Electrical Engineering is a free introductory textbook to the basics of electrical engineering. See the editorial for more information....

# Classes of Circuits

Author: E.E. Kimberly

Circuits energized by one alternating emf are called single-phase circuits. Circuits of more than two lines energized by two or more electromotive forces not all of which are in time phase with each other are called polyphase circuits. The use of polyphase circuits is justifiable for economic reasons and also because of the superior performance of machines operating on them. Industrial polyphase circuits are usually three-phase.

 Fig. 9-1. Relationships of Three-Phase emf's

#### The Use of the Double Subscript

In the analysis of single-phase circuits the use of the single subscript, as in E1 or E2 is sufficient and simple for designating sinusoidal and vector quantities. In polyphase circuits it is advantageous to use a double-subscript notation which aids in describing such quantities. For example, in Fig. 9-1 the vector E02 represents the voltage generated in conductor 2 with point 0 as reference. Likewise, a vector E20 would represent the same voltage but with reference to the near end of the conductor. Therefore, E20=-E02. The double subscript is then derived from the diagram of connections; and the vector point of origin is indicated by the first letter or figure of the subscript, and the termination of the vector is indicated by the second letter or figure of the subscript.

Last Update: 2010-10-05