Laboratory work studying a constant electric motor. We understand the principles of operation of electric motors: the advantages and disadvantages of different types
Laboratory work→ number 10
Study of a DC electric motor (on a model).
Purpose of the work: Familiarize yourself with the basic parts of a DC electric motor using a model of this motor.
This is perhaps the easiest 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, semi-rings, winding, shaft).
The electric motor offered to you by your teacher may be similar to the one shown in the figure, or it may have a different appearance, since there are many options for school electric motors. This is not of fundamental importance, since the teacher will probably tell you in detail and show you how to handle the model.
Let us list the main reasons why a properly connected electric motor does not work. Open circuit, lack of contact of brushes with half rings, damage to the armature winding. If in the first two cases you are quite capable of handling it on your own, if the winding breaks, you need to contact a teacher. Before turning on the engine, you should make sure that its armature can rotate freely and nothing interferes with it, otherwise when turned on, the electric motor will emit a characteristic hum, but will not rotate.
To use presentation previews, create a Google account and log in to it: https://accounts.google.com
Slide captions:
In the pictures, determine the direction of the Ampere force, the direction of the current in the conductor, the direction of the magnetic field lines, and the poles of the magnet. N S F = 0 Let's remember.
Laboratory work No. 11 Study of a DC electric motor (on a model). Purpose of the work: to get acquainted with a model of a DC electric motor with its structure and operation. Equipment and materials: electric motor model, laboratory power supply, key, connecting wires.
Safety regulations. There should be no foreign objects on the table. Attention! Electric current! The insulation of the conductors must not be damaged. Do not turn on the circuit without the teacher's permission. Do not touch the rotating parts of the electric motor with your hands. Long hair must be removed so that it does not get caught in the rotating parts of the engine. After completing the work, put the workplace in order, open the circuit and disassemble it.
The order of work. 1. Consider the model of the electric motor. Indicate its main parts in Figure 1. 1 2 3 Fig.1 4 5 1 - ______________________________ 2 - ______________________________ 3 - ______________________________ 4 - ______________________________ 5 - ______________________________
2. Assemble an electrical circuit consisting of a current source, an electric motor model, a key, connecting everything in series. Draw a diagram of the circuit.
3. Rotate the motor. If the engine does not work, find the reasons and eliminate them. 4. Change the direction of current in the circuit. Observe the rotation of the moving part of the electric motor. 5.Draw a conclusion.
Literature: 1. Physics. 8th grade: studies. for general education institutions/A.V. Peryshkin. - 4th ed., finalized. - M.: Bustard, 2008. 2 . Physics. 8th grade: studies. For general education institutions / N.S. Purysheva, N.E. Vazheevskaya. - 2nd ed., stereotype. - M.: Bustard, 2008. 3. Laboratory work and test assignments in physics: Notebook for 8th grade students. - Saratov: Lyceum, 2009. 4. Notebook for laboratory work. Sarahman I.D. Municipal educational institution secondary school No. 8 in Mozdoka, North Ossetia-Alania. 5. Laboratory work at school and at home: mechanics / V.F. Shilov.-M.: Education, 2007. 6. Collection of problems in physics. Grades 7-9: a manual for general education students. institutions / V.I. Lukashik, E.V. Ivanova.-24th ed.-M.: Education, 2010.
Preview:
Laboratory work No. 11
(on model)
Purpose of the work
Devices and materials
Work progress.
Laboratory work No. 11
Studying DC Electric Motor
(on model)
Purpose of the work : get acquainted with a model of a DC electric motor with its structure and operation.
Devices and materials: electric motor model, laboratory power supply, key, connecting wires.
Safety regulations.
There should be no foreign objects on the table. Attention! Electric current! The insulation of the conductors must not be damaged. Do not turn on the circuit without the teacher's permission. Do not touch the rotating parts of the electric motor with your hands.
Practice tasks and questions
1.What physical phenomenon is the action of an electric motor based on?
2.What are the advantages of electric motors over thermal ones?
3. Where are DC electric motors used?
Work progress.
1. Consider the model of the electric motor. Indicate its main parts in Figure 1.
2. Assemble an electrical circuit consisting of a current source, an electric motor model, a key, connecting everything in series. Draw a diagram of the circuit.
Fig.1
Draw a conclusion.
3. Rotate the motor. If the engine does not work, find the reasons and eliminate them.
4. Change the direction of current in the circuit. Observe the rotation of the moving part of the electric motor.
Fig.1
Condition of the task: Laboratory work No. 10. Study of a DC electric motor (on a model).
Problem from
Physics textbook, 8th grade, A.V. Peryshkin, N.A. Rodina
for 1998
Online physics workbook
for 8th grade
Laboratory work
- number
10
Study of a DC electric motor (on a model).
Purpose of work: To become familiar with the main parts of an electric DC motor using a model of this motor.
This is perhaps the easiest 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, semi-rings, winding, shaft).
The electric motor offered to you by your teacher may be similar to the one shown in the figure, or it may have a different appearance, since there are many options for school electric motors. This is not of fundamental importance, since the teacher will probably tell you in detail and show you how to handle the model.
Let us list the main reasons why a properly connected electric motor does not work. Open circuit, lack of contact of brushes with half rings, damage to the armature winding. If in the first two cases you are quite capable of handling it on your own, if the winding breaks, you need to contact a teacher. Before turning on the engine, you should make sure that its armature can rotate freely and nothing interferes with it, otherwise when turned on, the electric motor will emit a characteristic hum, but will not rotate.
Don't know how to solve? Can you help with a solution? Come in and ask.
←Laboratory work No. 9. Assembling an electromagnet and testing its action. Laboratory work No. 11. Obtaining an image using a lens.-
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, it 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 an EMF in its 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 a 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 finished accelerating to the rated value of the armature rotation 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.
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 a given supply voltage value
.
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.
current"Place of the lesson in the 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 a 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.
