On what physical principle the asynchronous motor operates. AC induction motor

Electric motors are power machines used to convert electrical energy into mechanical energy. The general classification separates them according to the type of supply current to the motors of a constant and alternating current. The article below discusses electric motors with an AC specification, their types, distinctive characteristics and advantages.

AC electric motor of industrial type

The principle of energy conversion

Among the electric motors used in all industries and household appliances, AC motors are most common. They are found almost in every sphere of life - from children's toys and washing machines to cars and powerful production machines.

The principle of operation of all electric motors is based on Faraday's law of electromagnetic induction and Ampere's law. The first of them describes the situation when an electromotive force is generated on a closed conductor located in a changing magnetic field. In motors, this field is created through the stator windings, through which an alternating current flows. Inside the stator (which is the body of the device) is the movable element of the engine - the rotor. A current flows on it.

The rotation of the rotor is explained by Ampere's law, which states that electric charges, flowing along the conductor located inside the magnetic field, the force acting in the plane perpendicular to the lines of force of this field acts. Simply put, the conductor, which in the engine design is the rotor, starts to rotate around its axis, and it is fixed to the shaft to which the working mechanisms of the equipment are connected.

Types of engines and their devices

AC electric motors have a different device, thanks to which it is possible to create machines with the same rotor speed relative to the stator magnetic field, and such machines where the rotor "lags" behind the rotating field. According to this principle, these engines are divided into the corresponding types: synchronous and asynchronous.

Asynchronous

The basis of the design of an asynchronous electric motor is a pair of important functional parts:

1. The stator is a block of cylindrical shape, made of steel sheets with pazanmi for laying conductive windings, whose axes are located at an angle of 120 ° relative to each other. The poles of the windings go to the terminal box, where they are connected in different ways, depending on the required parameters of the electric motor.
2. Rotor. The design of asynchronous electric motors uses two types of rotors:
   • Short-circuited. It is called so, because it is made of several aluminum or copper rods, short-circuited with the help of end rings. This design, which is the current-carrying winding of the rotor, is called in the electromechanics a "squirrel cage".
   • Phase. On rotors of this type is established three-phase winding, similar to the stator winding. Most often the ends of its conductors go to the terminal pad, where they connect with a "star", and the free ends are connected to the contact rings. The phase rotor allows using brushes to add an additional resistor in the winding circuit, which allows changing the resistance to reduce the starting currents.

In addition to the described key elements of the asynchronous motor, its design also includes a fan for cooling the windings, a terminal box and a shaft transmitting the generated rotation to the operating mechanisms of the equipment, the operation of which is provided by this engine.

The work of asynchronous electric motors is based on the law of electromagnetic induction, which states that the electromotive force can arise only under conditions of the difference in the rotor speed and the stator magnetic field. Thus, if these speeds were equal, the EMF could not appear, but the impact on the shaft of such "braking" factors as load and friction of bearings always creates sufficient conditions for work.

Synchronous

The construction of synchronous AC motors is somewhat different from that of asynchronous analogs. In these machines, the rotor rotates around its axis at a speed equal to the rotational speed of the stator magnetic field. The rotor or armature of these devices is also equipped with windings, which are connected to one another with one ends, and others to a rotating collector. The contact areas on the collector are mounted so that at a certain moment of time it is possible to supply power through the graphite brushes to only two opposite contacts.

Principle of operation of synchronous electric motors:

1. When the magnetic flux interacts in the stator winding with the rotor current, a torque appears.
2. The direction of motion of the magnetic flux varies simultaneously with the direction of the alternating current, so that the output shaft is rotated in one direction.
3. Adjustment of the desired speed is carried out by adjusting the input voltage. Most often, in high-speed equipment, for example, perforators and vacuum cleaners, this function is performed by the rheostat.

Most often, the reasons for the output of synchronous motors are:

• wear of graphite brushes or weakening of the clamping spring;
• wear of the shaft bearings;
• contamination of the collector (cleaned with sandpaper or alcohol).

Three-phase alternator

History of invention

The invention of the simplest way of converting energy from electrical to mechanical belongs to Michael Faraday. In 1821, this great English scientist conducted an experiment with a conductor dropped into a vessel with mercury, at the bottom of which lay a permanent magnet. After the supply of electricity to the conductor, it came into motion, rotating, respectively, by magnetic field lines. Nowadays this experience is often spent in physics lessons, replacing mercury with brine.

Further study of the issue led to the creation of Peter Barlow in 1824, a unipolar engine, called the wheel Barlow. Its design includes two gears made of copper, located on one axis between permanent magnets. After the current is applied to the wheels, as a result of its interaction with the magnetic fields, the wheels begin to rotate. During the experiments, the scientist established that the direction of rotation can be changed by changing the polarity (by permuting the magnets or contacts). Practical application of the "wheel Barlow", but played an important role in the study of the interaction of magnetic fields and charged conductors.

The first working model of the device, which became the progenitor of modern engines, was created by the Russian physicist Boris Semenovich Jacobi in 1834. The principle of using a rotating rotor in a magnetic field, demonstrated in this invention, is practically unaltered using modern engines direct current.

But the creation of the first engine with asynchronous principle   The work belongs at once to two scientists - Nicola Tesla and Galileo Ferraris, who successfully demonstrated their inventions in one year (1888). A few years later, the two-phase brushless AC motor, created by Nikola Tesla, has already been used in several power plants. In 1889, the Russian electrical engineer Mikhail Osipovich Dolivo-Dobrovolsky perfected Tesla's invention for working in a three-phase network, so he was able to create the first asynchronous motor   AC power more than 100 watts. He also owns the invention of the methods of connecting phases in three-phase electric motors: "Star" and "triangle", starting rheostats and three-phase transformers.

The alternating current system proposed by Westinghouse

Connection to single-phase and three-phase power supplies

According to the type of supply network AC motors are classified into single- and three-phase.

Connecting asynchronous single-phase motors   performs very easily - for this it is sufficient to connect the phase and neutral wire   single-phase 220V network. Synchronous motors can also be powered from this type of network, but the connection is a little more difficult - you need to connect the windings of the rotor and the stator so that their single-pole magnetization contacts are located opposite each other.

Connection to a three-phase network is somewhat more complicated. First of all, it should be noted that the terminal box contains 6 pins - in pairs for each of the three windings. Secondly, it makes it possible to use one of the two connection methods ("star" and "triangle"). Incorrect connection may cause the motor to malfunction due to the meltdown of the stator windings.

The main functional difference between the "star" and the "triangle" is the different power consumption, which is done to enable the machine to be connected to three-phase networks with different line voltages - 380V or 660V. In the first case, it is necessary to connect windings according to the "triangle" scheme, and in the second one - "star". This inclusion rule allows in both cases to have a voltage of 380V on the windings of each phase.

In the connection panel, the winding leads are arranged in such a way that the jumpers used for switching are not crossed. If the motor terminal box contains only three clamps, then it is designed to operate on the same voltage as specified in the technical documentation, and the windings are interconnected internally.

Advantages and disadvantages of electric AC motors

Today, among all electric motors, AC devices occupy a leading position in terms of the volume of use in power plants. They have a low cost, easy to maintain design and efficiency of at least 90%. In addition, their device allows you to smoothly change the speed of rotation, without resorting to additional equipment like gearboxes.

The main disadvantage of AC motors with an asynchronous operation principle is the fact that it is only possible to adjust their shaft rotation frequency by changing the input frequency of the current. This does not allow for a constant rotation speed, and also reduces power. Asynchronous motors are characterized by high starting currents, but low starting torque. To correct these shortcomings, a frequency drive is used, but its price is contrary to one of the main advantages of these engines - low cost.

  Weak Point synchronous motor   is its complex construction. Graphite brushes quite quickly fail under load, and also lose tight contact with the collector due to the weakening of the clamping spring. In addition, these motors, like the asynchronous analogs, are not protected against wear of the shaft bearings. The disadvantages also include a more complex start-up, the need for a constant-current source and an exceptionally frequency-controlled speed control.

Application

To date, electric motors with specification for AC are common in all areas of industry and life. At power plants they are installed as generators, used in production equipment, automotive and even household appliances. Today in each house you can find at least one device with an electric AC motor, for example, a washing machine. The reasons for such great popularity are the universality, durability and ease of maintenance.

Among asynchronous electric machines   The most widely used devices with a three-phase specification. They are the best option   For use in many power units, generators and high-power plants, whose work is related to the need to monitor the rotation speed of the shaft.

The DC electric machine consists of a stator, an armature, a collector, a brush holder and bearing shields (Figure 1). The stator consists of a frame (body), main and additional poles, which have excitation windings. This fixed part of the machine is sometimes called an inductor. Its main purpose is to create a magnetic flux. The frame is made of steel, the main and additional poles, as well as bearing shields, are bolted to it. On top of the frame there are rings for transportation, from below - paws for fixing the machine to the foundation. The main poles of the machine are recruited from sheets of electrical steel 0.5-1 mm thick in order to reduce the losses that arise from the pulsations of the magnetic field of the poles in the air gap under the poles. The steel sheets of the pole core are pressed and fastened with rivets.

Figure 1 - DC machine:
  I - shaft; 2 - front bearing shield; 3 - collector; 4 - brush holder; 5 - armature core with winding; b - core of the main pole; 7 - pole coil; 8 - bed; 9 - rear bearing shield; 10 - the fan; 11 - paws; 12 - bearing

Figure 2 - The poles of the DC machine:
  a is the main pole; b - additional pole; c - winding of the main pole; r - winding of the additional pole; 1 - pole piece; 2 - core
  In the poles, the core and the tip are distinguished (Figure 2). The core is put on the excitation winding, along which current flows, creating a magnetic flux. The field winding is wound on a metal frame covered with an electrocardboard (in large machines), or placed on an isolated electrocardboard core (small machines). For better cooling, the coil is divided into several parts, between which the ventilation ducts are left. Additional poles are set between the main poles. They serve to improve switching. Their windings are connected in series to the armature circuit, so the winding conductors have a large cross section.
  The DC machine anchor consists of a shaft, a core, a winding and a collector. The core of the anchor is assembled from the stamped sheets of electrical steel with a thickness of 0.5 mm and is pressed on both sides by means of pressure washers. In machines with a radial ventilation system, the core sheets are assembled into separate packs of 6-8 cm thick, between which 1 cm wide ventilation ducts are made. In case of axial ventilation, a hole is made in the core for air to travel along the shaft. On the outer surface of the armature there are grooves for winding.

Figure 3 - Location of the armature winding section in the grooves of the core
  The armature winding is made of copper wires round or rectangular section in the form of pre-made sections (Figure 3). They fit into the grooves, where they are carefully insulated. The winding is made two-layered: place in each groove two sides of different anchor coils - one above the other. The winding is fixed in the grooves with wedges (wooden, getinax or textolite), and the frontal parts are fixed with a special wire bandage. In some designs wedges are not used, and the winding is fixed with a bandage. The bandage is made of non-magnetic steel wire, which is wound with pre-tension. In modern machines for bandaging anchors use a glass tape.
  The collector of the DC machine is assembled from wedge-like plates of cold-rolled copper. The plates are isolated from each other by spacers made of collector micanite with a thickness of 0.5-1 mm. The lower (narrow) edges of the plates have cutouts in the form of a "swallowtail", which serve for fixing copper plates and mikanite insulation. Collectors fasten the pressure cones in two ways: at one of them the force from the clamp is transmitted only to inner surface   "swallowtail", the second - on the "swallowtail" and the end of the plate.
  Collectors with the first method of fastening are called arched, with the second - wedge. The most common arched collectors.
  In the collector plates on the anchor side, with a small difference in the diameters of the collector and the anchor, protrusions are made in which the slits are milled. They lay the ends of the armature winding and are soldered with tin solder. With a large difference in diameters, solder to the collector is done with the help of copper strips, which are called "cockerels".
  In high-speed high-speed machines, external insulated shroud rings are used to prevent the plates from bulging under the action of centrifugal forces.
  The brush machine consists of a traverse, brush fingers (bolts), brush holders and brushes. The traverse is designed for fastening the brushes of the brush holders on it, forming an electric circuit.
  The brush holder consists of a cage into which a brush is placed, a lever for pressing the brush to the collector and springs. The pressure on the brush is 0.02-0.04 MPa.
  To connect the brush to the electrical circuit   there is a flexible copper wire.
  In machines of low power, tubular brush holders are used, which are fixed in the bearing shield. All brush holders of the same polarity are connected together by busbars that are connected to the terminals of the machine.
  Brushes (Figure 4), depending on the composition of the powder, the method of manufacture and physical properties are divided into six main groups: coal-graphite, graphite, electrographic, copper-graphite, bronze graphite and silver-graphite.
  The bearing shields of the electric machine serve as the connecting parts between the frame and the armature, and also the supporting structure for the armature, whose shaft rotates in the bearings installed in the shields.

Figure 4 - Brushes:
  a - for machines of small and medium power; b - for high-power machines; 1 - brush wire; 2 - tip
  There are conventional and flanged bearing shields.
  Bearing shields are made of steel (less often from cast iron or aluminum alloys) by casting, as well as welding or stamping. In the center of the shield is a boring under the bearing of rolling: ball or roller. In large-capacity machines, in many cases, sliding bearings are used.
  In recent years, stator DC motors are assembled from individual sheets of electrical steel. In the sheet at the same time, the yoke, grooves, main and additional poles are stamped.

The main purpose of any engine is the communication (transmission) of mechanical energy to the working bodies of the production mechanisms necessary for them to perform certain technological operations. This mechanical energy is generated by the electric motor due to the electric energy consumed by it from electrical network, to which it is connected. In other words, the electric motor converts electric power   in the mechanical.
The amount of mechanical energy produced by the engine per unit time is called its power. The mechanical power on the motor shaft is determined by the product of the motor torque and its rotational speed. Note that some engines have forward motion, so their mechanical power depends on the engine's developed effort and speed of this translational motion.
   Depending on the nature of the supply voltage, DC and AC motors are distinguished. Among the most common DC motors are, for example, motors with independent, sequential and mixed excitation, and examples of AC motors are asynchronous and synchronous motors.
   Despite the variety of existing electric motors (including special purpose), the action of any of them is based on the interaction of the magnetic field and the conductor with electric shock   or a magnetic field and a ferromagnetic body or a permanent magnet.
   Let us consider the interaction of a magnetic field and a conductor with an electric current. Suppose that B is the magnetic field of a magnet with N-S poles (Figure 1),
   Fig. I. Interaction of a magnetic field and a conductor with a current.
   whose lines of force are shown in fine lines, the conductor is placed perpendicular to these lines by the drain I. Then, according to the known physical law, the force F (Ampere force) acts on this conductor, which is proportional to the induction of the magnetic field B, the length of the conductor I, and the current I:
   F = BlI. (1)
   The direction of the force acting on the conductor F can be determined by the so-called left-hand rule: if the fingers of the left arm are pulled in the direction of the current I, and the palm is arranged so that the lines of the magnetic field enter it, the bent thumb will show the direction of the action of the force F.
   We note that, in accordance with the law of electromagnetic induction, the current passing through the conductor will create its own magnetic field with concentric lines of force around the conductor (in Figure 1 this field is not shown), and therefore the picture of the magnetic field between the poles of the magnet will change slightly. However, this circumstance does not change the essence of the phenomenon under consideration.
   The one shown in Fig. 1 the circuit can serve as the simplest model of the motor of translational motion, because under the action of the force F the conductor with current tends to make a rectilinear displacement in the direction of the action of this force.
   To explain the principle of the generation of torque in rotational motion engines, let us consider the behavior in a field of the same magnet of a frame with a current consisting of conductors A and B (Fig. 2, a). The current to the conductors of the frame is fed from an external DC source through two contact rings K, fixed on the axis of rotation of the frame 00 ".

In the cases shown in Fig. 2, and the frame position and current and magnetic field directions on the conductors of the frames A and B will be acted upon by forces F having the directions indicated in the figure in accordance with the left hand rule. These forces will create a torque M relative to the axis of the frame 00 ", under which the frame will rotate counterclockwise.
   In the physics course it is shown that this moment is directly proportional to the current strength I, the induction of the magnetic field B, the area of ​​the frame with the current S, and depends on the angle a between the lines of the magnetic field and the axis of the frame aa, perpendicular to its plane:
   M-BIS sin a-Mmax sin a, (2)
   where Mmax = BIS is the maximum moment developed by the frame. With the position of the frame shown in Fig. 2, a, angle a-90 °, therefore the moment acting on the frame is maximal.



   Fig. 2. The principle of the DC motor. a is the formation of the moment at a = 90 °; b - formation of the moment at a = 270 ": e - formation of a constant torque direction.
Let us now consider another position of the frame when it turns half a turn and the conductor A is already under the pole 5, and the conductor B is under the pole N (Figure 2.6). Since the direction of the current in the conductors was the same, it is possible to determine by the same rule of the left hand that in this position of the frame the force F acting on its conductors changed its direction to the opposite direction. Accordingly, the opposite direction and the direction of the torque M will change, which will tend to rotate the frame already in the other direction, clockwise. The same conclusion can be made easily on the basis of an analysis of formula (2): since the angle a became 270 ° (90 ° +/- 180 °) or, which is the same, -90 °, then sin a = -1 and the moment changed its sign to the opposite.
   Thus, the frame under the action of a direction-changing torque will oscillate about its axis of rotation 00. "Such a device obviously can not be placed in the basis of a constant-direction rotary-motion motor, which usually requires a constant direction and a constant direction of rotation.
   What should be done to ensure that the resulting torque on the frame has a constant direction? It is not difficult to see that for this there are two principal possibilities:
   1) to change the current direction in the conductors of the frame when the position of the conductors under the poles of the magnetic system changes;
   2) change the direction of the magnetic field as the frame rotates and the current direction in it is constant, the gili, in other words, create a rotating magnetic field.
   The first of these principles is used in direct current engines, the second one is the basis for the operation of AC motors.
   Let us first consider the formation of a constant in the direction of the torque by changing the direction of the current in the frame and, thus, we will explain the principle of the action of DC motors.
   To change the current direction in the conductors of the frame, it is obviously necessary to have a device that changes the direction of the current in the frame, depending on the position of its conductors.
   The simplest possible mechanical device of this type can be realized by a simple change in the construction of the sliding contacts K (Fig. 2, a, b) serving to feed the current to the frame. This transformation consists in replacing two contact rings with one, but consisting of two halves (segments) isolated from each other, to which the conductors of frames A and B are connected (Fig. 2, c). In this case, when the frame is rotated by half a turn, the current direction in the conductors changes to the opposite one, so the torque will retain its direction and the frame will continue to rotate in the same direction. A similar mechanical switching device, called a collector, is used in conventional DC motors. In some special engine designs discussed below, this switching device is made non-contact (electronic).
   A real DC motor, a simplified diagram of which is shown in Fig. 3, has, of course, a much more complicated construction than that shown in Fig. 2, c. To obtain a large torque, usually several dozen frames are used, which form the winding 1 of the armature. The conductors of the armature winding are located in the grooves of the cylindrical ferromagnetic core 2, and their ends are connected to a corresponding number of isolated segments of the ring forming the collector (not shown in the figure).



   Fig. 3. Diagram of the DC motor.
   Fig. 4 The principle of the synchronous motor. a is the equilibrium position; б - formation of the torque
   The core, winding and collector form the motor armature, which rotates in the bearings installed in the engine casing. The current to the armature conductors is fed from the DC network by means of sliding brush contacts.
The magnetic field is created by the poles 3 of the magnet located in the motor housing 4. This magnetic field is usually called the excitation field. For its formation, permanent magnets or electromagnets can be used.
   The winding of the electromagnet is usually called the excitation winding (position 5 in Figure 3). The field winding is connected to the DC network and can be switched on independently of the armature winding or in series with it. In the first case, the engine is called an engine with independent excitation, in the second case - with consecutive excitation.
   Some DC motors have two excitation windings - independent and consistent. Such engines are called engines with mixed excitation. The number of poles of the magnetic excitation field can be more than two, for example four, as shown in Fig. 3.
   Let us now turn to the consideration of AC motors.
   Let us turn again to the experiments with the frame and consider its position shown in Fig. 4, a. Note that this figure is a simplified front view of the circuit in Fig. 2a, the direction of the current in the conductor flowing into the plane of the drawing is indicated by a cross, and the flow from the plane of the drawing is a point.
   It follows from formula (2) that in the horizontal frame shown, the torque acting on the frame is zero (a = 0), although the forces acting on the conductors A and B are nonzero. The explanation of this situation is that the direction of the action of these forces passes through the axis of rotation of the frame 00 ", therefore the arm of forces F relative to this axis is zero and no torque is created.
   This position of the frame is balanced, and it maintains a state of rest.
   Let's turn now somehow magnet N-S   clockwise at some angle a, without changing the direction of the current in the conductors, as shown in Fig. 4.6. It is easy to see that such a rotation of the magnet will cause a change in the direction of action of the forces F and the appearance of the arm of applying these forces relative to the axis of rotation of the frame. As a result, a torque will act on the frame in accordance with formula (2), tending to return the frame to the equilibrium position, and the frame will then turn after the magnet by the same angle a.
   If we now begin to rotate the NS magnet uniformly, the frame will also rotate in the same direction synchronously with the rotation of the magnetic field, since when the "non-synchronism" appears between the rotation of the field 12 and the frame (a = / = 0) , seeking to synchronize this rotation. Motors using this principle, therefore, received the name of synchronous motors, and their moment, determined by means of formula (2), is often called the synchronizing moment.
   So, for the synchronous motor to work, it is necessary to create a rotating magnetic field and place conductors in it, streamlined unchanged in the direction of the current.
   Let us consider how a rotating magnetic field is obtained in real AC motors. The rotating magnetic field of a synchronous motor is formed by a system of windings connected to an alternating current network. Typically, synchronous motors use three-phase windings stacked in the grooves of the stator core with a certain spatial shift along the circumference. In the theory of electrical machines it is shown that if such a winding is connected to a three-phase alternating current network, the currents form a magnetic field rotating in the air gap of the motor, whose rotation frequency n0 is determined by the frequency of the current in the network f and the number of pole pairs of the motor p formed by the stator winding:

   The interaction of this rotating magnetic field with current in the conductors of the rotor winding and will cause the rotation of the synchronous motor, which will occur synchronously with the rotation of the stator magnetic field.
In the absence of the moment of loading on the shaft of the synchronous motor, the axes of the magnetic fields of the stator and the rotor coincide (cc = 0), the motor does not develop a torque and rotates with a frequency n0. If a motor appears at the moment of resistance (load), the axis of the rotor field begins to lag behind the axis of the stator field, and this process will occur until, at some angle a0, the rotating (synchronizing) torque of the motor becomes equal to the moment of loading. The synchronous motor will continue to rotate at a frequency of n, overcoming the moment of resistance on its pal.
   This position will be maintained until the value of the maximum motor torque corresponding to the angle «= 90 °. With further increase in the load moment, the synchronous motor is said to "fall out of synchronism" and stop. Thus, the synchronous motor can overcome only a certain, nominal resistance moment, which corresponds to the angle a = 20-30 ° for synchronous motors.
   A simplified diagram of the synchronous motor is shown in Fig. 5. In the engine casing in the grooves of the core I, a three-phase winding of alternating current 2 is laid, which, when connected to an alternating current network, forms a rotating magnetic field. A core with a winding forms a fixed part of the motor - a stator.
   The field current is played by the excitation winding 3 of the motor located on the ferromagnetic core 4. The field winding has several tens of turns (frames) and is connected to the DC network through the contact rings and brush contact (in Figure 5 these parts of the motor are not shown).
   The excitation winding, the core and the contact rings together with the motor shaft form the rotor of the motor - its rotating part.
   The synchronous motor, constructed according to the scheme of Fig. 5, is usually called the pole pole, which is due to the presence of poles at the rotor core. Along with this, there are so-called non-pole-pole synchronous motors, in which the core of the rotor does not have pronounced poles.

   Fig. 5. Diagram of a synchronous motor with electromagnetic excitation.
   The action of the synchronous motor can be based, in addition to the above-discussed principle of the interaction of a magnetic field and a conductor with a current, also on the principle of interaction of a magnetic field with a permanent magnet or a ferromagnetic body. To illustrate this principle, we consider the behavior of a permanent magnet 2 placed in the field of a magnet 1, as shown in Fig. 6. It is known from the course of physics that the opposite poles of two magnets are always attracted, while the same poles are repulsed. Accordingly, the magnet 2 will occupy a position at which its north pole will be at the south pole of magnet 1, and the south pole at the north pole. This position will be equilibrium for the system of two magnets in question.



   Fig. 6. Diagram of the synchronous motor.
   Fig. 7. The principle of the asynchronous motor.
   We note here a very important fact: the equilibrium position simultaneously corresponds to the minimum magnetic resistance in the path of the magnetic flux and to the minimum curvature of the magnetic field lines. In other words, the magnets tend to occupy such a mutual position, in which the magnetic field lines are slightly curved, and the magnetic resistance to the magnetic flux is minimal.
   Now it is easy to find out what will happen to the magnet 2 if we start to rotate the magnet I. Obviously, it will also begin to rotate together with the magnet I, trying to maintain the equilibrium position, and the rotation frequencies of both magnets will be the same (synchronous). Synchronous motors, whose rotors are permanent magnets, are called synchronous motors with permanent magnets.
The same synchronous rotation of the rotor can also be obtained if, instead of a permanent magnet 2, a ferromagnetic body of the same shape is placed in the field of a permanent magnet I. Once placed in a magnetic field, the ferromagnetic rotor will be magnetized, with the south pole at the north pole of the magnet, and the north pole of the ferromagnetic body at the south pole of the magnet. Such a position the ferromagnetic rotor will tend to retain even when the magnetic field rotates, which determines the operation of the synchronous motor with the rotor in the form of a ferromagnetic body. This type of engine was called a synchronous motor with a reactive rotor. Note that, for the operation of such an engine, its rotor should in principle have pronounced poles, and their number (not necessarily two) must be equal to the number of poles of the rotating magnetic field.
   The formation of a rotating magnetic field of a synchronous jet engine with permanent magnets occurs in the same way as in a conventional synchronous motor, using a stator winding connected to an alternating current network.
   To explain the operation principle of another, very common type of AC-asynchronous motor, we turn again to experiments with a frame placed in a magnetic field. However, this time we will not draw current to the frame, but make it closed, as shown in Fig. 7. Let's find out what will happen with such a frame if we start rotating the poles of the magnet again, say, with the clock speed clockwise.
   Since the frame is initially immobile, when the magnet rotates, the magnetic flux passing through the frame begins to change. Then, in accordance with the law of electromagnetic induction (Faraday's law), the electromotive force (EMF) of induction will be induced (induced) in the frame, under which action current will flow through the conductors of the frame. The interaction of this current with magnetic field   will lead to the appearance of torque, under the action of which the frame will begin to rotate. This is the principle of the asynchronous motor.
   To determine the direction of rotation of the frame, we apply Lenz's law, according to which the currents flowing in the frame when the magnetic flux changes through its contour have a direction in which they prevent this change. And since in the conducted experiment this change is caused by the rotation of the magnetic field, the currents in the frame will have a direction in which the resulting torque will rotate the frame in the same direction as the field, since only in this case there will be a decrease in the change in the magnetic flux through the contour of the frame. Thus, the frame will begin to rotate in the same direction as the field, but with the speed of rotation n.
   We note here one important fact: the frequency of rotation of the frame n will always be somewhat less than the rotation frequency of the magnetic field n0. Indeed, if we assume the opposite, ie, that the frame and field rotation frequencies are the same, then the magnetic flux through the contour of the frame will not change, the EMF and currents in the frame will not be induced accordingly, and the torque disappears.
   Thus, to create a torque frame, a fundamental difference is needed between the rotational frequencies of the magnetic field n0 and the frame n, that is, the asynchrony (nonsynchronism) of their rotation, which is reflected in the name of this type electric motor. The degree of difference between these frequencies and rotations is numerically characterized by the so-called slip of an asynchronous motor s, determined by formula

It should be noted here that when the load moment appears on the frame axis due to the reduction in the frame speed n (the frame is braked), the motor slip will increase and the magnetic flux through the contour of the frame will start to change more strongly. At the same time, the EMF and currents in the frame and, correspondingly, the torque of the motor will begin to increase. This process will occur until, at a certain rotation speed of the frame, the torque of the frame does not balance the moment of loading and a new steady-state operating mode will not occur. When the load decreases, the reverse process occurs.
   So, for the operation of an asynchronous motor it is necessary to have a rotating magnetic field and closed frames (contours) on the rotating part of the motor-rhetor.
   The rotating magnetic field of an asynchronous motor (Figure 8) is formed in the same way as in a synchronous motor, with the help of windings 2 located in the grooves of the stator package I and connected to an alternating current network.
   The windings of the 3 rotors of an asynchronous motor consist usually of several tens of closed frames (contours) and have two basic designs: short-circuited and phase.
   When carrying out a short-circuited winding, the conductors laid in the grooves of the ferromagnetic package 4 of the rotor are short-circuited. Usually such winding is obtained by pouring molten aluminum into the grooves of the bag and is called a "squirrel cage".
   When making a "phase" winding, the ends of the winding phases are output through the sliding contacts (rings), which allows to include in the rotor circuit various additional resistors, necessary, for example, to start the engine or regulate its speed.

   Fig. 8. Scheme of an asynchronous motor.
   It should be noted that to obtain the torque of an asynchronous motor, it is not necessary to place a winding from the rotor electrical conductors. It is possible to produce the rotor simply as a solid ferromagnetic cylinder and place it in a conventional stator of an asynchronous motor. Then, when the stator windings are connected to the grid and a rotating magnetic field appears in the massive body of the rotor, so-called eddy currents (Foucault currents) will be induced, the direction of which is also determined by the Lenz law. When these currents interact with the magnetic field, a torque is created, under which the solid rotor starts to rotate in the direction of rotation of the magnetic field, just like a conventional rotor with a winding. Such engines are called asynchronous motors with a massive rotor.
   We note that eddy currents arise, of course, and 9 cores of a conventional rotor with a winding, but in this case they are harmful, since they cause additional heating of the rotor. Usually their action is tried to weaken, for which the core of the rotor is collected (blended) from separate electrical steel sheets isolated from each other, thereby creating a large electrical resistance for the eddy currents. In this case, the core is often called a packet.
   Reviewed in this section general principles   The work of DC and AC motors is the physical basis of work and engines for special purposes.
   Electric motors of both general and special purpose are characterized by nominal data, which include power on the motor shaft, voltage, current, speed, efficiency and some other values. The main nominal data are regulated state standards   (GOST) for electric machines and are indicated in the passport.
   The nominal data of the engine corresponds to the normal thermal mode of its operation, in which the temperature of all parts of the engine does not exceed the permissible level. To ensure this mode, the engine is appropriately calculated and has a cooling (ventilation) system.
   The cooling method distinguishes:
   engines with natural cooling, in which there are no special devices for ventilation;
motors with internal and external self-ventilation, cooled by a fan located on the motor shaft and ventilating respectively the internal cavity or the external surface of the engine;
   motors with independent cooling, which are cooled with a separate fan ("rider"), which has its own drive.
   The operation of the engines is also characterized by some other values ​​that are not directly indicated in its passport - the nominal torque corresponding to the nominal data of the engine and the starting torque and current that correspond to the moment of starting (connecting to the network) of the engine. When analyzing the operation of the engine, the values ​​of the starting torque and current are usually compared with the corresponding nominal values. The torque and current of the motor at start-up must not exceed certain permissible values ​​determined by the conditions of engine heating and the normal operation of its collector-brush assembly.

The electric motor is a device that converts the energy of electricity into mechanical energy. Electric motors have become widespread, in almost all areas of daily life. Before considering the types of electric motors, we should briefly discuss the principle of their operation. All the action takes place according to Ampere's law, when a magnetic field is formed around the wire where the electric wire flows. When this wire rotates inside the magnet, each side of it will be alternately attracted to the poles. Thus, the wire loop will rotate. Electric motors are divided among themselves, depending on the current used, which can be variable or constant.

AC electric motors

The peculiarity of the alternating current is the change of its direction a certain number of times within a second. As a rule, an alternating current with a frequency of 50 hertz is used.

With the connection, the current first starts flowing in one direction, and then its direction is completely changed. Thus, the sides of the loop, receiving a jerk, are attracted alternately to different poles. That is, in fact, their ordered attraction and repulsion take place. Therefore, with a change in direction, the wire loop will rotate around its axis. With the help of these circular motions, the energy is converted from electrical to mechanical.

AC motors have many designs and are represented by a wide variety of models. This makes it possible to widely use them not only in industry, but also in everyday life.

Direct current electric motors

The first invented engines were still direct-current devices. The alternating current was still unknown at that time. Unlike a variable, the DC motion is always in the same direction. Rotation of the rotor stops after a turn of 90 degrees. The direction of the magnetic field coincides in the direction of the electric current.

Therefore, a metal ring connected to a DC source is cut into two parts and is called a ring commutator. At the beginning of the rotation, the current flows through the first side of the switch and through the wires. The electric current flowing through the wire loop creates a magnetic field in it. With further rotation of the loop, the switch also rotates. After the empty space has passed through the ring, it moves to the other part of the switch. Further, the effect of an alternating electric current occurs, thanks to which the rotation of the loop continues.

All DC motors are used in conjunction with AC devices in production and transport.

Classification of electric motors

Electric motors found applications not only in the industrial sphere, but also in the household. Motors of the asynchronous type, as well as synchronous ones, are distinguished by such property as reversibility. They can function not only in the generator mode, but also in the motor. Read what is the corrugation for the cable and wires and how to choose.

It is necessary to go more closely to the study of electrical machines, so consider the principle of the asynchronous engine:

• After this motor is connected to the network, its windings must be fixed by means of a triangle.
• When there is no marking on the terminals of the terminal board, the beginning and end part of the winding must be determined independently.
• After turning on the windings of the engine, the rotating field of the driving part will form.
• Connect the engine in three-phase network   alternating electricity. The field penetrates not only into the winding of the fixed part, but also into the winding part of the rotor.
• A moving field induces an electromotive force. In the winding of the fixed part, the electromotive force of self-induction is induced. Its direction is oriented towards the voltage, it also plays the role of a current limiter in the stator winding.
• The winding of the motor is short-circuited. In motors with this type of rotor, under the influence of EMF, a current appears in the winding. Due to the fact that the current in the winding interacts with the magnetic field, a force Fem is created, which is electromagnetic.

Collector motors are the same as asynchronous machines, to engines of universal type.

Collector electromotors can function both from permanent and from alternating electricity.

To develop high speed, such engines do not need high loads. In household systems, the starting of collector motors is most often performed under load.

For example, you can consider a fan of a conventional vacuum cleaner. Those parts of the machine that are driven are usually fixed to the motor shaft. Collector universal engines have some drawbacks. In addition to producing an unpleasant noise, they can interfere with various radio devices. Such motors require special care. Read the manual how to select a detector hidden wiring   and how to use it.

Collector universal machines have their own merits. In household machines they are used much more often than asynchronous motors. The speed of rotation of the collector engines can reach 25,000 rpm. Despite this, they are distinguished by a smooth adjustment of the speed regimes. This is their universality.

The principle of functioning of a collector electric motor is as follows: a rectangular frame with an axis of rotation, which is a current conductor, placed between the poles of an electromagnet, will certainly begin to move. From where the current flows in the frame, the course of its rotation will also depend.

The principle of operation of different types of engines

See the principle of the asynchronous motor on video:

The principle of operation of electric motors will differ depending on the type of engine. Thanks to electric motors, man was able to achieve such high technological progress. People learn about how the electric DC engine works from school. Virtually all DC machines function due to magnetic attraction and reverse process - repulsion.

If you place a wire between two poles of a magnet and pass a current through this wire, it will be pushed out from the inside. The current at the time it passes through the wire forms a magnetic field along its entire length. When the field of the magnet and the conductor are connected, the magnetic field from one pole will increase, and on the other hand it will weaken on the other hand. The wire will be pushed at a right angle and in a specific direction, which can be calculated by the rule of the left hand. This phenomenon was used not only for the work of the earliest electric engines, but also used in modern devices. Read.

Electric motor modern type   has not one frame, but an anchor with a large number of conductors. They are packed in special grooves. A stator with a winding is used instead of an electromagnet. The principle of this engine is as follows:

• if we let the current through the upper wires of the armature at the rate "from us," and in the opposite direction, let the current flow through the lower wires,
• the upper conductors will begin to be pushed to the right side, and the lower ones to the left.
• the force of the impact will be sent to the armature wire.
• thanks to this process, the anchor will begin to rotate.
• the torque is transmitted to the motor shaft, and the motor starts to drive various machinery mechanisms.

AC motors are most often used in domestic conditions. Their necessity is due to the fact that they can provide a permanent constant speed of rotation, and in addition provide the opportunity to adjust this speed.
  Look at the video for a detailed explanation of AC and DC:

In the case of such an engine windings wound around the anchor or rotor are located. The leads of the windings are soldered to the parts of the collector or the current collector ring. The voltage is fed through graphite brushes. They must apply voltage to only one pair of pins. The rotating phase in the motor of this type appears as a result of the interaction between the current of the armature and the flow in the winding where the excitation process takes place. Since the course of alternating current will be transformed, there will be an exchange and direction of flow in the anchor and the hull. Read.

Look at the video of how the Electrolux inverter engine works:

Video

Watch a video about motors:

Constantly rotation will be a one-sided type. To change the speed of movement, you need to change the voltage. In the case of drills or, for example, in vacuum cleaners, a variable-type resistance is used to adjust the speed.

Nov 22 2015 Tatyana Sumo