Three-phase electric power

Three-phase power transformer which is the sole source of 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.

The three phases are typically labelled by colors which vary by country. In the UK they were traditionally red, yellow and blue but it is in the process of harmonising to brown black and grey right now (april 2004-april 2006 window). In the USA they are black, red and blue. The current European standard is brown, black and grey but this is a very recent introduction. Brown and two blacks was common in Europe before and other combinations were in use before that.

Three phase systems may or may not have a neutral core. The advantage of having one is it allows a higher voltage to be used for the three phase system whilst supporting the lower voltage of single phase appliances. However in high voltage distribution situations it is common not to have a neutral as the loads can simply be connected phase-phase (and they have to be designed for the high voltage system anyway)

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, separated from each other by 120° (which is the maximum phase separation possible). 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 up to 30,000 volts. This voltage is usually "stepped-up" to a higher voltage with a transformer. The reason the voltage is increased is to reduce losses. Power essentially is equal to the product of voltage and current - so as you increase the voltage, you will reduce the current for a given value of power. Heating losses in a transmission line are proportional to the square of the current so if you can halve the current in a line, you will reduce the losses by four. For this reason some transmission lines operate at voltages in excess of 500,000 volts.

Generally voltage is changed several times during distriburion being changed up on the way to the grid main lines and down again from there to the destination.

At the destination, a transformer supplies the power stepped down from the high-voltage transmission line to three sinusoidally varying electric currents of 120 V (in the US) or 230 V (in Europe) alternating current (Vac). Including the neutral this gives four conductors. This 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 countries standard single phase voltage (120 Vac or 230 Vac) to the circuit.

The power transmission grid is organised 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 used a lot for heaters in america, such as, for example, a 2kW, 208 volt baseboard heater.

Three Phase Loads

All three phases are typically used in large industrial motors, or efficient air conditioners (e.g. most York units above 2.5 tons are available in 3 phase) as this is the most efficient way to transmit large amounts of electrical power. The greatest power demand is when starting the motor.

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.

Two phase

Sometimes single phase center-tapped (split phase) 240 Vac is incorrectly referred to as "two-phase". It should be noted that a two phase system is a system in which the two voltages are 90° out of phase. For example, if one is <math>cos(2\pi\times 60t)<math> and the other is <math>\sin(2\pi\times 60t)<math>, where t is time, then you have a two phase system, also known as a quadrature system (one being referred to as the real part and the other being referred to as the imaginary part). A two phase system for 120 Vac line to neutral will measure approximately 169.7 Vac line to line. Two phase systems are seldom used for high power because they require the same number of hookup wires as three-phase (i.e. one for sine, one for cosine, and a common wire) delta connection, and the two phase system also does not balance the same amount of electricity in each of the three wires (although the cosine and sine are balanced, the neutral is not the same as the other two). A two-phase system is said to provide complex power and such systems are used at lower voltages (e.g. for communications applications, or to run stepper motors, and the like) but not commonly distributed at high power levels.

Complex Power

If we plot phasors of a two phase or three-phase system around the unit circle in the complex plane, we have a form of complex power.

A single phase 240 Vac split phase (center-tapped) power system, when plotted as phasors on the complex plane, can exist entirely along the real axis. It is this lack of complex power capability that impairs its ability to create a rotating magnetic field, and it is the rotating magnetic field that makes motors run very efficiently. For heating, such power is fine, but for example, running an air conditioner, it is far better to use complex power.

Testing

Take GREAT CARE when testing anything at mains voltages (or higher) and if you are unsure what you are doing and how to use your equipment safely then DON'T DO IT.

How to test three-phase electrical supply

A three-phase electrical supply consists of three active conductors and an earth ground.

A three-phase induction motor cannot function correctly if its electrical supply is not within certain parameters.

Typical parameters are 208 or 415 volts between phases, 120 or 240 volts from any phase to earth or ground, voltage within 12% of nominal values, and each phase within 5% of each other.

In a typical three-phase induction motor circuit, an appropriate place to test is at the line side of the direct-on-line motor starter.

Figure 1:

      A  B  C        earth/
      O  O  O        ground
       /  /  /
      /  /  /
      O  O  O

Tests should be made between A and B, A and C, B and C, A and earth, B and earth, and C and earth.

Note that listed voltages are for countries which use 120 or 240 volts only!

How to test three-phase devices

Three-phase pumps, compressors, and the like, must be connected in proper phase sequence to avoid damage. Typically such devices will draw less current when connected wrong, and can be easily checked with an amprobe (clip on ammeter) for current draw. For example, testing an air conditioner that has a scroll compressor, one will find that if it's hooked up in the wrong phase sequence, it will draw too little current, and thus any two wires can be switched to change the phase sequence.

There are special small pocket sized motors that are used to check phase sequence by checking the direction of rotation of the motor. These are expensive. A cheaper alternative is to use three neon lights, and whip the head (or eyes) in a quick motion past them to see which way the phase sequence is going.

Topics including testing motor coil resistance and testing earth fault resistance are covered separately.

3 Phase electrical outlets

The image on the left shows a 3 phase socket. The one on the right a triplex outlet with neons in the top sockets for phase indication. Both are american. if you look carefully you can see that underneath the three phase socket on the left there are breaker positions marked. 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 2pole 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 3phase supply. Typically, a 3 pole common trip 15Amp 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.