Laboratory work study of the electric motor. We understand the principles of operation of electric motors: the advantages and disadvantages of different types. Features of the use of asynchronous motors in a single-phase circuit

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 a washing machine, refrigerator, juicer, food processor, fans, electric shavers, etc. Electric motors set in motion devices and mechanisms connected to it.

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

How an electric motor works

The engine works based on the effect discovered by Michael Faraday in 1821. He made the discovery that when an electric current in a conductor interacts with a magnet, continuous rotation can occur.

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 be repelled from one, and attracted to the other.

As a result, the frame will turn to a horizontal position, in which there will be zero effect of the magnetic field on the conductor. In order for the rotation to continue, you need to add another frame at an angle or change the direction of the current in the frame at the right time.

In the figure, this is done using two half-rings, to which the contact plates from the battery adjoin. As a result, after a half-turn is completed, 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 coiled coils of wire coated with insulating varnish. These turns are an electromagnet or, as they are also called, an excitation winding.

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

In other more powerful motors use only electromagnets or windings. The rotating part with them is called the rotor, and the fixed part is called the stator.

Types of electric motors

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

Alternating current operating directly from the mains.
Direct current that run on batteries, batteries, power supplies or other DC sources.

According to the principle of work:

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

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

The principle of operation and the device of an asynchronous electric motor

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

Rotor, which are one 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-feeding brushes, reliability, durability and reliability are greatly increased.

Usually, main cause of failure 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 occurs) in the rotor. Here is an 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 turn slower.

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

For example, how does a circular saw work. She 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 greater EMF value begins to be induced in it. The current consumed by the motor increases and it starts to work at full power. If the load on the shaft is so great that it stalls, 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 of suitable power. If you take more, then 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 mains 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 are served by adjusting 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 speed.

The principle of operation and the device of a synchronous AC motor

This type of electric motor is used in everyday life where a constant rotation speed is required, the possibility of its adjustment, as well as if a rotation speed of more than 3000 rpm is required (this is the maximum for asynchronous).

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

In the case of a synchronous AC motor windings are located (3 in the figure), which are also wound on the rotor or armature (1). Their conclusions are soldered to the sectors of the slip ring or collector (5), which are energized with the help of graphite brushes (4). Moreover, the conclusions are arranged so that the brushes always supply voltage to only one pair.

The most frequent breakdowns collector motors is:

Brush wear or their poor contact due to the weakening of the clamping spring.
Collector pollution. Clean with either alcohol or zero sandpaper.
Bearing wear.

Principle of operation. The torque in the 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 will also change simultaneously in the body and armature, due to which the rotation will always be in the same direction.

Condition of the problem: Laboratory work No. 10. The study of an electric DC motor (on a model).

Task from
Reshebnik in Physics, Grade 8, A.V. Peryshkin, N.A. Rodina
for 1998
Online Physics Resource Book
for grade 8
Laboratory works
- room
10

The study of the DC electric motor (on the model).

The purpose of the work: To get acquainted with the main details 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.

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

←Lab No. 9. Assembling an electromagnet and testing its operation.Lab No. 11. Taking an image with a lens.-

Laboratory works→ number 10

The study of the DC electric motor (on the model).

Objective: Familiarize yourself with the main parts of the DC electric motor on the model of this motor.

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 the 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 power washing machines, vacuum cleaners, computers, hair dryers, children's toys, watches, and many other devices.

Basic physical processes and principle of operation

Electric charges moving inside, which are called electric current, are always affected by a mechanical force that tends to deflect their direction in a plane located perpendicular to the orientation of the magnetic lines of force. When an electric current passes through a metal conductor or a coil made of it, 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, which creates a rotational movement.

This property of the interaction of electric and magnetic energy based on the creation of an electromotive force in a closed current-carrying circuit is put into operation of any electric motor. Its design includes:

winding through which electric current flows. It is placed on a special core-anchor and fixed in rotation bearings to reduce the counteraction of friction forces. This design is called a rotor;

a stator that creates a magnetic field that, with its lines of force, permeates electric charges passing through the turns of the rotor windings;

housing for the stator. Inside the housing, special landing sockets are made, inside of which the outer cages of the rotor bearings are mounted.

Simplified, the design of the simplest electric motor can be represented by a picture of the following form.

When the rotor rotates, a torque is created, the power of which depends on the overall design of the device, the magnitude of the applied electrical energy, and its losses during transformations.

The value of the maximum possible torque of the engine is always less than the electrical energy applied to it. It is characterized by the value of the efficiency factor.

Types of electric motors

According to 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

They have a stator magnetic field created by permanently fixed or special electromagnets with excitation windings. The armature winding is rigidly mounted in the shaft, which is fixed in bearings and can freely rotate around its own axis.

The principal device of such an engine is shown in the figure.

On the core of the armature made of ferromagnetic materials there is a winding consisting of two parts connected in series, which at one end are connected to conductive collector plates, and at the other end are commutated with each other. Two graphite brushes are located at diametrically opposite ends of the armature and are pressed against the contact pads of the collector plates.

A positive potential of a constant current source is supplied to the lower brush of the pattern, and negative to the upper brush. The direction of the current flowing through the winding is shown by the dotted red arrow.

The current causes the magnetic field of the north pole in the lower left part of the armature, and the south pole in the upper right (gimlet rule). This leads to repulsion of the rotor poles from the stationary ones of the same name and attraction to opposite 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 pass to other collector plates. The direction of the current in them is reversed. The rotor continues further rotation.

The simple design of such a collector device leads to large losses of electrical energy. Such motors work in devices of a simple design or toys for children.

DC motors involved in the production process have a more complex design:

the winding is not divided into two, but into more parts;

each winding section is mounted on its own pole;

the collector device is made with a certain number of contact pads according to the number of winding sections.

As a result of this, a smooth connection of each pole through its contact plates to the brushes and the current source is created, and power losses are reduced.

The device of such an anchor is shown in the picture.

With electric DC motors, the direction of rotation of the rotor can be reversed. To do this, it is enough to change the movement of the current in the winding to the opposite 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, the voltage is supplied from generators with a sign-variable value.

The stator of such motors is made by a magnetic circuit. It is made from ferromagnetic plates with grooves into which winding turns are placed with a frame (coil) configuration.

Synchronous 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, winding conductors are placed, schematically shown in the form of a frame through which alternating current flows.

Consider the case for the moment of time corresponding to the passage of the positive part of its half-wave.

In the bearing cages, a rotor with a built-in permanent magnet rotates freely, in which the north “N mouth” and the south “S mouth” poles are pronounced. 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 the stator (the poles of the same name repel, and the opposite poles attract), which tend to turn the armature of the electric motor from an arbitrary position to the final one, when the opposite poles are as close as possible to each other.

If we consider the same case, but for the moment of 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 give continuous movement to the rotor in the stator, not one frame winding is made, but a certain number of them, so that each of them is powered by a separate current source.

The principle of operation of a three-phase AC motor with synchronous rotation electromagnetic fields of the rotor and stator is 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, winding B is green, and winding C is 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 indicated by the "+" and "-" signs.

With the passage of a positive half-wave in phase A in the forward direction, the axis of the rotor field 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 goes in phase C, the armature will rotate 60 degrees clockwise. After current is applied to phase B, a similar rotation of the armature will occur. Each next current flow in the next phase of the next winding will rotate the rotor.

If a voltage of a three-phase network shifted along an angle of 120 degrees is brought to each winding, then alternating currents will circulate in them, which will unwind the armature and create its synchronous rotation with the supplied electromagnetic field.

The same mechanical design has been successfully applied in three-phase stepper motor. Only in each winding, with the help of control, DC pulses are supplied and removed according to the algorithm described above.

Their launch starts a rotational movement, and the termination at a certain point in time ensures a dosed rotation of the shaft and a stop at a programmed angle to perform certain technological operations.

In both described three-phase systems, 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 another, 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 an acceleration of rotation. The amplitude of the current in the phase depends on the internal resistance of the winding and the value of the voltage applied to it. It determines the amount of torque and power of the electric motor.

Asynchronous motors

These motor designs have the same stator magnetic circuit with windings as in the previously considered single-phase and three-phase models. They got their name because of the non-synchronous rotation of the electromagnetic fields of the armature and stator. This was done by improving the configuration of the rotor.

Its core is made of plates of electrical grades of steel with grooves. Aluminum or copper conductors are mounted in them, which are closed by conductive rings at the ends of the armature.

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 rotation of the motor shaft begins.

With this design, the movement of the rotor 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 bodies of industrial mechanisms perform reciprocating or translational movement in one plane, which is necessary for the operation of metalworking machines, vehicles, hammer blows when driving piles ...

Moving such a working body with the help of gearboxes, ball screws, belt drives and similar mechanical devices from a rotary electric motor complicates the design. The 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, as in rotary electric motors.

The principle of operation is to impart a reciprocating linear movement to the runner-rotor due to the transfer of electromagnetic energy from a fixed stator with an open magnetic circuit of a certain length. Inside it, by turning on the current in turn, a traveling magnetic field is created.

It acts on the armature winding with the collector. 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 on direct or alternating current, and can operate in synchronous or asynchronous mode.

The disadvantages of linear motors are:

technology complexity;

high price;

low energy performance.

Lab #9

Topic. The study of the DC motor.

Objective: 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. Assemble the 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.