X Международная студенческая научная конференция Студенческий научный форум - 2018

The principle of converting electrical energy into mechanical energy by an electromagnetic field was demonstrated by the British scientist Michael Faraday in 1821 and consisted of a free-hanging wire dipped in mercury. A permanent magnet was installed in the middle of the bath with mercury. When current was passed through the wire, the wire rotated around the magnet, indicating that the current caused a cyclic magnetic field around the wire. This is the simplest kind from the class of electric motors.

The greatest technical achievement of the late 19th century was the invention of an industrial electric motor. This compact, economical, convenient motor soon became one of the most important elements of production, replacing other types of engines from everywhere where it was only possible to deliver electric current. Electric motors appeared in the second quarter of the XIX century, but several decades passed before favorable conditions for their widespread introduction into production were created.

One of the first perfect electric motors, powered by a direct current battery, was created in 1834 by the Russian electrical engineer Jacobi. This engine had two groups of U-shaped electromagnets, of which one group was located on a stationary frame. Their pole pieces were arranged asymmetrically - elongated in one direction. The motor shaft consisted of two parallel brass discs connected by four electromagnets, placed at an equal distance from each other. When the shaft rotated, the movable electromagnets passed against the poles of the stationary. In the latter polarities they went alternately: either positive or negative. Conductors, reinforced on the shaft of the machine, retreated to the electromagnets of the rotating disk. A commentator was attached to the motor shaft, which changed the direction of the current in the moving electromagnets during each quarter of the shaft rotation. The windings of all electromagnets in the fixed frame were connected in series and flowed around the battery in one direction. The windings of the electromagnets of the rotating disk were also connected in series, but the direction of the current in them varied eight times in one revolution of the shaft. Consequently, the polarity of these electromagnets also changed eight times in one revolution of the shaft, and these electromagnets were alternately attracted and repelled by the electromagnets of the fixed frame.

The Jacobi engine for its time was the most perfect electrical device. In the same year of 1834, a detailed report on the principles of his work was presented to the Paris Academy of Sciences.

In 1838, Jacobi perfected his electric motor and, installing it on a rowing boat, with ten satellites made a small voyage along the Neva at a speed of 4.5 km / h. The source of the current was a powerful battery of galvanic cells.

Until the perfect electric generator was invented and introduced into production, the electric motors could not be widely used, since it was too expensive and unprofitable to feed them from a battery. In addition, for various reasons, direct current motors have received only limited application. A much more important role is played in the production of electromotor, working on alternating current, to the consideration of which we now turn.

For an alternating current, a special motor design is required. The inventors did not immediately find it. First of all, a model of the so-called synchronous alternating current motor was developed. One of the first such engines was built in 1841 by Charles Wheatstone.

His system had big disadvantages: in addition to the fact that the synchronous engine required an additional accelerating engine for its launch, it also had another flaw: in the event of an overload, the synchronism of its course was violated, the magnets started to break the rotation of the shaft and the engine stopped. Therefore, synchronous motors are not widely used. Genuine revolution in electrical engineering

It occurred only after the invention of an induction motor. A similar device was invented by Bailey in 1879.

In 1888, the Italian physicist Ferraris and the Yugoslav inventor Tesla (who worked in the US) discovered the phenomenon of a rotating electromagnetic field. Tesla's invention marked the beginning of a new era in electrical engineering and aroused a keen interest throughout the world. Already in June 1888, Westinghouse Electric Company bought from him for a million dollars all patents for a two-phase system and offered to organize the manufacture of asynchronous motors at their plants. Soon the induction motor of Tesla was significantly redesigned and perfected by the Russian electrical engineer Dolivo-Dobrovolsky. The first important innovation that Dolivo-Dobrovolsky introduced into the asynchronous motor was the creation of a rotor with a "squirrel cage" winding. In all the early models of asynchronous motors, the rotors were very unsuccessful, and therefore the efficiency of these motors was lower than that of other types of electric motors. Great importance was played here by the material from which the rotor was made, since it had to satisfy two conditions at once: to have low electrical resistance and to have good magnetic permeability. In terms of reducing electrical resistance, the best design could be a rotor in the form of a copper cylinder. But copper is a bad conductor for the magnetic flux of the stator, and the efficiency of this engine was very low. If the copper cylinder was replaced by steel, the magnetic flux increased sharply, but since the electrical conductivity became less than copper, the efficiency was again low.

Dolivo-Dobrovolsky found a way out of this contradiction: he performed the rotor in the form of a steel cylinder, and in the drilled along the periphery of the last channel began to lay copper rods. On the frontal parts of the rotor, these rods are electrically connected to each other. The decision of Dolivo-Dobrovolsky was the best. After he received a patent for his rotor in 1889, his device did not fundamentally change until now.

Subsequently, Dolivo-Dobrovolsky began to think over the design of the stator - the stationary part of the engine. Dolivo-Dobrovolsky saw two tasks before him: to increase the efficiency of the engine and to achieve greater uniformity of its operation.

Dolivo-Dobrovolsky built his first three-phase induction motor in the winter of 1889. As a stator, a circular anchor of a direct current machine with 24 half-closed slots was used in it.

Given Tesla's mistakes, Dolivo-Dobrovolsky distributed the windings in the grooves along the entire circumference of the stator, which made the distribution of the magnetic field more favorable. The rotor was cylindrical with windings "in the shape of a squirrel cage". The air gap between the rotor and the stator was only 1 mm, which at that time was a bold decision, as usually a gap was made larger. The squirrel cages had no isolation. As a source of three-phase current, a standard direct current generator was used, which was reconstructed into a three-phase generator in the same way as described above.

The impression made by the first launch of the engine on the management of the AEG was huge. For many it became obvious that the long thorny path of creating an industrial electric motor was finally passed to the end. In their technical performance, the engines of Dolivo-Dobrovolsky surpassed all existing electric motors - they had very high efficiency, they worked without fail in any modes, they were reliable and easy to handle. Therefore, they immediately became widespread throughout the world. From this time, the rapid introduction of electric motors into all spheres of production and the widespread electrification of industry began.

Operating principle

The principle of electromagnetic induction is based on the work of any electric machine. The electric machine consists of a stationary part - a stator (for asynchronous and synchronous alternating current machines) or an inductor (for direct current machines) and a moving part - a rotor (for asynchronous and synchronous alternating current machines) or an armature (for direct current machines). In the role of an inductor on low-power direct current motors, permanent magnets are often used.

The rotor of an asynchronous motor can be:

• Short-circuited;

• phase (with winding) - used where it is necessary to reduce the starting current and adjust the speed of the induction motor

• Anchor is a moving part of direct current machines (engine or generator) or a so-called universal motor working on the same principle. In fact, the universal motor is the same direct current motor with sequential excitation (armature winding and inductor are connected in series). The difference is only in the calculation of the windings. At a constant current there is no reactive (inductive or capacitive) resistance

Classification of electric motors

By the principle of the appearance of torque, electric motors can be divided into hysteresis and magneto electric. In the engines of the first group, the torque is created due to hysteresis during magnetization reversal of the rotor. These engines are not traditional and not widely used in industry.

The most common magneto electric motors, which by type of energy consumed are divided into two large groups - direct current motors and alternating current motors (there are also universal motors that can feed on both types of current).

Direct current motors

Direct current motor in section. On the right is a collector with brushes.

The direct current motor is an electric motor powered by a direct current. This group of engines, in turn, by the presence of a brush-collector unit is divided into: collector motors, brushless motors.

The brush-collector unit provides electrical connection of the rotating and stationary parts of the machine and is the most unreliable and difficult to maintain structural element.

As a type of excitation, collector motors can be divided into: motors with independent excitation from electromagnets and permanent magnets, motors with self-excitation.

Motors with self-excitation are divided into: parallel excitation engines, series excitation engines, mixed excitation engines, brushless motors - electric motors made in the form of a closed system using a rotor position sensor, a control system (coordinate converter) and a power semiconductor converter (inverter).

Alternating Current Motors

The alternating current motor is an electric motor powered by alternating current. According to the principle of operation, these engines are divided into synchronous and asynchronous motors. The principal difference is that in synchronous machines the first harmonic of the magneto motive force of the stator moves with the speed of rotation of the rotor (due to what the rotor itself rotates with the speed of rotation of the magnetic field in the stator), and for asynchronous ones there is always a difference between the speed of rotation of the rotor and the speed of rotation of the rotor fields in the stator (the field rotates faster than the rotor).

Synchronous electric motor is an alternating current motor whose rotor rotates synchronously with the magnetic field of the supply voltage.

Synchronous motors are divided into:

• Synchronous motor with excitation windings;

• synchronous motor with permanent magnets;

• Synchronous jet engine;

• hysteresis motor;

• Stepper motor;

• hybrid synchronous jet engine with permanent magnets;

• Reactive hysteresis motor.

There are synchronous motors with discrete angular displacement of the rotor - stepper motors. At them the set position of the rotor is fixed by supplying power to the corresponding windings. The transition to another position is carried out by removing the supply voltage from one winding and transferring it to the other.

An asynchronous electric motor is an alternating current motor in which the rotational speed of the rotor is different from the frequency of the rotating magnetic field generated by the supply voltage. These engines are the most common nowadays.

The number of phases of alternating current motors are divided into:

• single-phase - start manually, or have a starting winding, or have a phase-shifting circuit;

• two-phase - including capacitor ones;

• three-phase;

• multiphase;

Universal collector motor

Universal collector motor - collector motor, which can work on both a constant and an alternating current. It is made only with a serial excitation winding at a power of up to 200 W. The stator is made of special electro technical steel. The excitation winding is switched in partially at an alternating current and completely at constant. For alternating current nominal voltage of 127, 220 V, for constant 110, 220 V. It is used in household appliances, power tools.

Alternating current motors with power from the industrial network of 50 Hz do not allow obtaining a speed above 3000 rpm. Therefore, to obtain high frequencies, a collector motor is used, which, moreover, is obtained more easily and less than an alternating current motor of the same power, or special transmission mechanisms are used that change the kinematic parameters of the mechanism to the required (multipliers). When using frequency converters or the presence of a high frequency network (100, 200, 400 Hz), alternating current motors are lighter and smaller than collector motors (the collector node sometimes occupies half the space). The resource of alternating current induction motors is much higher than that of collector motors, and is determined by the condition of the bearings and the insulation of the windings.

A synchronous motor with a rotor position sensor and an inverter is an electronic analog of a direct current collector motor. Strictly speaking, the universal collector motor is a direct current collector motor with series-connected excitation windings (stator), optimized for operation on alternating current of the household electrical network. This type of motor, regardless of the polarity of the applied voltage, rotates in one direction, because due to the serial connection of the stator and rotor windings, the change of the poles of their magnetic fields occurs simultaneously and the resultant moment remains directed to one side. For the possibility of working on alternating current, a stator is used from a soft magnetic material having a small hysteresis (resistance to magnetization reversal). To reduce the losses due to eddy currents, the stator is made of a set of isolated plates.

The use of electric motors in our time

Nowadays, electric motors are used in any production. No plant can do without them. The work of many household appliances is possible only thanks to electric motors. The most widely used three-phase electric motors.

Systems of universal drives are also common now. Since electric motors are the basis of modern household appliances, and also, without them, there is no way to do in any sphere of modern production, the main requirements imposed on them are quality and reliability.

Electric motors are divided into two groups: direct and alternating current. They are synchronous and asynchronous. A wide use of synchronous motors occurs in such installations as air ducts, hydraulic pumps. Asynchronous electric motors have been used in household appliances and in production.

Electric motors, which have received the widest distribution - are three-phase. Without them, no sector of the national economy can do. Among the areas of their application - telemechanics, automation. And in general, the work of household appliances is impossible without a working electric motor. In our time, the demand for electric motors is constantly growing.

Temporary current electric motors have been widely used in industry, transport, aviation, and automatic control systems. And also in many areas of the national economy, where it is impossible to do without their use. Therefore, their reliability and quality are so important.

The source of information:

1) https://ru.wikipedia.org/wiki/Электрический_двигатель

2) http://etosibir.ru/e-lektrodvigateli-shiroko-primenyayutsya-v-nashe-vremya/

Код для цитирования: Скопировать