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Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by electrical grids worldwide to transfer power. It is also used to power large motors and other heavy loads. A three-phase system is generally more economical than others because it uses less conductor material to transmit electric power than equivalent single-phase or two-phase systems at the same voltage. The three-phase system was invented by Galileo Ferraris and Nikola Tesla in 1887 and 1888.

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 electric 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 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 electric power systems:

• 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 or reduce the size of the neutral conductor; all the phase conductors carry the same current and so can be the same size, for a balanced load.
• Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations.
• Three-phase systems can produce a magnetic field that rotates in a specified direction, which simplifies the design of electric motors.

Most household loads are single-phase. In North American single-family dwellings, three-phase power generally does not enter the home; multiple-unit apartment blocks may have three-phase power but three-phase power is not used for household appliances. Areas that supply three-phase power for homes typically distribute only one phase to individual loads. Some large European appliances may be powered by three-phase power, such as electric stoves and clothes dryers.

Wiring for the three phases is typically identified by color codes which vary by country. Connection of the phases in the right order is required to ensure correct rotation of three-phase motors. For example, pumps and fans may not work in reverse. Maintaining the identity of phases is required if there is any possibility two sources can be connected at the same time; a direct interconnection between two different phases is a short-circuit.

Generation and distribution

At the power station, an electrical generator converts mechanical power into a set of three AC electric currents, one from each coil (or winding) of the generator. The windings are arranged such that the currents vary sinusoidally at the same frequency but with the peaks and troughs of their wave forms offset to provide three complementary currents with a phase separation of one-third cycle (120° or 2π⁄3 radians). The generator frequency is typically 50 or 60 Hz, varying by country.

At the power station, transformers change the voltage from generators to a level suitable for transmission.

After further voltage conversions in the transmission network, the voltage is finally transformed to the standard utilization before power is supplied to customers.

Transformer connections

A "delta" connected transformer winding is connected between phases of a three-phase system. A "wye" ("star") transformer connects each winding from a phase wire to a common neutral point.

In an "open delta" or "vee" system, only two windings are used. Such a connection is sometimes useful if a three-phase bank of transformers have had one unit fail, or when growth in the system is expected. Since out-of-phase currents flow in the windings, the rating of two transformers connected in open delta is only 57.7 % of their combined rating.

Where a delta-fed system must be grounded for protection from surge voltages, a grounding transformer (usually a zigzag transformer) may be connected to all three phases; this allows ground fault currents to return from any phase to ground. Another variation is a "corner grounded" delta system, in which one phase conductor is deliberately connected to ground.

Three-wire delta versus four-wire wye

Three-phase circuits occur in two basic configurations: three-wire (delta or wye) and four-wire wye. Both types have three energized ("hot" or "live") wires, but the four-wire wye circuit also has a neutral wire connected to the common wye point of each phase. The '3-wire' and '4-wire' designations do not count the ground wire used on many transmission lines which is solely for fault protection and does not deliver power.

A four-wire system with symmetrical voltages between phase and neutral is obtained when the neutral is connected to the "star point" of a wye winding. All three phases will have the same magnitude of voltages to the neutral in such a system. Other non-symmetrical systems have been used. In a high-leg delta system, one winding of a delta transformer feeding the system is center-tapped. This center tap is grounded.

Three-wire distribution systems need one less conductor and tend to redistribute unbalanced loading during transformation going back to the generation source. Faults on one phase to ground may go undetected until a second phase becomes faulted to ground. This can stress old insulation on conductors and equipment connected to the two remaining phases. When a second phase fault occurs problems can errupt mysteriously and seeming unrelated to any recent events at the location of the first fault . This could be on a different property or building.

The four-wire wye neutral wire is preferred when designers want ground referenced voltages and the flexibility of more voltage selections. Faults on one phase to ground will cause a protection event (fuse or breaker open) locally and not involve other phases or other connected equipment. A common example is seen in local distribution in Europe, where each house is connected to just one of the live wires, but each house's neutral wire is connected to one common neutral. When neighbouring houses draw unequal currents, the common neutral wire carries a current as a result of the imbalance. Hence electrical engineers work to make sure that the power is divided equally, so the neutral wire carries as little current as possible and therefore wastes little power. By distrbuting a large number of houses over all three phases, on average a nearly balanced load is seen at the point of supply.

Four-wire wye systems can also supply three wire loads. Neutrals do not need to be connected from the system to delta configured loads. Voltage specifications still must be observed.

In a three-phase, four-wire, delta (high-leg delta system, the neutral is a center tap in one of the delta phase supply windings. This can also be supplied by two single-phase transformers in a V formation (open delta).

Single-phase loads

Single-phase loads may be connected to a three-phase system in two ways. Load may be connected across any two phases, or a load can be connected from phase to neutral, if neutral is available.

Single-phase loads should be distributed evenly between the phases of the three-phase system for efficient use of the supply transformer and supply conductors. If the line-to-neutral voltage is a standard load voltage, single-phase loads can connect to a phase and the neutral. Loads can be distributed over three phases to balance the load.

In a symmetrical three-phase four-wire, wye system, the three phase conductors have the same voltage to the system neutral. The voltage between line conductors is ${\displaystyle {\sqrt {3}}}$ times the phase conductor to neutral voltage.

${\displaystyle V_{L-L}={\sqrt {3}}*V_{L-N}}$

The currents returning from the customers' premises to the supply transformer all share the neutral wire. If the loads are evenly distributed on all three phases, the sum of the returning currents in the neutral wire is approximately zero. Any unbalanced phase loading on the secondary side of the transformer will use the transformer capacity inefficiently.

If the supply neutral is broken, phase-to-neutral voltage is no longer maintained. Phases with higher relative loading will experience reduced voltage and phases with lower relative loading will experience elevated voltage, up to the phase-to-phase voltage.

A wye system provides phase-to-neutral relationship of ${\displaystyle V_{L-L}={\sqrt {3}}*V_{L-N}}$

A high-leg delta provides provides phase-to-neutral relationship of ${\displaystyle V_{L-L}=2*V_{L-N}}$, however, L-N load is imposed on one phase. A transformer manufacturer's page suggests that L-N loading to not exceed 5% of transformer capacity.

In some multiple-unit residential buildings of North America, three-phase power is supplied to the building but individual units have only single-phase power formed from two of the three supply phases. Lighting and convenience receptacles are connected from either phase conductor to neutral, giving the usual 120 V required by typical North American appliances. The phase-to-phase voltage is 208 volts.

Unbalanced loads

When the currents on the three live wires of a three-phase system are not equal or do not have the correct phases, the power-loss is greater than for a perfectly balanced system. The degree of imbalance is expressed by symmetrical components. Three-phase systems are evaluated at generating stations and substations in terms of these three components, of which two are zero in a perfectly balanced system.

Non-linear loads

With linear loads, the neutral only carries the current due to imbalance between the phases. Devices that utilize rectifier-capacitor front-end such as switch-mode power supplies, computers, office equipment and such produce third order harmonics that are in-phase on all the supply phases. Consequently, such harmonic currents add in the neutral which can cause the neutral current to exceed the phase current.

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, electric vehicles and many other kinds of motor-driven equipment. A three-phase motor is 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. Three-phase motors also vibrate less and hence last longer than single-phase motors of the same power used under the same conditions.

Line frequency flicker in light can be reduced by evenly spreading three phases across line frequency operated light sources so that illuminated area is provided light from all three phases. The effect of line frequency flicker is detrimental to super slow motion cameras used in sports event broadcasting. Three phase lighting has been applied successfully at the 2008 Beijing Olympic to provide consistent light level for each frame for SSM cameras. Resistance heating loads such as electric boilers or space heating may be connected to three-phase systems. Electric lighting may also be similarly connected.

Rectifiers may use a three-phase source to produce a six-pulse DC output.The output of such rectifiers is much smoother than rectified single phase and, unlike single-phase, does not drop to zero between pulses. Such rectifiers may be used for battery charging, electrolysis processes such as aluminium production or for operation of DC motors. The majority of automotive alternators produce power as three phase AC power which is rectified to DC through a diode bridge.

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

In Germany, a 1965 publication shows some "full size" stoves are designed for a three-phase feed. However, the individual heating units may be connected between phase and neutral to allow for connection by three individual circuits on the same single-phase supply. In many areas of Europe, single-phase power is the only source available.

Phase converters

Phase converters are utilized when three phase equipment needs to be operated on single phase power source. They're utilized by customers when three phase power is not available or cost is not justifiable. Such converters may also allow the frequency to be varied (resynthesis) allowing speed control. Some railway locomotives utilize single phase source to drive three phase motors fed through an electronic drive

Mechanical

One method to generate three phase power from a single phase source is the rotary phase converter, essentially a three-phase motor with special starting arrangements and power factor correction that produces balanced three-phase voltages. When properly designed, these rotary converters can allow satisfactory operation of three-phase motor on a single phase source. In such a device, the energy storage is performed by the mechanical inertia (flywheel effect) of the rotating components. An external flywheel is sometimes found on one or both ends of the shaft.

A three-phase generator can be driven by a single-phase motor. This motor-generator combination can provide a frequency changer function as well as phase conversion, but requires two machines with all their expense and losses. The motor-generator method can also form an uninterruptable power supply when used in conjunction with a large flywheel and a standby generator set.

Non-mechanical

A second method that was popular in the 1940s and 1950s was the transformer method. At that time, capacitors were more expensive than transformers, so an autotransformer was used to apply more power through fewer capacitors. Separated it from another common method, the static converter, as both methods have no moving parts, which separates them from the rotary converters.

Another method often attempted is with a device referred to as a static phase converter. This method of running three-phase equipment is commonly attempted with motor loads though it only supplies ⅔ power and can cause the motor loads to run hot and in some cases overheat. This method does not work when sensitive circuitry is involved such as CNC devices or in induction and rectifier-type loads.

Variable-frequency drives (also known as solid-state inverters) are used to provide precise speed and torque control of three-phase motors. Some models can be powered by a single-phase supply. VFDs work by converting the supply voltage to DC and then converting the DC to a suitable three-phase source for the motor.

Digital phase converters are designed for fixed-frequency operation from a single-phase source. Similar to a variable-frequency drive, they use a microprocessor to control solid-state power switching components to maintain balanced three-phase voltages.

Alternatives to three-phase

• Split-phase electric power is used when three-phase power is not available and allows double the normal utilization voltage to be supplied for high-power loads.

• Two-phase electric power, like three-phase, gives constant power transfer to a linear load. For loads that connect each phase to neutral, assuming the load is the same power draw, the two-wire system has a neutral current which is greater than neutral current in a three-phase system. Also motors are not entirely linear, which means that despite the theory, motors running on three-phase tend to run smoother than those on two-phase. The generators in the Adams Power Plant at Niagara Falls which were installed in 1895 were the largest generators in the world at the time and were two-phase machines. True two-phase power distribution is basically obsolete. Special-purpose systems may use a two-phase system for control. Two-phase power may be obtained from a three-phase system (or vice versa) using an arrangement of transformers called a Scott-T transformer.

• Monocyclic power was a name for an asymmetrical modified two-phase power system used by General Electric around 1897, championed by Charles Proteus Steinmetz and Elihu Thomson. This system was devised to avoid patent infringement. In this system, a generator was wound with a full-voltage single-phase winding intended for lighting loads and with a small fraction (usually ¼ of the line voltage) winding which produced a voltage in quadrature with the main windings. The intention was to use this "power wire" additional winding to provide starting torque for induction motors, with the main winding providing power for lighting loads. After the expiration of the Westinghouse patents on symmetrical two-phase and three-phase power distribution systems, the monocyclic system fell out of use; it was difficult to analyze and did not last long enough for satisfactory energy metering to be developed.

• High-phase-order systems for power transmission have been built and tested. Such transmission lines use six (two-pole, three-phase) or twelve (two-pole, six-phase) lines and employ 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 high-voltage direct current (HVDC) converter at each end of the line.

Color codes

Conductors of a three-phase system are usually identified by a color code, to allow for balanced loading and to assure the correct phase rotation for induction motors. Colors used may adhere to International Standard IEC 60446, older standards or to no standard at all and may vary even within a single installation. For example, in the U.S. and Canada, different color codes are used for grounded (earthed) and ungrounded systems.

• ^1 In Australia and New Zealand, active conductors can be any color except green/yellow, green, yellow, black or light blue. Yellow is no longer permitted in the 2007 revision of wiring code ASNZS 3000. European color codes are used for all IEC or flex cables such as extension leads, appliance leads etc. and are equally permitted for use in building wiring per AS/NZS 3000:2007.
• ^2 The international standard green-yellow marking of protective-earth conductors was introduced to reduce the risk of confusion by color blind installers. About 7% to 10% of men cannot clearly distinguish between red and green, which is a particular concern in older schemes where red marks a live conductor and green marks protective earth or safety ground.
• ^3 In Europe, there still exist installations with older colors for protective earth but, since the early 1970s, all new installations use green/yellow according to IEC 60446.
• ^4 See Paul Cook: Harmonised colours and alphanumeric marking. IEE Wiring Matters, Spring 2006.
• ^5 Since 1975, the U.S. National Electric Code has not specified coloring of phase conductors. It is common practice in many regions to identify 120/208Y conductors as black, red, and blue. Local regulations may amend the N.E.C. The U.S. National Electric Code has color requirements for grounded conductors, ground and grounded-delta 3-phase systems which result in one ungrounded leg having a higher voltage potential to ground than the other two ungrounded legs. Orange is only appropriate when the system has a grounded delta service, regardless of voltage.
• ^6 The U.S. National Electric Code does not specify coloring of phase conductors, other than orange for grounded delta. It is common practice in many regions to identify 277/480Y conductors as brown, orange and yellow (delta) or brown, violet and yellow (wye), with orange always being the center phase. Local practice may amend the N.E.C. The US N.E.C. rule 517.160 (5) states these colors are to be used for isolated power systems in health care facilities. Color of conductors does not identify voltage of a circuit, because there is no formal standard.
• ^7 In the U.S., a green/yellow striped wire may indicate an isolated ground. In most countries today, green/yellow striped wire may only be used for protective earth (safety ground) and may never be unconnected or used for any other purpose.

See also

• Three-phase AC railway electrification
• Charging station
• Frequency converter
• High-leg delta
• Industrial & multiphase power plugs & sockets
• International Electro-Technical Exhibition - 1891
• John Hopkinson
• Mikhail Dolivo-Dobrovolsky
• Nikola Tesla
• Y-Δ transform

v t e Nikola Tesla
Career and inventions	Patents Plasma lamp plasma globe Polyphase system Alternating-current commutatorless induction motor Tesla Experimental Station Teleforce Telegeodynamics Tesla coil history Wireless power Resonant inductive coupling Radio control Tesla turbine Tesla's oscillator Tesla valve Three-phase electric power Wardenclyffe Tower Tesla Electric Light and Manufacturing
Writings	The Inventions, Researches, and Writings of Nikola Tesla Colorado Springs Notes, 1899–1900 "Fragments of Olympian Gossip" My Inventions: The Autobiography of Nikola Tesla
Other	War of the currents Westinghouse Electric World Wireless System In popular culture
Related	Nikola Tesla Museum Nikola Tesla Memorial Center Tesla Science Center at Wardenclyffe IEEE Nikola Tesla Award Tesla Satellite Award Belgrade Nikola Tesla Airport New Yorker Hotel The Secret of Nikola Tesla (1980 film) Tesla – Lightning in His Hand (2003 opera) The Prestige (2006 film) Tower to the People (2015 documentary) Tesla (2016 film) The Tesla World Light (2017 film) The Current War (2019 film) "Nikola Tesla's Night of Terror" (2020 TV episode) Tesla (2020 film) Tesla: Man Out of Time Wizard: The Life and Times of Nikola Tesla The Man Who Invented the Twentieth Century Boat SI derived unit Lunar crater Asteroid

References

1. William D. Stevenson, Jr. Elements of Power System Analysis Third Edition, McGraw-Hill, New York (1975). ISBN 0-07-061285-4. Page 2.
2. http://www.allaboutcircuits.com/vol_2/chpt_10/2.html
3. Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed., 1917, vol. 4, Ch. 46: Alternating Currents, p. 1026, fig. 1260.
4. Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed., 1917, vol. 4, Ch. 46: Alternating Currents, p. 1026, fig. 1261.
5. H. W. Beaty, D.G.Fink (ed) Standard Handbook for Electrical Engineers Fifteenth Edition,McGraw-Hill, 2007 ISBN 0-07-144146-8 , page 10-11
6. ^ Lowenstein, Michael. "The 3rd Harmonic Blocking Filter: A Well Established Approach to Harmonic Current Mitigation". IAEI Magazine. Retrieved 24 November 2012.
7. Federal pacific
8. Enjeti, Prasad. "Harmonics in Low Voltage Three-Phase Four-Wire Electric Distribution Systems and Filtering Solutions" (PDF). Texas A&M University Power Electronics and Power Quality Laboratory. Retrieved 24 November 2012.
9. Hui, Sun. "Sports Lighting – Design Considerations For The Beijing 2008 Olympic Games" (PDF). GE Lighting. Retrieved 18 December 2012.
10. IEEE
11. www.rle.mit.edu/per/conferencepapers/cpconvergence00p583.pdf
12. "British and European practices for domestic appliances compared", Electrical Times, volume 148, page 691, 1965.
13. Japan Railway & Transport Review. No. 58: 58. 2011 http://www.jrtr.net/jrtr58/pdf/51-60web.pdf. {{cite journal}}: |volume= has extra text (help); Cite has empty unknown parameter: |1= (help); Missing or empty |title= (help); Unknown parameter |month= ignored (help)
14. Canadian Electrical Code Part I, 23rd Edition, (2002) ISBN 1-55324-600-X, rule 4-036 (3)
15. Canadian Electrical Code 23th edition 2002, rule 24-208(c)

• Electric power
• Electrical engineering
• Electrical wiring
• Nikola Tesla