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Three-phase electric power
Three-phase transformer with four wire output for 208Y/120 volt service: one wire for neutral, others for A, B and C
phases
Three-phase electric power transmission
Three-phase electric power is a common method of alternating-current electric
power generation, transmission, anddistribution.[1] 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 usually more economical than an equivalent single-phase or twophase system at the same voltage because it uses less conductor material to transmit electrical
power.[2] The three-phase system was independently invented by Galileo Ferraris, Mikhail DolivoDobrovolsky and Nikola Tesla in the late 1880s.

Details[
In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency)
which reach their instantaneous peak values at one third of a cycle from each other. Taking one current 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 anelectric 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 loads. In high-voltage distribution situations,
it is common not to have a neutral wire as the loads can simply be connected between phases (phasephase 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 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 threephase power is not used for household appliances. Utilities that supply three-phase power for lower-loaddensity area 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
Animation of three-phase current flow
Left: Elementary six-wire three-phase alternator, with each phase using a
separate pair of transmission wires.[3] Right: Elementary three-wire threephase alternator, showing how the phases can share only three wires. [4]
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.
Further information: Mains power systems
At the power station, transformers change the voltage from generators to a level suitable
for transmission minimizing losses.
After further voltage conversions in the transmission network, the voltage is finally transformed to the
standard utilization before power is supplied to customers.
The majority of automotive alternators produce power as three phase AC power which is rectified to DC
through a diode bridge.[5]
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 "V" system, only two sets of transformers are used. A closed delta system can operate
as an open delta if one of the transformers has failed or needs to be removed.[6] In open delta, each
transformer must carry current for its respective phases as well as current for the third phase, therefore
capacity is reduced to 87%. With one of three transformers missing and the remaining two at 87%
efficiency, the capacity is 58% ((2/3)
× 87%).[7][8]
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[according to whom?]; this allows ground fault
currents to return from any phase to ground.[according to whom?] Another variation is a "corner grounded" delta
system, which is a closed delta that is grounded at one of the junctions of transformers.[9]
Three-wire and four-wire circuits
Wye (Y) and Delta (Δ) circuits
There are two basic three-phase configurations: delta or wye (star). Either type can be wired for three or
four wires. The fourth wire is provided as a neutral. 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 "common star point" of an all supply windings. 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 setup
produces three voltages. If the voltage between center tap and the two adjacent phases is 100%, the
voltage across any two phases is 200% and neutral to "high leg" is ≈ 173%.[6]
Three-wire distribution systems need one less conductor and help redistribute unbalanced loading during
transformation going back to the generation source.[citation needed]
The four-wire wye system is used when ground referenced voltages or the flexibility of more voltage
selections are required. 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.[citation needed] An example of application is
a local distribution in Europe, where each customer is fed a phase and a neutral. When other customers
sharing the neutral draw unequal currents, the common neutral wire carries a current as a result of the
imbalance. Electrical engineers try to design the system so the loads are balanced as much as possible. By
distributing a large number of houses over all three phases, on average a nearly balanced load is seen at
the point of supply.[citation needed]
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. [10]
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.
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 √3 times the phase conductor to neutral
voltage.
VL-L = √3 VL-N[11]
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.[citation needed]
A high-leg delta provides phase-to-neutral relationship of VL-L =
2 VL-N , however, L-N load is imposed
on one phase.[6] A transformer manufacturer's page suggests that L-N loading to not exceed 5% of
transformer capacity.[12]
√3 is ≈ 1.73, so if VL-N was defined as 100%, VL-L would be ≈
as 100%, then VL-N ≈
100% × 1.73 = 173% If VL-L was set
57.7%
Unbalanced loads
When the currents on the three live wires of a three-phase system are not equal or are not at an exact
120° phase angle, 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.[citation needed]
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.[10][13]
Three-phase loads
A transformer for a high-leg delta system; 200 V 3-phase motors would be connected to L1, L2 and L3. 200 V
Single-phase load would be connected L1 and L2. Single phase 100 V load between either L1 or L2 and
neutral (N). L3 (wild or high leg) will be 173.2 V to neutral.
An 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.[citation needed] Such motors are applied
in industry for many applications. 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.[citation needed] Three-phase motors also vibrate less and hence last longer than singlephase motors of the same power used under the same conditions.[citation needed]
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 Olympics to provide consistent light
level for each frame for SSM cameras.[14]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.[15] 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.
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.[16]
Phase converters
Phase converters are used when three-phase equipment needs to be operated on a single-phase
power source. They are used 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 use a single-phase source to drive three-phase motors fed through an electronic
drive.[17]
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 factorcorrection that
produces balanced three-phase voltages. When properly designed, these rotary converters can allow
satisfactory operation of a 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 2/3
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 twowire system has a neutral current that 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 threephase tend to run smoother than those on two-phase. The generators in the Adams Power
Plant at Niagara Falls that 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, invented by Charles F. Scott.[18]

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 1/4
of the line voltage) winding that 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 typically would use six phases or twelve phases. High-phase-order transmission lines allow
transfer of slightly less than proportionately higher power through a given volume without the
expense of a high-voltage direct current (HVDC) converter at each end of the line. However, they
require correspondingly more pieces of equipment.
Color codes
See also: Electrical wiring#Color code
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.
Country
L1
L2
L3
Neutral
Ground /
protective earth
Australia and New Zealand as per
AS/NZS 3000:2007 Figure 3.2 (or as
per IEC 60446 as approved by
AS:3000)
White (or
Red (or
black)[note
brown)[note 1]
(prev.
1]
yellow)
Dark
Black
Green/yellow
blue (or (or
striped (green on
grey)[note blue)[note very old
1]
1]
installations)
Canada (mandatory)[19]
Red
Black
Blue
White or Green or bare
Grey
copper
Canada (isolated three-phase
installations)[20]
Orange
Brown
Yellow
White
Green
European Union and all countries who
use European CENELEC standards
April 2004 (IEC 60446), Hong Kong
from July 2007, Singapore from March
2009
Brown
Black
Grey
Blue
Green/yellow
striped[note 2]
Older European (IEC 60446, varies by
country[note 3]
Black or
brown or
grey
Black or
brown or
grey
Black or
brown or Blue
grey
Green/yellow
striped[note 3]
UK until April 2006, Hong Kong until
April 2009, South Africa, Malaysia,
Singapore until February 2011
Red
Yellow
Blue
Black
Green/yellow
striped (green on
installations
before c. 1970)
India and Pakistan
Red
Yellow
Blue
Black
Green/yellow
striped, or green
Former USSR (Russia, Ukraine,
Kazakhstan) and People's Republic of
China (per GB 50303-2002 Section
15.2.2)
Yellow
Green
Red
Sky blue
Green/yellow
striped
Brown
Blue
Yellow/green
striped, older
may be only
yellow or bare
copper
Blue
White,
or grey
Green,
green/yellow
striped,[note 5]or a
bare copper wire
Norway
United States (common practice)[note 4]
Black
Black
United States (alternative practice)[note 6] Brown

Three-phase AC railway electrification

Charging station

Frequency converter

Industrial & multiphase power plugs & sockets

International Electrotechnical Exhibition

John Hopkinson

Y-Δ transform
White/Grey
Red
Orange
(delta), violet Yellow
(wye)
Grey, or
Green
white