Generating sets produce single-phase or three-phase current at industrial frequency, while chemical sources produce direct current. In this case, in practice, situations often arise when one type of electricity is not enough for the operation of certain devices and it is required to convert it.
For this purpose, the industry produces a large number of electrical devices that operate with different parameters of electrical energy, converting them from one type to another with different voltages, frequency, number of phases and waveforms. According to the functions they perform, they are divided into conversion devices:
simple;
with the ability to adjust the output signal;
endowed with the ability to stabilize.
Classification methods
By the nature of the operations performed, the converters are divided into devices:
straightening;
inverting in one or more stages;
changes in signal frequency;
conversion of the number of phases of the electrical system;
modification of the type of voltage.
According to the control methods of the occurring algorithms, the adjustable converters operate on:
pulse principle used in DC circuits;
phase method used in harmonic oscillation circuits.
The simplest converter designs may not be equipped with a control function.
All conversion devices can use one of the following types of electrical circuit:
pavement;
zero;
with or without a transformer;
with one, two, three or more phases.
Rectifier devices
This is the most common and old class of converters that allow you to receive rectified or stabilized direct current from an alternating sinusoidal, usually industrial frequency.
Rare exhibits
Low-power devices
Just a few decades ago, selenium structures and vacuum-based devices were still used in radio and electronic devices.
Such devices were based on the principle of current rectification by one single element from a selenium plate. They were sequentially assembled into a single structure through mounting adapters. The higher the voltage required for rectification, the more such elements were used. They were not very powerful and could withstand a load of several tens of milliamperes.
A vacuum was created inside the sealed glass case of the lamp rectifiers. It housed electrodes: an anode and a cathode with a filament providing for thermionic emission.
Such lamp devices provided direct current power for various circuits of radio receivers and televisions until the end of the last century.
Ignitrons are powerful devices
In industrial devices, ionic mercury devices with an anode and a cathode, operating on the principle of a controlled arc charge, were widely used in the past. They were used where it was required to operate with a DC load with a force of hundreds of amperes at a rectified voltage of up to five kilovolts inclusive.
An electron flow was used for the flow of current from the cathode towards the anode. It was created by an arc discharge caused at one or more areas of the cathode, called luminous cathode spots. They were formed when the auxiliary arc was turned on from the ignition electrode until the main arc was ignited.
For this, short-term pulses of several milliseconds were created with a current strength of up to tens of amperes. Changing the shape and strength of the pulses made it possible to control the operation of the ignitron.
This design provided good voltage maintenance during rectification and a fairly high efficiency. But, the technical complexity of the design and the difficulties of operation led to the rejection of its use.
Semiconductor devices
Diodes
Their work is based on the principle of current conduction in one direction due to properties p-n a transition formed by contacts between semiconductor materials or metal and semiconductor.
Diodes pass current only in a certain direction, and when a variable sinusoidal harmonic passes through them, they cut off one half-wave and, due to this, are widely used as rectifiers.
Modern diodes are produced very wide range and are endowed with a variety of technical characteristics.
Thyristors
The thyristor uses four conduction layers, which form a more complex semiconductor structure than a diode with three series-connected p-n transitions J1, J2, J3. Contacts with the outer layer "p" and "n" are used as the anode and cathode, and with the inner one - as the control electrode of the UE, which is used to turn the thyristor into operation and perform regulation.
The rectification of the sinusoidal harmonic is done according to the same principle as with a semiconductor diode. But, for the thyristor to work, it is necessary to take into account a certain feature - the structure of its internal transitions must be open for the passage of electric charges, and not closed.
This is done by passing a current of a certain polarity through a gate electrode. The picture below shows the ways to open the thyristor, which are used at the same time to adjust the amount of current passed at different times.
When the current is applied through the RE at the moment of transition of the sinusoid through the zero value, a maximum value is created, which gradually decreases at points "1", "2", "3".
In this way, the current is rectified in conjunction with thyristor regulation. Triacs and power MOSFETs and / or AGBTs in power circuits work in a similar way. But, they do not perform the function of rectifying the current, passing it in both directions. Therefore, their control schemes use an additional pulse interruption algorithm.
DC / DC converters
These designs perform the reverse operation of the rectifiers. They are used to generate alternating sinusoidal current from direct current obtained from chemical current sources.
Rare development
Since the end of the 19th century, electrical machine structures have been used to convert direct voltage to alternating voltage. They consisted of a direct current electric motor, which received energy from a battery or a set of batteries, and an alternating voltage generator, the armature of which rotated from the motor drive.
In some devices, the generator winding was wound directly on the common rotor of the engine. With this method, not only the signal shape was changed, but, as a rule, the voltage amplitude or frequency was increased.
If three windings spaced 120 degrees apart are wound on the generator armature, then with its help an equivalent symmetrical three-phase voltage was obtained.
Umformers were widely used until the 70s for vacuum tubes, equipment for trolley buses, trams, electric locomotives before the massive introduction of semiconductor elements.
Inverter converters
Principle of operation
As a basis for consideration, we take the circuit for checking the KU202 thyristor from a battery and a light bulb.
A normally closed contact of the SA1 button and a low-power incandescent lamp are embedded in the circuit for supplying the positive potential of the battery to the anode. The control electrode is connected through a current-limiting resistor and an open contact of the SA2 button. The cathode is rigidly connected to the minus of the battery.
If at time t1 you press the SA2 button, then a current will flow to the cathode through the control electrode chain, which will open the thyristor and the lamp included in the anode branch will light up. It, due to the design feature of this thyristor, will continue to burn even when the SA2 contact is opened.
Now at the moment of time t2 we press the button SA1. The power supply circuit of the anode will be de-energized, and the light will go out due to the fact that the passage of current through it stops.
The graph of the presented picture shows that a direct current passed within the time interval t1 ÷ t2. If you switch buttons very quickly, then you can form with a positive sign. Similarly, you can create a negative impulse. For this purpose, it is enough to slightly change the circuit for the passage of current in the opposite direction.
A sequence of two pulses of positive and negative values \u200b\u200bcreates a waveform called a square wave in electrical engineering. Its rectangular shape rather roughly resembles a sinusoid with two half-waves of opposite signs.If, in the considered scheme, we replace the SA1 and SA2 buttons with relay contacts or transistor switches and switch them according to a certain algorithm, then it will be possible to automatically create a current with a meander shape and adjust it to a certain frequency, duty cycle, period. Such switching is handled by a special electronic control circuit.
Block diagram of the power unit
As an example, consider the simplest primary circuit system of a bridge inverter.
Here, instead of a thyristor, specially selected field-effect transistor switches are engaged in the formation of a rectangular pulse. The load resistance Rн is included in the diagonal of their bridge. The power electrodes of each transistor "source" and "drain" are oppositely connected to shunt diodes, and the output contacts of the control circuit are connected to the "gate".
Due to the automatic operation of control signals, voltage pulses of different duration and sign are output to the load. Their sequence and characteristics are tailored to the optimal parameters of the output signal.
Under the action of the applied voltages on the diagonal resistance, taking into account the transient processes, a current arises, the shape of which is already more close to a sinusoid than that of a meander.
Difficulties in technical implementation
For the good functioning of the power circuit of the inverters, it is necessary to ensure the reliable operation of the control system, which is based on switching the keys. They are endowed with two-sided conduction properties and are formed by shunting transistors by connecting reverse diodes.
In order to regulate the amplitude of the output voltage, it is most often used by choosing the area of \u200b\u200bthe pulse of each half-wave by controlling its duration. In addition to this method, there are devices that operate on an amplitude pulse conversion.
In the process of forming the output voltage circuits, a violation of the symmetry of the half-waves occurs, which adversely affects the operation of inductive loads. This is most noticeable in transformers.
When the control system is operating, an algorithm for generating the keys of the power circuit is set, which includes three stages:
1. straight;
2. short-circuited;
3.inverse.
On the load, not only pulsating currents, but also currents changing in direction are possible, which create additional noise at the source terminals.
Typical designs
Among the many different technological solutions used to create inverters, three schemes are common, considered in terms of the degree of increase in complexity:
1. bridge without a transformer;
2. with zero terminal of the transformer;
3. bridge with a transformer.
Output waveforms
Inverters are designed to supply voltages:
rectangular;
trapezoid;
stepped alternating signals;
sinusoids.
Phase converters
The industry produces electric motors for operation in specific operating conditions, taking into account power from certain types of sources. However, in practice, situations arise when, for various reasons, it is necessary to connect a three-phase asynchronous motor to a single-phase network. For this, various electrical circuits and devices have been developed.
Energy-consuming technologies
The stator of a three-phase asynchronous motor includes three windings that are wound in a certain way, spaced 120 degrees apart, each of which, when a current of its voltage phase is applied to it, creates its own rotating magnetic field. The direction of the currents is chosen so that their magnetic fluxes complement each other, providing mutual action for the rotation of the rotor.
When there is only one phase of the supply voltage for such a motor, it becomes necessary to form three current chains from it, each of which is also displaced by 120 degrees. Otherwise, the rotation will not work or will be defective.
In electrical engineering, there are two easy ways rotation of the current vector relative to the voltage by the connection method by:
1. an inductive load when the current starts to lag behind the voltage by 90 degrees;
2. Capacitance to create a 90 degree current lead.
The above picture shows that from one phase of the voltage Ua, you can get a current shifted in an angle not by 120, but only by 90 degrees forward or backward. Moreover, for this, it will also be necessary to select the ratings of capacitors and chokes in order to create an acceptable operating mode of the engine.
In practical solutions of such circuits, most often they stopped at the capacitor method without using inductive resistances. For this, the supply phase voltage was applied to one winding without any transformations, and to the other, shifted by capacitors. The result was an acceptable torque for the engine.
But in order to spin the rotor, it was required to create additional torque by connecting the third winding through starting capacitors. It is impossible to use them for permanent operation due to the formation of large currents in the starting circuit, which quickly create increased heating. Therefore, this chain was switched on for a short time to gain the moment of inertia of the rotor rotation.
Such circuits were easier to implement due to the simple formation of capacitor banks of certain ratings from individual available elements. The chokes had to be calculated and wound independently, which is difficult to perform not only at home.
But, best conditions for the operation of the engine, they were created with the complex connection of the capacitor and the choke in different phases with the selection of the directions of the currents in the windings and the use of current-suppressing resistors. With this method, the engine power loss was up to 30%. However, the designs of such converters were not economically viable because they consumed more electricity for operation than the motor itself.
The capacitor starting circuit also consumes an increased rate of electricity, but to a lesser extent. In addition, the motor connected to its circuit is capable of generating a power slightly exceeding 50% of that which was created with a normal three-phase power supply.
Due to the difficulties of connecting a three-phase motor to a single-phase power supply circuit and large losses of electricity and output power, such converters have shown their low efficiency, although they continue to work in separate installations and machine tools.
Inverter devices
Semiconductor elements have made it possible to create more rational phase converters produced on an industrial basis. Their designs are usually designed to operate in three-phase circuits, but they can be designed to operate with a large number of strings spaced at different corners.
When the converters are powered from one phase, the following sequence of technological operations is performed:
1. rectification of single-phase voltage by diode assembly;
2. smoothing the ripple by the stabilization circuit;
3. conversion of direct voltage to three-phase due to the inversion method.
In this case, the power circuit can consist of three single-phase parts that operate autonomously, as discussed earlier, or one common, assembled, for example, according to an autonomous three-phase inverter conversion system using a neutral common wire.
Here, for each phase load, its own pairs of semiconductor elements work, which are controlled from a common control system. They create sinusoidal currents in the phases of the resistances Ra, Rb, Rc, which are connected to the common supply circuit through the neutral wire. It adds the vectors of currents from each load.
The quality of the approximation of the output signal to the form of a pure sine wave depends on the overall design and the complexity of the circuit used.
Frequency converters
On the basis of inverters, devices are created that allow changing the frequency of sinusoidal oscillations within a wide range. To do this, the 50 hertz electricity supplied to them undergoes the following changes:
straightening;
stabilization;
high frequency voltage conversion.
The work is based on the same principles of the previous designs, except that the control system based on microprocessor boards generates an output voltage of an increased frequency of tens of kilohertz at the converter output.
Frequency conversion based on automatic devices allows you to optimally regulate the operation of electric motors at the moments of starting, braking and reversing, as well as conveniently changing the rotor speed. At the same time, the harmful effect of transient processes in the external power supply network is sharply reduced.
Welding inverters
The main purpose of these voltage converters is to maintain a stable burning of the arc and easy control of all its characteristics, including ignition.
For this purpose, several blocks are included in the design of the inverter, which carry out sequential execution:
rectification of three-phase or single-phase voltage;
stabilization of parameters by filters;
inversion of high-frequency signals from a stabilized constant voltage;
conversion to / h voltage by a step-down transformer to increase the value of the welding current;
secondary rectification of the output voltage for the formation of an arc at welding.
By using high-frequency signal conversion, the dimensions of the welding transformer are significantly reduced and materials for the entire structure are saved. have great advantages in operation in comparison with their electromechanical counterparts.
Transformers: voltage converters
In electrical engineering and power engineering, transformers operating on the electromagnetic principle are still most widely used to change the amplitude of the voltage signal.
They have two or more windings and, through which magnetic energy is transmitted to convert the input voltage into an output voltage with a changed amplitude.
Direct use of natural energy sources.
Conversion using a steam engine
Conversion using electricity
Energy Conversion in Industrial Energy
As mentioned above, electricity generation is a separate industry. Currently, the largest share of electricity is produced at three types of power plants:
1. HPP (hydroelectric power plant)
2. TPP (thermal power plant)
3. NPP (nuclear power plant)
Consider the conversion of energy in these types of power plants:
HPP
CHP
When using thermal energy of steam in energy conversion chains, it becomes possible to use part of the thermal energy for heating (shown by a dotted line) or for production needs.
NPP (with a single-loop reactor)
Thermal circuit.
Basic concepts
Earlier we considered the types of energy and the possibilities of its transformation from one type to another, let us dwell on thermal energy in more detail, since it plays a very important role in the processes occurring at nuclear power plants.
As stated earlier, thermal energy, this is the energy of the chaotic movement of molecules or atoms in liquids and gases and the vibrational movement of molecules or atoms in a solid. The higher the speed of this movement, the more thermal energy the body has.
We all encounter in our daily life the processes of transferring thermal energy from one body to another (hot tea heats a glass, a heating radiator in an apartment heats the air, etc.) based on the definition of thermal energy, one can define heat exchange.
Definition: The process of transferring energy as a result of the exchange of chaotic movement of molecules, atoms or microparticles is called heat exchange.
It is known from everyday experience that heat energy or heat is transferred from a hotter body to a colder one, and it seems quite logical to take temperature as a measure of thermal energy, but this is a gross mistake. Body temperature is a measure of the ability to heat exchange with surrounding bodies. Knowing the temperatures of the two bodies, we can only say about the direction of heat transfer. A body with a higher temperature will give off heat and cool down, and a body with a lower temperature will receive heat and heat up, but the amount of transmitted energy cannot be determined based on temperature alone. You don't have to look far for an example: try pouring an equal amount of boiling water into an aluminum mug and a ceramic one. Aluminum will almost instantly heat up, almost without cooling the water, and the ceramics will heat up much less and much longer, and the initial boiling water temperature in both cases is 100 ° C. Hence the conclusion: different amounts of heat energy are required to heat different substances to the same temperature , each substance has its own heat capacity
Definition:the specific heat of a substance is the amount of energy required to heat one kilogram of a given substance by one degree.
where: Q-energy; C - heat capacity; m is the mass; dT-heating;
Heat transfer methods.
As a rule, in industrial power plants, the process of converting source energy into thermal energy occurs in one place (a boiler for a thermal power plant, a reactor for a nuclear power plant), and the process of converting thermal energy into mechanical energy and then into electrical energy in another, therefore, there is a problem of transferring thermal energy in space. How can you transfer thermal energy from one point in space to another?
Thermal conductivity
When heating one end of a metal wire, one can notice that the temperature rises along its entire length, and the shorter the wire, the faster the opposite, not directly heated, part will heat up. By heating the wire from one side, we make the atoms and electrons vibrate more strongly at the heating site, the vibrating atoms and electrons involve neighboring atoms and electrons in the vibration, and heat energy spreads in a solid, in our case, in a metal wire. This method of transferring thermal energy is called thermal conductivity.
Definition: Thermal conductivity is a process of heat transfer in a continuous medium through the chaotic movement of micro particles.
The amount of heat transferred due to thermal conductivity depends on the physical properties of the medium in which heat transfer takes place. Each substance has its own coefficient of thermal conductivity l (A metal rod about a meter long, placed with one end in a fire, cannot be held in bare hands, a wooden stick of the same shape will burn more than half of it before it heats up significantly).
The greater the temperature difference dT between the hot and cold points of the medium, the more heat is transferred per unit of time. The larger the cross-sectional area, the more heat is transferred, per unit time.
Probably everyone knows how to boil water with a fire in a wooden bowl. It is necessary to throw stones red-hot in the fire into the water. The heated stones are immediately moistened with water and give it their warmth. The process of transferring heat from stones to the surrounding water is similar to thermal conductivity, but the distribution of heat energy over the volume of water is different.
Convective heat transfer
Consider what happens in the volume of cold water when hot stones heat part of it around them. It is known from physics that when heated, bodies expand, in other words they increase their volume, and since the mass remains constant, the density decreases. As Archimedes' law says, a body with a density greater than the density of a liquid submerges, and with less it floats. Same
one can say about a heated liquid, having a lower density, it will begin to rise, mixing with cold layers in the upper part of the vessel, which, in turn, will begin to fall, after a while the temperature throughout the volume will become the same.
Definition:Convective heat transfer - heat transfer during mixing of more heated particles of the medium with less heated ones.
In the example above, the motion was caused by the difference in density between the hot and cold parts of the liquid, such convection is called natural or free. If the movement is caused by the operation of a pump or fan, then convection is called forced.
Convective heat exchange occurs in gases in the same way as in liquids.
In many modern nuclear power plants, heat is removed from the reactor by forcibly pumping water, gas, or liquid metal through the core. A substance that, when heated, takes heat from a source is called a heat carrier.
Heat transfer by radiation
Experiments show that heat exchange between bodies is possible even if they are in a vacuum without touching each other. In this case, the types of heat exchange described above cannot be carried out. How does the transfer of heat energy take place in this case?
A heated body emits electromagnetic waves which, as you know, can spread in airless space; a less heated body absorbs these waves and heats up.
Definition: Heat transfer by radiation is the transfer of thermal energy using electromagnetic waves.
In modern nuclear power plants, during normal operation, the heat exchange by radiation is negligible compared to convective heat exchange.
Thermal circuit
Having considered the methods of possible heat exchange, let us return to the issue of heat energy transfer in the conditions of a nuclear power plant or thermal power plant. As you know, at operating stations, the process of converting the source energy into heat occurs continuously and in case of termination of heat removal, inevitable overheating of the installation will occur. Therefore, along with the source, a consumer of thermal energy is needed, which will take heat and either convert it into other forms of energy or transfer it to other systems. Heat transfer from the source to the consumer is carried out using a heat carrier. Based on the above, it is possible to depict the simplest thermal circuit containing an energy source, an energy consumer, and coolant paths.
There are three main ways to convert energy. The first of them consists in obtaining thermal energy by burning fuel (fossil or plant origin) and consuming it for direct heating of residential buildings, schools, enterprises, etc. The second method is to convert the thermal energy contained in the fuel into mechanical work, for example, when using the products of the distillation of oil to ensure the movement of various equipment, cars, tractors, trains, airplanes, etc. The third method is the conversion of heat released during the combustion of fuel or fission of nuclei into electrical energy with its subsequent consumption or for production heat, or to perform mechanical work.
Electricity is also obtained by converting the energy of the falling water. Electricity thus plays the role of a kind of intermediary between energy sources and its consumers (Fig. 9.1). Just as the middleman in the market drives up prices, so the consumption of energy in the form of electricity drives up prices because of the waste of converting one type of energy to another. At the same time, converting various forms of energy into electrical energy is convenient, practical, and sometimes it is the only possible way to actually consume energy. In some cases, it is simply impossible to efficiently use energy without turning it into electrical energy. Before the discovery of electricity, the energy of falling water (hydropower) was used to provide the movement of mechanical devices: spinning machines, mills, sawmills, etc. After the conversion of hydropower into electrical energy, the scope of application expanded significantly, it became possible to consume it at considerable distances from the source. The fission energy of uranium nuclei, for example, cannot be used directly without converting it into electrical energy.
Fossil fuels, unlike hydrosources, have long been used only for heating and lighting, and not for the operation of various mechanisms. Firewood and coal, and often dried peat, were burned to heat residential buildings, public and industrial buildings. Coal, in addition, has been and is used for metal smelting. Coal oil, obtained by distilling coal, was poured into lamps. Only after the invention of the steam engine in the 18th century. the potential of this fossil fuel was truly revealed, which became a source of not only heat and light, but also the movement of various mechanisms and machines. Steam locomotives, steam-powered steamboats, fueled by coal appeared. At the beginning of the XX century. coal began to be burned in power plant boilers to generate electricity.
Fossil fuels play an extremely important role today. It provides heat and light, is one of the main sources of electricity and mechanical energy to provide a huge fleet of numerous machines and different types transport. It should not be forgotten that fossil organic raw materials are consumed in huge quantities by the chemical industry to produce a wide variety of useful and valuable products.
Electric machines are divided into two main types by purpose: electric generators and electric motors... Generators are designed to generate electrical energy, and electric motors are designed to drive wheel pairs of locomotives, rotate the shafts of fans, compressors, etc.
In electrical machines, a process of energy conversion takes place. Generators convert mechanical energy into electrical energy. This means that for the generator to work, it is necessary to rotate its shaft with some kind of engine. On a diesel locomotive, for example, a generator is driven by a diesel engine, at a thermal power plant by a steam turbine, at a hydroelectric power plant by a water turbine. Electric motors, on the other hand, convert electrical energy into mechanical energy. Therefore, for the engine to work, it must be connected with wires to a source of electrical energy, or, as they say, included in the electrical network.
The principle of operation of any electric machine is based on the use of the phenomena of electromagnetic induction and the occurrence of electromagnetic forces during the interaction of conductors with current and a magnetic field. These phenomena occur during the operation of both the generator and the electric motor. Therefore, they often talk about generator and motor modes of operation of electric machines.
In rotating electric machines, two main parts are involved in the process of energy conversion: the armature and the inductor with their own windings, which move relative to each other. The inductor creates a magnetic field in the machine; e is induced in the armature winding. etc. with. and a current arises. When the current interacts in the armature winding with a magnetic field, electromagnetic forces are created, through which the process of converting energy in the machine is realized.
The principle of operation of an electric generator. The simplest electric generator is a loop rotating in a magnetic field (Fig. 67, a). In this generator, turn 1 is the armature winding. The inductor are permanent magnets 2, between which the armature 3 rotates. When the loop rotates at a certain frequency of rotation, its sides (conductors) intersect the magnetic lines of force of the Thebes flux, each conductor induces e. etc. with. e... With the adopted in Fig. 67, and the direction of rotation of the armature e. etc. with. in a conductor located under the south pole, according to the rule right hand is directed from us, and the e.m.f. in a conductor located under the North Pole - towards us. If you connect a receiver of electrical energy 4 to the armature winding, then an electric current i will flow in a closed circuit. In the conductors of the armature winding, current I will be directed in the same way as e. etc. with. e.
Let us find out why for the rotation of the armature in a magnetic field it is necessary to expend mechanical energy obtained from a diesel engine or a turbine (prime mover). As it was established in Chapter II, when current I passes through conductors located in a magnetic field, an electromagnetic force F acts on each conductor. When indicated in Fig. 67, and the direction of the current according to the rule of the left hand, the force F directed to the left will act on the conductor located under the south pole, and the force F directed to the right will act on the conductor located under the north pole. These forces create together an electromagnetic moment M, directed clockwise.
Considering Fig. 67, but it is clear that the electromagnetic moment M, arising when the generator gives off electrical energy, is directed in the direction opposite to the rotation of the conductors, therefore it is a braking torqueseeking to slow down the rotation of the generator armature. In order to prevent the armature from stopping, it is required to apply an external torque M external to the armature shaft, opposite to the moment M and equal to it in magnitude. Taking into account the friction and other internal losses in the machine, the external torque must be greater than the electrical
magnetic moment M created by the generator load current. Therefore, in order to continue the normal operation of the generator, it is necessary to supply mechanical energy to it from the outside - to rotate its armature with any engine 5.
In the absence of load (with an open external circuit of the generator), the generator is in idle mode. In this case, only the amount of mechanical energy is required from the diesel or turbine, which is necessary to overcome friction and compensate for other internal energy losses in the generator. With an increase in the generator load, that is, the electric power P el given by it, the current i passing through the conductors of the armature winding and the braking moment M created by it increase.Therefore, the mechanical power P mx, which the generator must receive from diesel engine or turbine to continue normal operation.
Thus, the more electrical energy is consumed, for example, by electric motors of a diesel locomotive from a diesel locomotive generator, the more mechanical energy it takes from the diesel engine rotating it and the more fuel must be supplied to the diesel engine.
From the conditions of operation of an electric generator considered above, it follows that it is characteristic of it:
coincidence in the direction of the current i and e. d. with e in the conductors of the armature winding; this indicates that the machine is giving off electrical energy;
the occurrence of an electromagnetic braking torque M, directed against the rotation of the armature; from this follows the need for the machine to receive mechanical energy from the outside.
The principle of the electric motor. In principle, the electric motor is designed in the same way as the generator. The simplest electric motor is a turn 1 (Fig. 67.6), located on an anchor 3, which rotates in the magnetic field of poles 2. The conductors of the turn form an armature winding. If you connect the coil to a source of electrical energy, for example, to electrical network 6, then an electric current i will start to flow through each of its conductors. This current, interacting with the magnetic field of the poles, creates electromagnetic forces F. At the indicated in Fig. 67, b, the direction of the current on the conductor located under the south pole will be acted upon by the force F directed to the right, and the force F directed to the left will act on the conductor lying under the north pole. As a result of the combined action of these forces, an electromagnetic torque M is created, directed counterclockwise, which drives the armature with the conductor into rotation at a certain frequency n. If you connect the armature shaft with any mechanism or device 7 (wheelset of a diesel or electric locomotive, machine tool, etc. .), then the electric motor will drive this device into rotation, i.e., give it mechanical energy. In this case, the external moment M vn created by this device will be directed against the electromagnetic moment M.
Let us find out why electrical energy is consumed during the rotation of the armature of an electric motor operating under load. As it was found, when the armature conductors rotate in a magnetic field, e is induced in each conductor. d. s, the direction of which is determined by the rule of the right hand; therefore, with the indicated in Fig. 67, b direction of rotation e. etc. with. e, induced in the conductor located under the south pole will be directed away from us, and e. etc. with. e, induced in the conductor located under the north pole, will be directed towards us. Fig. 67, b it is seen that e. etc. with. That is, those induced in each conductor are directed against the current i, that is, they prevent it from passing through the conductors.
In order for the current i to continue to pass through the armature conductors in the same direction, that is, so that the electric motor continues to work normally and develop the required torque, it is necessary to apply an external voltage U to these conductors, directed towards the e. etc. with. and larger than the total e. etc. with. E induced in all series-connected conductors of the armature winding. Therefore, it is necessary to supply electrical energy to the electric motor from the network.
In the absence of a load (external braking torque applied to the motor shaft), the electric motor consumes a small amount of electrical energy from an external source (network) and a small no-load current flows through it. This energy is used to cover the internal power losses in the machine.
As the load increases, the current consumed by the electric motor and the electromagnetic torque it develops increase. Consequently, an increase in the mechanical energy given off by the electric motor when the load increases, automatically causes an increase in the electricity it takes from the source.
From the operating conditions of the electric motor considered above, it follows that it is characteristic of it:
coincidence in the direction of the electromagnetic moment M and rotation frequency n; this characterizes the return of mechanical energy by the machine;
the appearance in the conductors of the armature winding e. etc. with. e, directed against the current i and external voltage U. This implies the need for the machine to receive electrical energy from the outside.
The principle of reversibility of electrical machines. Considering the principle of operation of the generator and the electric motor, we found that they are arranged in the same way and that there is much in common in the basis of the operation of these machines. The process of converting mechanical energy into electrical energy in the generator and electrical energy into mechanical energy in the engine is associated with the induction of e. etc. with. in the conductors of the armature winding rotating in a magnetic field and the occurrence of electromagnetic forces as a result of the interaction of the magnetic field and conductors with current. The difference between a generator and an electric motor lies only in the mutual direction of e. d. with, current, electromagnetic torque and speed.
Summarizing the considered processes of operation of the generator and electric motor, it is possible to establish the principle of reversibility of electrical machines. According to this principle any electric machine can operate both as a generator and an electric motor and switch from generator mode to motor mode and vice versa.
To clarify this position, consider the operation of a direct current electric machine under various conditions. If the external voltage U is greater than the total e. etc. with. G. in all series-connected conductors of the armature winding, then the current I will flow in the one indicated in Fig. 68, and the direction and the machine will operate as an electric motor, consuming electrical energy from the network and giving mechanical energy. However, if, for whatever reason, e. etc. with. E becomes greater than the external voltage U, then the current I in the armature winding will change its direction (Fig. 68, b) and will coincide with e. etc. with. E. In this case, the direction of the electromagnetic moment M will also change, which will be directed against the rotation frequency n. Coincidence in the direction of e. etc. with. E and current I means that the machine began to give electrical energy to the network, and the appearance of a braking electromagnetic moment M indicates that it must consume mechanical energy from the outside. Therefore, when e. etc. with. E, induced in the conductors of the armature winding, becomes greater than the voltage of the network U, the machine switches from the motor mode of operation to the generator mode, i.e., at E< U машина работает двигателем, при Е > U - generator.
The transfer of an electric machine from a motor mode to a generator mode can be carried out different ways: decreasing the voltage U of the source to which the armature winding is connected, or increasing the e. etc. with. E in the armature winding.
Energy and its types. Purpose and use
Energy plays an important role in the development of human civilization. Energy consumption has approximately the same character of change over time with the accumulation of information. Production volume is closely related to energy consumption.
By definition from physical science, energy is the ability of a body or system of bodies to do work. The classifications of forms and types of energy are varied. The following types of energy are most common in everyday life:
- electric
- mechanical
- internal
- electromagnetic.
Internal energy is chemical, thermal, atomic. It is determined by the potential energy of interaction between the particles constituting the body or the kinetic energy of their random motion.
The energy received as a result of a change in the state of motion of material points or bodies is called kinetic. Kinetic energy includes thermal energy due to the movement of molecules, and mechanical energy body movements.
Definition 1
Potential energy is the energy received as a result of a change in the relative position of parts of the system or position in relation to other bodies. TO potential energy include the energy of masses, which are attracted by the law of gravitation, chemical energy and the energy of the position of homogeneous particles.
The main source of energy is the sun. When exposed to sunlight, the chlorophyll in plants decomposes carbon dioxide from the air into carbon and oxygen. Carbon builds up in plants.
Solar energy also produces water energy. Solar energy evaporates water and lifts the steam into the higher layers of the atmosphere.
As a result of the different degrees of heating by the sun of the earth, wind is generated in different places, which is used in wind turbines.
Remark 1
The energy contained in natural energy resources can be converted into mechanical, electrical, chemical. This energy is called primary.
Traditional types of primary energy are:
- fossil fuels - gas, oil, coal, etc.
- hydropower
- nuclear fuel - uranium, thorium, etc.
Remark 2
The energy obtained as a result of the transformation of primary energy is called secondary energy. This includes electricity, hot water energy, dad, etc.
Currently, methods are being developed for the use of unconventional energy sources, which include the energy of the sun, wind, heat of the earth, the energy of sea waves and tides. The listed energy sources are renewable, they are environmentally friendly, and there is no pollution of the environment during their use.
In the process of energy consumption from its supply to consumers, five stages are distinguished:
- Obtaining energy resources. This is the extraction of fuel and its enrichment, the concentration of water pressure with the help of hydraulic engineering structures, etc.
- Transfer of the extracted energy resources to special installations that transform it. This is done by transportation by land and water, as well as by pipeline transmission of gas, oil, water, etc.
- At this stage, primary energy is converted into secondary energy, which is most optimal for specific conditions. Most often it is electrical and thermal energy.
- Transmission and distribution of converted energy
- Power consumption
Energy conversion
Energy conversion is carried out using various energy converters. Such converters are special devices designed to convert natural energy into a form that is convenient for use.
One of the types of converters that are effective are heat pumps. They are a refrigerator-like device with a freezer submerged in the sea.
Solar energy converters are efficient enough for some parts of the world. Solar energy converters are used for spacecraft. The energy of these elements is sufficient to maintain the operability of devices located on spacecraft.
The main purpose of thermionic energy conversion is to generate electricity for use in remote areas, in space and under water. When developing such a converter, a number of problems arise:
- regulation and maintenance of the required vacuum
- development of a corrosion-resistant shell of the converter housing and others.
Thermionic converters work well with a nuclear reactor. TPP, HPP, NPP
Not all types of energy sources are convenient and can be used. The most widespread, convenient and affordable forms of energy are oil, gas and water. Several decades ago, nuclear energy was added to them.
Thermal power plants convert heat energy released during fuel combustion into electrical energy. Among such power plants, steam turbine power plants constitute the main part. On them, thermal energy is used in a generator to produce high-pressure steam, which drives a steam turbine rotor connected to the rotor of an electric generator.
In hydroelectric power plants, the energy of the flow of water is converted into electricity. A hydroelectric power plant consists of a chain of structures that concentrate water flows and create a pressure, and power equipment, which converts the energy of the water pressure into mechanical energy, and the mechanical energy is converted into electrical energy. Hydraulic resources. Compared to fuel and energy, they are renewable.
In nuclear power plants, the power generator is a nuclear reactor. Nuclear power plants operate on nuclear fuel, the reserves of which exceed those of fossil fuel.
With the help of wind power plants, wind energy is converted into electricity. The average annual wind speed in many areas is 6 m / s, which makes it possible to effectively use this method of energy conversion.
Tidal energy uses the tidal energy of the World Ocean. The disadvantage of this method is the high cost of construction and uneven energy production.
Solar energy uses solar radiation to convert it into electrical energy. For this, a solar battery is installed, the basis of which is photocells.
Geothermal power plants convert the Earth's internal heat into electricity. To build such a power plant, special geological conditions are required, which limits the use of this method of energy conversion.
Bioenergy uses bacteria to recycle organic compounds - waste and debris.
