We understand the principles of operation of electric motors: the advantages and disadvantages of different types. We understand the principles of operation of electric motors: the advantages and disadvantages of different types Laboratory work studying a DC electric motor

1. Purpose of the work: Study starting features, mechanical characteristics and methods of regulating engine speed direct current with mixed excitement.

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). Her graphic image 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 implies three possible ways engine speed control:

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.

An electric motor is an electrical device for converting electrical energy into mechanical energy. Today, electric motors are widely used in industry to drive various machines and mechanisms. In the household they are installed in washing machine, refrigerator, juicer, food processor, fans, electric shavers, etc. Electric motors drive the devices and mechanisms connected to it.

In this article I will talk about the most common types and operating principles of AC electric motors, widely used in the garage, household or workshop.

How does an electric motor work?

The engine works based on the effect, discovered by Michael Faraday back in 1821. He made the discovery that when interacting electric current Continuous rotation may occur in the conductor and magnet.

If in a uniform magnetic field Place the frame in a vertical position and pass current through it, then an electromagnetic field will arise around the conductor, which will interact with the poles of the magnets. The frame will repel from one, and attract to the other.

As a result, the frame will rotate to a horizontal position, in which the effect of the magnetic field on the conductor will be zero. In order for the rotation to continue, it is necessary to add another frame at an angle or change the direction of the current in the frame at the appropriate moment.

In the figure, this is done using two half-rings, to which the contact plates from the battery are adjacent. As a result, after completing a half-turn, the polarity changes and the rotation continues.

In modern electric motors Instead of permanent magnets, inductors or electromagnets are used to create a magnetic field. If you disassemble any motor, you will see wound turns of wire coated with insulating varnish. These turns are the electromagnet or, as they are also called, the field winding.

At home Permanent magnets are used in battery-powered children's toys.

In others, more powerful Motors use only electromagnets or windings. The rotating part with them is called the rotor, and the stationary part is the stator.

Types of electric motors

Today there are quite a lot of electric motors of different designs and types. They can be separated by type of power supply:

Alternating current, operating directly from the mains.
Direct current that operate on batteries, rechargeable batteries, power supplies or other direct current sources.

According to the operating principle:

Synchronous, which have windings on the rotor and a brush mechanism to supply electric current to them.
Asynchronous, the simplest and most common type of motor. They do not have brushes or windings on the rotor.

A synchronous motor rotates synchronously with the magnetic field that rotates it, while an asynchronous motor rotates slower than the rotating magnetic field in the stator.

Operating principle and design of an asynchronous electric motor

In the asynchronous case motor, stator windings are laid (for 380 Volts there will be 3 of them), which create a rotating magnetic field. Their ends are connected to a special terminal block for connection. The windings are cooled thanks to a fan mounted on the shaft at the end of the electric motor.

Rotor, which is one piece with the shaft, is made of metal rods that are closed to each other on both sides, which is why it is called short-circuited.
Thanks to this design, there is no need for frequent periodic maintenance and replacement of current supply brushes, and reliability, durability and reliability increase many times over.

Usually, main cause of failure of an asynchronous motor is the wear of the bearings in which the shaft rotates.

Principle of operation. In order for an asynchronous motor to work, it is necessary that the rotor rotates slower than the electromagnetic field of the stator, as a result of which an EMF is induced (an electric current arises) in the rotor. Here important condition, if the rotor rotated at the same speed as the magnetic field, then, according to the law of electromagnetic induction, no EMF would be induced in it and, therefore, there would be no rotation. But in reality, due to bearing friction or shaft load, the rotor will always rotate more slowly.

Magnetic poles are constantly rotating in the motor windings, and the direction of the current in the rotor constantly changes. At one point in time, for example, the direction of currents in the stator and rotor windings is depicted schematically in the form of crosses (current flows away from us) and dots (current towards us). The rotating magnetic field is shown as a dotted line.

For example, how does a circular saw work. It has the highest speed without load. But as soon as we start cutting the board, the rotation speed decreases and at the same time the rotor begins to rotate more slowly relative to the electromagnetic field and, according to the laws of electrical engineering, an even larger EMF begins to be induced in it. The current consumed by the motor increases and it begins to operate at full power. If the load on the shaft is so great that it stops, then damage to the squirrel-cage rotor may occur due to the maximum value of the EMF induced in it. That is why it is important to select an engine with suitable power. If you take a larger one, then the energy costs will be unjustified.

Rotor speed depends on the number of poles. With 2 poles, the rotation speed will be equal to the rotation speed of the magnetic field, equal to a maximum of 3000 revolutions per second at a network frequency of 50 Hz. To reduce the speed by half, it is necessary to increase the number of poles in the stator to four.

A significant disadvantage of asynchronous motors is that they can adjust the speed of rotation of the shaft only by changing the frequency of the electric current. And so it is not possible to achieve a constant shaft rotation speed.

Operating principle and design of an AC synchronous electric motor

This type of electric motor is used in everyday life where a constant rotation speed is required, the ability to adjust it, and also if a rotation speed of more than 3000 rpm is required (this is the maximum for asynchronous ones).

Synchronous motors are installed in power tools, vacuum cleaners, washing machines, etc.

In a synchronous housing In the AC motor there are windings (3 in the figure), which are also wound on the rotor or armature (1). Their leads are soldered to the sectors of the slip ring or collector (5), to which voltage is applied using graphite brushes (4). Moreover, the terminals are located so that the brushes always supply voltage to only one pair.

Most common breakdowns commutator motors are:

Brush wear or their poor contact due to weakening of the pressure spring.
Collector contamination. Clean with either alcohol or grit sandpaper.
Bearing wear.

Principle of operation. The torque in an electric motor is created as a result of the interaction between the armature current and the magnetic flux in the field winding. With a change in the direction of the alternating current, the direction of the magnetic flux in the housing and armature will also change simultaneously, due to which the rotation will always be in one direction.

Laboratory work No. 9

Subject. Study of DC electric motor.

Goal 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.

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 the 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.

Condition of the task: Laboratory work No. 10. Study of an electric DC 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 works
- 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.

Don't know how to solve? Can you help with a solution? Come in and ask.

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