Three-phase electric power

Three-phase power transformer which is the sole transferpoint for electricity to a suburban shopping mall in Canada. Note the four wires used for the 208V/120Y service: one is for the neutral, and the other three are for the X, Y, and Z phases.

Three-phase is a common method of electric power transmission in industrialised countries. It is a type of polyphase system.

This article deals with where, how and why three phase is used. For information on the basic maths and principles of three phase please see three-phase. For information on testing three phase kit please see three-phase testing.

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 high voltage 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 distribution. Firstly all three wires carry the same current. Secondly power transfer into a linear balanced load is constant.

Most domestic loads are single phase. Generally three phase power either doesn't enter homes 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 the table for more information.

Colour Codes

Three phase color codes. WARNING - CODES ARE NOT ALWAYS FOLLOWED. Colours used may adhere to old standards or to no standard at all, and may vary even within a single installation.

	L1	L2	L3	Neutral	Earth
North America	Black	Red	Blue	White	Green
UK until April 2006	Red	Yellow	Blue	Black	green/yellow striped (green on very old installs)
Europe (including UK) from April 2004	Brown	Black	Grey	Blue	green/yellow striped
Previous European (varies by country)	Brown or black	Brown or black	Brown or black	Blue	green/yellow striped

Generation and Distribution

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 with different phases. In a three-phase system the phases are spaced equally, giving a phase separation of 120°. The frequency is typically 50 Hz in Europe and 60 Hz in the US (see List of countries with mains power plugs, voltages & frequencies).

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 transmisison and distribution network the power is finally transformed to the standard mains voltage. The power may already have been split into single phase at this point or it may still be three phase. Where the stepdown is 3 phase the output of this transformer is nearly always star connnected with the standard mains voltage (120 V (in the US) or 230 V (in Europe)) being the phase-neutral voltage.

Where three phase at low voltage is in use it may be split out into single phase service cables through joints in the supply network or it may be delivered to a master distribution board (breaker panel) at the customer's premises. Connecting an electrical circuit from one phase to the neutral supplies the country's standard single phase voltage (120 Vac or 230 Vac) to the circuit.

The power transmission grid is organized so that each phase carries the same magnitude of current out of the major parts of the transmission system. The currents returning from the customers' premises to the last supply transformer all share the neutral wire, but the three-phase system ensures that the sum of the returning currents is approximately zero. The delta wiring of the primary side of that supply transformer means that no neutral is needed in the high voltage side of the network.

Connecting Phase-Phase

Connecting between two phases provides √3 or 173% of the single-phase voltage (208 VAC in US; 400 VAC in Europe) because the out-of-phase waveforms add to provide a higher peak voltage in the resulting waveform. Such connection is referred to as a line to line connection and is usually done with a two pole circuit breaker. This kind of connection is typically used for high power appliances, such as (in the US) a 2kW, 208 volt baseboard heater.

Three Phase Loads

The most important class of three-phase load is the electric motor. A three phase induction motor has a simple design, inherently high starting torque, and high efficiency. Such motors are applied in industry for pumps, fans, blowers, compressors, conveyor drives, and many other kinds of motor-driven equipment. A three-phase motor will be more compact and less costly than a single-phase motor of the same voltage class and rating; and single-phase AC motors above 10 HP (7.5 kW) are uncommon.

Large air conditioning equipment (e.g. most York units above 2.5 tons (8.8 kW) output power) use three-phase motors for reasons of efficiency and economy.

Resistance heating loads such as electric boilers or space heating may be connected to three-phase systems. Electric lighting may also be similarly connected. These types of loads do not require the revolving magnetic field characteristic of three-phase motors but take advantage of the higher voltage and power level usually associated with three-phase distribution.

Large rectifier systems may have three-phase inputs; the resulting DC current is easier to filter (smooth) than the output of a single-phase rectifier. Such rectifiers may be used for battery charging, electrolysis processes such as aluminum production, or for operation of DC motors.

An interesting example of a three-phase load is the electric arc furnace used in steelmaking and in refining of ores.

In much of Europe stoves are designed to allow for a three phase feed. Usually the individual heating units are connected between phase and neutral to allow for connection to a single phase supply where this is all that is available.

Phase Converters

Occasionally the advantages of three-phase motors make it worth-while to convert single-phase power to three phase. Small customers, residential or farm properties may not have a three-phase supply or may not want to pay for the extra cost of a three-phase service, but may still wish to use three-phase equipment.

One method is with a rotary converter, essentially a three-phase motor with special starting arrangements that produces a three-phase system. When properly designed these rotary converters can allow satisfactory operation of three-phase equipment such as machine tools on a single phase supply.

Some devices are made which create an imitation three-phase from single phase center-tapped power (240 volts AC in the United States; with phase separation of 180°). This is done by creating a third "subphase" between the two polarities, resulting in a phase separation of 180° - 90° = 90°. Many three-phase devices will run on this configuration, but at lower efficiency.

Solid-state inverters also can be used to power three-phase motors from a single-phase supply.

Alternatives to three-phase

Split phase (single phase centre tapped) is useful when high voltage three phase is not available. It gives some of the cable saving and efficiency advantages of a higher voltage whilst still allowing normal voltage single phase loads to be powered directly.

Two phase Like three phase, gives constant power transfer to a linear load. But in a three wire system it has a neutral current which is greater than the phase currents. Also motors aren't entirely linear and this means that despite the theory motors running on three phase tend to run smoother than those on two phase. True two-phase power distribution is essentially obsolete. Special purpose systems may use a two-phase system for control.

High phase order systems for power transmission have been built and tested. Such transmission lines use 6 or 12 phases and design practices characteristic of extra-high voltage transmission lines. High-phase order transmission lines may allow transfer of more power through a given transmission line right-of-way without the expense of a HVDC converter at each end of the line.

Three phase electrical outlets

see main article Industrial & multiphase power plugs & sockets

The image on the left shows a 3 phase socket. The one on the right shows a triplex outlet with neons in the top sockets for phase indication. Both are American. Underneath the three phase socket on the left there are breaker positions marked in small writing. These are 8, 10 and 12 representing the red, black and blue phases respectively.

Three phase power can be supplied either by the use of a three phase socket, or by triplexing. Most North American receptacles are duplex receptacles. The top and bottom sockets can also be separated, if desired, and, for example, supplied by separate breakers with a common neutral. This is typically done in kitchens where a high load will likely be placed on both sockets. In this case, a common trip 2-pole breaker is often used.

The concept of duplexing can be generalized to triplexing, so that three duplex receptacles can be supplied by a common neutral, from a 3-phase supply. Typically, a 3 pole common trip 15 A breaker is used to supply such a socket. This enables three single phase loads to be supplied in a phase-sequenced manner. An example of such a load is a light fixture having three bulbs. For flicker-free operation, three bulbs are each fitted with a separate plug, and driven 120 degrees out of phase with one another, from a triplex receptacle. The top receptacles shown in the figure, are fitted with neon night lights to indicate phase sequence, for triplex loads where proper phase sequence is desired.

This can, however, create problems in enviroments that have large quantities of dimmed circuits, such as theatres. Harmonic 'noise' created by dimming equipment can effectivley overload a combined neutral and result in a fire. Combined neutrals can also cause inconsistent response from individual circuits in this situation. Although not the case in older facilites, in all new installs of large-scale theatrical dimming equipment, manufacturers require individual neutrals to maintain warranty status of the equipment.