Three-phase power supply system- a special case of multiphase systems of electrical circuits, in which sinusoidal EMFs of the same frequency created by a common source act, shifted relative to each other in time by a certain phase angle. In a three-phase system, this angle is 2π / 3 (120 °).
The multi-wire (six-wire) three-phase AC system was invented by Nikola Tesla. A significant contribution to the development of three-phase systems was made by M.O.Dolivo-Dobrovolsky, who was the first to propose three- and four-wire AC transmission systems, revealed a number of advantages of low-wire three-phase systems in relation to other systems, and conducted a number of experiments with an asynchronous electric motor.
Animated image of the flow of currents in a symmetrical three-phase circuit with a "star" connection
Vector diagram of phase currents. Symmetrical mode.
Advantages
Possible wiring diagram of a three-phase network in multi-apartment residential buildings
- Profitability.
- Economical transmission of electricity over long distances.
- Less material consumption of 3-phase transformers.
- Less material consumption of power cables, since at the same power consumption, the currents in the phases are reduced (compared to single-phase circuits).
- Balance of the system. This property is one of the most important, since in an unbalanced system an uneven mechanical load occurs on the power generating unit, which significantly reduces its service life.
- The ability to easily obtain a circular rotating magnetic field required for the operation of an electric motor and a number of other electrical devices. 3-phase motors (asynchronous and synchronous) are simpler than DC motors, single or 2-phase, and have high efficiency rates.
- Possibility of obtaining in one installation two operating voltages - phase and linear, and two power levels when connected to a "star" or "triangle".
- Possibility of a sharp reduction in flickering and stroboscopic effect of luminaires based on fluorescent lamps by placing three lamps (or groups of lamps) powered from different phases in one luminaire.
Due to these advantages, three-phase systems are the most common in modern power generation.
Connection diagrams of three-phase circuits
Star
Existing types of line voltage protection that can be found on sale in electrical stores. As required by modern standards, installation takes place on a DIN rail.
A star is such a connection when the ends of the phases of the generator windings (G) are connected to one common point, called a neutral point or neutral... The ends of the phases of the receiver windings (M) are also connected to a common point. The wires connecting the beginning of the phases of the generator and receiver are called linear... The wire connecting two neutrals is called neutral.
Busbars for distributing neutral wires and ground wires when connected with a star. One of the advantages of a star connection is savings on the neutral wire, since only one wire is required from the generator to the point of separation of the neutral wires near the consumer.
A three-phase circuit that has a neutral wire is called a four-wire circuit. If there is no neutral wire - three-wire.
If the resistances Z a, Z b, Z c of the receiver are equal to each other, then such a load is called symmetrical.
The relationship between linear and phase currents and voltages.
The voltage between the line wire and neutral (U a, U b, U c) is called phase... The voltage between two line wires (U AB, U BC, U CA) is called linear... To connect the windings with a star, with a symmetrical load, the relationship between linear and phase currents and voltages is true:
Consequences of burnout (breakage) of the neutral wire in three-phase networks
With a symmetrical load in a three-phase system, supplying the consumer with line voltage is possible even in the absence of a neutral wire. However, when the load is supplied with phase voltage, when the load on the phases is not strictly symmetrical, the presence of a neutral wire is required. When it breaks down or a significant increase in resistance (poor contact), the so-called "phase imbalance" occurs, as a result of which the connected load, calculated for the phase voltage, may be at an arbitrary voltage in the range from zero to linear (the specific value depends on the distribution of the load over the phases at the moment of a break in the neutral wire). This is often the reason for the failure of consumer electronics in apartment buildings. Since the resistance of the consumer remains constant, then, according to Ohm's law, with an increase in voltage, the current passing through the consumer device will be much higher than the maximum allowable value, which will cause combustion and / or failure of the supplied electrical equipment. Undervoltage can also cause equipment failure. Sometimes a burnout (break) of the neutral wire at a substation can cause a fire in apartments.
The problem of harmonics that are multiples of the third
Modern technology is increasingly equipped with impulse network. A switching source without a power factor corrector consumes current in narrow pulses near the peak of the supply voltage sine wave, at the time of charging the input rectifier capacitor. A large number of such power sources in the network creates an increased current of the third harmonic of the supply voltage. The currents of harmonics that are multiples of the third, instead of mutual compensation, are mathematically summed up in the neutral conductor (even with symmetrical load distribution) and can lead to its overload even without exceeding the permissible power consumption in phases. This problem exists, in particular, in office buildings with a large number of office equipment operating simultaneously.
The existing installations for reactive power compensation are not able to solve this problem, since the decrease in the power factor in networks with a predominance of switching power supplies is not associated with the introduction of a reactive component, but is due to the nonlinearity of current consumption. The solution to the problem of the third harmonic is the use of a power factor corrector (passive or active) as part of the circuit of manufactured switching power supplies.
The requirements of the IEC 1000-3-2 standard impose restrictions on the harmonic components of the load current of devices with a power of 50 W. In Russia, the number of harmonic components of the load current is standardized by the standards GOST 13109-97, OST 45.188-2001.
Triangle
A triangle is such a connection when the end of the first phase is connected to the beginning of the second phase, the end of the second phase is connected to the beginning of the third, and the end of the third phase is connected to the beginning of the first.
The relationship between line and phase currents and voltages
To connect the windings with a triangle, with a symmetrical load, the relationship between linear and phase currents and voltages is true:
Common voltage standards
Marking
Conductors belonging to different phases are marked with different colors. Neutral and protective conductors are also marked with different colors. This is to provide adequate protection against electric shock and to facilitate the maintenance, installation and repair of electrical installations and electrical equipment. In different countries, the marking of conductors has its own differences. However, many countries adhere to the general principles of conductor color coding as set out in the International Electrotechnical Commission standard IEC 60445: 2010.
Three Phase Double Circuit Power Line |
At the present time all over the world is the most widespread three-phase alternating current system.
Three-phase electrical circuit system is called a system consisting of three circuits in which variables act, EMF of the same frequency, phase-shifted relative to each other by 1/3 of the period (φ = 2π / 3). Each individual circuit of such a system is briefly called its phase, and the system of three phase-shifted alternating currents in such circuits is simply called three-phase current.
Almost all generators installed in our power plants are generators of three-phase current... In essence, each such generator is a connection in one electrical machine of three alternators, designed in such a way that the induced ones are shifted relative to each other by one third of the period, as shown in Fig. one.
Rice. 1. Graphs of the dependence of the EMF on time, induced in the armature windings of a three-phase current generator
How such a generator is implemented is easy to understand from the circuit in Fig. 2.
Rice. 2. Three pairs of independent wires connected to three armatures of a three-phase current generator supply the lighting network
There are three independent armatures located on the stator of an electric machine and offset by 1/3 of the circle (120 o). In the center of the electric machine, an inductor common to all armatures rotates, shown in the diagram in the form.
In each coil of the same frequency, but the moments of passage of these EMF through zero (or through the maximum) in each of the coils will be shifted by 1/3 of the period relative to each other, because the inductor passes by each coil 1/3 of the period later than past the previous one.
Each winding of a three-phase generator is an independent current generator and a source of electrical energy. By connecting the wires to the ends of each of them, as shown in fig. 2, we would get three independent circuits, each of which could power certain electrical receivers, for example.
In this case, six wires would be required to transfer all the energy that is being absorbed. It is possible, however, to interconnect the windings of a three-phase current generator in such a way as to get by with four or even three wires, i.e., significantly save wiring.
The first of these ways is called star connection(fig. 3).
Rice. 3. Four-wire wiring system when connecting a three-phase generator with a star. Loads (groups of electric lamps I, II, III) are supplied with phase voltages.
We will call the terminals of the windings 1, 2, 3 the beginnings, and the terminals 1 ", 2", 3 "- the ends of the corresponding phases.
The connection of stars is that we connect the ends of all the windings to one point of the generator, which is called the zero point or neutral, and we connect the generator to the receivers of electricity with four wires: three so-called line wires coming from the beginning of windings 1, 2, 3, and neutral or neutral wire going from the zero point of the generator. This wiring system is called four-wire.
The voltages between the zero point and the beginning of each phase are called phase voltages, and the voltages between the beginnings of the windings, that is, points 1 and 2, 2 and 3, 3 and 1, are called linear. Phase voltages are usually denoted by U1, U2, U3, or in general form U f, and line voltages - U12, U23, U31, or in general form U l.
Between the amplitudes or rms values when connecting the generator windings with a star, there is a relationship U l =√3 U f ≈ 1.73U f
Thus, for example, if the phase voltage of the generator is U f = 220 V, then when the generator windings are connected with a star, the line voltage U l is 380 V.
In the case of a uniform load of all three phases of the generator, that is, at approximately the same currents in each of them, the current in the neutral wire is zero... Therefore, in this case, you can eliminate the zero wire and go to an even more economical three-wire system. All loads are connected in this case between the corresponding pairs of line wires.
With an unbalanced load, the current in the neutral wire is not zero, but, generally speaking, it is weaker than the current in the linear wires. Therefore, the neutral wire can be thinner than the linear ones.
When operating a three-phase alternating current, they strive to make the load of the various phases as equal as possible. Therefore, for example, when arranging the lighting network of a large house with a four-wire system, a neutral wire and one of the linear ones are introduced into each apartment in such a way that, on average, each phase has approximately the same load.
Another way of connecting the generator windings, which also allows three-wire wiring, is the delta connection, shown in fig. 4.
Rice. 4. Diagram of connecting the windings of a three-phase generator with a triangle
Here, the end of each winding is connected to the beginning of the next, so that they form a closed triangle, and line wires are connected to the vertices of this triangle - points 1, 2 and 3. When connected with a triangle, the line voltage of the generator is equal to its phase voltage: U l = U f.
In this way, switching the generator windings from star to delta leads to a decrease in the line voltage by √3 ≈ 1.73 times... A delta connection is also permissible only with the same or almost the same phase load. Otherwise, the current in the closed loop of the windings will be too strong, which is dangerous for the generator.
When using three-phase current, individual receivers (loads) powered by separate pairs of wires can also be connected either by a star, that is, so that one end of them is connected to a common point, and the remaining three free ends are connected to the line wires of the network, or with a triangle, that is, so that all loads are connected in series and form a common circuit, to points 1, 2, 3 of which the line wires of the network are connected.
In fig. 5 shows the connection of loads with a star for a three-wire wiring system, and in fig. 6 - with a four-wire wiring system (in this case, the common point of all loads is connected to the neutral wire).
Rice. 7. Delta connection of loads with three-wire wiring system
It is practically important to keep in mind the following. When the loads are connected with a triangle, each load is under line voltage, and when connected with a star, under voltage, in√3 times smaller. For the case of a four-wire system, this is clear from Fig. 6. But the same is the case in the case of a three-wire system (fig. 5).
Between each pair of line voltages, two loads are connected in series here, the currents in which are phase-shifted by 2π / 3. The voltage across each load is equal to the corresponding line voltage divided by√3 .
Thus, when switching loads from a star to a triangle, the voltages at each load, and therefore the current in it, increase by√3
≈
1.73 times. If, for example, the line voltage of a three-wire network was 380 V, then when connected with a star (Fig. 5), the voltage on each of the loads will be 220 V, and when switched on with a triangle (Fig. 7) it will be equal to 380 V.
In preparing the article, information from a physics textbook edited by G. S. Landsberg was used.
Three-phase voltage is an electrical power system that uses three phase lines, with a phase shift of 120 degrees. This provides a level playing field for many applications and improves efficiency.
The emergence of the three-phase voltage concept
Dolivo-Dobrovolsky in Russia and Nikola Tesla in the rest of the world are considered the father of three-phase voltage. Events related to the era of the emergence of the subject of the dispute took place in the 80s of the XIX century. Nikola Tesla demonstrated the first two-phase motor while working for a company where he set up electrical installations for various purposes. Interest in the phenomenon of electrifying the fur of a domestic cat led the scientist to great discoveries. Walking in the park with a friend, Nikola Tesla realized that he would be able to put into practice Arago's theory of a rotating magnetic field, and he would need:
- Two phases.
- Offset between them at an angle of 90 degrees.
To show the great significance of the discovery, we note that Yablochkov's transformer at the indicated time did not gain mass fame, and Faraday's experiments on magnetic induction were safely forgotten, having written down only the formula of the law. The world did not want to know about:
- alternating current;
- phase;
- reactive power.
Generators (alternators) and dynamos straightened the voltage using a mechanical switch. In a similar way, the entire electricity industry, which was scarce at that time, was vegetating. Edison was just beginning to invent, no one really knew about it yet. By the way, in the Russian Federation they believe that the device was invented by Lodygin.
Tesla's idea looked revolutionary, it remained unknown how to get two phases with a given interphase shift. The young scientist was not very interested in the question. He read about the reversibility of electrical machines and exuded the confidence that he could easily build a generator by properly positioning the windings. There were no difficulties in the drive. At the beginning of the 80s, steam was actively used, the demonstration model was supposed to be powered by a dynamo.
Tesla was not concerned with the need to obtain a specific frequency. Research was not carried out, it was simply required to make the rotor rotate. The idea was realized through slip rings. At that time, DC collector motors were supplied with similar contacts, Tesla's conclusion is not surprising. It is more interesting to explain the choice of the number of phases.
The advantage of three phases
experimenters aloud assert the advantage of three phases over two, but an explanation is required. Thoughts about efficiency, torque and so on immediately pop into my head. But Tesla drew hundreds of constructions in a notebook, obviously, he would be able to arrange the poles in order to achieve the necessary parameters. Conclusion - it's not about the design of the devices.
Now 380 V is transmitted only through three wires. This could not have been achieved in the original version of Nikola Tesla. In 1883, Edison spent a lot of energy trying to use a three-core wire. Obviously, I heard about the demonstration organized by Nikola Tesla, and understood the danger of the situation. In the civilized world, the main profit goes to the owner of the patent, why would a famous inventor bring out a capable engineer?
Edison's logic is simple: users will see that three-core cables are cheaper than four-core cables, and will refuse to use Nikola Tesla's new products. It’s not hard to guess that the ingenious plan of the inventor of the base for incandescent light bulbs failed. And with a bang. And the fault was ... Dolivo-Dobrovolsky. Nikola Tesla's system required four wires to create two phases. At the same time, Dolivo-Dobrovolsky proposed to transfer more energy through three.
It's about symmetry. Linear voltages of 380 V at every moment leave an alternative to choose from. For example, the current from the first phase is capable of leakage to the second or third. Depending on the presence of a suitable potential. The result is balance. If you combine the two phases of the Nikola Tesla system, you get a vinaigrette. As a result, it is permissible to remove the neutral in the Dolivo-Dobrovolsky system if the load is symmetrical - as often happens in practice.
As a result, more voltage is obtained between the wires, which reduces the passing current for each at the same power. Moreover, sometimes it is possible to use only three lines, this applies to most enterprises. The benefits are obvious when creating local substations: the neutral of the secondary winding is grounded right there, there is no need to pull an extra wire from the hydroelectric power station. These reasons have become the dominant advantages of three-phase voltage networks. Tesla wires are easily retrofitted into three phases.
Reason for losing Edison
It is often believed that Tesla's system turned out to be better, so Edison lost. It is difficult to say how many dollars the latter lost, but Nicola was swapped by $ 4.5 million by modern standards. Inflation! Authors tend to believe that Edison got his own. Nikola Tesla was able to prove the advantages of direct current. For example, the latter is less prone to corona on wires, the amplitude does not contain sharp emissions.
Today it has been proven that it is more profitable to transmit direct current over long distances. This excludes from consideration the reactances of the network - inductance and capacitance. This greatly reduces the unstable reactive power. The XXI century can become the second birth of direct current for its transmission over long distances. But laughter causes Edison's inability to transmit energy. Tesla had the right to help, then DC devices today would be used on an equal basis with AC consumers. This is better for brushed motors - efficiency and torque increase.
It turns out that direct current is beneficial to transmit. Edison simply could not find the right solution, he tried to take the problem in impetuity, without plunging into the rear. Edison was a pure practitioner and did not know how to find such clever solutions as transformers. But all generators of the mid-19th century had a built-in switch for rectification. It only remained to connect to the line, and on the receiving side to carry out the conversion. And that's it! Nicola punished Edison brilliantly, proving the existence of a certain power in the world that controls the course of history.
alternating current was chosen because of its powerful transmission medium. It's about the transformer. Designed for the first time in 1831 (or earlier) by Michael Faraday, this irreplaceable element of modern technology has gone without the attention it deserves. Interest in the device was returned by Heinrich Rumkorf fifteen years later, using a dynamo to generate a discharge in a spark gap. The step-up transformer greatly enhanced the effect. This directly opened the way for scientists to conduct experiments, but the essence of the transformation did not receive the attention it deserved.
Instead, scientists fought hard with direct current. By creating engines, lighting devices and generators for him. Surprisingly, knowing about the reversibility of electric machines, they did not figure out how to create a unipolar motor, which is used today in hand mixers and blenders. In fact, household motors are single-phase. And only a small part works on direct current.
Let's point out an implicit advantage. DC has a higher safety limit. It seems possible to make industrial networks harmless to people. Let's consider the statement in more detail, the arguments are not obvious to an inexperienced reader.
Why DC is safer
Burnt electricians say that a shock with a current of 220 V is not too dangerous, the main thing is not to fall under a three-phase linear voltage. It is approximately three times higher by the root (within 1.7). Linear voltage is the voltage between two phases. Due to the shift between them by 120 degrees, this curious effect is obtained. The ignorant ask what the difference is at a 90 degree shift. The answer was given at the beginning - the three phases form a symmetrical system. With a shift of 90, it would take four.
As a result, each line voltage is supplied along a pole, which greatly simplifies their multiplication when it is required to achieve high power. For example, in traction motors of steamers, where it is required to change the force extremely smoothly and it is necessary to use shaft rotation. It happens that three or even six poles are not enough. Only two vacuum cleaner collector motor is enough.
So, there is 308 V between the phases. It looks safe if you increase the frequency of the transmission line to 700 Hz. Tesla found that from the indicated value, the skin effect is clearly manifested, the current does not penetrate deeply into the body. Therefore, it does not cause significant damage to humans. The scientist demonstrated tongues of lightning on the body at much higher voltages and said that it is good for health, cleanses the skin well.
The frequency of 700 Hz (or higher) was not put into use - at the same time, the losses of transformers increased significantly. At the time of the decision on the ratings of the first AC hydroelectric power station, there were no developments in the manufacture of electrical materials. We suggest reading more in the topic. There is no need to duplicate information. Due to the lack of the necessary materials, the magnetization reversal losses increased strongly with increasing frequency. Today, this does not cause difficulties at the level of technology.
There is a difficulty - shielding. In the years of the first attempts to transfer energy, they did not know about radiation. Radio made its first steps in the 90s of the XIX century. In fact, the increase in frequency is accompanied by a sharp increase in the release of energy into space. And the wires had to be shielded, it is expensive, requires powerful dielectrics. It is not a fact that modern networks would be able to solve the problem.
Tesla suggested transmitting energy through the ether. Why did he build the Wardencliff tower? But ... the industrialists turned out to be interested in selling copper for the manufacture of wires and on this basis they refused to finance the scientist. But the main thing is that the time is coming when the three-phase voltage will go into oblivion or will be obtained from converters, and Tesla himself will give an answer on how to do this.
More precisely, the answer will be given by the numerous patents and ideas of the inventor. It is not for nothing that the records were immediately confiscated after the death of the scientist and were carefully classified. We recommend that you take up the study. It's time to dream that cars will run on vegetable oil without polluting the environment with disgusting smoke and fumes. Please note that all secrets lie on the surface and are waiting for those who want to reveal them. Perhaps one of the readers will be able to do it first?
In the electrical equipment of residential apartment buildings, as well as in the private sector, three-phase and single-phase networks are used. Initially, the electrical network comes from the power plant with three phases, and most often the three-phase power network is connected to residential buildings. Further, it has branches into separate phases. This method is used to create the most efficient transmission of electrical current from the power plant to its destination, as well as to reduce transportation losses.
To determine the number of phases in your apartment, you just need to open the switchboard located on the staircase, or right in the apartment, and see how many wires enter the apartment. If the network is single-phase, then there will be 2 wires. A third wire is also possible - grounding.
Three-phase networks in apartments are rarely used, in cases of connecting old electric stoves with three phases, or powerful loads in the form of a circular saw or heating devices. The number of phases can also be determined by the magnitude of the input voltage. In a 1-phase network, the voltage is 220 volts, in a 3-phase network between phase and zero it is also 220 volts, between 2 phases - 380 volts.
Differences
If you do not take into account the difference in the number of wires of the networks and the connection diagram, then you can determine some other features that have three-phase and single-phase networks.
- In the case of a three-phase power supply network, phase imbalance is possible due to uneven phase separation of the load. On one phase, a powerful heater or stove can be connected, and on the other a TV and a washing machine. Then this negative effect arises, accompanied by an asymmetry of voltages and currents in phases, which leads to malfunctions of household devices. To prevent such factors, it is necessary to distribute the load in advance in phases before laying the wires of the electrical network.
- A 3-phase network requires more cables, conductors and switches, which means that you will not save too much money.
- The power capabilities of a single-phase household network are significantly less than a three-phase one. If you plan to use several powerful consumers and household devices, power tools, then it is preferable to supply a three-phase power network to the house or apartment.
- The main advantage of a 3-phase network is the low voltage drop compared to a 1-phase network, provided that the power is the same. This can be explained by the fact that in a 3-phase network the current in the phase conductor is three times less than in a 1-phase network, and there is no current on the wire at all.
Advantages of a 1-phase network
The main advantage is the cost-effectiveness of its use. In such networks, three-wire cables are used, compared to the fact that in 3-phase networks, five-wire cables are used. To protect equipment in 1-phase networks, you need to have single-pole protective ones, while in 3-phase networks you cannot do without three-pole circuit breakers.
In this regard, the dimensions of the protection devices will also differ significantly. Even on one electrical machine there is already a savings of two modules. And in terms of dimensions, this is about 36 mm, which will significantly affect the placement of machines in. And during installation, the space saving will be more than 100 mm.
Three-phase and single-phase networks for a private house
Electricity consumption by the population is constantly increasing. In the middle of the last century, there were relatively few household appliances in private homes. Today, in this regard, the picture is completely different. Household consumers of energy in private houses multiply by leaps and bounds. Therefore, in their own private estates, there is no longer the question of which power supply networks to choose for connection. Most often, in private buildings, power networks with three phases are performed, and a single-phase network is abandoned.
But is a three-phase network worth such superiority in installation? Many believe that by connecting three phases, it will be possible to use a large number of devices. But this does not always work out. The highest permissible power is determined in the technical conditions for the connection. Usually, this parameter is 15 kW for all private households. In the case of a single-phase network, this parameter is approximately the same. Therefore, it is clear that there is no particular benefit in terms of power.
But, it must be remembered that if three-phase and single-phase networks have equal power, then for a 3-phase network it can be used, since the power and current are distributed over all phases, therefore, it loads the individual phase conductors less. The rated current of the circuit breaker for a 3-phase network will also be lower.
Of great importance is the size, which for a 3-phase network will have a noticeably larger size. It depends on the size of the three-phase, which has dimensions larger than the single-phase, and the automatic input will also take up more space. Therefore, a switchboard for a three-phase network will consist of several tiers, which is a disadvantage of this network.
But the three-phase power supply also has its advantages, expressed in the fact that you can connect three-phase current receivers. They can be other powerful devices, which is the advantage of a three-phase network. The operating voltage of a 3-phase network is 380 V, which is higher than in a single-phase type, which means that more attention will have to be paid to electrical safety issues. This is also the case with fire safety.
Disadvantages of a three-phase network for a private house
As a result, there are several disadvantages of using a three-phase network for a private house:
- It is necessary to obtain technical specifications and permission to connect the network from the power supply.
- There is an increased risk of electric shock and fire hazard due to overvoltage.
- Significant overall dimensions of the power supply switchboard. For the owners of country houses, such a disadvantage is not of great importance, since they have enough space.
- Installation in the form of modules on the lead-in box is required. In a three-phase network, this is especially true.
Benefits of three-phase power supply for private houses
- It is possible to distribute the load evenly across the phases, in order to avoid phase imbalance.
- Powerful three-phase energy consumers can be connected to the network. This is the most tangible benefit.
- Decrease in nominal values of protection devices at the input, as well as decrease in input.
- In many cases, it is possible to obtain permission from the sales company to increase the permissible maximum power consumption level of electricity consumption.
As a result, we can conclude that it is recommended to practically carry out the input of a three-phase power supply network for private buildings and houses with a living area of more than 100 m 2. Three-phase power supply is especially suitable for those owners who are going to install a circular saw, a heating boiler, various drives of mechanisms with three-phase electric motors.
The rest of the owners of private houses do not have to switch to three-phase power supply, as this can only create additional problems.
Phase voltage and line voltage, star and delta connection. You can often hear these words in the conversations of professional electricians. But not even every electrician knows their exact meaning. So what do these terms mean? Let's try to figure it out.
At the dawn of the development of electrical engineering, the energy of electric generators and batteries was transmitted to consumers via direct current networks. In the United States, the main apologist for this idea was the famous inventor Thomas Edison and the largest energy companies at that time, obeying the authority of the "engineering giant", unquestioningly implemented it.
However, when the question arose of creating an extensive electrical network of consumers powered by a generator located at a great distance, which required the creation of the first power line, the project won by the then unknown Serbian emigrant Nikola Tesla.
He radically changed the very idea of a power supply system, using a generator and alternating current electric lines instead of a constant one. which made it possible to significantly reduce energy losses, material consumption and improve energy efficiency.
In this system, a three-phase alternator created by Tesla was used, and the transmission of energy was carried out using voltage transformers invented by the Russian scientist P.N. Yablochkov.
Another Russian engineer M.O.Dolivo-Dobrovolskiy a year later not only created a similar power supply system in Russia, but also significantly improved it.
Tesla used six wires to generate and transmit energy, Dobrovolsky suggested reducing this number to four by modifying the connection of the generator.
Experimenting with the creation of a generator, he simultaneously invented an asynchronous electric motor with a squirrel-cage rotor, which is still the most widely used in industry.
The concept of a phase exists only in sinusoidal alternating current circuits. Mathematically, such a current can be represented and described by the equations of a rotating vector, fixed at one end at the origin. The change in the voltage value of the circuit over time will be the projection of this vector onto the coordinate axis.
The value of this value depends on the angle at which the vector is to the coordinate axis. Strictly speaking, the angle of the vector is the phase.
The voltage value is measured relative to the earth potential, which is always zero. Therefore, the wire in which there is an alternating current voltage is called phase, and the other, grounded, is called zero.
The phase angle of a single vector does not represent a great practical value - in electrical networks, it makes a full revolution of 360 ° in 1/50 sec. Far more useful is the relative angle between two vectors.
In circuits with so-called reactive elements: coils, capacitors, it is formed between the vectors of voltage and current values. This angle is called phase shift.
If the values of reactive loads do not change over time, then the phase shift between current and voltage will be constant. And already with its help, you can analyze and calculate electrical circuits.
In the 19th century, when there was still no scientific theory of electricity, and all the development of new equipment was carried out empirically, experimenters noticed that a coil of wire rotating in a constant magnetic field creates an electric voltage at its ends.
Then it turned out that it changes according to a sinusoidal law. If you wind a coil with many turns, the voltage will increase proportionally. This is how the first electric generators appeared that could provide consumers with electrical energy.
Tesla in the generator, being developed for the then largest in the United States, the Niagara hydroelectric power station, for more efficient use of the magnetic field, placed in it not one coil, but three.
In one revolution of the rotor, the stator magnetic field was crossed by three coils at once, due to which the output of the generator increased by the root of three times and it was possible to power three different consumers simultaneously from it.
While experimenting with such generators, early electrical engineers noticed that the voltages in the windings did not change simultaneously. When, for example, in one of them it reaches a positive maximum, in the other two it will be equal to half of the negative minimum, and so periodically for each winding, and for the mathematical description of such a system, a system of three rotating vectors with a relative angle between them of 120 ° was already needed ...
Later it turned out that if the loads in the winding circuits were very different from each other, this significantly worsened the operation of the generator itself. It turned out that in large branched networks it is more profitable not to drag three different power lines to consumers, but to bring one three-phase power line to them, and already at the end of it, ensure an even distribution of loads on each phase.
It was such a scheme that Dolivo-Dobrovolsky proposed, when one terminal from each of the three windings of the generator is connected together and grounded, as a result of which their potential becomes the same and equal to zero, and electrical voltages are removed from the other three windings.
This scheme is called "star connection". It is still the main scheme for organizing three-phase electrical networks.
Let's figure out what phase voltage is
To create such networks, it is required to conduct a power transmission line from the generator to the consumers, consisting of three phase wires and one zero wire. Of course, in real networks, to reduce losses in wires, step-up and step-down transformers are also connected at both ends of the lines, but this does not change the real picture of the network operation.
A zero wire is needed to fix the potential of the generator's common output to the consumer, because it is in relation to it that voltage is created in each phase wire.
Thus, the phase voltage is generated and measured relative to the common point of connection of the windings - the neutral wire. In a three-phase network that is well balanced in terms of loads, a minimum current flows through the neutral wire.
At the output of a three-phase power line there are three phase wires: L1, L2, L3 and one zero - N. According to the existing European standards, they must be color coded:
- L1 - brown;
- L2 - black;
- L3 - gray;
- N - blue;
- Yellow-green for protective earth.
Such lines are supplied to large serious consumers: enterprises, urban areas, etc. But low-power end users, as a rule, do not need three voltage sources, so they are connected to single-phase networks, where there is only one phase and one neutral wire.
The uniform distribution of loads in each of the three single-phase lines ensures the phase balance in the three-phase power supply system.
Thus, for the organization of single-phase networks, the voltage of one of the phase wires relative to zero is used. This voltage is called phase voltage.
According to the standard adopted in most countries for end users, it should be 220 V. Almost all household electrical equipment is calculated and produced for it. In the United States and some Latin American countries, a standard voltage of 127 V is adopted for single-phase networks, and in some places 110 V.
What is line voltage
The advantages of a single-phase network are that one of the wires has a potential close to that of the earth.
This, firstly, helps to ensure the electrical safety of the equipment when the risk of electric shock is only one, phase conductor.
Secondly, such a scheme is convenient for wiring networks, calculating and understanding their work, and making measurements. So, to find the phase wire, no special measuring devices are needed, it is enough to have an indicator screwdriver.
But one more voltage can be obtained from three-phase networks if the load is connected between two phase wires. It will be higher in value than the phase voltage, because it will represent a projection onto the coordinate axis of not one vector, but two, located at an angle of 120 ° to each other.
This "appendage" will give an increase of about 73%, or √3-1. According to the existing standard, the line voltage in a three-phase network should be equal to 380 V.
What is the main difference between these voltages
If a corresponding load is connected to such a network, for example, a three-phase electric motor, it will give a mechanical power that is significantly greater than a single-phase one of the same size and weight. But there are two ways to connect a three-phase load. One, as already mentioned, is a "star".
If the initial terminals of all three windings of the generator or linear transformer are not connected together, but each of them is connected to the final terminal of the next, creating a serial chain from the windings, such a connection is called a "triangle".
Its peculiarity is the absence of a neutral wire, and to connect to such networks, you need appropriate three-phase equipment, whose loads are also connected by a "triangle".
With such a connection, only line voltages of 380 V operate in the load. One example: an electric motor connected to a three-phase network according to the "star" circuit, with a current in the windings of 3.3 A, will develop a power of 2190 W.
The same motor, switched on by a "triangle", will be three times more powerful at the root - 5570 W by increasing the current to 10 A.
It turns out that, having a three-phase network and the same electric motor, we can get a much greater power gain than when using single-phase, and simply by changing the connection scheme, we will triple the output power of the motor. True, its windings must also be rated for increased current.
Thus, the main difference between the two types of voltages in alternating current networks, as we found out, is the value of the line voltage, which is 3 times greater than the phase voltage. The absolute value of the potential difference between the phase wire and the Earth is taken as the value of the phase voltage. Linear voltage is the relative value of the potential difference between the two phase wires.
Well, at the end of the article, there are two videos about the connection with a star and a triangle, for those who want to understand in more detail.
