Distant current electric motors are not used as often as AC motors. Below we give their advantages and disadvantages.
In everyday life engines direct current We found use in children's toys, as the sources for their power are batteries. They are used in transport: in the metro, trams and trolley buses, cars. In industrial enterprises, DC electric motors are used in the actuators of the units, for uninterrupted power supply of which batteries are used.
DC Engine Construction and Maintenance
The main winding of the DC motor is anchorconnected to the power supply through brush apparatus. Anchor rotates in a magnetic field created by poles of the Stator (excitation windings). Face parts of the stator are closed with shields with bearings in which the engine anchor shaft rotates. On the one hand, on the same shaft set fan Cooling, running the air flow through the internal cavities of the engine at its operation.
The brush apparatus is a vulnerable element in the engine design. Brushes are carved to the collector to repeat it as possible, pressed against it with a constant force. During the work of the brush, the conductive dust from them settles on the fixed parts, it periodically needs to be deleted. The brushes themselves sometimes need to move in the groove, otherwise they are stuck in them under the action of the same dust and "hang" above the collector. The characteristics of the engine depends on the position of the brushes in the space in the plane of rotation of the anchor.
Over time, brushes wear out and replaced. The collector in contact places with brushes is also grieving. Periodically, the anchor dismantle and drag the collector on the lathe. After pulling out, the isolation between the collector lamellas is cut into some depth, since it is a stronger collector material and will destroy brushes with further work out.
DC motor turning circuit
Availability of excitation windings - distinctive feature DC machines. From the methods of their connection to the network depend on electrical and mechanical properties electric motor.
Independent arousal
The excitation winding is connected to an independent source. Engine characteristics are the same as the engine with permanent magnets. The rotational speed is regulated by resistance in the chain of the anchor. It is regulated by it and the restatatom (adjusting resistance) in the excitation winding circuit, but with an excessive decrease in its value or when the current, the anchor increases to dangerous values. Engines with independent excitation can not be launched at idle or with low load on the shaft. The rotational speed will increase dramatically, and the engine will be damaged.
The remaining schemes are called schemes with self-excitation.
Parallel arousal
Rotor and excitation windings are connected in parallel to one power source. With this turn on the current through the excitation winding several times less than through the rotor. The characteristics of the electric motors are obtained with rigid, allowing them to be used to drive machines, fans.
Adjusting the speed of rotation is ensured by the inclusion of rotor chain or sequentially with an excitation winding.
Sequential arousal
The excitation winding is turned on consistently with anchor, the same current flows on them. The speed of such an engine depends on its load, it cannot be turned on at idle. But it has good starting characteristics, so a sequential excitation scheme is applied on electrified transport.
Mixed arousal
With this scheme, two excitation windings are used, located pairwise on each of the electric motor poles. They can be connected so that their streams either fold are either subtracted. As a result, the engine may have characteristics as a sequential or parallel excitation scheme.
To change the direction of rotation Change the polarity of one of the excitation windings. To manage the start of the electric motor and the speed of its rotation, step-down resistant switching is used.
Natural speed and mechanical characteristics, scope
In the sequential excitation engines, the anchor current is also an excitation current: i. in \u003d. I. A \u003d. I.. Therefore, the flow F Δ changes wide limits and can write that
  | (3) |
 | (4) |
The speed characteristic of the engine [see expression (2)], shown in Figure 1, is soft and has a hyperbolic character. For k. F \u003d const view of the curve n. = f.(I.) Showing a stroke line. With small I. The engine speed becomes unacceptable large. Therefore, the operation of sequential excitation engines, with the exception of the smallest, at idle is not allowed, and the use of belt transmission is unacceptable. Usually minimal permissible load P. 2 = (0,2 – 0,25) P. n.
Natural characteristic of the engine of sequential excitation n. = f.(M.) In accordance with relation (3), shown in Figure 3 (curve 1 ).
Since in parallel excitation engines M. ∼ I., and in engines of consistent excitement approximately M. ∼ I. ² and when starting is allowed I. = (1,5 – 2,0) I. n, the sequential excitation engines develop a significantly larger starting point compared to parallel excitation engines. In addition, in parallel excitation engines n. ≈ const, and in sequential excitation engines, according to expressions (2) and (3), approximately (with R. a \u003d 0)
n. ∼ U. / I. ∼ U. / √M. .
Therefore, in parallel excitation engines
P. 2 \u003d Ω × M. \u003d 2π × n. × M. ∼ M. ,
and in sequential excitation engines
P. 2 \u003d 2π × n. × M. ∼ √ M. .
Thus, in sequential excitation engines when changing the torque M. st \u003d. M. In wide limits, power changes in smaller limits than in the engines of parallel excitation.
Therefore, for sequential excitation engines less dangerous overload on the moment. In this regard, sequential excitation engines have significant advantages in the case of severe starting conditions and change the torque of the load over wide limits. They are widely used for electric traction (trams, metro, trolley buses, electric locomotives and diesel locomotives railways) and in lifting and transport installations.
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| Figure 2. Schemes for adjusting the speed of rotation of the sequential excitation engine by shunting the excitation winding ( but), anchor shunt ( b.) and the inclusion of resistance to the chain of anchor ( in) |
Note that with increasing the speed of rotation, the sequential excitation motor into the generator mode does not switch. Figure 1 is obvious from the fact that the characteristic n. = f.(I.) Does not cross the ordinate axes. It is physically explained by the fact that when switching to the generator mode, at a given direction of rotation and a given polarity of the voltage, the current direction should change to the opposite, and the direction of the electromotive force (er. S.) E. And the polarity of the poles should be maintained unchanged, however, the last when the current direction changes in the excitation winding is impossible. Therefore, to translate the sequential excitation engine to the generator mode, you must switch the ends of the excitation winding.
Speed \u200b\u200bcontrol by weight weakening
Regulation n. Through the attenuation of the field, it is made either by shunting the excitation winding by some resistance R. Sh.V (Figure 2, but), or a decrease in the number of coat winding included in the work. In the latter case, appropriate conclusions from the excitation winding should be provided.
As the resistance of the excitation winding R. in and the drop in the voltage on it is small, then R. S.V. should also be little. Resistance loss R. Sh.V. Therefore, small, and the total losses for excitation during shunting are even decreasing. As a result, the efficiency (k. P. D.) The engine remains high, and this method of regulation is widely applied in practice.
When shunting the excitation winding of the excitation current with the value I. Reduced before
and speed n. accordingly increases. Expressions for high-speed and mechanical characteristics at the same time we obtain if in equalities (2) and (3) replace k. F. k. F. k. OV, where
it is an excitation attenuation coefficient. When adjusting the speed, the change in the number of turns of the excitation winding
k. OV \u003d. w. V. Brab / w. V.Pill.
Figure 3 shows (curves 1 , 2 , 3 ) characteristics n. = f.(M.) For this occasion of speed control at several values k. O.V (meaning k. OV \u003d 1 corresponds to the natural characteristic 1 , k. OV \u003d 0.6 - curve 2 , k. OV \u003d 0.3 - curve 3 ). Characteristics are given in relative units and correspond to the case when k. F \u003d const and R. A * \u003d 0.1.
 |
| Figure 3. Mechanical characteristics of the engine of sequential excitation when different methods Rotation speed control |
Speed \u200b\u200bcontrol by shunting anchor
When shunting anchor (Figure 2, b.) Current and the excitation flow increase, and the speed decreases. Since the voltage drop R. in × I. little and therefore you can take R. in ≈ 0, then resistance R. S.A. is practically under the total voltage of the network, its value should be significant, the loss in it will be great and to. p. d. much will decrease.
In addition, the shunting anchor is effective when the magnetic circuit is not saturated. In this regard, the shunting of an anchor in practice is rarely used.
Figure 3 Curve 4 n. = f.(M.) As
I. Sh.A ≈ U. / R. Sh.A \u003d 0.5 I. n.
Speed \u200b\u200bcontrol by turning the resistance to the anchor chain
Speed \u200b\u200bcontrol by turning the resistance to the anchor chain (Figure 2, in). This method allows you to adjust n. Down from the nominal value. Since simultaneously at the same time significantly decreases to. P. D., Then such a method of regulation finds limited applications.
Expressions for high-speed and mechanical characteristics in this case are obtained if in equalities (2) and (3) replace R. A. R. a +. R. ra. Characteristic n. = f.(M) for this method of speed control R. Ra * \u003d 0.5 is shown in Figure 3 as a curve 5 .
 |
| Figure 4. Parallel and sequential switching on sequential excitation engines to change the speed of rotation |
Voltage change speed control
This way you can adjust n. Down from the nominal value with the preservation of the high to. p. d. The regulation method under consideration is widely used in the transport installations, where a separate engine is installed on each master axis, and the control is carried out by switching the engines from parallel inclusion in the network to the sequential (Figure 4). Figure 3 Curve 6 It is a characteristic n. = f.(M.) for this case when U. = 0,5U. n.
17. characteristics of a mixed excitation engine.
The concept of a mixed excitation electric motor is shown in Fig. 1. In this engine there are two excitation windings - parallel (shunt, sho), connected parallel to the anchor chains, and serial (serial, CO), connected sequentially the anchor chain. These magnetic flux windings can be included according to or meeting.
Fig. 1 - Mixed excitation motor circuit.
With the consistent, the excitation windings of their MDCs are also folded and the resulting flow F is approximately equal to the amount of threads generated by both windings. In the oncoming turn on the resulting stream is equal to the difference in the flow of parallel and serial windings. In accordance with this, the properties and characteristics of the mixed excitation electric motor depend on the method of inclusion of the windings and on the ratio of their MDS.
Speed \u200b\u200bcharacteristic N \u003d F (Ia) at u \u003d UAN and IV \u003d const (here IV - current in parallel winding).
With an increase in the load, the resulting magnetic flux increases with a consistent turning on the windings, but to a lesser extent than that of the engine of the sequential excitation, therefore the speed characteristic in this case turns out to be softer than the engine of parallel excitation, but more rigid than the engine of sequential excitation.
The ratio between MDS windings may vary widely. Engines with a weak serial winding have a weakly incident speed characteristic (curve 1, Fig. 2).
Fig. 2 - high-speed characteristics of the engine of mixed excitation.
The greater the proportion of consistent winding in the creation of MDS, the closer the speed characteristic is approaching the characteristic of the sequential excitation engine. In Fig. 2, line 3 depicts one of the intermediate characteristics of the mixed excitation engine and for comparison, a sequential excitation engine characteristic (curve 2) is given.
With the ongoing turn on the sequential winding with an increase in the load, the resulting magnetic flux decreases, which leads to an increase in the engine speed (curve 4). With such an extreme characteristic, the engine operation may be unstable, because The stream of serial winding can significantly reduce the resulting magnetic flux. Therefore, engines with counter-inclusion of windings do not apply.
Mechanical characteristic n \u003d f (m) at u \u003d UAN and IV \u003d const. Mixed excitation motor is shown in Fig.3 (line 2).
Fig. 3 - mechanical characteristics of the engine of mixed excitation.
It is located between the mechanical characteristics of the parallel engines (curve 1) and the sequential (curve 3) of excitation. Picking up the MDS of both windings accordingly, you can get an electric motor with a characteristic close to the characteristic of the engine of parallel or sequential excitation.
Scope of engines of sequential, parallel and mixed excitation.
Therefore, for sequential excitation engines less dangerous overload on the moment. In this regard, sequential excitation engines have significant advantages in the case of severe starting conditions and change the torque of the load over wide limits. They are widely used for electric traction (trams, metro, trolley buses, electric locomotives and diesel locomotives) and in lifting installations.
Natural speed and mechanical characteristics, scope in parallel excitation engines.
Natural high-speed and mechanical characteristics, scope of use in mixed excitation engines.
Mixed excitation engine
The mixed excitation engine has two excitation windings: parallel and serial (Fig. 29.12, a). The rotation frequency of this engine
, (29.17)
where and - streams of parallel and consistent excitation windings.
The plus sign corresponds to the agreed inclusion of excitation windings (MDS windings fold). In this case, with an increase in the load, the overall magnetic flow increases (due to the stream of serial winding), which leads to a decrease in the engine speed. With the ongoing turning on the windings, the flow with an increase in the load demagnetizes the machine (minus sign), which, on the contrary, increases the speed of rotation. The operation of the engine becomes unstable, since with an increase in the load, the speed of rotation is inconsistently growing. However, with a small number of turns of the serial winding with an increase in the load, the speed of rotation does not increase and the load remains almost unchanged in the entire range.
In fig. 29.12, B shows the operating characteristics of the engine of mixed excitation with the agitated turning on the excitation windings, and in Fig. 29.12, B - mechanical characteristics. In contrast to the mechanical characteristics of the sequential excitation engine, the latter have a more severe look.
Fig. 29.12. Mixed excitation engine scheme (a), its workers (b) and mechanical (c) characteristics
It should be noted that in its form, the characteristic of the mixed excitation engine occupies an intermediate position between the corresponding characteristics of the engines of parallel and sequential excitation, depending on which MDS prevails in which of the excitation windings (parallel or sequential).
The mixed excitation engine has advantages compared to the sequential excitation engine. This engine can operate, as the parallel winding stream limits the engine speed in H.H. And eliminates the danger of "spreading". You can adjust the speed of rotation of this engine with a retail in the parallel excitation winding circuit. However, the presence of two excitement windings makes the mixed excitation engine more expensive compared to the types of types discussed above, which limits its use. Mixing and exclusion engines are usually used where significant starting moments are required, rapid acceleration during acceleration, steady operation and permissible only a small decrease in the speed of rotation with an increase in the load on the shaft (rolling mills, trucks, pumps, compressors).
49. Starting and overload properties of DC motors.
Starting DC motor direct turning on it to the network voltage is allowed only for small power engines. In this case, the current peak at the beginning of the start can be about 4 - 6-fold nominal. The direct start of the DC motors of considerable power is completely unacceptable, because the initial current peak here will be equal to 15 - 50 times nominal. Therefore, the start of medium and large power engines are produced using a starting row, which limits the current when starting to allowed by switching and mechanical strength of values.
Starting Pasteat is performed from a wire or tape with high resistivity divided into sections. The wires are attached to copper button or flat contacts in the transition places from one section to the other. Contacts moves the copper brush of the rotting lever of the rod. Reostats may have another execution. The excitation current when starting a parallel excitation engine is set by the corresponding normal workThe excitation chain is activated directly to the network voltage so that there is no voltage reduction due to the voltage drop in the retain (see Fig. 1).
The need to have a normal excitation current is related to the fact that when starting the engine should develop a greater permissible MEAM, which is necessary to ensure rapid acceleration. Starting DC motor is made with a sequential decrease in the resistance of the rheostat, usually - by transferring the river lever from one fixed contact of the restart to another and off sections; Reducing the resistance can be carried out and by closing the sections of the sections by contactors, triggered by the specified program.
When starting manually or automatically, the current varies from the maximum value equal to 1.8 -2.5 to a multiple nominal at the beginning of work when this resistance Reostata, to a minimum value equal to 1.1 - 1.5-fold nominal at the end of work and before switching to another position of the starting row. Anchor current after turning on the engine with RP resistance is RP
where the UC is the network voltage.
After switching on, the engine acceleration begins, while the anti-EDC E has occurs and the anchor current decreases. If we consider that the mechanical characteristics of N \u003d F1 (MN) and N \u003d F2 (II) are practically linear, then when overclocked, an increase in the rotational speed will occur according to the linear law, depending on the current anchor (Fig. 1).
Fig. 1. DC engine starting diagram
The launcher (Fig. 1) for different resistance in the chain of the anchor is the segments of linear mechanical characteristics. With a decrease in the anchor current to the IMIN value, the R1 resistance section is turned off and the current increases to the value
where E1 - EDC at the point A characteristics; R1 resistance of the off section.
Then the engine is accelerated again to the point B, and so on until the engine is released when the engine is turned directly to the UC voltage. Starting reasons are designed to heat up on 4 -6 launches in a row, so you need to monitor that at the end of the startup retaining, it was completely removed.
When the engine is stopped, it turns off from the source of energy, and the launcher is fully turned on - the engine is ready for the next start. To eliminate the possibility of the appearance of large emf self-induction when the excitation circuit breaks and, when it is disconnected, the circuit may close to the discharge resistance.
IN adjustable drives Starting DC motors are made by gradually increasing the power supply voltage so that the current when the start is supported in the required limits or remained for more than the start time is approximately unchanged. The latter can be carried out by automatic control The process of changing the power supply voltage in feedback systems.
Start and stop MPT
Direct inclusion of it on the network voltage is allowed only for small power engines. In this case, the current peak at the beginning of the start can be about 4 - 6-fold nominal. The direct start of the DC motors of considerable power is completely unacceptable, because the initial current peak here will be equal to 15 - 50 times nominal. Therefore, the start of medium and large power engines are produced using a starting row, which limits the current when starting to allowed by switching and mechanical strength of values.
DC motor startit is performed with a sequential decrease in the resistance of the rheostat, usually by transferring the river lever from one fixed root of the restart to another and off sections; Reducing the resistance can be carried out and by closing the sections of the sections by contactors, triggered by the specified program.
When you start manually or automatically, the current varies from the maximum value equal to 1.8 -2.5 to a multiple nominal at the beginning of operation at a given resistance of the rheostat, to the minimum value equal to 1.1 - 1.5 times the nominal value at the end of work and before Switching to another position of starting row.
Braking It is necessary in order to reduce the time of engine lowers, which, in the absence of braking, may be unacceptably large, as well as to fix the driven mechanisms in a certain position. Mechanical braking DC motors are usually performed when applied. brake shoes on brake pulley. Disadvantage mechanical brakes It is that the braking moment and the inhibition time depend on random factors: oils or moisture on the brake pulley and others. Therefore, such braking is applied when the time and braking path are not limited.
In some cases, after pre-electric braking at low speed, it is possible to accurately stop the mechanism (for example, a lift) in a given position and secure its position in a certain place. Such braking is also applied in emergency cases.
Electric braking Provides fairly accurate obtaining the desired burning point, but cannot provide a fixation of the mechanism in a specified location. Therefore, electric braking, if necessary, is complemented by mechanical, which is enforced after the end of the electric.
Electric braking occurs when the current flows according to the motor EDC. Three ways of braking are possible.
DC motion braking with energy return to the network.In this case, the EDC e should be greater than the power supply voltage of the UC and the current will flow in the EMF direction, being a generator current. Spare kinetic energy will be transformed into electrical and partially returned to the network. The inclusion scheme is shown in Fig. 2, a.
Fig. 2. Circuit diagrams of DC motors: I - with the return of energy to the network; b - when opposing; B - dynamic braking
DC motor braking can be performed when the power supply voltage decreases so that UC< Е, а также при спуске грузов в подъемнике и в других случаях.
Braking in antique It is performed by switching the rotating motor to the opposite direction of rotation. In this case, the EDC E and the voltage of the UC is anchored, and to limit the current I should include a resistor with initial resistance.
where IMAs are the greatest permissible current.
Braking is associated with big energy loss.
Dynamic DC Engine Braking It is performed when the rotating excited motor of the RT resistor (Fig. 2, B) is turned on on the clamps. Spare kinetic energy is transformed into electrical and dissipated in an anchor chain as a heat. This is the most common way of braking.
Schemes for switching on the DC motor of parallel (independent) excitation: A - engine power circuit, b - inclusion circuit with dynamic braking, in - circuit for countercluded.
Transient processes in MTT
In the general case, in the electrical circuit, transition processes may occur if there are inductive and capacitive elements in the circuit, which have the ability to accumulate or give the energy of a magnetic or electric field. At the moment of switching, when the transition process begins, the energy is redistributed between inductive, capacitive elements of the chain and external energy sources connected to the chain. In this case, part of the energy is irretrievably converted into other types of energies (for example, to thermal on active resistance).
After the transition process is completed, a new installed mode is established, which is determined only by external sources of energy. When the external sources of energy are disconnected, the transition process may occur due to the energy of the electromagnetic field accumulated prior to the start of the transition mode in inductive and capacitive elements of the chain.
Changes in the energy of magnetic and electric fields cannot occur instantly, and therefore cannot instantly flow processes at the time of switching. In fact, the hopping (instantaneous) change in energy in the inductive and capacitive element leads to the need to have infinitely high power P \u003d DW / DT, which is almost impossible, because in real electrical circuits there is no infinitely high power.
Thus, transient processes cannot occur instantly, as it is impossible in principle instantly change the energy accumulated in the electromagnetic field of the chain. Theoretically transient processes end in time t → ∞. Almost the transient processes are fast, and their duration is usually fragmented by a second. Since the magnetic w m and electric fields w e is described by expressions
the current in inductance and the voltage on the tank cannot be changed instantly. This is based on the laws of switching.
The first law of switching is that the current in the branch with an inductive element at the initial moment of time after switching is the same meaning as it has been directly before switching, and then it starts to change smoothly from this value. The said is usually written in the form I L (0 -) \u003d i L (0 +), believing that the switching occurs instantly at the time T \u003d 0.
The second switching law is that the voltage on the capacitive element at the initial moment after switching is the same value as it has directly before switching, and then it starts to change smoothly from this value: UC (0 -) \u003d UC (0 +) .
Consequently, the presence of a branch containing inductance in the circuit is included in the voltage is equivalent to the break of the chain in this place at the time of switching, since I L (0 -) \u003d i L (0 +). The presence in the circuit is included in the voltage, branch containing a discharged condenser, is equivalent to a short circuit in this place at the time of switching, since U C (0 -) \u003d U C (0 +).
However, voltages on inductors and currents in tanks are possible in the electrical circuit.
In electrical circuits with resistive elements, the energy of the electromagnetic field is not covered, as a result of which transient processes do not occur in them, i.e. In such circuits, stationary modes are installed instantly, jump.
In fact, any element of the chain has some kind of resistance R, the inductance L and the capacity C, i.e. In real electrical devices, there are thermal losses due to current passage and the presence of resistance R, as well as magnetic and electric fields.
Transient processes in real electrical devices can be accelerated or slowed down by selecting the appropriate parameters of the chains elements, as well as through the use of special devices.
52. Magnitohydrodynamic DC machines. Magnetic hydrodynamics (MHD) is an area of \u200b\u200bscience that studies the laws of physical phenomena in electrically conductive liquid and gas environments when they are moved in a magnetic field. On these phenomena, the principle of action of various magnetohydrodynamic (MHD) of Machines of direct and alternating current is founded. Some MHD machines are used in various fields of technology, while others have significant prospects for applications in the future. Below are the principles of the device and actions of the MHD of DC machines.
Electromagnetic pumps for liquid metals
Figure 1. Principle of the device of the electromagnetic pump of DC
In the DC pump (Figure 1), the channel 2 with a liquid metal is placed between the poles of the electromagnet 1 and using the electrodes 3 welded to the channel walls, the constant current from the external source is passed through the liquid metal. As the current to the liquid metal in this case It is summed up with a conductive path, then such pumps are also called conduction.
When the fields of poles are interacted with a current in a liquid metal on metal particles, electromagnetic forces act, the pressure and the liquid metal develops. The currents in the liquid metal distort the field of the poles ("anchor reaction"), which leads to a decrease in the effectiveness of the pump. Therefore, in powerful pumps between the pole tips and the channel, the tires are placed ("compensation winding"), which are switched on sequentially into the channel circuit in the counter direction. The electromagnet excitation winding (not shown in Figure 1) is usually turned on sequentially in the channel circuit of the channel and has only 1 to 2 turns.
The use of conductive pumps is possible for low-breeding liquid metals and at such temperatures when the channel walls can be made of heat-resistant metals (non-magnetic stainless steel and so on). Otherwise, the induction pumps of alternating current are more suitable.
The pumps of the described type began to be used about 1950 in research purposes and in such installations with nuclear reactors, in which liquid-metal carriers are used to remove heat from reactors: sodium, potassium, their alloys, bismuth and others. The temperature of the liquid metal in the pumps is 200 - 600 ° C, and in some cases up to 800 ° C. One of the performed sodium pumps has the following calculated data: temperature 800 ° C, pressure of 3.9 kgf / cm², consumption 3670 m³ / h, useful hydraulic power 390 kW, consumed current 250 ka, 2.5 V voltage, power consumption 625 kW, efficiency ratio of 62.5%. Other characteristic data of this pump: Channel cross-section 53 × 15.2 cm, flow rate in a channel 12.4 m / s, active channel length 76 cm.
The advantage of electromagnetic pumps is that they do not have moving parts and the liquid metal path can be sealed.
DC pumps require for powering sources with high current and low voltage. For nutrition power pumps Rectifier installations are unsuitable, as they are obtained bulky and with a small efficiency. More appropriate in this case are unipolar generators, see the article "Special Types of Generators and DC Converters".
Plasma rocket engines
Considered electromagnetic pumps are peculiar DC motors. Such devices are also suitable for overclocking, accelerating or moving plasma, that is, high-temperature (2000 - 4000 ° C and more) ionized and therefore electrically conductive gas. In this regard, the development of jet plasma engines for cosmic missiles is made, and the problem of obtaining plasma expiration rates of up to 100 km / s is set. Such engines will not have a large strength of thrust and therefore will be suitable for work away from the planets, where the fields are weak; However, they have the advantage that mass flow Substances (plasma) small. Necessary for their nutrition electric energy It is assumed to be obtained with nuclear reactors. For plasma direct current engines, the difficult problem is the creation of reliable electrodes for the flow of current to the plasma.
Magnitohydrodynamic generators
MHD machines, like all electrical machines, reversible. In particular, the device shown in Figure 1 can also work in the generator mode, if it is driving a conductive fluid or gas through it. In this case, it is advisable to have independent arousal. The generated current is removed from the electrodes.
In such principle, electromagnetic water flowmeters are constructed, alkali and acid solutions, liquid metals, and the like. The electromotive force on the electrodes at the same time is proportional to the speed of movement or flow of fluid.
MHD generators are of interest from the point of view of creating powerful electrical generators to directly convert thermal energy into electric. To do this, through the device of the form shown in Figure 1, it is necessary to skip at a speed of about 1000 m / s conductive plasma. Such a plasma can be obtained by burning ordinary fuels, as well as by heating the gas in nuclear reactors. To increase the conductivity of the plasma, it is possible to introduce small additives easily ionized alkali metal.
Plasma electrical conductivity at temperatures of order 2000 - 4000 ° C relative to small (resistivity about 1 Ohm × cm \u003d 0.01 Ohm × m \u003d 104 oh mm² / m, that is, about 500,000 times more than in copper). Nevertheless, in powerful generators (about 1 million kW) it is possible to obtain acceptable technical and economic indicators. MHD generators with a liquid metal working fluid are also being developed.
When creating plasma MHD, direct current generators arise difficulties with the choice of materials for electrodes and with the manufacture of securities in the work of the channels. IN industrial installations Also, a complex task is a DC conversion relative to low voltage (several thousand volts) and great strength (hundreds of thousands of amps) in alternating current.
53. Unipolar cars. The primary generator invented Michael Faraday. The essence of the effect, open by the Faraday, is that when the disk is rotated in a transverse magnetic field, the Lorentz power acts on electrons in the disk, which shifts them to the center or peripherals, depending on the direction of the field and rotation. Due to this, there is an electromotive force, and through the current-piece brushes concerning the axis and periphery of the disk, a significant current and power can be removed, although the voltage is small (usually, the shares of the volt). Later, it was found that the relative rotation of the disk and the magnet is not a prerequisite. Two magnets and a conductive disc between them, rotating together, also show the presence of the effect of unipolar induction. The magnet made of electrically conductive material, during rotation, can also work as a unipolar generator: it itself is and the disk from which the electrons are removed, and it is the source of the magnetic field. In this regard, the principles of unipolar induction develop within the framework of the concept of movement of free charged particles relative to the magnetic field, and not relative to magnets. The magnetic field, in this case, is considered to be fixed.
Disputes about such cars walked for a long time. It could not understand that the field is the property of the "empty" space, physics that deny the existence of the ether could not. That's right, since the "space is not empty", it has ether, and it is it that provides a medium of the magnetic field of the magnetic field relative to which the magnets and the disc is rotated. The magnetic field can be understood as a closed stream of ether. Therefore, the relative rotation of the disk and the magnet is not a prerequisite.
In the works of the Tesla, as we have already noted, the improvements of the scheme were made (the size of the magnets was increased, and the disc is segmented), which allows you to create self-predatory Tesla unipolar machines.
In the motors under consideration, the excitation winding is performed with a small number of turns, but is designed for large currents. All features of these engines are associated with the fact that the excitation winding is turned on (see Fig. 5.2, in) Consistent with an anchor winding, as a result of which the excitation current is equal to the current anchor and the created flow rate F proportional to the current anchor:
where but \u003d / (/ I) - nonlinear coefficient (Fig. 5.12).
Nonlinearity but Related to the form of the engine magnetization curve and the demagnetizing effect of the anchor reaction. These factors manifest themselves with / I\u003e, / Yang (/ Yang - rated anchor current). With smaller currents but It can be considered a permanent value, and with / me\u003e 2 / I n engine is saturated and the stream has little depends on the current anchor.
Fig. 5.12.
The main equations of the sequential excitation engine, in contrast to the equations of independent excitation engines are nonlinear, which is associated, first of all, with the product of variables:
When the current changes in the anchor circuit, the magnetic flow F is changed, leaving the vortex currents in the massive parts of the magnetic pipeline. The effect of vortex currents can be taken into account in the engine model in the form of an equivalent short-circuit contour described by the equation
and the equation for the chain of the anchor has the form:
where W B, W B T is the number of turns of the excitation winding and the equivalent number of turns of the vortex currents.
In the steady mode
From (5.22) and (5.26) we obtain expressions for the mechanical and electromechanical characteristics of the DC motor of the sequential excitation:
In the first approximation, the mechanical characteristics of the sequential excitation engine, without taking into account the saturation of the magnetic chain, can be represented as hyperboles that do not cross the ordinate axis. If put L J. C \u003d /? I + /? B \u003d 0, the characteristic will not cross the abscissa axis. Such a characteristic is called perfect.The real natural characteristic of the engine crosses the abscissa axis and due to the saturation of the magnetic pipeline at the moments more M N. Styling (Fig. 5.13).
Fig. 5.13.
A characteristic feature of the characteristics of the engine of sequential excitation is the absence of a point of perfect idle move. When the load decreases, the speed increases, which can lead to an uncontrolled motor acceleration. It is impossible to leave such an engine without load.
An important advantage of sequential excitation engines is a large reloading capacity at low speeds. When current overload 2-2.5 times, the engine develops a moment of 3.0 ... 3.5 M n. This circumstance has determined the widespread use of sequential excitation engines as an electric drive vehicleFor which the maximum moments are needed when moving from place.
The change in the direction of rotation of the sequential excitation engines cannot be achieved by changing the polarity of the emergence of the anchor chain. In the sequential excitation engines, when reversing, it is necessary to change the current direction in one part of the anchor chain: either in the anchor winding, or in the excitation winding (Fig. 5.14).
Fig. 5.14.
Artificial mechanical characteristics for speed control and torque can be obtained in three ways:
- introducing additional resistance to the engine anchor chain;
- changing the supply voltage motor;
- By shunting winding anchor with additional resistance. With the introduction of additional resistance to the anchor chain, the stiffness of the mechanical characteristics is reduced and the starting point is reduced. This method is used when starting the sequential excitation engines that receive power from the sources with an unregulated voltage (from contact wires, etc.) in this case (Fig. 5.15) The required value of the starting point is achieved by the sequential shorting of the sections of the starting resistor by means of contactors K1-KZ.
Fig. 5.15.Reostate mechanical characteristics of a sequential excitation engine: /? 1do - R IAO. - resistant steps of an additional resistor in anchor chain
The most economical way to regulate the speed of the sequential excitation engine is the change in the supply voltage. The mechanical characteristics of the engine are shifted down parallel to the natural characteristic (Fig. 5.16). In shape, these characteristics are similar to the rigging mechanical characteristics (see Fig. 5.15), however, there is a fundamental difference - when adjusting the voltage change, there are no losses in additional resistors and adjustment is performed smoothly.
Fig. 5.1
Sequential excitation engines When used as a drive mobile units, in many cases, power from a contact network or other power sources with a constant voltage value supplied to the engine are obtained, in which case the regulation is made by means of a pulse voltage controller (see § 3.4). This scheme is shown in Fig. 5.17.
Fig. 5.17.
Independent adjustment of the excitation motor of the sequential excitation engine is possible if the anchor winding is accumulated (Fig. 5.18, a). In this case, the excitation current B \u003d I + / W, i.e. Contains a constant component that does not depend on the engine load. In this case, the engine acquires the properties of the mixed excitation engine. Mechanical characteristics (Fig. 5.18,6) acquire greater rigidity and intersect the ordinate axis, which makes it possible to obtain a stable reduced rate at low loads on the motor shaft. A significant drawback of the scheme is large energy losses in the shunt resistance.
Fig. 5.18.
Two brake modes are characterized for direct current and sequential excitation engines: dynamic brakingand anti-influx.
Dynamic braking mode is possible in two cases. In the first - anchor winding closes to resistance, and the excitation winding is powered by a network or other source through the adding resistance. In this case, the engine characteristics are similar to the characteristics of an independent excitation engine in dynamic braking mode, (see Fig. 5.9).
In the second case, the diagram of which is shown in Fig. 5.19, the engine when the KM contacts are turned off and the KV contacts are closed as a self-excitation generator. When moving from the motor regime in the brake, it is necessary to maintain the direction of the current in the excitation winding to avoid demagnetization of the machine, since the machine goes into self-excitation mode. The mechanical characteristics of this mode are presented in Fig. 5.20. There is a boundary speed with f, below which self-excitation of the machine does not occur.
Fig.5.19.
Fig. 5.20.
In the anti-conference, the anchor chain includes additional resistance. In fig. 5.21 shows the mechanical characteristics of the engine for two options for opposition. Characteristic 1 is obtained if when the engine is running in the direction "Forward" in (point from) Change the current direction in the excitation winding and enter the anchor addition resistance to the anchor chain. The engine switches to the counterclude mode (point but) with brake torque M brass.
Fig.5.21.
If the drive works in loading mode, When the task of actoring the lifting mechanism when operating in the "Back" direction, the engine is turned on in the direction "Forward" B, but with high email resistance in the chain of the anchor. Drive operation corresponds to the point b. In a mechanical characteristic 2. Operation in the mode of the opposition is conjugate with large energy loss.
The dynamic characteristics of the sequential excitation DC motor describes the system of equations arising from (5.22), (5.23), (5.25) during the transition to the operator form of recording:
In the structural scheme (Fig. 5.22) coefficient but \u003d D / I) reflects the machine saturation curve (see Fig. 5.12). Influence of vortex currents neglect.
Fig. 5.22.
To determine the transfer functions of the sequential excitation engine by the analytical method is rather difficult, so the analysis of transient processes is made by computer simulation based on the diagram shown in Fig. 5.22.
Mixed excitation DC motors have two excitation windings: independent and consistent. As a result, their static and dynamic characteristics combine the characteristic properties of the two previously considered DC motors. Which of the species more belongs to one or another engine of mixed excitation depends on the ratio of the magnetizing forces created by each of the windings: B / P.V. \u003d B / P.V.\u003e where in the 'B - the number of turns of the winding of independent and consistent excitation .
Source Equations of Mixed Excitation Engine:
where in, R b W B - current, resistance and number of turns of the winding of independent excitation; L M - Mutual inductance of excitation windings.
The equations of the steady mode:
From where the electromechanical characteristic equation can be written in the form:
In most cases, a sequential excitation winding is performed by 30 ... 40% MD C, then the speed of perfect idling exceeds the nominal speed of the engine by about 1.5 times.
