Editing Three-phase electric power (section)

== Principle ==
[[File:3 phase AC waveform.svg|thumb|Normalized [[waveform]]s of the instantaneous voltages in a three-phase system. The graph maps voltages over time for one whole cycle of the system. Time starts from left, and increases towards the right side. Each of the three phases starts and ends the cycle in the same value, as each cycle is (ideally) identical. The phase order is 1–2–3. This sequence repeats each cycle, and thus the [[rotational frequency]] of the generator sets the  [[utility frequency|frequency]] of the power system. Ideally, each phase's [[voltage]], current, and power is offset from the others' by 120°, thus having all at equidistance. This symmetry can also be recreated in converters.]]

[[File:Three Phase Electric Power Transmission.jpg|thumb|Three-phase electric power transmission lines]]

[[File:Transzformator-allomas.jpg|thumb|Three-phase transformer (Békéscsaba, Hungary): On the left are the primary wires, and on the right are the secondary wires.]]

In a symmetric three-phase power supply system, three conductors each carry an [[alternating current]] of the same frequency and voltage amplitude relative to a common reference, but with a phase difference of one third of a cycle (i.e., 120 degrees out of phase) between each. The common reference is usually connected to ground and often to a current-carrying conductor called the neutral. Due to the phase difference, the [[voltage]] on any conductor reaches its peak at one third of a cycle after one of the other conductors and one third of a cycle before the remaining conductor. This phase delay gives constant power transfer to a balanced linear load. It also makes it possible to produce a rotating magnetic field in an [[electric motor]] and generate other phase arrangements using transformers (for instance, a two-phase system using a [[Scott-T transformer]]). The amplitude of the voltage difference between two phases is <math>\sqrt{3} = 1.732\ldots</math> times the amplitude of the voltage of the individual phases.

The symmetric three-phase systems described here are simply referred to as ''three-phase systems'' because, although it is possible to design and implement asymmetric three-phase power systems (i.e., with unequal voltages or phase shifts), they are not used in practice because they lack the most important advantages of symmetric systems.

In a three-phase system feeding a balanced and linear load, the sum of the instantaneous currents of the three conductors is zero. In other words, the current in each conductor is equal in magnitude to the sum of the currents in the other two, but with the opposite sign. The return path for the current in any phase conductor is the other two phase conductors.

Constant power transfer is possible with any number of phases greater than one.  However, two-phase systems do not have neutral-current cancellation and thus use conductors less efficiently, and more than three phases complicates infrastructure unnecessarily.
Additionally, in some practical generators and motors, 
two phases can result in a less smooth (pulsating) torque.<ref>{{Cite book|last=von Meier|first=Alexandra|title=Electric Power Systems|publisher=John Wiley & Sons, Inc.|year=2006|isbn=978-0-471-17859-0|location=Hoboken, New Jersey|pages=160|quote=We also stated one rationale for this three-phase system; namely, that a three-phase generator experiences a constant torque on its rotor as opposed to the pulsating torque that appears in a single- or two-phase machine, which is obviously preferable from a mechanical engineering standpoint.}}</ref>  

Three-phase systems may have a fourth wire, common in low-voltage distribution. This is the [[ground and neutral|neutral]] wire. The neutral allows three separate single-phase supplies to be provided at a constant voltage and is commonly used for supplying multiple [[Single-phase electric power|single-phase]] loads. The connections are arranged so that, as far as possible in each group, equal power is drawn from each phase. Further up the [[electric power distribution|distribution system]], the currents are usually well balanced. Transformers may be wired to have a four-wire secondary and a three-wire primary, while allowing unbalanced loads and the associated secondary-side neutral currents.

=== Phase sequence ===
{{see also|Electric machine#Sequence}}
Wiring for three phases is typically identified by colors that vary by country and voltage. The phases must be connected in the correct order to achieve the intended direction of rotation of three-phase motors. For example, pumps and fans do not work as intended in reverse. Maintaining the identity of phases is required if two sources could be connected at the same time. A direct connection between two different phases is a [[short circuit]] and leads to flow of unbalanced current.