The three-phase AC system is widespread and used all over the world. With the help of a three-phase system, optimal conditions are provided for the transmission of electricity through wires over long distances, the ability to create electric motors that are simple in design and convenient in operation.
Three-phase AC system
A system consisting of three circuits with acting electromotive forces (EMF) of the same frequency is called. These EMFs are phase shifted relative to each other by one third. Each individual circuit in the system is called a phase. The whole system of three alternating currents, phase shifted, is called three-phase current.
Almost all generators that are installed in power plants are three-phase current generators. In the design, three are connected in one unit. The electromotive forces induced in them, as mentioned earlier, are shifted by one third of the period relative to each other.
How does the generator work?
The three-phase generator has three separate armatures located on the stator of the device. They have an offset of 1200 between them. An inductor rotates in the center of the device, which is common to the three armatures. A variable EMF of the same frequency is induced in each coil. However, the moments of passage of these electromotive forces through zero in each of these coils are shifted by 1/3 of the period, since the inductor passes near each coil 1/3 of the time later than the previous one.
All windings are independent current generators and power sources. If you connect wires to the ends of each winding, you get three independent circuits. In this case, six wires are required to transmit all the electricity. However, with other connections of the windings with each other, it is quite possible to do with 3-4 wires, which gives great savings in wire.
Star connection
The ends of all windings are connected at one point of the generator, the so-called zero point. Then a connection is made with consumers using four wires: three - linear wires that go from the beginning of windings 1, 2, 3, one is a zero (neutral) wire coming from the zero point of the generator. This system is also called four-wire.
Delta connection
In this case, the end of the previous winding is connected to the beginning of the next one, thereby forming a triangle. Linear wires are connected to the vertices of the triangle - points 1, 2, 3. With this connection, they coincide. Compared to star connection, delta connection reduces line voltage by about 1.73 times. It is allowed only under the condition of the same phase load, otherwise it can increase in the windings, which poses a danger to the generator.
Individual consumers (loads), which are powered by separate pairs of wires, can also be connected in the same way either by a star or a triangle. As a result, a situation similar to a generator is obtained: when connected by a triangle, the loads are under line voltage, when connected by a star, the voltage is 1.73 times less.
Most alternators and power transmission lines use three-phase systems. The current is transmitted through three lines (or four) instead of two. Three-phase current is a system of alternating electric current, where the values \u200b\u200bof currents and voltages change according to a sinusoidal law. The frequency of sinusoidal current oscillations in Russia and Europe is 50 Hz.
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Three-phase transmission line
Why use three-phase current
Transportation of electricity from power plants to distant points involves the use of very long wires and cables with high resistance. This means that some of the energy will be lost, dissipating as heat. By reducing the currents transmitted through power lines, losses can be significantly reduced.
The most common form of power generation is three-phase generation. In industry, three-phase alternating current is often used to drive electric motors.
The advantages of a three-phase system:
- Possibility of the presence of phase and line voltages in three-phase circuits of two different values: high - for powerful consumers, low - for the rest;
- Reduced energy transmission losses, hence the use of cheaper wires and cables;
- Three-phase machines have a more stable torque than single-phase machines (higher performance);
- Best performance in three phase generators;
- In some cases, DC must be derived from AC. At the same time, the use of 3-phase current is a significant advantage, since the ripple of the rectified voltage is much lower.
What is three phase current
A three-phase AC system consists of three sinusoidal current signals that differ by a third of a cycle or 120 electrical degrees (360 ° full cycle). They pass their maximums in a regular order called a phase sequence. The sinusoidal voltage is proportional to the cosine or sine of the phase.
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Three-phase current
Three phases are usually supplied over three (or four) wires, and the phase and line voltages in three-phase circuits represent the potential differences between the pairs of conductors. Phase currents are current quantities in each conductor.
Three-phase circuit diagrams
In a star configuration, there are three phase conductors. If the zero points of the power supply and the receiver are connected, then a four-wire "star" is obtained.
The circuit distinguishes between the phase-to-phase voltage between the phase conductors (it is also called linear), and the phase voltage between the individual phase conductors and the N-conductor.
What is the phase voltage is most clearly defined by constructing vectors - these are three symmetric vectors U (A), U (B) and U (C). Here you can see what the line voltage is:
- U (AB) \u003d U (A) - U (B);
- U (BC) \u003d U (B) - U (C);
- U (CA) \u003d U (C) - U (A).
Important! Vector constructions give an idea of \u200b\u200bthe shift between the matching phase and phase-to-phase voltage - 30 °.
Therefore, the line voltage for a star circuit with uniform loads can be calculated as follows:
Uab \u003d 2 x Ua x cos 30 ° \u003d 2 x Ua x √3 / 2 \u003d √3 x Ua.
Other indicators of phase voltage are similarly found.
Linear and phase voltages, if we sum up the vector values \u200b\u200bof all phases, are equal to zero:
- U (A) + U (B) + U (C) \u003d 0;
- U (AB) + U (BC) + U (CA) \u003d 0.
If an electrical receiver with resistance identical in each phase is connected to the "star":
then you can calculate the linear and phase currents:
- Ia \u003d Ua / Za;
- Ib \u003d Ub / Zb;
- Ic \u003d Uc / Zc.
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Plotting vectors in the "Y" scheme
As applied to the general cases of a "star" system, the linear current values \u200b\u200bare identical to the phase ones.
It is usually assumed that the source supplying power to the loads is balanced, and only impedance determines the operation of the circuit.
Since the summing current index corresponds to zero (Kirchhoff's law), then in the case of a four-wire system, no current flows in the neutral conductor. The system will behave the same whether there is a neutral conductor or not.
For the active power of a three-phase receiver, the following formula is valid:
P \u003d √3 x Uph I x cos φ.
Reactive power:
Q \u003d √3 x Uph I x sin φ.
"Y" at asymmetric load
This is such a circuit configuration, where the current value of one phase differs from another, or the phase shifts of currents are different compared to voltages. The phase-to-phase voltages will remain symmetrical. By vector constructions, the appearance of a shift of the zero point from the center of the triangle is determined. The result is an asymmetry of phase voltages and the appearance of Uo:
Uo \u003d 1/3 (U (A) + U (B) + U (C)).
Despite the asymmetric load, the summing current indicator is zero.
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"Y" without N-conductor with unbalanced load
Important! The operation of an asymmetric load circuit depends on whether or not an N-conductor is present.
The circuit behaves differently when an N-conductor with an insignificant impedance Zo \u003d 0 is connected. The zero points of the MT and the electric receiver are galvanically connected and have the same potential. The phase voltage of different phases takes on an identical value, and the current value inN-conductor:
Io \u003d I (A) + I (B) + I (C).
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Four-wire "Y" circuit
When transmitting power, it is common to use three-wire systems at high and medium voltage levels. At low voltage levels where unbalanced loads are difficult to avoid, four-wire systems are used.
Diagram "Δ"
By connecting the end of each phase of the electrical receiver to the beginning of the next, a three-phase current with phases connected in series can be obtained. The resulting circuit configuration is called a "triangle". In this form, it can only work as a three-wire.
With the help of vector constructions, understandable even for dummies, phase and line voltages and currents are illustrated. Each phase of the electrical consumer is connected to a line voltage between two conductors. Line and phase voltages are identical at the power receiver.
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Diagram "Δ" and construction of vectors
Phase currents for the "triangle" - I (A), I (B), I (C). Phase - I (AB), IBS), I (CA).
Linear currents are found from vector constructions:
- I (A) \u003d I (AB) - I (CA);
- I (B) \u003d I (BC) - I (AB);
- I (C) \u003d I (CA) - I (BC).
The summing current value in a symmetrical system corresponds to zero. RMS values \u200b\u200bof phase currents:
I (AB) \u003d I (BC) \u003d I (CA) \u003d U / Z.
Since the phase shift between U and I is 30 °, line current in this configuration will be equal to:
I (A) \u003d I (AB) - I (CA) \u003d 2 x I (AB) x cos 30 ° \u003d 2 x If x √3 / 2 \u003d √3 x If.
Important! The effective value of the line current is √3 times the effective value of the phase current.
Three-phase and single-phase current
Schematic configuration "Y" makes it possible to use two different voltages when powering consumers of a household and industrial network: 220 V and 380 V. 220 V is obtained using two conductors. One of them is phase, the other is N-conductor. The voltage between them corresponds to the phase voltage. If you take 2 conductors, both of which are phases, then the voltage between the phases is called linear and is equal to 380 V. All 3 phases are used for connection.
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Voltage distribution in single-phase and three-phase systems
The main differences between single-phase and three-phase systems:
- Single-phase current assumes power supply through one conductor, three-phase current - through three;
- To complete the single-phase power supply circuit, 2 conductors are required: one more neutral, for three-phase - 4 (plus neutral);
- The greatest power is transmitted in three phases, in contrast to a single-phase system;
- Single-phase network is simpler;
- In the event of a phase wire failure in a single-phase network, the power is completely lost, in a three-phase network, it is supplied through the two remaining phases.
Interesting. Nikola Tesla, the discoverer of multiphase currents and the inventor of the asynchronous motor, used a two-phase current with a phase difference of 90 °. Such a system is suitable for creating a rotating magnetic field more than a single-phase, but less than a three-phase one. The two-phase system first became widespread in the United States, but then completely disappeared from use.
Today, almost all power supply is based on low frequency three-phase currents with the parallel use of individual phases. Almost all power plants have generators that produce three-phase current. Transformers can be operated with three-phase or single-phase current. The presence of reactive power in such networks requires the installation of compensating equipment.
Video
Many have heard such mysterious words as one phase, three phases, zero, grounding or landand know that these are important concepts in the world of electricity. However, not everyone understands what they mean and how they relate to the surrounding reality. Nevertheless, you must know this.
Without going into technical details that are not needed by the DIYer, we can say that three-phase network - this is a method of transferring electric current, when alternating current flows through three wires, and one returns back. The above needs to be explained a little. Any electrical circuit consists of two wires. One by one, the current goes to the consumer (for example, to the kettle), and the other goes back. If you open such a circuit, then the current will not flow. That's the whole description of a single-phase network (Fig. 1).
Figure: 1. Single-phase circuit diagram
The wire through which the current flows is called phase, or simply phase, and through which it returns - zero, or zero. A three-phase circuit consists of three phase conductors and one return conductor. This is possible because the AC phase in each of the three wires is shifted with respect to the adjacent one by 120 ° C (Fig. 2). A textbook on electromechanics will help answer this question in more detail.
Figure: 2. Three-phase circuit diagram
AC transmission occurs precisely with the help of three-phase networks. This is economically beneficial - two more neutral wires are not needed. Approaching the consumer, the current is divided into three phases, and each of them is given a zero. So he gets into apartments and houses. Although sometimes a three-phase network is brought directly into the house. As a rule, we are talking about the private sector, and this state of affairs has its pros and cons. This will be discussed later.
Earth, or, more correctly, grounding - the third wire in single-phase network... In fact, it does not carry a workload, but serves as a kind of fuse.
This can be explained with an example. In the event that electricity goes out of control (for example, a short circuit), there is a danger of fire or electric shock. To prevent this from happening (that is, the current value should not exceed a level safe for humans and devices), grounding is introduced. Through this wire, the excess electricity literally goes into the ground (Fig. 3).
Figure: 3. The simplest
One more example. Suppose a small breakdown occurs in the operation of the electric motor of the washing machine and part of the electric current enters the outer metal shell of the appliance. If there is no ground, this charge will continue to wander through the washing machine. When a person touches it, he will instantly become the most convenient outlet for this energy, that is, he will receive an electric shock. If there is a ground wire in this situation, the excess charge will drain through it without causing harm to anyone. In addition, it can be said that neutral conductor it can also be grounding and, in principle, it is, but only at a power plant.
Some craftsmen, relying on basic knowledge of electrical engineering, establish neutral wire as grounding. Never do that. If the neutral wire breaks, the cases of grounded devices will be energized by 220 V.
In 99% of cases, a single-phase network is installed for an apartment. It is very simple to distinguish it from a three-phase one. If the incoming cable has 3 or 2 wires, then the network is single-phase, when 5 or 4 is three-phase (Fig. 4).
Figure: 4. A four-core or two-core cable becomes if the ground wire is removed
As you know, a three-phase current flows through wires that transmit energy over a distance - this is more profitable. He enters the apartment single-phase. The splitting of a three-phase circuit into 3 single-phase circuits occurs during VRU... A five-core cable enters there, and a three-core cable goes out (Fig. 5).
Figure: 5. Scheme of splitting a three-phase network into single-phase consumers
When asked where the 2 more go, the answer is simple: they feed other apartments. This does not mean that there are only 3 apartments, there can be as many of them as you want, if only the cable can withstand. It's just that inside the switchboard, a three-phase circuit is disconnected into single-phase circuits (Fig. 6).
Figure: 6. Single-phase electrical network
For each phase that goes to the apartment is added zero and grounding, this is how you get a three-core cable.
Ideally in three-phase network only one zero. More is not necessary, since the current is phase shifted relative to each other by one third. Zero is a neutral conductor in which there is no voltage. It has no potential with respect to the ground, in contrast to the phase, in which the voltage is 220 V... In a pair "phase - phase" voltage 380 V... In a three-phase network to which nothing is connected, there is no voltage in the neutral conductor. The most interesting thing starts to happen when the network is connected to a single phase circuit. One phase enters the apartment, where there are 2 bulbs and a refrigerator, and the second - where there are 5 air conditioners, 2 computers, a shower cabin, an induction cooker, etc. (Fig. 7).
Figure: 7. Three-phase electrical network
It is clear that the load on these two phases is not the same and there is no longer any talk of any neutral conductor. Voltage also appears on it, and the more uneven the load, the greater it is.
The phases no longer compensate each other, so that the total is zero.
Recently, the situation with non-compensation of currents in such a network has been aggravated by the fact that new electrical appliances have appeared, which are called pulse. At the moment of switching on, they consume much more energy than during normal operation. These impulse devices, coupled with different phase loads, create such conditions that a voltage arises in the neutral conductor (zero), which can be 2 times more than on any phase. However, the neutral is the same cross-sectionsas the phase wire, and the load is greater.
That is why in recent years, a phenomenon called burning off zero - the neutral conductor simply cannot cope with the load and burns out. It is not easy to deal with such a phenomenon: it is necessary either to increase the cross-section of the neutral wire (which is expensive), or to distribute the load evenly between the 3 phases (which is impossible in an apartment building). At worst, you can buy a step-down isolation transformer, it is voltage regulator.
In private home the situation is better, since the owner is one and it is much easier to distribute the electricity in phases. It's even an exciting activity - calculate power electrical appliances and distribute them in phases so that the load is the same. All calculations are done approximately, and does not mean at all that it is necessary to turn on the light and 2 TVs, and if the carpentry machine is working on the street, this is overkill. It all depends on the desire of the owner of the house: to carry out a three-phase network or a single-phase one. This has its pros and cons.
Cons of a three-phase network 2.
1. The tension in a particular area is highly dependent on the work of others. If one of the phases is overloaded, the others may not work correctly. It can manifest itself as you like. To prevent this from happening, you need a stabilizer - not a cheap thing.
2. Equipment in a switchboard, designed specifically for a three-phase network, is required, as well as the costs of a three-phase network device. They will be larger than for single-phase. In addition, you need to know the rules for operating three-phase networks.
The advantages of a three-phase network also 2.
1. A three-phase network allows you to get more power. If a single-phase network with a total power of 10 kW devices is already experiencing overloads, then a three-phase network copes well with 30 kW. The example is very simple. If only 1 phase enters the house from the power line, then with a cross-section of the incoming conductor of 16 mm2, the maximum power will be only 14 kW, and if all 3 phases are already 42 kW. The difference is quite tangible.
2. It becomes unusually easy to connect electrical appliances with a three-phase power supply (electric stoves). The most important thing in the case of a private house is the three-phase electric motors that are on many machines.
The advantages of three-phase current are obvious only to electricians. What is a three-phase current for the layman seems very vague. Let's clear up the uncertainty.
Three-phase alternating current
Most people, with the exception of electricians, have a very vague idea of \u200b\u200bwhat the so-called "three-phase" alternating current is, and in terms of current strength, voltage and electric potential, as well as power, they are often confused.
Let's try to give initial concepts about this in simple language. To do this, let us turn to analogies. Let's start with the simplest - the flow of direct current in conductors. It can be compared to water flow in nature. As you know, water always flows from a higher point on the surface to a lower one. Always chooses the most economical (shortest) path. The analogy with the flow of current is complete. Moreover, the amount of water flowing per unit of time through some section of the flow will be similar to the current in the electric circuit. The height of any point of the river channel relative to the zero point - sea level - will correspond to the electric potential of any point in the chain. And the difference in the height of any two points of the river will correspond to the voltage between the two points of the chain.
Using this analogy, you can easily imagine in your mind the laws of the flow of a direct electric current in a circuit. The higher the voltage - the difference in altitude, the greater the flow rate, and, therefore, the amount of water flowing through the river per unit of time.
A water flow, just like an electric current, during its movement, experiences the resistance of the channel - water will flow violently along a rocky channel, changing direction, warming up a little from this (violent flows do not freeze even in severe frosts due to heating from the resistance of the channel). In a smooth canal or pipe, water will flow quickly and, as a result, much more water will pass through the canal per unit of time than a winding and rocky channel. The resistance to water flow is exactly the same as the electrical resistance in a circuit.
Now imagine a closed bottle with some water in it. If we start to rotate this bottle around the transverse axis, then the water in it will flow alternately from the neck to the bottom and vice versa. This view is analogous to alternating current. It would seem that the same water flows back and forth, so what? However, this variable flow of water is capable of doing work.
Where did the concept of alternating current come from?
Yes, since the very time when humanity, having learned that the movement of a magnet near a conductor causes an electric current in the conductor. It is the movement of the magnet that causes the current, if the magnet is placed next to the wire and not moved, this will not cause any current in the conductor. Further, we want to receive (generate) current in the conductor in order to use it in the future for any purpose. To do this, we will make a coil of copper wire and start moving the magnet near it. The magnet can be moved near the coil as you like - move in a straight line back and forth, but in order not to move the magnet with your hands, it is technically more difficult to create such a mechanism than just start rotating it around the coil, similar to the rotation of the water bottle from the previous example. This is exactly how - for technical reasons - we got the sinusoidal alternating current, which is now used everywhere. A sinusoid is a time-based description of rotation.
Later it turned out that the laws of the flow of alternating current in the circuit differ from the flow of direct current. For example, for direct current flow, the resistance of the coil is simply the ohmic resistance of the wires. And for alternating current, the resistance of the coil of wires increases significantly due to the appearance of the so-called inductive resistance. Direct current does not pass through a charged capacitor; for it, a capacitor is an open circuit. And the alternating current is able to flow freely through the capacitor with some resistance. Further, it turned out that alternating current can be converted using transformers into alternating current with a different voltage or current strength. Direct current does not lend itself to such a transformation, and if we turn on any transformer into a direct current network (which absolutely cannot be done), then it will inevitably burn out, since only the ohmic resistance of the wire will resist direct current, which is made as small as possible, and through the primary winding a large current will flow in the short-circuit mode.
Note also that electric motors can be designed to operate on both DC and AC. But the difference between them is this - DC electric motors are more difficult to manufacture, but they allow you to smoothly change the rotation speed with a conventional rheostat regulating the current strength. And AC motors are much simpler and cheaper to manufacture, but rotate at only one speed, due to the design. Therefore, both are widely used in practice. Depending on the destination. DC motors are used for control and regulation purposes, and AC motors are used as power plants.
Further, the design idea of \u200b\u200bthe generator's inventor moved in approximately the following direction - if it is most convenient to use the rotation of a magnet next to the coil to generate current, then why not place several coils around the rotating magnet instead of one generator coil (there is so much space around)?
It will turn out right away, as it were, several generators working from one rotating magnet. Moreover, the alternating current in the coils will differ in phase - the maximum current in the subsequent coils will be somewhat delayed relative to the previous ones. That is, the sinusoids of the current, if they are graphically depicted, will be shifted, as it were, among themselves. This important property is the phase shift, which we will discuss below.
Thinking about this, the American inventor Nikola Tesla first invented alternating current, and then a three-phase current generation system with six wires. He placed three coils around the magnet at an equal distance at angles of 120 degrees, if the axis of rotation of the magnet is taken as the center of the corners.
(The number of coils (phases) can actually be any, but to get all the advantages that a polyphase current generation system gives, at least three are enough).
Further, the Russian electrical engineer Mikhail Osipovich Dolivo-Dobrovolsky developed N. Tesla's invention, first proposing a three- and four-wire transmission system for three-phase alternating current. He proposed connecting one end of all three windings of the generator to one point and transmitting electricity through only four wires. (Savings on expensive non-ferrous metals are significant). It turned out that with a symmetrical load of each phase (equal resistance), the current in this common wire is zero. Because when summing (algebraically, taking into account the signs) currents shifted in phase by 120 degrees, they mutually annihilate. This common wire was called so - zero. Since the current in it occurs only when the phase loads are uneven and it is numerically small, much less than the phase currents, it became possible to use a smaller wire as a "zero" wire than for phase wires.
For the same reason (phase shift by 120 degrees), three-phase ones turned out to be much less material-intensive, since in the magnetic circuit of the transformer there is a mutual absorption of magnetic fluxes and it can be done with a smaller cross section.
Today, the three-phase power supply system is carried out by four wires, three of them are called phase and are designated by Latin letters: on the generator - A, B and C, at the consumer - L1, L2 and L3. The zero wire is denoted as - 0.
The voltage between the neutral wire and any of the phase wires is called phase and is 220 volts in consumer networks.
There is also a voltage between the phase wires, and it is much higher than the phase voltage. This voltage is called linear and is 380 volts in consumer circuits. Why is it more than phase? Yes, all this is due to a phase shift of 120 degrees. Therefore, if on one wire, for example, at a given time the potential is equal to plus 200 volts, then on the other phase wire at the same time the potential will be minus 180 volts. Voltage is the potential difference, that is, it will be + 200 - (-180) \u003d + 380 V.
The question arises, if the current does not flow through the zero wire, then can it be removed at all. Can. And we will get a three-wire power supply system. With the connection of consumers by the so-called "triangle" - between the phase wires. However, it should be noted that with an uneven load on the sides of the "triangle", destructive loads will act on the generator, therefore this system can be used with a huge number of consumers, when the uneven loads are leveled. Electricity transmission from large power plants at high phase and line voltages (hundreds of thousands of volts) is carried out in this way. Why is such a high voltage applied? The answer is simple - to reduce heating losses in the wires. Since the heating of the wires (energy loss) is proportional to the square of the flowing current, it is desirable that the flowing current be minimal. Well, to transfer the required power at a minimum current, you need to increase the voltage. (Power lines) and are designated, for example, power lines - 500 - this is a power line under a voltage of 500 kilovolts.
By the way, losses in power transmission lines can be further reduced by using high voltage direct current transmission (the capacitive component of losses acting between the wires ceases to act), even such experiments were carried out, but such a system has not yet become widespread, apparently due to greater savings in wires with three-phase generation system.
Conclusions: the advantages of a three-phase system
In conclusion of the article, we will summarize - what are the advantages of a three-phase generation and power supply system?
- Saving on the number of wires required to transmit electricity. Considering the considerable distances (hundreds and thousands of kilometers) and the fact that non-ferrous metals with low electrical resistivity are used for wires, the savings are very significant.
- Three-phase transformers, with the same power as single-phase ones, have a much smaller magnetic circuit. That allows you to get significant savings.
- It is very important that a three-phase electric power transmission system creates a rotating electromagnetic field when the consumer is connected to three phases. Again, due to the phase shift. This property made it possible to create extremely simple and reliable three-phase electric motors, which do not have a collector, and the rotor, in fact, is a simple "blank" in the bearings, to which no wires need to be connected. (In fact, the design of a squirrel-cage rotor has its own characteristics and is not a blank at all) These are the so-called three-phase asynchronous electric motors with a squirrel-cage rotor. Very widespread today as power plants. A wonderful property of such motors is the ability to reverse the direction of rotation of the rotor by simply switching any two phase wires.
- Possibility of obtaining two operating voltages in three-phase networks. In other words, change the power of an electric motor or heating installation by simply switching the supply wires.
- Possibility of significant reduction of flicker and stroboscopic effect of luminaires based on fluorescent lamps by placing three lamps in the luminaire powered from different phases.
Thanks to these advantages, three-phase power supply systems are widely used in the world.
Three-phase connection makes it possible to turn on generators and electric motors of increased power, as well as the ability to work with different voltage parameters, it depends on the type of load connection to the electrical circuit. To work in a three-phase network, you need to understand the ratio of its elements.
Elements of a three-phase network
The main elements of a three-phase network are a generator, an electric power transmission line, and a load (consumer). To consider the question of what is the linear and phase voltage in a circuit, we will give a definition of what a phase is.
A phase is an electrical circuit in a polyphase electrical circuit system. The beginning of the phase is the terminal or end of the conductor of electricity, through which the electric current enters it. Experts have always differed in the number of phases in electrical circuits: single-phase, two-phase, three-phase and multiphase.
The most commonly used three-phase switching of objects, which has a significant advantage, both over polyphase circuits and over a single-phase circuit. The differences are as follows:
- lower costs for the transportation of electrical energy;
- the ability to create EMF for the operation of asynchronous motors is the operation of elevators in multi-storey buildings, equipment in the office and in production;
- this type of connection makes it possible to simultaneously use both line and phase voltage.
What is phase and line voltage?
Phase and line voltages in three-phase circuits are important for manipulations in electrical power panels, as well as for the operation of equipment powered by 380 volts, namely:
- What is phase voltage? This voltage, which is determined between the beginning of the phase and its end, in practice it is determined between the neutral wire and the phase.
- Line voltage is when the value is measured between two phases, between the terminals of different phases.
In practice, the phase voltage differs from the line voltage by 60%, in other words, the line voltage parameters are 1.73 times higher than the phase voltage. Three-phase circuits can have a line voltage of 380 volts, which makes it possible to obtain a phase voltage of 220 V.
What's the Difference?
For society, the concept of "phase-to-phase voltage" is found in apartment buildings, high-rise buildings, when the ground floors are intended for office premises, as well as in shopping centers, when the building objects are connected by several power cables of a three-phase network, which provide a voltage of 380 volts. This type of connection at home ensures the operation of asynchronous motors for lifts, the operation of an escalator, and industrial refrigeration equipment.
In practice, wiring a three-phase circuit is quite simple, given that a phase and zero go to the apartment, and all three phases + a neutral wire go to the office space.
The complexity of the linear connection scheme lies in the difficulty of determining during the installation of the conductor, which can lead to equipment failure. The circuit differs mainly between phase and line connections, connections of load windings and power supply.
Connection diagrams
There are two schemes for connecting voltage sources (generators) to the network:
- "Triangle";
- "Star".
When a star connection is made, the beginning of the generator windings are connected at one point. It does not provide the ability to increase power. And the connection according to the "triangle" scheme is when the windings are connected in series, namely, the beginning of the winding of one phase is connected to the end of the winding of the other. This gives the ability to triple stress.
For a better understanding of the connection diagrams, experts define what phase and line currents are:
- linear current is the current that flows in the submarine of the connection between the source of electrical energy and the receiver (load);
- phase current is the current flowing in each winding of an electrical power source or in load windings.
Linear and phase currents matter when there is an unbalanced load on the source (generator), this is often found in the process of connecting objects to the power supply. All parameters related to the line are line voltages and currents, and those related to the phase are parameters of phase quantities.
From the "star" connection it can be seen that the line currents have the same parameters as the phase ones. When the system is symmetrical, there is no need for a neutral conductor; in practice, it maintains source symmetry when the load is unbalanced.
Due to the asymmetry of the connected load (and in practice this happens with the inclusion of lighting devices in the circuit), it is necessary to ensure the independent operation of three phases of the circuit, this can also be done in a three-wire line, when the phases of the receiver are connected in a triangle.
Experts pay attention to the fact that when the line voltage decreases, the phase voltage parameters change. Knowing the value of the phase-to-phase voltage, you can easily determine the value of the phase voltage.
How to calculate line voltage?
and Ohm's law:
When a branched system for supplying an object with electricity is performed, sometimes there is a need to calculate the voltage between two wires "zero" and "phase": IF \u003d IL, which indicates the equality of the phase and linear parameters. The relationship between phase wires and line wires can be found using the formula:
The finding element of the voltage ratios and the assessment of the power supply system by specialists is performed according to linear parameters when their value is known. In power supply systems from four wires, 380/220 volts are marked.
Conclusion
Using the capabilities of a three-phase circuit (four-wire circuit), connections can be made in different ways, which makes it possible for its widespread use. Experts consider the three-phase voltage for connection to be a universal option, since it makes it possible to connect high-power loads, living quarters, office buildings.
In apartment buildings, the main consumers are household appliances designed for a 220 V network, for this reason it is important to make an even distribution of the load between the phases of the circuit, this is achieved by connecting apartments to the network according to a checkerboard principle. The distribution of the load of private houses differs, in them it is performed according to the values \u200b\u200bof the load on each phase of all home equipment, by the currents in the conductors passing during the period of maximum switching on of the devices.
