The electrical industry is currently producing electrical machines direct current for work in various conditions. Ship vehicles have design features of individual units, but the general design of these vehicles is the same. Figure (1.4) shows a longitudinal and cross section of a normal machine. DC machine consists of 2 main parts: fixed - stator and rotating - armature. There is always an air gap between them.
Rice. 1.4 - DPT in the context
The stator, which is an inductor, i.e. such a part of the machine, in which the magnetic field is induced, consists of the frame I, the main 2 and additional 3 poles. The stator also includes bearing shields 7 with bearings 11. A brush set 9 and a terminal box 10 are mounted on the stator.
The armature consists of an armature core 4 and a collector 8, mounted on a shaft 6. In machines with self-ventilation, a fan 12 is mounted on the shaft.
Stanina- serves as a magnetic circuit and at the same time is a structural basis to which the main and additional strips and end shields are attached. It is a hollow cylinder cast or welded from cast iron or steel. For large machines, the bed is split. On ships, for the convenience of maintenance and repair, machines with a rotary bed are also used. The part of the bed, along which the magnetic fluxes of the main and auxiliary poles are closed, is called yoke 1. Together with the bed, paws 13 are cast to fasten the machine to the foundation. One or more lifting eyes 14 are installed on the bed for lifting the machine.
Major poles are designed to create a magnetic flux of the required magnitude in the machine. The main pole (Figure 1.5) consists of a core 1 and field winding coils 2,3. On the side facing the armature, the core ends with a pole piece 4, with the help of which the required distribution of magnetic induction in the air gap is ensured.
Figure 1.5
Hearts to poles is made of electrical steel sheets with a thickness of 0.5 ÷ 1.0 mm. Coated with insulating varnish to reduce losses from eddy currents caused by pulsation of the magnetic flux due to the serration of the armature. Sheets of steel are pressed and fastened with pins. The field winding coils are wound on the insulating frame 5, and then put on the core. In relation to the armature winding, the field windings can be connected in parallel or in series. The coils of the parallel winding 2 consist of a large number of turns of small wire. Coils of series winding 3 consist of a small number of turns of wire of large cross-section, through which a large armature current flows. To improve the insulation, the coils are compounded, i.e. impregnated with insulating varnishes (compounds) in vacuum at an elevated temperature, and then dried in special ovens. The assembled pole is attached to the frame with bolts 6.
Additional poles serve to improve the commutation of the machine, i.e. provide sparkless operation of brushes and collector. They consist of a core 1 and a pole coil 5 (Figure 1.6) and are installed between the main poles along the geometric neutral line. The core has a tip 2 of a certain shape. The coil is made of strip copper of large cross-section, since it is connected in series to the armature circuit and a large current passes through it. The size of the gap δ between the pole and the armature is adjusted when setting up the machine using magnetic and non-magnetic spacers 4 between the pole and the bed. Additional poles are attached to the frame with bolts 3.
Figure 1.6
Anchor consists of a core of the magnetic circuit, winding 5, shaft 6 and structural parts for their fastening.
Anchor core is a steel cylinder made of stamped sheets 1 (Figure 1.7) of electrical steel with a thickness of 0.5 mm, which are insulated from each other with varnish to reduce losses from eddy currents.
Figure 1.7
In the sheets, grooves are stamped to accommodate the armature winding and holes for placing the core on the armature shaft, for tie rods and axial ventilation. The iron package of the anchor is attached to the shaft with a key, and is pulled from the ends by pressure rings. In large machines, the anchor consists of several packs of stamped sheets, between which gaps are made for better cooling of the machine (radial ventilation). The part of the armature core occupied by the slots is called the toothed zone.
Armature winding made of insulated wire of round or rectangular cross-section. It consists of separate elements - sections (Figure 1.8), formed from one or more turns.
Figure 1.8
Sections are made according to templates. The part of section 1, embedded in the slots of the armature core, is called the slot or active part. The part of section 2, located outside the core - in the air and connecting the active parts, is called the frontal part (frontal connections). The ends of the sections are soldered to the manifold plates. For fastening the sections in the grooves, wooden, getinax or textolite wedges are used. In addition to the turn insulation, the winding has groove insulation from the core. The frontal parts are secured with a wire band.
Electrical insulating materials used to insulate windings, according to the degree of heat resistance, are divided into classes that allow a certain heating temperature. In DC machines, classes A, B, C and H are mainly used. The collector (Figure 1.9) is assembled from copper plates I, isolated from each other and from the shaft on which it is attached, using mikanite gaskets 8 and cuffs 5.7 ... On the side facing the shaft, the plates are in the form of a dovetail 2. Insulated pressure cones 3,4 are inserted into the two cone-shaped recesses of the collector, which tighten the collector plates in the axial direction. When assembled, the collector is pressed hot and then grinded to give it a strictly cylindrical shape. Depending on the size of the armature and the collector, the ends of the winding sections are soldered into the collector plates directly or through special
Collector(Figure 1.9) is recruited from copper plates I, isolated from each other and from the shaft on which it is attached, using mikanite spacers 8 and collars 5,7. On the side facing the shaft, the plates are in the form of a dovetail 2.
Figure 1.9
Insulated pressure cones 3,4 are inserted into two cone-shaped recesses of the collector, which tighten the collector plates in the axial direction. When assembled, the collector is pressed hot and then grinded to give it a strictly cylindrical shape. Depending on the size of the armature and the collector, the ends of the winding sections are soldered into the collector plates directly or through special copper connections - cock 9. The collector is rigidly attached to the rotor shaft next to the armature core.
Brushing devices O- designed to provide electrical connection between the fixed clamps connected to the external circuit and the rotating armature winding (through the collector) (Figure 1.10).
Figure 1.10
It consists of brushes 1, brush holders 3, fingers 5, traverse 6 and connecting busbars. The brush has direct contact with the collector 2. It is usually made from a specially processed mixture of coal, graphite and other components in the form of a rectangular prism and is placed in the holder of the brush holder 4. The brush can move in the holder in a radial direction with respect to the collector and for a snug fit is pressed against it by a spring through a pressure lever. The brush holders are attached to the pins 5, which are embedded in the traverse 6 through the insulating bushings 7. One finger can have from 2 to 10 brushes, which for uniform wear of the collector are staggered on its surface and are connected to the fingers with copper flexible cables. The number of fingers is always equal to the number of the main poles. The fingers having the same polarity are connected by means of a connecting bus, from which a tap is made to the terminal box of the machine or to the winding of the additional pole.
The traverse can be attached to end shields, frame or base plate. The mount allows the entire brush system to be rotated relative to the frame.
Terminal box... An insulating panel with terminals is installed in the terminal box, to which the winding leads of the machine are connected for connection to the external electrical network.
DC electric machines.
The device of electrical machines
Direct current. Reversibility of machines
By designation, DC electric machines are divided into generators and motors.
Generators generate electrical energy entering the power system; motors create mechanical torque on the shaft, which is used to drive various mechanisms and Vehicle.
Electric cars are reversible. This means that one and the same machine can work as both a generator and an engine. Therefore, we can talk about the device of DC machines without considering separately the device of the generator or engine.
The property of reversibility should not be opposed to the specific purpose of a machine, which is usually designed and used either as a motor or as a generator. Machines designed to operate both in generator and motor modes are much less commonly used. These are the so-called starter-generators, which are installed on some moving objects.
The generator and engine differ in design and design features... Therefore, using the engine as a generator or a generator as a motor leads to deterioration performance characteristics machines, in particular to a decrease in the efficiency.
In any DC machine, moving and stationary parts are clearly distinguished. The movable (rotating) part of the machine is calledrotor , motionless -stator .
The part of the machine in which the electromotive force, it is customary to call the anchor, and the part of the machine in which the magnetic field of excitation is created is called the inductor. Typically, in a DC machine, the stator serves as the inductor and the rotor as the armature.
DC machine stator is also calledbed.The frame is made of a magnetically conductive material (usually cast steel); it performs two functions, being, firstly, a magnetic circuit through which the magnetic flux of excitation of the machine passes, and, secondly, the main structural detail, in which all other parts are placed. Poles are attached to the frame from the inside. The pole of the machine consists of a core, a pole piece and a coil. When passing through the DC coils, a magnetic field flux is induced at the poles. Beyond the main poles in machines increased power(more than 1 kW), additional poles of smaller sizes are installed, designed to improve the operation of the machine. The coils of the additional poles are connected in series with the armature winding.
Armature core and manifold roll on the same shaft. The steel armature shaft is supported by bearings mounted in the side guards of the machine. In turn, the side shields are bolted to the stator.
To reduce eddy currents and associated heat losses, the armature core is made of thin sheets of electrical steel, insulated from each other with a varnish. Ventilation ducts are drilled in the anchor body, through which cooling air passes. The conductors of the armature winding, connected to the collector plates, are laid in the grooves of the armature core.The collector is made up of separated copper plates. micanite gaskets. The surface of the copper plates is specially treated to increase their abrasion resistance.
The electrical connection of the rotating armature winding with the fixed terminals of the machine is carried out with aOby the power of brushes sliding over the collector.
The brushes are inserted into special holders of the brush holder and pressed against the collector by spiral or leaf springs. The brush holders are attached to the traverse, which, together with the brushes, can be rotated relative to the stator by a certain angle in one direction or the other.Graphite is used as the basis for the manufacture of the brush. To obtain the desired properties (a certain electrical conductivity, increased resistance to abrasion), powders of metals (copper, lead) are added to the brush.
In fig.5 .1. shows the appearance of a DC machine of the P series, produced by the domesticindustry. The machines of this series are designed for various powers from 0.3 to 200 kW. The P series motors are rated for 110 or 220 V, and the generators for 115 or 230 V.
Rice.5 .1. Appearance DC machines
Rice. 9.2. DC Machine Cross Section:
1 - armature core with winding conductors; 2 - coil of the excitation winding;3 - shaft; 4 - the main pole; 5 - additional pole; 6 - stator
A cross section of a DC machine is shown schematically in Fig.5 .2, where the stator, creating the magnetic field flux, and the rotor, in the grooves of which the conductors of the armature winding are placed, are visible. There is an air gap between the pole piece and the armature, which excludes friction between the rotor and the stator (Fig.5 .3, a). The magnetic induction in the air gap changes along the circumference according to the law, which is called trapezoidal (Fig.5 .3, b).
The device of a DC machine is shown in Fig.5 .4.
DC machines usually have a forced air cooling carried out by a fan mounted on the armature shaft. Hydrogen and water cooling systems have been developed for powerful machines.
Rice.5 .3. Schematic representation of an air gap1 between pole piece 2 and armature 3 (a) and magnetic induction in the air gap (b)
To protect the machine from dust and moisture, the structural windows providing access to the collector and brushes are closed with removable steel strips or plates.
Rice.5 .4. DC machine device:
1 - collector; 2 - brushes; 3 - anchor core; 4 - core of the main pole; 5 - pole coil; 6 - stator; 7 - bearing shield; 8 - fan; 9 - armature winding
DC motor. If you connect a DC machine to the mains, current will flow through the armature winding. In accordance with Ampere's law, mechanical forces act on the armature winding conductors in a magnetic field of excitation. These forces create a torque, under the influence of which the armature begins to spin.
The rotating armature shaft is used to drive various mechanisms: lifting and transport vehicles, machine tools, sewing machines etc.
Based on the law of conservation of energy, it can be assumed that the power consumed by the motor from the network is the greater, the greater the mechanical load on its shaft. However, to understand the essence of the operation of an electric motor, it is important to trace how a change in mechanical load affects the electrical power consumed by the motor.
Let's figure it out. The armature winding of the motor rotates in a magnetic field of excitation. Under these conditions, in accordance with the law of electromagnetic induction, an EMF occurs in the armature winding. Applying the rule of the right hand, it is not difficult to establish that it is directed towards the applied mains voltage. Therefore, it was called the counter-EMF. It is the back-EMF that is the factor that regulates the consumption of electrical power from the network.
According to the law of electromagnetic induction, the back-EMF is directly proportional to the rate of change of the magnetic flux penetrating the turns of the armature winding. Consequently, with a decrease in the frequency of rotation of the armature, the back-EMF also decreases.
If there is no mechanical load on the motor shaft (the motor is idling), only frictional torques impede the motor torque and the armature speed reaches its maximum value. In this case, the back-EMF almost completely compensates for the mains voltage and a minimum current passes through the armature winding. Accordingly, the electrical power consumed from the network is minimal.
Engine speed control
direct current independent and parallel excitation
Let's turn again to the basic equation of the electric motor. The expression for the EMF of the engine is no different from the expression for the EMF of the generator. This is understandable: in both cases, the winding conductors cross lines of force magnetic field. The fact that the armature of the generator is spun mechanically, and the armature of the engine - by electromagnetic forces, from the point of view of the law of electromagnetic induction does not matter.
From a practical point of view, it is important to understand the conditions and methods for controlling the engine speed. The derived formula allows us to solve this problem. First of all, we note that in order to reduce power losses, the resistance of the armature winding is sought to be made as small as possible (in real machines it is hundredths or thousandths of an ohm).
Thus, there are two ways to smoothly change the engine speed over a wide range: 1) change and voltage U supplied to the engine armature; 2) change in the magnetic flux of excitation Ф (excitation current Iv).
The second method of controlling the engine speed is preferable, since it is associated with lower energy losses: the excitation current is tens of times less than the armature current, and the losses in the control rheostat are proportional to the square of the current. However, if it is necessary to change the engine speed within a very wide range, both methods are used simultaneously.
The ability to smoothly and economically control the speed over a wide range is the most important advantage of DC motors.
In many cases, it becomes necessary to change the direction of rotation of the electric motor armature. Changing the direction of rotation is called reversing.
To reverse the DC motor, you must change the direction of the field magnetic flux or armature current. With a simultaneous change in the direction of the excitation flow and the armature current due to a change in the polarity of the power supply voltage, the direction of rotation of the motor armature does not change.
The reversing of the motors is carried out using switches in the armature circuit or in the excitation circuit.
The expression for the engine speed shows that as the magnetic field flux decreases, the frequency increases indefinitely. From this point of view, a break in the excitation circuit of the motor is dangerous, in which the magnetic flux sharply decreases to the residual magnetization flux, and the motor is "peddling". The "runaway" mode is especially probable for an unloaded engine. The "runaway" mode is emergency: centrifugal forces deform the armature winding, the armature gets wedged, and in some cases even collapses.
Educational institution
"Brest State University named after A.S. Pushkin"
abstract
Topic: "DC machines"
Completed by a student of the 3rd year of the group
Supervisor:
Brest, 2009
General information
Anchor reaction
Commutation
General information
In modern electric power industry, alternating current is mainly used, but direct current is also widely used. This is due to the advantages of direct current, which made it indispensable in solving many practical problems. So, among electric machines, DC motors occupy a special position. DC motors allow smooth control of rotation speed within any limits, while creating a large starting torque. This property of DC motors makes them indispensable as traction motors for urban and railway transport(tram, trolleybus, metro, electric locomotive, diesel locomotive). DC motors are also used in the electric drive of some metal-cutting machines, rolling mills, hoisting-and-transport machines, excavators. Direct current is also used to power electrolytic baths, electromagnets for various purposes, control and monitoring equipment, and to charge batteries. This power is provided by DC generators, usually driven by asynchronous and synchronous AC motors. However, generators are often replaced by rectifiers (semiconductor diodes and thyristors) and direct current is obtained from alternating current.
DC machines are also included in the electrical equipment of cars, ships, aircraft and missiles.
The principle of operation and device of the DC generator. types of armature windings
The principle of operation of a DC generator is based on the occurrence of an EMF in a frame rotating in a magnetic field (Fig. 6-1, a). For one revolution in each working (active) part of the frame, the EMF changes sign twice. In order for the current in the external circuit to have only one direction (constant), a collector is used - two half rings connected to the ends of the frame, and the frame is connected to the external circuit through a rotating collector and fixed brushes. As soon as the active side of the frame begins to cross the magnetic induction lines in the opposite direction compared to the previous ones, the semi-ring of the collector connected to this side will begin to come into contact with the other brush. Thanks to such a device, the direction of the current in the external circuit remains unchanged, although its value changes (pulsates, Fig. 6-1, b).
The structure of an industrial DC generator is shown in Figure 6-2. On the inner surface of the frame I, made of one-piece cast iron, the main poles 2 with field windings and additional poles with windings are rigidly fixed to compensate for the EMF of self-induction and armature reaction.
In most cases, electromagnets are powered by the generator itself. An anchor 3 is placed inside the bed, which is a metal cylinder assembled from stamped plates of electrical steel. In the longitudinal grooves on the surface of the armature, the armature winding is placed, consisting of interconnected sections. To smooth out the pulsations of the EMF and current, the armature winding is evenly placed over the entire surface, the magnetic resistance between the poles is reduced due to the steel core of the armature. The conclusions of the windings are soldered to the copper plates of the collector 4, isolated from each other and from the machine body, and the end of one section and the beginning of the next are soldered to the same plate. The collector is rigidly mounted on the armature shaft, and the fan is also attached to the same shaft. The armature shaft is placed in the bearings of the end shields 5, which are mounted on the sides of the frame. A small air gap is formed between the armature and the stator poles, due to which the armature can rotate freely. Carbon brushes inserted into the brush holders 6 are superimposed on the cylindrical surface of the collector. To reduce the resistance, the brushes are often pressed from a mixture of carbon and copper powder.
DC machines are often made multi-pole (Fig. 6-3), while in each section of the winding in one revolution, the value and sign of the EMF change as many times as there are poles. The magnetic circuit of such a machine is more complex, with the number of pairs of brushes being equal to the number of pairs of poles, and brushes of the same polarity are connected together.
Let's consider the principles of operation of a DC generator in more detail.
If the anchor is made in the form of a ring and a winding in the form of a closed toroid is placed on it, then such an anchor is called annular, and the winding is called spiral. When this armature rotates in a magnetic field, EMF will be induced in the turns of its winding (Fig. 6-4, a). It turns out that in the turns of one half of the winding, the EMF has one sign, in the turns of the other half - the opposite.
If the turns are evenly distributed over the surface of the armature, then there will be no current in the winding, since the EMF action of both halves is mutually compensated. If, for example, the insulation is partially removed from the outer side of the turns and two stationary brushes (a and b) are placed on two opposite sides so that when the armature rotates they can touch each turn, then it is easy to see that the entire winding will seem to split in half and when the armature rotates, the turns of one half of the winding will gradually pass into the other, while the number of turns of each half, the polarity and the value of the EMF will remain low. If you now connect the load to the brushes, then a constant current will be established in the external circuit and in each half of the winding.
Obviously, for a more complete use of the EMF, the brush windings must be connected at those points where the EMF is not induced. A straight line passing through two such points is called geometric neutral (GN). With this arrangement of the brushes, the winding is divided into two parallel branches, connected to each other and by an external chain by brushes. If the brushes are displaced relative to the geometric neutral, then in part of the turns of each parallel branch the EMF will have the opposite polarity, and sparking may begin under the brushes, since in the turns (sections) wrapped by the brushes, the EMF is different from zero.
The annular armature can be improved if you do not remove the insulation from the winding turns, but make taps from them connected to the collector plates, and put the brushes on the collector (Figure 6-4, b). If such a machine has four poles, then the winding will be divided into four parts (Fig. 6-5, a). If, instead of two brushes, we put four and the same ones connected to each other (Fig. 6-5, b), then the winding will have four parallel branches. It is easy to see that with an increase in the number of parallel branches, the load current can be correspondingly increased. The annular armature with a spiral winding discussed above has significant drawbacks. First, the magnetic flux is closed through the wall of the ring (armature), bypassing the inner cavity, therefore, the active side of each turn of the winding is the one located on the surface, and the inner part of the coil is not used to obtain EMF and serves only as a connecting conductor. This circumstance leads to irrational consumption of copper. Secondly, the spiral winding cannot be made according to a template, therefore, machines with an annular armature are not manufactured at present.
Bypassing the inner cavity, therefore, the active side of each turn of the winding is the one that is located on the surface, and the inner part of the turn is not used to obtain EMF and serves only as a connecting conductor. This circumstance leads to irrational consumption of copper. Secondly, the spiral winding cannot be made according to a template, therefore, machines with an annular armature are not manufactured at present.
The disadvantages of the annular armature are eliminated by replacing it with a drum armature. The drum armature windings (Fig. 6-6) are laid in special grooves on the surface of the cylinder (armature) in the form of separate sections, connected in a certain way to the collector plates and to each other. A section is the part of the winding between two adjacent collector taps. Both sides of each section are active; sections are made according to a template.
EMF and electromagnetic moment of the DC generator
Let us derive the dependence of the EMF of the generator on the parameters of the machine, the speed of rotation of the armature and the magnetic flux.
The EMF induced in each turn of the winding can be determined by the formula. (1). As applied to a DC machine, this formula (and the entire subsequent derivation) is greatly simplified by the introduction of the concept of average induction. Let the magnetic flux created by the main pole, Ф, then at 2 p poles the total magnetic flux is equal to 2р F.
where d is the diameter of the armature core, l is the generatrix of the armature cylinder (armature length). Then the average EMF of one conductor of the winding at = 90 ° is equal to
where l is the length of the active part of the conductor (equal to the generatrix of the armature cylinder); v - linear (circumferential) speed of movement of the conductor.
Let us substitute in the formula (3) the value of the average induction B cf and the linear velocity and after the transformation we get:
where n is the speed of rotation of the armature.
Let the winding contain 2a parallel branches, then there will be active conductors in each parallel branch. Since the EMF of the generator is equal to the EMF of the parallel branch, it can be written:
where is the EMF of the generator.
Substitute expression (4) into equation (3), after reduction we get:
. (6)
In the resulting formula, the highlighted fraction contains parameters that depend on the design of the machine. For a given machine design, this value is constant. We denote this fraction through c, then for the EMF of the generator we finally have:
Thus, the EMF of a direct current generator is proportional to the value of the magnetic flux Ф and the speed of rotation of the armature n. Therefore, to maintain a constant voltage at the terminals of the generator, the EMF can be changed either by the value of the magnetic flux or by the speed of rotation of the rotor (or both). In practice, the rotor of the generator is driven by a motor operating normally at a certain speed of rotation of the shaft, and the magnetic flux is changed by changing the current in the field winding. The power of a direct current generator can be represented by the formula of mechanical power (P =), and work A should be understood as the work spent on overcoming the braking torque developed by the armature in one revolution when the armature rotates at a speed n (without losses). Then this formula can be written like this:
where F is the force acting on the anchor. With this interaction, a force acts on each conductor of the armature winding with a current I, and on N conductors of the winding
Taking into account relation (2), the last equation can be written as follows:
, (10)
Substituting equation (10) into equation (8), we obtain an expression for the power:
Since Ф, we finally have:
For the total moment of the machine M, you can write:
,
where is a constant coefficient for a given machine, depending on the features of its design. Thus, the electromagnetic moment of the machine is expressed by the formula M = cF1 i. (13)
Anchor reaction
In the mode idle move of a DC generator, only EMF is induced in its winding, and there is no current in the winding, since the EMF of the parallel branches is mutually compensated. In this case, the machine has only one magnetic flux - the flux of the poles. But as soon as the load is turned on, a current will appear in the armature winding and, as you know, this current will create its own magnetic flux, which will begin to superimpose on the current of the poles, i.e. there is a phenomenon called an anchor reaction
If we separately depict the pictures of the fields of the poles (Fig. 6-7, a) and the anchor (Fig. 6-7, b) and compare them, then we can see that the field of the anchor is transverse with respect to the poles. Obviously, as a result of their interaction (overlapping), as in a synchronous generator with an active load, under the incident edges of the poles with an unsaturated magnetic system of the machine, the induction will decrease, and under the receding edges, it will increase, while the total flux will not change. At high loads, saturation of the magnetic circuit occurs. Then, as a result of the armature reaction, not only will the field picture be distorted (Fig. 6-7, c), but the total magnetic flux and the associated EMF will decrease, while the magnetic resistance of the pole piece and the armature teeth located under this pole will, as it were, increase. As a result, the flux of excitation passing through them will decrease. The reaction of the armature will lead to the fact that in the sections located on the geometric neutral, the EMF is different from zero. Consequently, when the sections are short-circuited by the brushes, currents may appear that generate sparking and burning of the collector and brushes. This undesirable phenomenon can be eliminated by moving the brushes along the collector in the direction of its rotation at a certain angle (from the geometric neutral nn 'to the physical neutral mm'), where the EMF in the sections is zero. If we take into account that the position of the physical neutral changes with a change in the load (with an increase in the load, the angle P increases), then it will not be possible to completely eliminate the sparking in this way (you will have to continuously rotate the brushes simultaneously with the load changes). In practice, the brushes are set at the lowest arcing at rated load. When the machine is running in engine mode, the physical neutral is shifted against the direction of rotation.
The influence of the armature reaction can be weakened by increasing the air gap between the poles and the armature, but this will lead (as in a synchronous machine) to unnecessary copper consumption and an increase in the size of the machine. To weaken the influence of the armature reaction in DC machines, additional poles are used, which at the same time improve the switching of the current.
Commutation
During operation of the DC machine, the winding sections are continuously switched from one parallel branch to another, while the current in the switched sections changes its direction to the opposite. Since the time of this transition is very short, the rate of change of the current in the section is high. If we take into account that the section is placed on a steel core (the inductance is high), then the process of switching the section may be accompanied by the appearance in it of a significant EMF of self-induction and, possibly, sparking.
The process of switching sections of the winding from one parallel branch to another and all the phenomena accompanying this switching are called the switching process, and the duration of this process is called the switching period.
Let us consider this process in somewhat more detail using the example of a winding with two parallel branches.
Before the start of switching, when the brush comes into contact only with the collector plate 1 (Fig. 6-8, a), the load current I flows from plate 1 to point a, where it branches into both parallel branches. The section of interest to us (highlighted in the drawing with a bold line) is located in the right parallel branch. As soon as the right edge of the brush touches plate 2, the switching process begins, which will continue until the left edge of the brush leaves plate 1, while during the entire switching period the section we have selected will be short-circuited with a brush (Fig. 6-8, b) Since during the entire switching time the value and direction of the currents in wires 2 and 3 will not change, then as the brush of the collector plate 1 passes to plate 2, the current under the incident edge will increase, and under the running edge it will decrease, being distributed inversely proportional to the contact area, the current density in this case, it will be constant everywhere. But this would be the case with a very slow movement of the collector relative to the brush. In fact, the switching period lasts only thousandths of a second, during this time the current in the selected section (wires 1-4) changes from + to zero and from zero to -. Since the section has a large inductance, then under the action of the EMF of self-induction, an additional current will appear in it, the direction of which (according to Lenz's law) coincides with the decreasing current in the section. This additional current will greatly increase the current density under the running-off edge of the brush, and sparking will occur between the plate and the brush at the moment the brush comes off plate I.
Now, when the brush began to touch only plate 2 (Fig. 6-8, c), the section 1-4 highlighted by us turned out to be in the left parallel branch, the current in it changed its direction to the opposite. After that, switching of the next section will start, i.e. sparking will be observed under the brush again.
We examined the commutation under the brush of the same polarity. A brush of a different polarity is in exactly the same conditions, where the direction of currents in all conductors will be opposite. To reduce the additional current arising in the switched sections, solid carbon brushes are used in high voltage machines, which form large contact resistances in the closed sections. The improvement of the switching conditions in DC machines is mainly carried out with the help of additional poles. This method is based on the following.
EMF of self-induction in the switched sections occurs when these sections pass near the geometric neutral and depends on the value of the load current. If at this time an equal and opposite EMF is created by some additional field in the switched section, then the additional current may disappear. This is exactly what they do in practice. Additional poles are placed on the geometric neutral and provided with windings connected in series to the load circuit (Fig. 6-9). Additional poles by their field induce in the switching sections a switching EMF, proportional to the load current, and compensating for the EMF of self-induction in the section, while the field of additional
Fig. 6-9 poles also attenuates the influence of the armature reaction. For generators, an additional pole of opposite polarity is placed behind the main pole in the direction of its rotation, and for the engine - the same polarity. This condition is automatically fulfilled when the machine switches from the generator mode to the engine mode, since the direction of the current is reversed.
Most DC machines have two additional poles for each pair of main poles. In low-power machines (up to 5 kW), one additional pole is made for each pair of main poles.
Excitation methods for DC generators
Excitation of the generator is called the creation of a working magnetic flux, due to which an EMF is created in the rotating armature. DC generators, depending on the method of connecting the field windings, are distinguished: independent, parallel, series and mixed excitation.
Independent excitation generator has an OF excitation winding connected to an external current source through an adjusting rheostat (Fig. 6-10, a). The voltage at the terminals of such a generator (curve I in Fig. 6-11) with an increase in the load current decreases slightly as a result of a voltage drop across the internal resistance of the armature, and the voltages are always stable. This property turns out to be very valuable in electrochemistry (powering electrolytic baths).
Parallel excitation generator is a generator with self-excitation: the excitation winding of the OF is connected through an adjusting rheostat to the terminals of the same generator (Fig. 6-10, b). Such inclusion leads to the fact that with an increase in the load current I, the voltage at the terminals of the generator U "decreases due to the voltage drop across the armature winding. This, in turn, causes a decrease in the excitation current and EMF in the armature. Therefore, the voltage at the terminals of the generator UB decreases slightly faster (curve 2 in Fig. 6-11) than that of an independent excitation generator.
A further increase in the load leads to such a strong decrease in the excitation current that when the load circuit is short-circuited, the voltage drops to zero (a small short-circuit current is due only to residual induction in the machine). Therefore, it is believed that the parallel excitation generator is not afraid of a short circuit.
Generator sequential excitation has an OF excitation winding connected in series with the armature (Fig. 6-10, c). In the absence of a load (= 0), a small EMF is nevertheless excited in the armature due to the residual induction in the machine (curve 3 in Fig. 6-11). With an increase in the load, the voltage at the terminals of the generator first increases, and after reaching the magnetic saturation of the magnetic system of the machine, it begins to rapidly decrease due to the voltage drop across the armature resistance and due to the demagnetizing effect of the armature reaction.
Due to the large volatility of voltage with changing load, generators with series excitation are not currently used.
Mixed excitation generator has two windings: 0V U - connected in parallel with the armature, ОВ 2 (additional) - in series (Fig. 6-10, d). The windings are switched on so that they create magnetic fluxes of one direction, and the number of turns in the windings is chosen so that the voltage drop across the internal resistance of the generator and the EMF of the armature reaction would be compensated by the EMF from the flow of the parallel winding.
Reversibility of DC machines. engines
Electric DC machines, like AC machines, are reversible, i.e. they can work as generators and as motors. The transition of the generator to the engine operation mode can be explained as follows.
If the generator is connected to the DC network, then a current will be established in the windings of the armature and electromagnets, while the electromagnets will create a constant magnetic field and a force will act on each conductor of the armature winding with current, tending to turn the armature in the direction of the force (Fig. 6-12, and). Thus, the interaction of the magnetic field of the armature with the field of the excitation winding drives the armature into rotation.
Applying the rule of the left hand, you can easily notice that when the direction of the current changes only in the armature (Fig. 6-12, b) or only in the excitation winding (Fig. 6-12, b), the direction of rotation of the armature changes to the opposite, and the simultaneous change the direction of the current in both windings does not change the direction of rotation of the armature (Fig. 6-12, d)
Electric motors do not differ structurally from DC generators, i.e. they have exactly the same design (except for a few types of special-purpose motors).
Let's consider some of the features of the engines. If a DC motor with an armature winding resistance is connected to a network with a voltage U, then at the moment of start, a current will be established in the armature, the value of which can be determined by Ohm's law:
Since the resistance of the armature winding of powerful motors is only tenths and hundredths of an ohm, and the operating voltage is of the order of hundreds of volts, then starting current can be hundreds and thousands of amperes, exceeding the rated current for this engine 10-30 times. Such a current is not only undesirable, but also dangerous for the motor, since the collector can collapse and the motor winding burn out. Obviously, limiting the starting current can be carried out by including the starting rheostat in the armature circuit. Then the starting current will decrease and will be equal to:
The resistance of the starting rheostat is chosen so that the starting current does not exceed the rated current by more than 1.1 - 1.5 times.
As a result of the interaction of the armature with the field of the poles, the armature will begin to rotate, its winding will rotate in a magnetic field and an EMF of self-induction is induced in it, the polarity of which is opposite to the polarity of the mains voltage. This EMF causes a weakening of the current in the armature, and its value is proportional to the speed of rotation of the armature, i.e. as the motor accelerates, the current will decrease and the starting rheostat can be output.
In other words, for a normally rotating motor, the main part of the input voltage is balanced by the EMF of self-induction. The armature current with the starting rheostat removed can be expressed by the equation:
To clarify the role of self-induction EMF in the conversion of electrical energy into mechanical energy in a DC motor, equation (19) is presented in the following form:
The equation of electrical equilibrium was obtained, according to which the network voltage U applied to the motor terminals is balanced by the sum of the self-induction EMF and the voltage drop across the armature resistance
Multiplying both sides of equation (20) by I i, we get:
. (21)
In this new equation (21), the left side I and U represents nothing more than the electrical power consumed by the motor from the network, and the last term on the right side is the power absorbed by the armature resistance (electrical losses in the armature). Obviously, the term represents electrical power being converted to another type of energy. Consequently, there is that part of the electrical power consumed from the network, which is converted into mechanical power (including mechanical losses).
Thus, the EMF of self-induction in a DC motor affects the conversion of electrical energy consumed from the network into mechanical energy. With a fixed armature = 0, there is no (useful) conversion (= 0), although the power consumed from the network is maximum. On the contrary, at nominal operation of the motor (0), the power consumed from the mains () decreases, and the converted power becomes different from zero (0).
To obtain the formula for the engine speed, we substitute the EMF value from relation (7) into equation (19). After the transformation, we get:
Considering that the voltage drop across the armature resistance is much less than the mains voltage U, it can be assumed that the motor rotation speed is practically directly proportional to the applied voltage U and inversely proportional to the magnetic flux F. Hence, it follows that the motor rotation speed can be controlled by changing the resistance of the armature circuit (at constant mains voltage) or a change in the magnetic flux. At first glance, it may seem strange that an increase in the magnetic flux of a motor decreases its rotation speed (and vice versa).
Indeed, if the magnetic flux is reduced at a steady-state current in the armature and the speed of rotation, then the EMF of self-induction will decrease and the electrical equilibrium (20) will be violated. To restore this equilibrium at a lower magnetic flux, the armature will rotate faster, since the EMF of self-induction is proportional to its rotation speed. The value of the engine torque can be expressed by the same formula as for the generator (13).
Consuming electrical energy from the network, the DC motor develops a torque, which, in steady state, is always balanced by the braking torque generated by the load, therefore, with an increase in the mechanical load on the motor shaft, the torque turns out to be less than the braking torque. The motor decreases the rotation speed, and this leads to a decrease in the EMF of self-induction and an increase in the consumed current. With a constant magnetic flux, the load current increases until the equality of the torque and braking torques is restored.
Depending on the method of connecting the field winding to the armature, motors, like DC generators, distinguish between independent, parallel, series and mixed excitation.
Parallel and independent excitation motor
The circuit for switching on the DC motor, parallel excitation current through the PR starting rheostat is shown in Figure 6-13. If the excitation winding of such a motor is switched on through an adjustment rheostat PB to the voltage of another source, then an independent excitation motor will be obtained.
The speed characteristic n = f (I I) of such motors at U = const and I in = const is shown in Figure 6-14, for an explanation of which we turn to the formula for the motor speed (22):
A change in rotation speed can occur due to a change in load and magnetic flux. But a change in the load current only slightly changes the internal voltage drop due to the small resistance of the armature circuit, which is less, the more powerful the engine. The load current ultimately reduces the motor speed only slightly. As for the magnetic flux Ф, due to the reaction of the armature with an increase in the load current, it decreases slightly, which leads to a slight increase in the rotation speed. Thus, the rotational speed of the parallel excitation motor changes very little.
The rotation speed of the independent excitation motor can be adjusted by changing the resistance of the armature circuit or by changing the magnetic flux. Excessive decrease in the field current and especially the accidental breakage of this circuit are very dangerous for parallel and independent excitation motors, since
how the armature current rises to an unacceptably high value. In the case of an insignificant load (or at idle speed), the speed increases so much that it becomes dangerous for the integrity of the engine (occurs emergency mode- engine "runaway").
Independent excitation motors are widely used as executive motors in automation circuits, and sometimes as a so-called electromagnetic brake. When a large starting torque or short-term overload is required; the possibility of their complete unloading is excluded. They turned out to be indispensable as traction motors in electric transport (electric locomotive, subway, tram, trolleybus), in lifting and transport installations (cranes, etc.) and for starting engines internal combustion(starters) in automobiles and aviation.
Series excitation motors
Sequential excitation DC motor circuit. The field winding of the motor is connected in series with the armature, so the magnetic flux of the motor changes with the load. Since the load current is large, the excitation winding has a small number of turns, this makes it possible to somewhat simplify the design of the starting rheostat in comparison with the rheostat for a parallel excitation motor.
The speed characteristic can be obtained on the basis of the speed equation, which for a sequential excitation motor has the form:
n = 
,
where is the resistance of the excitation winding.
From the consideration of the characteristics, it can be seen that the engine speed is highly dependent on the load. With an increase in the load, the voltage drop across the resistance of the windings increases with a simultaneous increase in the magnetic flux, which leads to a significant decrease in the rotation speed. This is a characteristic feature of a sequential excitation motor.
A significant reduction in load will lead to an increase in engine speed, which is dangerous for the engine. At loads less than 25% of the nominal (and especially at idle), when the load current and magnetic flux, due to the small number of turns in the field winding, is so weak that the rotation speed rapidly increases to unacceptably high values (the motor can "spread"). For this reason, these motors are used only when they are connected to rotating machinery directly or via a gear train. The use of a belt drive is unacceptable, since the belt may break or come off, and the engine will be completely unloaded in this case.
The control of the rotational speed of the sequential excitation motor can be carried out by changing the magnetic flux or by changing the supply voltage.
Dependence of the torque on the load current (mechanical characteristic) of the motor
sequential excitation can be obtained if the magnetic flux is expressed in terms of the load current in the torque formula. In the absence of magnetic saturation, the flux is proportional to the excitation current, and the latter for a given motor is the load current, i.e.
On the graph, this characteristic has the shape of a parabola. The square-law dependence of the torque on the load current is the second characteristic feature series excitation motor, thanks to which these motors easily withstand large short-term overloads and develop a large starting torque.
The engine performance is shown in Figure 6-17.
From the consideration of all characteristics, it follows that sequential excitation motors can be adopted in cases where a large starting torque or short-term overloads are required; the possibility of their complete unloading is excluded. They turned out to be indispensable as traction motors in electric transport (electric locomotive, subway, tram, trolleybus), in lifting and transport installations (cranes, etc.) and for starting internal combustion engines (starters) in automobiles and aviation.
Economical regulation of the speed of rotation within a wide range is carried out in the case of simultaneous operation of several motors by means of various combinations of switching on the motors and rheostats. For example, at low speeds, they are switched on in series, and at high speeds, in parallel. The required switching is carried out by the operator (driver) by turning the switch knob.
Mixed excitation engine
The switching circuit of a mixed-excitation DC motor is shown in Figure 6-18. On each pole of such a motor there are windings - parallel and series. They can be turned on so that the magnetic fluxes are added (consonant) or subtracted (counter turn on).
The equations of rotation speed and torque for them are expressed as follows:
and M = c1 i (Φ pr ± Φ ps),
where the plus sign refers to the consonant switching on of the excitation windings, the minus sign to the opposite one. Depending on the ratio of the magnetic fluxes of both windings, in terms of properties, the motor approaches the motors of parallel or series excitation. As a rule, in mixed excitation motors, the series winding is the main (working) one, and the parallel one is the auxiliary one. Due to the magnetic flux of the parallel winding, the rotation speed of such a motor cannot increase infinitely at low loads (or at idle), i.e. the engine will not "run".
Engines with a consensus connection have found wide application in cases where a large starting torque and speed change are required under variable loads (including low loads and idling). Motors with opposite connection are used to obtain a constant speed with a varying load.
Figure 6-19 shows, for comparison, the load characteristics of motors with different ways excitement.
AC brushed motors
The simultaneous change in the current in the armature and the field winding of the DC motor does not change its direction of rotation. This property is used in AC collector motors, where the current with the mains frequency simultaneously changes its direction in both windings.
The design of AC brushed motors is much more complex than that of DC motors. The entire magnetic system is recruited from separate sheets of electrical steel isolated from each other in order to avoid its strong heating from such frequent magnetization reversal. To reduce the reactance of the motor, which degrades the cosnetwork, the frame is supplied with a compensation winding, located evenly around the circumference of the stator and connected in series with the armature. To improve the compensation of the EMF of self-induction in the armature sections, the stator is made implicitly. To obtain satisfactory switching, in which the short-circuited section turns out to be similar to the short-circuited winding of a transformer, the number of turns in the sections is reduced by increasing the number of sections, and the current is limited by switching on between the sections and the collector of special resistors. The presence of a large number of sections and plates of the collector greatly increases the size of the collector, which is an external distinguishing feature of AC collector motors from DC motors.
Almost all AC brushed motors are series excitation. Motors of parallel excitation, due to the high inductance of the excitation winding (large phase shift between the current in the armature and the flux), have a very small torque, therefore, in practice, such motors are not used.
Sometimes there are low-power so-called universal motors which operate on both direct and alternating current. In these motors, the winding is designed for direct current operation, and part of it (branch) for alternating current operation, since the resistance of the same winding is less for direct current than for alternating current.
Due to the complexity of the design and high cost, high-power brushed motors are used in rare cases where it is economically justified, for example, to drive one mechanism with wide speed control limits. Sometimes there are rotor-side three-phase collector motors in which moving the brushes on the collector gives wide speed control, but this motor is very expensive.
Low-power (up to 200 W) universal collector motors of series excitation have become widespread. They are used for the needs of a household electric drive (sewing machines, vacuum cleaners, for small electric drills, fans, etc.)
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CONSTRUCTION OF DC MACHINES
In fig. 3 given a drawing modern car direct current with longitudinal and cross sections. The stator consists of a frame 1 and the main 2 and additional 3 poles. The frame of machines of relatively low power is made from pieces of solid-drawn steel pipes, while those of powerful machines are welded from heavy-plate steel. To fix the machine on a foundation or an actuator, legs are welded to the bottom of the bed 4, and for transportation, eye bolts are screwed into the bed 5.
An excitation winding is placed on the cores of the main poles 6, which is made in the form of coils of copper insulated conductors of round or rectangular cross-section. The coils are insulated with tape and, after impregnation and drying, are pushed onto the pole core and secured with steel spring frames. Sometimes the coil is divided into two parts to increase the cooling surface. The pole with the coil on it is attached to the frame with bolts.
Additional poles are placed between the main poles and together with the coils 14 their field windings are also bolted to the frame.
The armature consists of a core 7, winding 8 and collector 9.
The core is assembled from individual sheets with a thickness of 0.5 mm, which are stamped from electrical steel. In the sheets of the anchor, grooves are cut into which the armature winding is laid. Laying the winding in the grooves ensures its reliable fastening to the rotating armature and reduces the air gap between the pole and the armature. The winding in the groove is fixed with a wedge made of fiberglass or with bandages located in the annular grooves of the armature core 13. Outside the grooves, in the frontal parts, the winding is fixed with bandages 12 made of wire or glass tape.
The assembled armature core is pressed between two thrust washers and fixed to the shaft with a sleeve or a spring split ring.
The frame, the cores of the pole and the armature are sections of the magnetic circuit, along which the magnetic flux created by the field windings is closed. To reduce the magnetic resistance along the path of this flow, all of these sections are made of steel. For the same purpose, the air gap between the armature and the poles is tried to be made smaller. Usually it is fractions of a millimeter for small cars and a few millimeters for more powerful ones.
When the armature rotates, the steel of its core will be magnetized, it will induce alternating currents- vortex, which will cause losses. To reduce eddy current losses, the core, as already mentioned, is assembled from separate sheets isolated from each other. For insulation, the sheets are varnished after stamping. Due to the toothed structure of the armature, flow pulsation will occur in the gap, as a result of which eddy currents will also be induced in the pole piece, to reduce which the tip and the entire pole are assembled from separate sheets.
Stationary brushes slide along the collector, which are placed in the brush holders. Brush holders are attached to cylindrical or prismatic pins 10, which, in turn, are fixed on the traverse 11. The fingers are made of getinax or steel, molded with plastic at the point of its articulation with the traverse. Usually the number of fingers is chosen equal to the number of poles.
The armature rotates in bearings 15, which are housed in end shields called end shields 16.
Let's consider some structural elements of the machine in more detail.
Major poles(Fig. 4) assembled from stamped sheets of electrical steel with a thickness of 1 mm. The sheets are pressed into a bag and fastened with steel rivets, the number of which is not less than four. The extreme sheets of the pole are made of thicker steel (4 - 10 mm) in order to avoid loosening of the sheets.
In order to obtain the required character of the distribution of the magnetic field in the air gap, the pole is terminated with a pole piece of a certain shape.
The air gap between the poles and the armature is either made the same over the entire width of the pole piece, or under the edges of the tip, due to its bevel, it is made larger. Sometimes an eccentric air gap is made, in which the centers of the radii of the armature and the pole tip do not coincide. In this case, the gap gradually increases from the middle of the pole to its edge (Fig. 5).
The pole has a threaded hole into which a bolt is screwed, with which the pole is attached to the bed. For more reliable fastening of the pole in large machines and machines operating in shaking conditions, the bolts are screwed into a special rod inserted into the pole (see Fig. 4).
Anchor core can consist of one or more packages. When the length of the core is less than 25 cm, it is made from one package (Fig. 6) and with a longer length - from several (Fig. 7). Ventilation channels are created between the packages with the help of special spacers, designed for better cooling of the anchor.
The shape of the grooves cut in the armature core is chosen oval semi-closed for machines of low power and rectangular open for machines of medium and high power (Fig. 8). Insulation (groove insulation) is placed between the walls of the groove and the winding conductors. In fig. 8 shows the fastening of the winding in the groove using a wedge.
Collector consists of a large number of plates electrically isolated from each other, which are stamped from shaped copper (Fig. 9). Insulation is carried out with thin gaskets, cut out of mikanite (pressed mica), which are placed between the copper plates. The gaskets are in the form of plates. The set of collector plates with gaskets must be firmly fixed and must have a strictly cylindrical shape.
According to the method of fixing the plates, there is a wide variety of collector designs, two of which are shown in Fig. 10. In fig. 10 and the manifold plates are clamped between the body and the pressure flange. The body and the pressure flange are made of steel, and micanite cuffs are put on them for insulation. In fig. 10, b shows the fastening of the plates using plastic. Currently, for machines of small and medium power, collectors with a plastic body are most commonly used.
The assembled collector is pushed onto the shaft and secured against turning with a key. Conductors from the sections that make up the armature winding are connected to each collector plate. To connect the conductors at the collector plates from the side facing the armature, protrusions, called cockerels, are made, in which the slots are milled. The conductors of the windings are laid in these slots and then sealed.
