asynchronous motors. Encyclopedia of radio electronics and electrical engineering

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Asynchronous electric motor, electrical asynchronous machine for converting electrical energy into mechanical energy. The principle of operation of A. e. is based on the interaction of a rotating magnetic field that occurs when a three-phase alternating current passes through the stator windings with a current induced by the stator field in the rotor windings, resulting in mechanical forces that cause the rotor to rotate in the direction of rotation of the magnetic field, provided that the rotor speed n is less than the rotation frequency of the field n₁. Thus, the rotor rotates asynchronously with respect to the field.

For the first time, the phenomenon called rotational magnetism was demonstrated by the French physicist D. F. Arago (1824). He showed that a copper disk mounted on a vertical axis begins to rotate if a permanent magnet is rotated over it. 55 years later, on June 28, 1879, the English scientist W. Bailey obtained the rotation of the magnetic field by alternately connecting the windings of 4 rod electromagnets to a direct current source. The works of M. Despres (France, 1880-1883), I. Thomson (USA, 1887) and others describe devices based also on the properties of a rotating magnetic field. However, a rigorous scientific exposition of the essence of this phenomenon was first, almost simultaneously and independently of each other, given in 1888 by the Italian physicist G. Ferraris and the Croatian engineer and scientist N. Tesla.

Biphasic A. e. was invented by N. Tesla in 1887 (English patent No. 6481), he made a public announcement of this invention in 1888. This type of A. e. not received mainly due to poor starting performance. In 1889, M. O. Dolivo-Dobrovolsky tested the world's first three-phase AE, which he designed, in which he used a "squirrel wheel" type rotor (German patent No. 51083), and placed the stator winding in grooves around the entire circumference of the stator. In 1890, Dolivo-Dobrovolsky invented a phase rotor with rings and starting devices (English patents No. 20425 and German No. 75361). After 2 years, he also proposed a rotor design called the "double squirrel cage", which, however, began to be widely used only from 1898 thanks to the work of the French engineer P. Bouchereau, who introduced A. e. with a rotor such as a motor with special starting characteristics.

The design of A. e., their power and dimensions depend on the purpose and working conditions.

For example, air-cooled and water-cooled engines (general applications); sealed, oil-filled (for electric drills) and explosion-proof (for work in mines, explosive areas, etc.); dust-, splash-proof (for use in marine conditions and tropical climates), etc. Some types of A. e. (for example, step, for servo systems, automation and telemechanics circuits, with step speed control, etc.) are developed and produced complete with control units and start-up equipment, with built-in gearboxes. Three-phase A. e. compared to single-phase ones, they have better starting and operating characteristics. The main structural elements of A. e.: the stator is a fixed part (Fig. 1 a) and the rotor is a rotating part (Fig. 1 b, c). In accordance with the method of performing the rotary winding of A. e. are divided into motors with slip rings and squirrel-cage motors. The air gap between the stator and the rotor in A. e. is made as small as possible (up to 0,25 mm). The frequency of rotation of the rotor A. e. depends on the frequency of rotation of the stator magnetic field and is determined by the frequency of the supply current and the number of pole pairs of the motor.

Fig. 1

When starting A. e. with a squirrel-cage rotor, a starting current occurs, the strength of which exceeds the strength of the rated current by 4-7 times. Therefore, direct connection to the network is only used for motors up to 200 kW. More powerful A. e. with a squirrel-cage rotor, they are first turned on at a reduced voltage so that the starting current decreases by 3-4 times. For the same purpose, start A. e. through an autotransformer connected during the start-up in series with the stator winding. The strength of the starting current of motors with a phase rotor is limited by the starting resistance in the rotor circuit, which is gradually reduced during the run-up of the rotor. After starting A. e. the rotor winding is short-circuited. To reduce friction losses and wear of the brushes, they are usually lifted by a brush-lifting device, which first short-circuits the rotor winding through the rings.

The frequency of rotation of A. e. they are mainly regulated by changing the number of pole pairs, the resistance included in the rotor circuit, changing the frequency of the supply current, as well as cascading several machines. Direction of rotation A. e. change by switching any two phases of the stator winding.

Due to the simplicity in production and reliability in operation, it is widely used in electric drive. The main disadvantages of A. e. - limited speed control range and significant reactive power consumption in low load mode. The creation of adjustable static semiconductor frequency converters significantly expands the scope of A. e. in automatic controlled electric drives.

Capacitor induction motor

1) an asynchronous electric motor powered by a single-phase network and having two windings on the stator, one of which is connected directly to the network, and the other is connected in series with an electric capacitor to form a rotating magnetic field. Capacitors create a phase shift between the winding currents, the axes of which are shifted in space. The greatest torque develops when the phase shift of the currents is 90°, and their amplitudes are chosen so that the rotating field becomes circular. When starting K. a. e. both capacitors are on, and after its acceleration, one of the capacitors is turned off; this is due to the fact that much less capacitance is required at rated speed than at start-up. K. a. in terms of starting and operating characteristics, it is close to a three-phase asynchronous motor. It is applied in electric drives of low power; at powers above 1 kW, it is rarely used due to the significant cost and size of capacitors.

2) A three-phase asynchronous electric motor, connected through a capacitor to a single-phase network. The working capacitance of a capacitor for a 3-phase motor is determined by the formula C_р = 2800  (µF) if the windings are connected in a star pattern, or C_р = 4800  (µF) if the windings are connected in a delta pattern. Starting capacitor C_п=(2,5 - 3)×С_р. The operating voltage of the capacitors must be 1,5 times higher than the mains voltage; capacitors are installed necessarily paper.

Scheme (a) and vector diagram (b) of a capacitor induction motor: U, U_Б, SHE IS_C - voltage; I_A, The_Б - currents; A and B - stator windings; B - centrifugal switch to disconnect C₁ after engine acceleration; C₁ and C₂ - capacitors.

Scheme of inclusion in a single-phase network of a three-phase asynchronous motor with stator windings connected according to the "star" (a) or "triangle" (b) scheme: B₁ Rotation direction switch (reverse), V₂ - Starting capacity switch; FROM_р - working capacitor; C_п - starting capacitor; HELL - asynchronous electric motor.

Publication: library.espec.ru

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