Three-Phase Circuit

three-phase circuit

[′thrē ¦fāz ′sər·kət]
A circuit energized by alternating-current voltages that differ in phase by one-third of a cycle or 120°.

Three-Phase Circuit


(or three-phase system), a combination of three single-phase AC circuits—the phases—in which there are three alternating voltages of identical frequency that

Figure 1. Three-phase circuits: (a), (b), and (c) with star-connected generator windings, (d) and (e) with delta-connected generator windings, (a), (b), and (d) with star-connected phase loads, (c) and (e) with delta-connected phase loads, (a) with neutral conductor, (b) without neutral conductor; (A), (B), and (C) line wires, (N) neutral conductor, (G) generator, (L) load, (Ul) and (Up) line and phase voltages, (ll) and (lp) line and phase currents

are shifted in phase with respect to one another. The three-phase circuit is a special case of a multiphase system.

Balanced three-phase systems, whose voltages are sinusoidal and of equal magnitude and have a phase shift of 120°, are the most common. Each phase of a three-phase system may constitute an independent electric circuit, or the phases may be electrically interconnected. Electrically interconnected systems are usually used in practice. In the simplest case, such systems consist of the phase windings of a three-phase generator, three power receivers (phase loads), and connecting line wires (see Figure 1). If the phase windings of the generators and the phase loads are star-connected, the circuit may also include a neutral conductor, which connects the neutral points of the generator and load. The neutral conductor reduces the influence of each phase’s operation on the other phases and makes possible the connection of single-phase loads to three-phase circuits.

The voltages between the line wires are called line voltages, and the currents flowing through the conductors are called line currents. Currents flowing through the phase windings of the generator and through the phase loads are called phase currents, and the voltages across the windings and loads are called phase voltages. Generally speaking, the line currents and voltages differ from the phase currents and voltages.

The three-phase system has become the most widely used system in power engineering. It was first proposed in 1891 by M. O. Dolivo-Dobrovol’skii, who developed all the basic components of the system, including generators, transformers, transmission lines, and three-phase electric motors. A three-phase circuit is more economical than a single-phase AC circuit, and it provides a substantial reduction in ripple of the rectified current. The use of three-phase circuits simplifies the generation of a rotating magnetic field in AC electric motors.


Obshchaia elektrotekhnika. Edited by V. S. Pantiushin. Moscow, 1970.
Teoreticheskie osnovy elektrotekhniki, vol. 1. Moscow, 1972.


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