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.


References in periodicals archive ?
It is clear from the frequency spectrum that the sideband harmonics components have greater values in the case of short circuit due to unbalance situation in the three-phase circuit of the stator winding.
Watt meters measuring only one phase of a three-phase circuit
A spokeswoman for power company Central Networks said: "These businesses need a larger power supply than homes and are run on a three-phase circuit.
The main types of construction works and approximate quantities, Including but not limited to: Dismantling of a reinforced concrete column 360 or 590,835 with the restoration of the ladder around the ladder and transporting the dismantled pillar to the warehouse of the contracting authority - 65; Dismantling of single-phase circuit breaker (automatic or load) -200; Dismantle of three-phase circuit breaker (automatic or load) -100; Installation of reinforced concrete column 250 / 9,5 - 600 pcs .; Installation of reinforced concrete column 835 / 9,5 - 90; Installation of reinforced concrete pillar ncg 951/13 or 952/13 -30pcs; Download vip for intercooler with delivery of lv fittings - 1350 pcs.
Developed equipment meets requirements and allows connecting to the network the motor or any three-phase circuit at the time instant of voltage crossing zero forward negative direction.
Proximity effect comes into play when there are multiple bus bars, as in a three-phase circuit. Searching the technical literature shows very little work in the area of measuring proximity effect, with the exception of round conductors.
In the traditional approach to modeling, based on the transition from the real three-phase circuit to the single-phase equivalents (in symmetrical components, d-q-0, [alpha]-[beta]-0 coordinates, etc.) such models in principle can not be implemented, as the transition itself strictly justified and is possible only when there is symmetry and sinusoidal [1].
The padmount capacitor banks shall be 1200 kVAR, dead front and ungrounded wye-connected to a 12,470-volt, three-wire, effectively grounded, three-phase circuit. (While the 12kV underground system has a concentric neutral on the underground power cables, it is not used as a current carrying conductor but is utilized as shield ground).
The three-phase circuits consist of output panels, each equipped with output receptacles protected by circuit breakers.
He covers magnetism, magnetic induction, inductance in alternating-current circuits, single-phase isolation transformers, autotransformers, current transformers, three-phase circuits, three-phase transformers, single-phase loads for three-phase transformers, transformer installation, transformer cooling, transformer maintenance, harmonics, direct-current generators, direct-current motors, alternators, three-phase motors, single-phase motors, motor maintenance and troubleshooting, motor nameplate data, and motor installation.
In addition, the device can accommodate 36 single-phase circuits, 12 three-phase circuits, or any combination in between.
Other topics include electric power systems, three-phase circuits, electromechanical energy conversion, synchronous machines, and direct-current machines.