrical from one another.Although several polyphase systems are possible, the 3-phase system is by far the most popular because it is the most e cient of all the supply systems. In this chapter, we shall confine our attention to 3-phase system only.