Article From : CableOrganizer.com

Three-phase electric power is a common method of electric power transmission. It is a type of polyphase system mainly used to power motors and many other devices. A three-phase system uses less conductor material to transmit electric power than equivalent single-phase, two-phase, or direct-current systems at the same voltage. 

In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency) which reach their instantaneous peak values at different times. Taking one conductor as the reference, the other two currents are delayed in time by one-third and two-thirds of one cycle of the electrical current. This delay between "phases" has the effect of giving constant power transfer over each cycle of the current, and also makes it possible to produce a rotating magnetic field in an electric motor.

Three phase systems may or may not have a neutral wire. A neutral wire allows the three phase system to use a higher voltage while still supporting lower voltage single phase appliances. In highvoltage distribution situations it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).

Three phase has properties that make it very desirable in electric power systems. First, the phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load. This makes it possible to eliminate the neutral conductor on some lines; all the phase conductors carry the same current and so can be the same size, for a balanced load. Second, power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations. Finally, three-phase systems can produce a magnetic field that rotates in a specified direction, which simplifies the design of electric motors. Three is the lowest phase order to exhibit all of these properties.

Most domestic loads are single phase. Generally three phase power either does not enter domestic houses at all, or where it does, it is split out at the main distribution board.

The three phases are typically indicated by colors which vary by country. See thetable for more information.

Animation of three-phase current flow




Elementary six-wire three-phase alternator, with each phase using a separate pair of transmission wires




Elementary three-wire three-phase alternator, showing how the phases can share only three transmission wires

At the power station, an electrical generator converts mechanical power into a set of alternating electric currents, one from each electromagnetic coil or winding of the generator. The currents are sinusoidal functions of time, all at the same frequency but offset in time to give different phases. In a three-phase system the phases are spaced equally, giving a phase separation of one-third cycle. The power frequency is typically 50 Hz in Asia, Europe, South America and Australia, and 60 Hz in the US and Canada (but see Mains power systems for more detail).

Generators output at a voltage that ranges from hundreds of volts to 30,000 volts. At the power station, transformers "step-up" this voltage to one more suitable for transmission.

After numerous further conversions in the transmission and distribution network the power is finally transformed to the standard mains voltage (i.e. the "household" voltage). The power may already have been split intosingle phase at this point or it may still be three phase. Where the stepdown is 3 phase, the output of this transformer is usually star connected with the standard mains voltage (120 V in North America and 230 V in Europe and Australia) being the phase-neutral voltage. Another system commonly seen in North America is to have a delta connected secondary with a center tap on one of the windings supplying the ground and neutral. This allows for 240 V three phase as well as three different single phase voltages (120 V between two of the phases and the neutral, 208 V between the third phase (known as a high leg) and neutral and 240 V between any two phases) to be made available from the same supply.