Electric motors are devices in which electrical energy is converted into mechanical energy. The principle of their operation is based on the phenomenon of electromagnetic induction.
However, the way the magnetic fields interact, causing the motor rotor to rotate, differ significantly depending on the type of supply voltage - alternating or direct.
The operating principle of an electric motor is based on DC lies the effect of repulsion of like poles of permanent magnets and attraction of unlike poles. 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 fundamental changes.
In low-power DC motors, one of the magnets is physically existing. It is attached directly to the machine body. The second is created in the armature winding after connecting a direct current source to it. For this purpose, a special device is used - a commutator-brush unit. 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 torque to occur, the poles of the permanent magnet of the armature must be continuously swapped. 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 alternately.
To connect the collector to the power supply, so-called brushes are used - graphite rods with high electrical conductivity and low sliding friction coefficient.The armature windings are not connected to the supply network, but are connected to the starting rheostat through a commutator-brush assembly. The process of turning on such a motor consists of connecting to the supply network and gradually reducing the active resistance in the armature circuit to zero. The electric motor turns on smoothly and without overload.
Features of using asynchronous motors in a single-phase circuit
Although the rotating magnetic field of the stator is most easily obtained from three-phase voltage, operating principle an asynchronous electric motor allows it to operate 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” winding. 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
Almost complete synchronism of magnetic fields allows the engine to gain speed even with significant loads on the shaft, which is what is required for the operation of drills, rotary hammers, vacuum cleaners, grinders or floor polishers.
If an adjustable one is included in the supply circuit of such an engine, then its rotation frequency can be smoothly changed. But the direction, when powered from an alternating current circuit, can never be changed.
Such electric motors are capable of developing very high speeds, are compact and have greater torque. However, the presence of a commutator-brush assembly reduces their service life - graphite brushes wear out quite quickly at high speeds, especially if the commutator 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 it is impossible to imagine life without them modern society, based on high technology, and something more effective has not yet been invented.
Synchronous principle of operation of an electric motor on video
study the device, principle of operation, characteristics of a DC electric motor;
acquire practical skills in starting, operating and stopping a DC electric motor;
Experimentally investigate theoretical information about the characteristics of a DC electric motor.
Basic theoretical principles
A DC electric motor is an electrical machine designed to convert electrical energy into mechanical energy.
The design of a DC electric motor is no different from a DC generator. This circumstance makes DC electric machines reversible, that is, allows them to be used in both generator and motor modes. Structurally, a DC electric motor has fixed and moving elements, which are shown in Fig. 1.
The fixed part - stator 1 (frame) is made of cast steel, consists of 2 main and 3 additional poles with 4 field windings 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 motionless in space is created. Additional poles are placed between the main poles and improve switching conditions.
The moving part of the DC electric 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 insulated from each other, thin sheets of electrical steel with a high silicon content, which reduces power losses. Windings 7 are pressed into the grooves of the armature, the terminals of which are connected to the collector plates 8, located on the same electric motor shaft (see Fig. 1).
Let's consider the principle of operation of a DC electric motor. Connecting a direct voltage to the terminals of an electric machine causes the simultaneous occurrence of current in the field (stator) windings and in the 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 Ampere's law . The direction of this force is determined by the left hand rule (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 influence of which the armature is rotated. The magnitude of the resulting 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 field windings retains the same direction, which determines the constant direction of the forces f, and therefore the torque.
Rotation of the armature in a magnetic field leads to the appearance of EMF winding, the direction of which is 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 induced current will arise, directed opposite to the main current. Therefore, the resulting EMF is called back EMF. Its value is equal
E = With e nФ,
Where n- rotation speed of the electric motor armature; With e is a coefficient depending on the structural elements of the machine. This EMF degrades the performance of the electric 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. When the machine is idling, 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 a load is connected to the motor, a magnetic field is created in the armature winding due to the current - the armature field. The axis of this field will be perpendicular to the axis of the main poles. Since when the armature rotates, the distribution of current in the armature conductors remains unchanged, the armature field remains motionless in space. The addition of this field with the field of the main poles gives the resulting field, which rotates through the angle against the direction of rotation of the armature. As a result, the torque decreases, since some of the conductors enter the zone of the pole of opposite polarity and create a braking torque. In this case, the brushes spark and the commutator burns, and 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 of 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 electric motors with parallel, series and mixed excitation are distinguished.
For motors with parallel excitation, the winding is designed for the full voltage of the supply network and is connected in parallel to the armature circuit (Fig. 3).
A series-wound motor has a field winding that is connected in series with the armature, so this winding is designed to carry the full armature current (Fig. 4).
Motors with mixed excitation have two windings, one is connected in parallel, the other in series with the armature (Fig. 5).
Rice. 3 Fig. 4
When starting DC electric motors (regardless of the method of excitation) by direct connection to the supply network, significant starting currents arise, 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 breakdown of its insulation. Therefore, DC motors are started using special starting devices. In most cases, the simplest starting device is used for these purposes - 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 Kirchhoff’s second law for the left side of the electrical circuit (see Fig. 3), the starting rheostat is completely withdrawn ( 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 rotation speed n= 0, therefore the counter-electromotive force induced in the armature winding, in accordance with the previously obtained expression, will also be equal to zero ( E= 0).
Armature winding resistance R I is a rather small quantity. In order to limit the possible unacceptably high current in the armature circuit during starting, a starting rheostat (starting resistance) is switched on in series with the armature, regardless of the method of excitation of the engine R start). In this case, the starting armature current
.
Starting rheostat resistance R The start is calculated to operate only for the start time and is selected in such a way that the starting current of the armature of the electric motor does not exceed the permissible value ( I i,start 2 I I,nom). As the electric 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, other things being equal, decreases. In this case, the resistance of the starting rheostat R start As the motor armature accelerates, it must be gradually reduced. After the engine has accelerated to the rated speed of the armature, 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.
Thus, the starting resistance R starting in the armature circuit is only necessary 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 will occur in it equal to R start I 2nd, significantly reducing the efficiency of the electric motor.
.
Taking into account the expression for EMF ( E=With e nФ), writing the resulting formula relative to the rotation speed, we obtain the equation for the frequency (speed) characteristics of the electric motor n(I I):
.
It follows from it that in the absence of load on the shaft and armature current I I = 0 motor rotation speed at given value 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 magnetic flux, other things being equal, the ideal idle speed increases. Therefore, in the event of a break in the excitation winding circuit, when the excitation current becomes zero ( Iв = 0), the motor magnetic flux is reduced to a value equal to the value of the residual magnetic flux F ost. In this case, the engine “goes into overdrive”, developing a rotation speed much higher than the nominal one, which poses a certain danger for both the engine and the operating personnel.
Frequency (speed) characteristic of a DC electric motor with parallel excitation n(I i) at a constant magnetic flux value F=const and constant value of the supplied voltage U = const looks like a straight line (Fig. 6).
Expressing the armature current in the equations of frequency characteristics through the electromagnetic torque of the motor M =With m I I F, we obtain the equation of the mechanical characteristic, i.e., the dependence n(M) at U = const for motors with parallel excitation:
.
Neglecting the influence of the armature reaction during the load change, we can assume that the electromagnetic torque of the motor is proportional to the armature current. Therefore, the mechanical characteristics of DC motors have the same form as the corresponding frequency characteristics. An electric motor with parallel excitation has a rigid mechanical characteristic (Fig. 7). From this characteristic it is clear that its rotation frequency decreases slightly with increasing load torque, since the excitation current when the field winding is connected in parallel and, accordingly, the magnetic flux of the motor remains 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 is clear that the rotation speed n of electric motors with parallel excitation decreases slightly with increasing load. Dependence of useful torque on the motor shaft on power R 2 is an almost straight line, since the torque of this motor is proportional to the load on the shaft: M=kР 2 / n. The curvature of this dependence is explained by a slight decrease in rotation speed with increasing load.
At R 2 = 0 the current consumed by the electric motor is equal to the no-load current. With increasing 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 eya = 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 losses; 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 rotation speed over a wide range (from zero to the nominal frequency value n nom). The disadvantages of this method include the fact that there are significant power losses in the additional resistance, which increase with decreasing rotation speed, as well as the need to use additional control equipment. In addition, this method does not allow adjusting the rotation speed of the electric motor upward from its nominal value.
A change in the rotation speed of a DC electric motor can also be achieved as a result of changing the value of the excitation magnetic flux. When the magnetic flux changes 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 armature circuit resistance, one can obtain a family of mechanical characteristics presented in Fig. 10.
The disadvantages of this method also include the relatively small control range due to limitations on the mechanical strength and switching of the electric motor. The advantage of this control method is its simplicity. For motors with parallel excitation, this is achieved by changing the resistance of the adjusting rheostat R r in the excitation circuit.
For DC motors with series excitation, a change in the magnetic flux is achieved by shunting the field winding with a resistance having the appropriate value, or by short-circuiting a certain number of turns of the field winding.
The method of regulating the rotation speed by changing the voltage at the motor armature terminals has become widely used, especially in electric drives built on the generator-motor system. With constant magnetic flux and armature circuit resistance, as a result of changing 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, it changes proportionally to the voltage. Since the resistance of the armature circuit remains unchanged, the rigidity of the family of mechanical characteristics does not differ from the rigidity of the natural mechanical characteristic at U=U nom.
The advantage of the considered control method is a wide range of rotation speed variations without increasing power losses. The disadvantages of this method include the fact that it requires a source of regulated supply voltage, and this leads to increase in weight, dimensions and cost of installation.
Any electric motor is designed to perform mechanical work due to the consumption of electricity applied to it, which is converted, as a rule, into rotational motion. Although in technology there are models that immediately create a translational movement of the working body. They are called linear motors.
In industrial installations, electric motors drive various machines and mechanical devices involved in the technological production process.
Inside household appliances, electric motors operate in washing machines, vacuum cleaners, computers, hair dryers, children's toys, watches and many other devices.
Basic physical processes and principle of operation
On the moving ones inside electric charges, which are called electric current, there is always a mechanical force that tends to deflect their direction in a plane located perpendicular to the orientation of the magnetic lines of force. When electric current passes through a metal conductor or a coil made of it, then this force tends to move/rotate each current-carrying conductor and the entire winding as a whole.
The picture below shows a metal frame through which current flows. The magnetic field applied to it creates a force F for each branch of the frame, creating a rotational motion.
This property of the interaction of electrical and magnetic energy based on the creation of an electromotive force in a closed conductive circuit is involved in the operation of any electric motor. Its design includes:
winding through which electric current flows. It is placed on a special anchor core and secured in rotation bearings to reduce the counteraction of friction forces. This structure is called a rotor;
a stator that creates a magnetic field, which with its lines of force penetrates the electric charges passing through the turns of the rotor winding;
housing for housing the stator. Special mounting sockets are made inside the housing, inside which the outer races of the rotor bearings are mounted.
A simplified design of the simplest electric motor can be represented by the following picture.
When the rotor rotates, a torque is created, the power of which depends on the general design of the device, the magnitude of the applied electrical energy, its losses during transformations.
The maximum possible torque power of the engine is always less than the electrical energy applied to it. It is characterized by the magnitude of the efficiency factor.
Types of electric motors
Based on the type of current flowing through the windings, they are divided into DC or AC motors. Each of these two groups has a large number of modifications using different technological processes.
DC motors
Their stator magnetic field is created by permanently mounted or special electromagnets with field windings. The armature winding is rigidly mounted in the shaft, which is fixed in bearings and can rotate freely around its own axis.
The basic structure of such an engine is shown in the figure.
On the armature core made of ferromagnetic materials there is a winding consisting of two series-connected parts, which are connected at one end to the current-conducting collector plates, and the other are connected to each other. Two graphite brushes are located at diametrically opposite ends of the armature and are pressed against the contact pads of the commutator plates.
The lower brush of the pattern is supplied with a positive potential of a constant current source, and the upper brush is supplied with a negative potential. The direction of current flowing through the winding is shown by a dotted red arrow.
The current causes a magnetic field of the north pole in the lower left part of the armature, and a south pole in the upper right (gimlet rule). This leads to repulsion of the rotor poles from like stationary poles and attraction to unlike poles on the stator. As a result of the applied force, a rotational movement occurs, the direction of which is indicated by the brown arrow.
With further rotation of the armature, by inertia, the poles move to other collector plates. The direction of the current in them changes to the opposite. The rotor continues to rotate further.
The simple design of such a collector device leads to large losses of electrical energy. Such engines operate in simple devices or toys for children.
DC electric motors involved in the production process have a more complex design:
the winding is sectioned not into two, but into more parts;
each winding section is mounted on its own pole;
The collector device is made of a certain number of contact pads according to the number of winding sections.
As a result, a smooth connection of each pole through its contact plates to the brushes and the current source is created, and electricity losses are reduced.
The device of such an anchor is shown in the picture.
For DC electric motors, the direction of rotation of the rotor can be reversed. To do this, it is enough to reverse the movement of current in the winding by changing the polarity at the source.
AC motors
They differ from previous designs in that the electric current flowing in their winding is described by periodically changing its direction (sign). To power them, voltage is supplied from alternating-sign generators.
The stator of such motors is made of a magnetic circuit. It is made of ferromagnetic plates with grooves into which winding turns with a frame (coil) configuration are placed.
Synchronous electric motors
The picture below shows working principle of single-phase AC motor with synchronous rotation of the electromagnetic fields of the rotor and stator.
In the grooves of the stator magnetic circuit at diametrically opposite ends there are winding conductors, schematically shown in the form of a frame through which alternating current flows.
Let us consider the case for the moment of time corresponding to the passage of the positive part of its half-wave.
A rotor with a built-in permanent magnet rotates freely in the bearing races, which has a clearly defined north “N mouth” and south “S mouth” pole. When a positive half-wave of current flows through the stator winding, a magnetic field with poles “S st” and “N st” is created in it.
Interaction forces arise between the magnetic fields of the rotor and stator (like poles repel, and opposite poles attract), which tend to rotate the armature of the electric motor from an arbitrary position to the final one, when the opposite poles are located as close as possible relative to each other.
If we consider the same case, but for the moment in time when the reverse - negative half-wave of current flows through the frame conductor, then the rotation of the armature will occur in the opposite direction.
To impart continuous movement to the rotor, not one winding frame is made in the stator, but a certain number of them, taking into account that each of them is powered by a separate current source.
Operating principle of three-phase synchronous rotation AC motor The electromagnetic fields of the rotor and stator are shown in the following picture.
In this design, three windings A, B and C are mounted inside the stator magnetic circuit, shifted at angles of 120 degrees to each other. Winding A is highlighted in yellow, B in green, and C in red. Each winding is made with the same frames as in the previous case.
In the picture, for each case, the current passes through only one winding in the forward or reverse direction, which is shown by the “+” and “-” signs.
When a positive half-wave passes through phase A in the forward direction, the rotor field axis takes a horizontal position because the magnetic poles of the stator are formed in this plane and attract the moving armature. Opposite rotor poles tend to approach the stator poles.
When the positive half-wave follows phase C, the armature will rotate 60 degrees clockwise. After current is supplied to phase B, a similar rotation of the armature will occur. Each successive flow of current in the next phase of the next winding will rotate the rotor.
If a three-phase network voltage shifted at an angle of 120 degrees is supplied to each winding, then alternating currents will circulate in them, which will spin the armature and create its synchronous rotation with the supplied electromagnetic field.
The same mechanical design has been successfully used in three-phase stepper motor
. Only in each winding, with the help of control, direct current pulses are supplied and removed according to the algorithm described above.
Their launch begins a rotational movement, and their cessation in certain moment time provides dosed rotation of the shaft and stopping at a programmed angle to perform certain technological operations.
In both three-phase systems described, it is possible to change the direction of rotation of the armature. To do this, you just need to change the phase sequence “A” - “B” - “C” to something else, for example, “A” - “C” - “B”.
The speed of rotation of the rotor is regulated by the duration of the period T. Its reduction leads to acceleration of rotation. The magnitude of the current amplitude in phase depends on internal resistance winding and the voltage applied to it. It determines the amount of torque and power of the electric motor.
Asynchronous electric motors
These motor designs have the same stator magnetic circuit with windings as in the previously discussed single-phase and three-phase models. They got their name due to the non-synchronous rotation of the electromagnetic fields of the armature and stator. This was done by improving the rotor configuration.
Its core is made of electrical grade steel plates with grooves. They are equipped with aluminum or copper current conductors, which are closed at the ends of the armature by conductive rings.
When voltage is applied to the stator windings, an electric current is induced in the rotor winding by an electromotive force and a magnetic field of the armature is created. When these electromagnetic fields interact, the motor shaft begins to rotate.
With this design, rotor movement is possible only after a rotating electromagnetic field has arisen in the stator and it continues in an asynchronous mode of operation with it.
Asynchronous motors are simpler in design. Therefore, they are cheaper and widely used in industrial installations and household appliances.
Linear motors
Many working parts of industrial mechanisms perform reciprocating or translational movement in one plane, necessary for the operation of metalworking machines, vehicles, hammer blows when driving piles...
Moving such a working body using gearboxes, ball screws, belt drives and similar mechanical devices from a rotary electric motor complicates the design. A modern technical solution to this problem is the operation of a linear electric motor.
Its stator and rotor are elongated in the form of strips, and not folded into rings, like those of rotational electric motors.
The principle of operation is to impart reciprocating linear movement to the runner-rotor due to the transfer of electromagnetic energy from a stationary stator with an open magnetic circuit of a certain length. Inside it, by alternately turning on the current, a running magnetic field is created.
It acts on the armature winding with the commutator. The forces arising in such an engine move the rotor only in a linear direction along the guide elements.
Linear motors are designed to operate at constant or alternating current, can work in synchronous or asynchronous mode.
The disadvantages of linear motors are:
complexity of technology;
high cost;
low energy levels.
Laboratory work No. 9
Subject. Study of DC electric motor.
Purpose of the work: study the structure and principle of operation of an electric motor.
Equipment: electric motor model, current source, rheostat, key, ammeter, connecting wires, drawings, presentation.
TASKS:
1 . Study the structure and principle of operation of an electric motor using a presentation, drawings and a model.
2 . Connect the electric motor to a power source and observe its operation. If the engine does not work, determine the cause and try to fix the problem.
3 . Indicate the two main elements in the design of an electric motor.
4 . What physical phenomenon is the action of an electric motor based on?
5 . Change the direction of rotation of the armature. Write down what you need to do to achieve this.
6. Collect electrical circuit, connecting in series an electric motor, a rheostat, a current source, an ammeter and a switch. Change the current and observe the operation of the electric motor. Does the speed of rotation of the armature change? Write down a conclusion about the dependence of the force acting on the coil from the magnetic field 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.
Please indicate 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 incorrect.
8 . List the advantages of an electric motor over a thermal engine.
current"Lesson location in work program: Lesson 55, one of the lessons on the topic “Electromagnetic phenomena”.
Objective of the lesson: Explain the structure and principle of operation of an electric motor.
Tasks:
study the electric motor using practical method– performing laboratory work.
learn to apply acquired knowledge in non-standard situations to solve problems;
To develop students’ thinking, continue to practice the mental operations of analysis, comparison and synthesis.
continue to develop students’ cognitive interest.
Methodological goal: the use of health-saving technologies in physics lessons.
Forms of work and types of activities in the lesson: testing knowledge, taking into account the individual characteristics of students; laboratory work is carried out in micro groups (pairs), updating students’ knowledge in a playful way; explanation of new material in the form of a conversation with a demonstration experiment, goal setting and reflection.
Lesson progress
1)Checking homework.
Independent work (multi-level) is carried out during the first 7 minutes of the lesson.
Level 1.
Level 2.
Level 3.
2). Learning new material. (15 minutes).
The teacher announces the topic of the lesson, the students formulate a goal.
Updating knowledge. Game of "yes" and "no"
The teacher reads the phrase; if the students agree with the statement, they stand up; if not, they sit.
The magnetic field is generated by permanent magnets or electric current.
There are no magnetic charges in nature.
The south pole of the magnetic needle indicates the south geographic pole of the Earth.
An electromagnet is a coil with an iron core inside.
The magnetic field lines are directed from left to right.
The lines along which magnetic arrows are installed in a magnetic field are called magnetic lines.
Presentation plan.
The effect of a magnetic field on a current-carrying conductor.
The dependence of the direction of movement of the conductor on the direction of the current in it and on the location of the poles of the magnet.
The design and operation of a simple commutator electric motor.
Movement of a conductor and frame with current in a magnetic field.
Design and principle of operation of a DC electric motor.
Safety briefing.
The work is carried out according to the description in the textbook p. 176.
4.The final stage of the lesson.
Task. Two electron beams repel, and two parallel wires carrying current in the same direction attract. Why? Is it possible to create conditions under which these conductors will also repel?
Reflection.
What new did you learn? Is this knowledge needed in everyday life?
Questions:
What determines the speed of rotation of the rotor in an electric motor?
What is an electric motor?
P . 61, create a crossword puzzle on the topic “electromagnetic phenomena.
Application.
Level 1.
1. How do opposite and like poles of magnets interact?
2. Is it possible to cut a magnet so that one of the resulting magnets has only a north pole, and the other has only a south pole?
Level 2.
Why is the compass body made of copper, aluminum, plastic and other materials, but not iron?
Why do steel rails and strips lying in a warehouse become magnetized after some time?
Level 3.
1.Draw the magnetic field of a horseshoe magnet and indicate the direction of the field lines.
2. Two pins are attracted to the south pole of the magnet. Why do their free ends repel each other?
Level 1.
1. How do opposite and like poles of magnets interact?
2. Is it possible to cut a magnet so that one of the resulting magnets has only a north pole, and the other has only a south pole?
Level 2.
Why is the compass body made of copper, aluminum, plastic and other materials, but not iron?
Why do steel rails and strips lying in a warehouse become magnetized after some time?
Level 3.
1.Draw the magnetic field of a horseshoe magnet and indicate the direction of the field lines.
2. Two pins are attracted to the south pole of the magnet. Why do their free ends repel each other?
MKOU "Allakskaya Secondary School"
Open physics lesson in 8th grade on the topic “ The effect of a magnetic field on a current-carrying conductor. Electric motor. Laboratory work No. 9 “Study of an electric DC motor current."
Prepared and conducted by: first category teacher Elizaveta Aleksandrovna Taranushenko.
