Why is 230 V preferred over 440 V.

Three-phase current

Lexicon> letter D> three-phase current

Definition: alternating electrical current, which oscillates in different lines (phases) with a time shift

More general terms: alternating current

Opposite term: single-phase alternating current

English: rotary current, polyphase current

Categories: electrical energy, physical principles

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: 04/26/2010; last change: 08/25/2020

URL: https://www.energie-lexikon.info/drehstrom.html

The term Three-phase current means in most cases Three-phase alternating current. Here, three (or four) instead of two lines transmit three alternating currents, which oscillate at different times (out of phase). In the European composite system, the frequency of this oscillation (i.e. the number of oscillations per second) is 50 Hz, in the USA it is 60 Hz.

There will be three in household three-phase current in Europe Phases supplied, i.e. three lines with a voltage of 230 V (previously 220 V) (effective value) compared to the earth potential, given by a neutral conductor (N). These tensions are called Star voltage or Line voltage designated. The phases are designated with L1, L2 and L3, often also after the older system with R, S and T. The corresponding connections to equipment are designated with U, V and W. The phase difference of the electrical voltage between two phases is 360 ° / 3 = 120 ° in each case.

The rms value of the voltage between two of the phase lines (the (External) conductor voltage or Triangle voltage) is approx. 400 V (previously 380 V) for three-phase three-phase current; this is the star voltage multiplied by the square root of 3. (The ratio of triangle voltage to star voltage is called Chaining factor and is the square root of 3, i.e. approx. 1.73 for three-phase current.) When the voltage of a three-phase line (e.g. a high-voltage line) is specified, the effective value of the phase-to-phase voltage is usually meant, not the star voltage.

For three-phase consumers (e.g. three-phase motors) and other equipment, the terminals that are connected to phases L1, L2 and L3 are labeled U, V and W.

Star connection and delta connection

Three-phase AC technology allows the same power to be transmitted with half as much conductor material as with single-phase AC technology.

If three consumers are each connected to a phase and to earth, one speaks of one Star connection (see Figure 3 on the left). Each consumer then has an effective voltage of Ueff = 230 V (in the low-voltage network, see Figure 2). If we assume that every consumer draws pure active power (i.e. does not cause reactive currents) at a current strengthI.effso is the performance per consumer P = Ueff · I.eff, and the overall performance is three times higher. In this symmetrical situation, the neutral conductor is not loaded with current, since the currents of the three consumers cancel each other out there. So only three lines are current-loaded, and the neutral conductor could also be omitted. If all three consumers were to be supplied with AC voltage separately, six lines would be required for the same total output, i.e. twice as much material. Conductor cables are used much better with three-phase current than with single-phase alternating current.

A Delta connection (Figure 3 right) means that three consumers are each connected with two phases (and not with the earth conductor). Each consumer then “sees” the higher rms voltage of 400 V, so that the total power is higher by the factor of the square root of 3 (approx. 1.732) with the same current intensity in the consumers. However, the current load on the supply lines is also higher by the same factor. (Note that in this case each phase line carries currents of two Consumers is loaded, even if there is at least a phase shift between serve.) Therefore, the transmission of the same power with delta or star connection requires the same current strength in the supply lines.

For more details, see the articles on star connection and delta connection.

Three-wire and four-wire systems

The line for the neutral conductor can often be omitted - especially with high-voltage lines.

If a three-phase consumer loads the three phases symmetrically, it occurs no Amperage in the neutral conductor. It is then often possible, even with a star connection, not to connect the neutral conductor at all, i.e. to lay only three instead of four lines. (The delta connection does not need the neutral conductor anyway.)

When it comes to power transmission, it is common to use high-voltage and medium-voltage levels (→Voltage levels) to use such three-wire systems. One tries to realize an operation which is as symmetrical as possible, i. H. with less Unbalanced load. At the low-voltage level, on the other hand, where significant unbalanced loads are difficult to avoid, four-wire systems are generally used, at least in Europe. These also have the advantage of offering two different voltages directly: the star voltage and the delta voltage. Most consumers in households use the star voltage.

One possible consequence of unbalanced loads is the creation of a Neutral point shiftas long as this is not prevented by grounding. A neutral point shift means that the neutral point is no longer at ground potential, but has a certain voltage against it. As a result, there is an overvoltage between the star point and at least one of the phases, which under certain circumstances can lead to the destruction of equipment.

Consistent overall performance

The total power transmitted in a three-phase system is constant over time with symmetrical operation and sinusoidal currents. The services in the individual phases pulsate, but shifted against each other in time so that the total output remains constant. That means z. B. that a three-phase motor, unlike a single-phase AC motor, can drive its load with approximately constant torque. This is a great advantage in a number of applications.

Direction of rotation

The arrangement of the three phases (if we assume the usual 3-phase system) corresponds to a certain direction of rotation. According to VDE 0100, Part 550, 1988-04, three-phase plug devices must be connected in such a way that a field urging to the right results when looking into a socket. This means that the phase of a conductor is shifted by 120 ° compared to that which is to the left of it in the direction of rotation, so that it always reaches the voltage maximum by a third period later.

For some consumers, such as ovens, the direction of rotation is irrelevant. In a three-phase motor, however, it is important because it determines the direction in which the rotor will rotate. The aforementioned standard is therefore necessary in order to achieve the desired direction of rotation in each case; it is not stipulated in the engine's design.

A rotating field measuring device can be used to check the direction of rotation, and with special adapters the direction of rotation can be changed without having to rework the connections. You just swap two of the phases with each other.

Complex number calculations

Complex numbers are often used to advantage for calculations in connection with three-phase current. Here a single complex number represents the complete sinusoidal voltage curve z. B. on a phase to earth or the voltage difference between two phases. It therefore contains the information about the amplitude and phase (position in time) of the respective oscillation. The same can be done for currents as long as these also have a sinusoidal curve. The connection between voltage and current strength is established with the help of complex impedances, which also deal with the phenomenon of reactive currents.

Rectification of three-phase current

In some cases direct current must be generated from three-phase current, for which a so-called rectifier is used. Compared to the use of a phase alternating current, an often significant advantage is that the ripple of the generated direct voltage is significantly lower: The direct voltage never drops to zero, but (with three-phase alternating current and the use of the usual six-pulse rectifier) ​​only by a few percent of the Peak voltage.

AC and three-phase sockets in households and businesses

Ordinary household sockets only ever have one of the three phases; Three-phase sockets offer all three.

The individual AC sockets in the household are only connected to one of the phases (outer conductor) and the neutral conductor as well as the protective conductor, while three-phase sockets (in Germany: CEE three-phase plug connectors according to IEC 60309) offer all three phases and the neutral conductor and protective conductor. They are intended for the connection of particularly powerful devices, e.g. B. of powerful electric motors and chargers for electric cars. There are different versions with different maximum currents (16 A, 32 A, 63 A, 125 A), which intentionally have slightly different geometrical details, so that only plugs and sockets (or couplings) that are designed for the same output fit together. The most common are the red plug connections with the connections 3L + N + PE, i.e. 3 phase conductors (L), neutral conductor (N) and protective conductor (earth potential, PE).

If a three-phase socket z. B. can deliver 32 A, the result is a maximum power of approx. 3 · 230 V · 32 A = 22 kW, six times more than for a single-phase AC socket with 16 A, which delivers a maximum of 3.7 kW. The fact that you have three phase lines alone brings a factor of 3, and a further doubling brings twice the current strength of 32 A. If the power is calculated using the phase-to-phase voltage instead of the star voltage, the prefactor is of course not 3, but the Square root of 3.

Electric stoves, electric heat pumps and electric storage heaters are also mostly connected to all three phases, but mostly permanently wired and not via a socket.

In addition to the neutral conductor, there is the three-phase and alternating current Protective conductorwhich, like the neutral conductor, is at ground potential, but has a different function. B. electrically conductive housing of devices connected so that even in the event of defects within the device, the housing can never receive a dangerous voltage to earth.

Three-phase current in the energy supply

Today almost the entire electrical energy supply is based on low-frequency three-phase current, the phases of which can be used individually as alternating current. This means that practically all power plants contain three-phase generators and feed the power grids with them. Most high-voltage lines transmit three-phase current, although high-voltage direct current transmission (HVDC) is increasingly being used for point-to-point connections with high power. Transformers can work with three-phase current or with alternating current for the individual phases.

Railways such as B. use the Deutsche Bahn none Three-phase current, but a separate single-phase alternating current network for traction current.

As with single-phase alternating current, the phenomenon of reactive currents occurs with three-phase current. This often makes additional technical equipment necessary, especially for reactive power compensation, and can lead to additional energy losses.

If possible, the three phases should all be equally loaded H. a strong Unbalanced load one tries to avoid. This is because it can lead to an increased load on components of the power supply, for example synchronous generators.

Three-phase current in North America

Three-phase current is also used a lot in North America for higher powers, but there are significant differences to the situation in Europe:

  • Most households are not supplied with three-phase current, but only with single-phase alternating current, sometimes with two opposite phases (single phase three-wire system). The same often applies to small businesses. The operation of three-phase motors is then at least not possible without additional technology. In addition, this creates greater unbalanced loads.
  • There are various modified three-phase systems (e.g. three phase delta, three phase high leg delta, TEE) in which Not how to work in Europe with a grounded star center. The individual phases can then have very different voltages against earth. In some cases, work is also carried out without a neutral conductor, i.e. with three-wire systems.

Due to the proliferation of different systems, installations and their maintenance can become quite complicated, and industrial plants cannot easily be put into operation at another location if the conditions there are again different. Often, additional technology is needed for customization.

Three-phase current for electric vehicles

With electric cars, three-phase current is encountered in two different contexts: within the vehicle (without direct relevance for the user) and when charging the battery.

The vehicle battery must be charged with direct current, which is obtained via a rectifier either in the car or in the charging station:

  • Simple chargers, such as those built into a number of electric cars, only work in a single phase. H. they only use one of the three phases, even if, for example, a type 2 charging cable is used. Since, for example, in Germany in these cases no more than 20 A can be charged due to the unbalanced load, the charging power is then limited to 230 V · 20 A = 4.6 kW. At a charging station that does not have this limitation, a vehicle can be charged faster under certain circumstances - but usually not massively faster, since even with 32 A only approx. 7.4 kW are possible.
  • Three-phase chargers are common for stationary charging stations, but they are also built into some vehicles. With the same conductor amperage, this enables three times the charging power - e.g. B. 20.7 kW with 30 A.

One or more three-phase electric motors are often used inside the car (Three-phase motors) a; these can be synchronous motors as well as asynchronous motors. Since the batteries supply direct current, the three-phase current must be generated using a suitable three-phase inverter. In contrast to power grids, a variable frequency and voltage of the three-phase current is used here, depending on the respective speed of the motor. With recuperation (braking energy recovery), direct current is generated again with the help of a rectifier to charge the battery.

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See also: alternating current, direct current, electrical energy, phase, neutral conductor, star connection, delta connection, three-wire and four-wire network, reactive current, unbalanced load, power network
as well as other articles in the categories of electrical energy, physical principles

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Question: Why can three-phase sockets in a house usually draw significantly more power than alternating current sockets?

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Question: Why are powerful electric motors often operated with three-phase current instead of single-phase alternating current?

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