Laboratory work 9 study of an electric motor. We understand the principles of operation of electric motors: the advantages and disadvantages of different types

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 the life of a 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

Laboratory works→ number 10

Study of a DC electric motor (on a model).

Goal 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.

Place of the lesson in the work program: lesson 55, one of the lessons on the topic “Electromagnetic phenomena”.

The purpose 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.

During the classes

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.

1. 
   The effect of a magnetic field on a current-carrying conductor.
2. 
   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.
3. 
   The design and operation of a simple commutator electric motor.

1. 
   Movement of a conductor and frame with current in a magnetic field.
2. 
   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?

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. Electrical engine. Laboratory work No. 9 “Study of an electric DC motor current."

Prepared and conducted by: first category teacher Elizaveta Aleksandrovna Taranushenko.

1. Purpose of the work: Study the starting features, mechanical characteristics and methods of regulating the rotation speed of a DC motor with mixed excitation.

Adaniye.

2.1. to independent work:

Study the design features, switching circuits of DC motors;

Study the method of obtaining the mechanical characteristics of a DC motor;

Familiarize yourself with the features of starting and regulating the rotation speed of a DC motor;

Draw circuit diagrams for measuring the resistance of the armature circuit and field windings (Fig. 6.4) and testing the motor (Fig. 6.2);

Using fig. 6.2 and 6.3 draw up an installation diagram;

Draw the forms of tables 6.1... 6.4;

Prepare oral answers to test questions.

2.2. to work in the laboratory:

Familiarize yourself with the laboratory setup;

Record in table 6.1. engine nameplate data;

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;

Construct a natural mechanical characteristic n=f(M) and speed characteristics n=f(I B) and n=f(U);

Draw conclusions based on the research results.

General information.

DC motors, unlike AC motors (primarily asynchronous), have a higher starting torque ratio and overload capacity, and provide smooth control of the rotation speed of the working machine. Therefore, they are used to drive machines and mechanisms with difficult starting conditions (for example, as starters in internal combustion engines), as well as when it is necessary to regulate the rotation speed within large limits (feed mechanisms of machine tools, running-brake stands, electrified vehicles).

Structurally, the engine consists of a stationary unit (inductor) and a rotating unit (armature). The field windings are located on the magnetic core of the inductor. There are two of them in a mixed-excitation motor: parallel with terminals Ш 1 and Ш2 and serial with terminals 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 of small cross-section wire with a large number of turns. The series winding has a low resistance R obc (usually from several Ohms to fractions of an Ohm), because consists of a small number of turns of large cross-section wire. The inductor is used to create a magnetic excitation flux when its windings are supplied with direct current.

The armature winding is placed in the grooves of the magnetic circuit and brought to the collector. Using brushes, its terminals I and I 2 are connected to a direct current source. The armature winding resistance 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 Iya with the magnetic excitation flux F:

М=К × Iя × Ф, (6.1)

where K is a constant coefficient depending on the engine design.

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 = C × n × Ф, (6.2)

where C is a constant coefficient depending on the engine design.

Armature circuit current:

I I =(U–E)/(R I +R OBC)=(U–С×n ×Ф)/(R I +R OBC), (6.3)

Solving expressions 6.1 and 6.3 together with respect to n, we find an analytical expression for the mechanical characteristics of the engine n=F(M). Its graphical representation is shown in Figure 6.1.

Rice. 6.1. Mechanical characteristics of a mixed-excitation DC motor

Point A corresponds to engine idling at rotation speed n o. With increasing mechanical load, the rotation speed decreases and the torque increases, reaching the nominal value M H at point B. In the aircraft section, the engine is overloaded. The current Iya exceeds the rated value, which leads to rapid heating of the armature and OVS windings, and sparking on the collector increases. The maximum torque Mmax (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 the torque axis at point D, we would obtain the value of the starting torque when the motor is directly connected to the network. The emf E is 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 is the rated network voltage;

I Y.N. - rated armature current.

Reducing the armature current to (1.3...2.5)×I Ya.N. provides sufficient initial starting torque MP (point D). As the engine accelerates, resistance Rx is reduced to zero, maintaining an approximately constant value of MP (section SD).

Rheostat R B in the circuit of the parallel excitation winding (Fig. 6.2) allows you to regulate the magnitude of the magnetic flux Ф (formula 6.1). Before starting the engine, it is completely withdrawn 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 constant quantities, and U, I I and Ф can be changed. This gives rise to three possible ways to regulate engine speed:

Changing the value of the supplied voltage;

By changing the value of the armature current using the adjusting rheostat Rx, which, unlike the starting rheostat, is designed for continuous operation;

By changing the magnitude of the excitation magnetic flux F, which is proportional to the current in the windings OVSh and OVS. In a parallel winding, it can be adjusted with a rheostat R b. Resistance R b is taken depending on the required speed control limits R B = (2...5) R obsh.

The motor nameplate indicates the rated rotation 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.