examine the device operating principle, characteristics of the DC motor;
acquire practical skills in starting, operating and stopping a DC electric motor;
experimentally explore theoretical information about the characteristics of the DC motor.
Basic theoretical provisions
A DC electric motor is an electrical machine designed to convert electrical energy into mechanical energy.
The device of the DC motor is no different from the DC generator. This circumstance makes DC electrical machines reversible, that is, it allows them to be used both in generator and motor modes. Structurally, a DC motor has fixed and movable elements, which are shown in Fig. 1.
Fixed part - stator 1 (frame) is made of cast steel, consists of main 2 and additional 3 poles with excitation windings 4 and 5 and a brush traverse with brushes. The stator performs the function of a magnetic circuit. With the help of the main poles, a magnetic field that is constant in time and immobile in space is created. Additional poles are placed between the main poles and improve switching conditions.
The movable part of the DC motor is the rotor 6 (armature), which is placed on a rotating shaft. The armature also plays the role of a magnetic circuit. It is made of thin, electrically isolated from each other, thin sheets of electrical steel with a high silicon content, which reduces power losses. Windings 7 are pressed into the armature grooves, the leads of which are connected to collector plates 8, placed on the same motor shaft (see Fig. 1).
Consider the principle of operation of a DC motor. Connecting a constant voltage to the terminals of an electrical machine causes the simultaneous occurrence in the excitation (stator) windings and in the current armature windings (Fig. 2). As a result of the interaction of the armature current with the magnetic flux created by the field winding, a force arises in the stator f, determined by Ampère's law . The direction of this force is determined by the rule of the left hand (Fig. 2), according to which it is oriented perpendicular to both the current i(in the armature winding), and to the magnetic induction vector IN(created by the excitation winding). As a result, a pair of forces acts on the rotor (Fig. 2). The force acts on the upper part of the rotor to the right, on the lower part - to the left. This pair of forces creates a torque, under the action of which the armature is driven into rotation. The magnitude of the emerging electromagnetic moment turns out to be equal to
M = c m I I F,
Where With m - coefficient depending on the design of the armature winding and the number of poles of the electric motor; F- magnetic flux of one pair of main poles of the electric motor; I I - motor armature current. As follows from Fig. 2, the rotation of the armature windings is accompanied by a simultaneous change in polarity on the collector plates. The direction of the current in the turns of the armature winding changes to the opposite, but the magnetic flux of the excitation windings retains the same direction, which causes the direction of forces to remain unchanged. f, and hence the torque.
The rotation of the armature in a magnetic field leads to the appearance of an emf in its winding, the direction of which is already determined by the right hand rule. As a result, for the one shown in Fig. 2 configurations of fields and forces in the armature winding, an induction current will occur, directed opposite to the main current. Therefore, the emerging EMF is called counter-EMF. Its value is
E = With e nФ,
Where n- frequency of rotation of the armature of the electric motor; With e is a coefficient depending on the structural elements of the machine. This EMF degrades the performance of the motor.
The current in the armature creates a magnetic field that affects the magnetic field of the main poles (stator), which is called the armature reaction. In the idle mode of the machine, the magnetic field is created only by the main poles. This field is symmetrical about the axes of these poles and coaxial with them. When connected to a load motor, due to the current in the armature winding, a magnetic field is created - the armature field. The axis of this field will be perpendicular to the axis of the main poles. Since the current distribution in the armature conductors remains unchanged during the rotation of the armature, the armature field remains stationary in space. The addition of this field to the field of the main poles gives the resulting field, which unfolds through an angle against the direction of armature rotation. As a result, the torque decreases, since part of the conductors enters the zone of the pole of opposite polarity and creates a braking torque. In this case, the brushes spark and the collector burns, a longitudinal demagnetizing field arises.
In order to reduce the influence of the armature reaction on the operation of the machine, additional poles are built into it. The windings of such poles are connected in series with the main winding of the armature, but a change in the direction of winding in them causes the appearance magnetic field directed against the magnetic field of the armature.
To change the direction of rotation of a DC motor, it is necessary to change the polarity of the voltage supplied to the armature or field winding.
Depending on the method of switching on the excitation winding, DC motors are distinguished with parallel, series and mixed excitation.
For motors with parallel excitation, the winding is designed for the full voltage of the supply network and is connected in parallel with the armature circuit (Fig. 3).
A motor with series excitation has a field winding that is connected in series with the armature, so this winding is designed for the full armature current (Fig. 4).
Motors with mixed excitation have two windings, one is connected in parallel, the other is connected in series with an armature (Fig. 5).
Rice. 3 Fig. 4
When starting DC motors (regardless of the method of excitation) by direct connection to the supply network, significant starting currents occur, which can lead to their failure. This occurs as a result of the release of a significant amount of heat in the armature winding and the subsequent violation of its insulation. Therefore, the start-up of DC motors is carried out by special starting devices. In most cases, for these purposes, the simplest starting device is used - a starting rheostat. The process of starting a DC motor with a starting rheostat is shown using the example of a DC motor with parallel excitation.
Based on the equation compiled in accordance with the second Kirchhoff law for the left side of the electrical circuit (see Fig. 3), the starting rheostat is completely removed ( R start = 0), armature current
,
Where U- voltage supplied to the electric motor; R i is the resistance of the armature winding.
At the initial moment of starting the electric motor, the armature speed n= 0, therefore, the counter-electromotive force induced in the armature winding, in accordance with the expression obtained earlier, will also be equal to zero ( E= 0).
Armature winding resistance R i is quite small. In order to limit the unacceptably large current in the armature circuit during start-up, a starting rheostat is switched on in series with the armature, regardless of the method of excitation of the engine (starting resistance R start). In this case, the armature starting current
.
Starting rheostat resistance R the start is calculated for operation only for the start time and is selected in such a way that the starting current of the motor armature does not exceed the allowable value ( I i, start 2 I i, nom). As the motor accelerates, the EMF induced in the armature winding due to an increase in its rotation frequency n increases ( E=With e nФ). As a result of this, the armature current, ceteris paribus, decreases. In this case, the resistance of the starting rheostat R start as the motor armature accelerates, it must be gradually reduced. After the end of the acceleration of the motor to the nominal value of the armature speed, the EMF increases so much that the starting resistance can be reduced to zero, without the danger of a significant increase in the armature current.
So the starting resistance R starting in the armature circuit is necessary only at start-up. During normal operation of the electric motor, it must be turned off, firstly, because it is designed for short-term operation during start-up, and secondly, if there is a starting resistance, thermal power losses equal to R start I 2 I, significantly reducing the efficiency of the electric motor.
.
Taking into account the expression for the EMF ( E=With e nФ), writing the resulting formula for the rotation frequency, we obtain the equation for the frequency (speed) characteristic of the electric motor n(I I):
.
It follows from it that in the absence of a load on the shaft and the armature current I I = 0 rotational speed of the electric motor at a given value of the supply voltage
.
Motor speed n 0 is the ideal idle speed. In addition to the parameters of the electric motor, it also depends on the value of the input voltage and magnetic flux. With a decrease in the magnetic flux, other things being equal, the rotational speed of an ideal idle speed increases. Therefore, in the event of an open circuit of the excitation winding, when the excitation current becomes zero ( I c = 0), the magnetic flux of the motor is reduced to a value equal to the value of the residual magnetic flux F rest. In this case, the engine “goes into overdrive”, developing a speed much higher than the nominal one, which poses a certain danger both for the engine and for the maintenance personnel.
Frequency (speed) characteristic of a DC motor with parallel excitation n(I i) at a constant value of the magnetic flux F=const and a constant value of the input voltage U = const looks like a straight line (Fig. 6).
Expressing in the equations of frequency characteristics the armature current through the electromagnetic torque of the motor M =With m I I F, we obtain the equation of the mechanical characteristic, i.e., the dependences n(M) at U = const for motors with parallel excitation:
.
Neglecting the influence of the armature reaction in the process of changing the load, it is possible to accept the electromagnetic torque of the motor as proportional to the armature current. Therefore, the mechanical characteristics of DC motors have the same form as the corresponding frequency characteristics. The shunt motor has a rigid mechanical characteristic (Fig. 7). From this characteristic it can be seen that its rotational speed decreases slightly with increasing load torque, since the excitation current when the excitation winding is connected in parallel and, accordingly, the magnetic flux of the motor remain practically unchanged, and the resistance of the armature circuit is relatively small.
The performance characteristics of a parallel-excited DC motor are shown in fig. 8. From these characteristics it can be seen that the rotational speed n motors with parallel excitation with increasing load decreases somewhat. The dependence of the useful moment on the motor shaft on the power R 2 is an almost straight line, since the moment of this motor is proportional to the load on the shaft: M=kP 2 / n. The curvature of this dependence is explained by a slight decrease in the rotational speed with increasing load.
At R 2 = 0 the current consumed by the electric motor is equal to the no-load current. With an increase in power, the armature current increases approximately according to the same dependence as the load torque on the shaft, since under the condition F=const the armature current is proportional to the load torque. The efficiency of an electric motor is defined as the ratio of the useful power on the shaft to the power consumed from the network:
,
Where R 2 - useful shaft power; R 1 =UI- power consumed by the electric motor from the supply network; R ey = I 2 i R i - electrical power losses in the armature circuit, R ev = UI in, = I 2 in R V - electrical power losses in the excitation circuit; R fur - mechanical power loss; R m - power losses due to hysteresis and eddy currents.
With a constant supply voltage and a constant magnetic flux, in the process of changing the resistance value of the armature circuit, a family of mechanical characteristics can be obtained, for example, for an electric motor with parallel excitation (Fig. 9).
The advantage of the considered control method lies in its relative simplicity and the ability to obtain a smooth change in the rotational speed over a wide range (from zero to the nominal value of the frequency n nom). The disadvantages of this method include the fact that there are significant power losses in the additional resistance, which increase with decreasing speed, as well as the need to use additional control equipment. In addition, this method does not allow you to adjust the speed of the motor up from its nominal value.
A change in the rotational speed of a DC motor can also be achieved as a result of changing the value of the excitation magnetic flux. When changing the magnetic flux in accordance with the frequency response equation for DC motors with parallel excitation at a constant value of the supply voltage and a constant value of the resistance of the armature circuit, a family of mechanical characteristics can be obtained, shown in fig. 10.
The disadvantages of this method should also include a relatively small range of regulation due to the presence of restrictions on the mechanical strength and switching of the electric motor. The advantage of this control method is its simplicity. For engines with parallel excitation, this is achieved by changing the resistance of the regulating rheostat R R in the excitation circuit.
For DC motors with series excitation, a change in the magnetic flux is achieved by shunting the excitation winding with a resistance of an appropriate value, or by short-circuiting a certain number of turns of the excitation winding.
Widespread use, especially in electric drives built according to the generator-motor system, has received a method of speed control by changing the voltage at the motor armature clamps. With a constant magnetic flux and resistance of the armature circuit, as a result of a change in the armature voltage, a family of frequency characteristics can be obtained.
With a change in the input voltage, the ideal idle speed n 0 in accordance with the expression given earlier, varies in proportion to the voltage. Since the resistance of the armature circuit remains unchanged, the stiffness of the family of mechanical characteristics does not differ from the stiffness of the natural mechanical characteristic at U=U nom.
The advantage of the considered method of regulation is a wide range of speed changes without increasing power losses. The disadvantages of this method include the fact that this requires a source of regulated supply voltage, and this leads to increase in weight, dimensions and cost of the installation.
Electric motors are devices that convert electrical energy into mechanical energy. The principle of their operation is based on the phenomenon of electromagnetic induction.
However, the methods of interaction of magnetic fields that make the motor rotor rotate differ significantly depending on the type of supply voltage - AC or DC.
The principle of operation of a DC electric motor is based on the effect of repulsion of the same poles of permanent magnets and attraction of opposite ones. The priority of its invention belongs to the Russian engineer B. S. Jacobi. The first industrial model of a DC motor was created in 1838. Since then, its design has not undergone major changes.
In low power DC motors, one of the magnets is physically present. It is attached directly to the body of the machine. The second is created in the armature winding after a DC source is connected to it. For this, a special device is used - a collector-brush assembly. The collector itself is a conductive ring attached to the motor shaft. The ends of the armature winding are connected to it.
In order for a torque to occur, it is necessary to continuously swap the poles of the armature's permanent magnet. This should happen at the moment the pole crosses the so-called magnetic neutral. Structurally, this problem is solved by dividing the collector ring into sectors separated by dielectric plates. The ends of the armature windings are connected to them in turn.
To connect the collector to the mains, so-called brushes are used - graphite rods with high electrical conductivity and a low coefficient of sliding friction.The armature windings are not connected to the mains, but are connected to the starting rheostat by means of a collector-brush assembly. The process of turning on such an engine consists of connecting to the mains and gradually reducing the active resistance in the armature circuit to zero. The electric motor turns on smoothly and without overloads.
Features of the use of asynchronous motors in a single-phase circuit
Despite the fact that the rotating magnetic field of the stator is easiest to obtain from a three-phase voltage, the principle of operation of an asynchronous electric motor allows it to work from a single-phase, household network, if some changes are made to their design.
To do this, the stator must have two windings, one of which is the "starting". The current in it is shifted in phase by 90 ° due to the inclusion of a reactive load in the circuit. Most often for this
The almost complete synchronism of the magnetic fields allows the engine to gain momentum even with significant loads on the shaft, which is required for the operation of drills, rotary hammers, vacuum cleaners, grinders or polishers.
If an adjustable one is included in the supply circuit of such an engine, then its rotational speed can be smoothly changed. And here is the direction, when powered by the circuit alternating current, can never be changed.
Such electric motors are capable of developing very high speeds, are compact and have a large torque. However, the presence of a collector-brush assembly reduces their motor resource - graphite brushes wear out quite quickly at high speeds, especially if the collector has mechanical damage.Electric motors have the highest efficiency (more than 80%) of all devices created by man. Their invention at the end of the 19th century can be considered a qualitative leap in civilization, because without them it is impossible to imagine life. modern society based on high technologies, and something more effective has not yet been invented.
Synchronous principle of operation of the electric motor on video
1. The purpose of the work: To study the features of starting, mechanical characteristics and methods for controlling the speed of a DC motor with mixed excitation.
Adania.
2.1. To independent work:
To study the design features, circuits for switching on DC motors;
To study the method of obtaining the mechanical characteristics of a DC motor;
Familiarize yourself with the features of starting and controlling the speed of a DC motor;
draw circuit diagrams to measure the resistance of the armature circuit and excitation windings (Fig. 6.4) and test the motor (Fig. 6.2);
Using fig. 6.2 and 6.3 draw up a wiring diagram;
Draw the forms of tables 6.1 ... 6.4;
Prepare oral answers to control questions.
2.2. to work in the laboratory:
Familiarize yourself with the laboratory setup;
Record in table 6.1. passport data of the engine;
Measure the resistance of the armature circuit and field windings. Record the data in table 6.1;
Assemble the circuit and conduct a study of the engine, write down the data in tables 6.2, 6.3, 6.4;
Build a natural mechanical characteristic n=f(M) and speed characteristics n=f(I B) and n=f(U);
Draw conclusions from the results of the study.
General information.
DC motors, in contrast to AC motors (primarily asynchronous ones), have a large starting torque ratio and overload capacity, and provide smooth control of the working machine speed. Therefore, they are used to drive machines and mechanisms with difficult starting conditions (for example, as starters in internal combustion engines), as well as, if necessary, to control the rotational speed over a wide range (machine tool feed mechanisms, running-brake stands, electrified vehicles).
Structurally, the engine consists of a fixed unit (inductor) and a rotating unit (armature). Excitation windings are located on the magnetic circuit of the inductor. There are two of them in a mixed excitation motor: parallel with pins Sh 1 and Sh2 and serial with pins C1 and C2 (Fig. 6.2). The resistance of the parallel winding R ovsh is, depending on the engine power, from several tens to hundreds of ohms. It is made with small wire a large number turns. The series winding has a low resistance R obc (usually from a few ohms to fractions of an ohm), because consists of a small number of turns of wire of a large cross section. The inductor serves to create a magnetic excitation flux when its windings are powered by direct current.
The armature winding is placed in the grooves of the magnetic circuit and brought to the collector. With the help of brushes, its conclusions I I and I 2 are connected to a direct current source. The resistance of the armature winding R I is small (Ohms or fractions of an Ohm).
The torque M of a DC motor is created by the interaction of the armature current Ia with the excitation magnetic flux Ф:
M \u003d K × Ia × F, (6.1)
where K is a constant coefficient depending on the design of the engine.
When the armature rotates, its winding crosses the excitation magnetic flux and an EMF E is induced in it, proportional to the rotation frequency n:
E \u003d C × n × F, (6.2)
where C is a constant factor depending on the design of the engine.
Armature current:
I I \u003d (U - E) / (R I + R OBC) \u003d (U - C × n × F) / (R I + R OBC), (6.3)
Solving together expressions 6.1 and 6.3 with respect to n, they find an analytical expression for the mechanical characteristics of the engine n = F (M). Her graphic image shown in Figure 6.1.
Rice. 6.1. Mechanical characteristics of a mixed-excitation DC motor
Point A corresponds to the engine idling with a speed of rotation n o. With an increase in the mechanical load, the rotational speed decreases, and the torque increases, reaching the nominal value M H at point B. In the BC section, the engine is operating with overload. The current Iya exceeds the nominal value, which leads to a rapid heating of the armature windings and the OBC, and the sparking on the collector increases. The maximum moment M max (point C) is limited by the operating conditions of the collector and the mechanical strength of the engine.
Continuing the mechanical characteristic until it intersects at point D "with the torque axis, we would get the value of the starting torque at direct connection motor to the network. EMF E is equal to zero and the current in the armature circuit in accordance with formula 6.3 increases sharply.
To reduce the starting current, a starting rheostat Rx (Fig. 6.2) with resistance is connected in series to the armature circuit:
Rx = U H / (1.3...2.5) ×I Ya.N. - (R I - R obc), (6.4)
where U h - rated voltage of the network;
I Ya.N. - rated armature current.
Armature current reduction to (1.3...2.5)×I Ya.N. provides sufficient initial starting torque Mn (point D). As the engine accelerates, the resistance Rx is reduced to zero, maintaining an approximately constant value of Mp (SD section).
The rheostat R B in the circuit of the parallel excitation winding (Fig. 6.2) allows you to adjust the magnitude of the magnetic flux Ф (formula 6.1). Before starting the engine, it is completely removed to obtain the required starting torque at a minimum armature current.
Using formula 6.3, we determine the engine speed
n = (U - I I (R I + R obc + Rx)) / (С Ф), (6.5)
in which R I, R obc and C are constants, and U, I I and F can be changed. From this follows three possible ways engine speed control:
Change in the magnitude of the input voltage;
By changing the value of the armature current using the adjusting rheostat Rx, which, unlike the starting one, is calculated for continuous operation;
By changing the magnitude of the excitation magnetic flux F, which is proportional to the current in the OVSH and OVSS windings. In a parallel winding, it can be adjusted with a rheostat R b. The resistance R b is taken depending on the required speed control limits R B =(2...5) R obsh.
The rating plate of the motor indicates the rated speed, which corresponds to the rated power on the motor shaft at the rated mains voltage and the output resistances of the rheostats R X and R B .
Laboratory work № 9
Subject. The study of the DC motor.
Goal of the work:
to study the device and the principle of operation of the electric motor.Equipment:
electric motor model, current source, rheostat, key, ammeter, connecting wires, drawings, presentation.
TASKS:
1
. Study the device and the principle of operation of the electric motor, using a presentation, drawings and a model.2
. Connect the motor to a power source and observe its operation. If the engine does not work, find the cause, try to fix the problem.3 . Indicate the two main elements in the device of the electric motor.
4 . On what physical phenomenon is the action of an electric motor based?
5 . Change the direction of rotation of the armature. Write down what needs to be done.
6. Gather electrical circuit by connecting in series an electric motor, a rheostat, a current source, an ammeter and a key. Change the current and observe the operation of the electric motor. Does the rotation speed of the armature change? Write down the conclusion about the dependence of the force acting on the side of the magnetic field on the coil, on the current strength in the coil.
7 . Electric motors can be of any power, because:
A) you can change the current strength in the armature winding;
B) you can change the magnetic field of the inductor.
Specify the correct answer:
1) only A is true; 2) only B is true; 3) both A and B are true; 4) both A and B are wrong.
8 . List the advantages of an electric motor over a heat engine.
Laboratory works→ number 10
The study of the DC electric motor (on the model).
Goal of the work: Familiarize yourself with the main parts of the DC electric motor on the model of this motor.
This is perhaps the most uncomplicated work for the 8th grade course. You just need to connect the motor model to a current source, see how it works, and remember the names of the main parts of the electric motor (armature, inductor, brushes, half rings, winding, shaft).
The electric motor offered to you by the teacher may be similar to the one shown in the figure, or it may have a different look, since there are many options for school electric motors. This is not of fundamental importance, since the teacher will probably tell in detail and show how to handle the model.
We list the main reasons that a properly connected electric motor does not work. Open circuit, lack of contact between the brushes and half rings, damage to the armature winding. If in the first two cases you are quite capable of coping on your own, in the event of a winding break, you need to contact the teacher. Before turning on the engine, make sure that its armature can rotate freely and nothing interferes with it, otherwise, when turned on, the electric motor will emit a characteristic buzz, but will not rotate.
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