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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Electric motors. asynchronous motors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric motors Asynchronous motors called electrical machines having at least two windings in which the alternating voltages are phase shifted relative to each other. Operating principle In asynchronous systems, it becomes possible to create a rotating magnetic field in a mechanically stationary device. A coil connected to an alternating current source produces a pulsating magnetic field, that is, a magnetic field that changes in value and direction.
In a cylinder with an inner diameter D, three coils are placed on the surface, spatially displaced relative to each other by 120°. The coils are connected to a three-phase voltage source (Figure 16.6). On fig. 16.7 shows a graph of instantaneous currents that form a three-phase system. Each of the coils creates a pulsating magnetic field. The magnetic fields of the coils, interacting with each other, form the resulting rotating magnetic field, characterized by the vector of the resulting magnetic induction On fig. 16.8 shows the magnetic induction vectors of each phase and the resulting vector At the moment t = t1 the current and magnetic induction in the A-X coil are positive and maximum, in the BY and CZ coils they are the same and negative. The vector of the resulting magnetic induction is equal to the geometric sum of the vectors of the magnetic inductions of the coils and coincides with the axis of the coil A-X. At the moment t = t2 the currents in coils A-X and CZ are the same in magnitude and opposite in direction. The current in phase B is zero. The resulting magnetic induction vector rotated clockwise by 30°.
At the moment t = t3 the currents in the coils A-X and BY are equal in magnitude and positive, the current in the CZ phase is maximum and negative, the vector of the resulting magnetic field is located in the negative direction of the CZ coil axis. For a period of alternating current, the vector of the resulting magnetic field will turn 360°. Linear speed of movement of the magnetic induction vector
where
field along the circumference of a cylinder with a diameter D. Linear Velocity
where n1 - synchronous frequency of rotation of a multi-pole magnetic field with the number of pairs of poles Р. The coils shown in fig. 16.6, create a bipolar magnetic field, with the number of poles 2P = 2. The field rotation frequency is 3000 rpm. To obtain a four-pole magnetic field, it is necessary to place six coils inside a cylinder with a diameter D, two for each phase. Then, according to formula (16.7), the magnetic field will rotate twice as slowly, with n1 = 1500 rpm. To obtain a rotating magnetic field, two conditions must be met:
Design An induction motor has a stationary part called the stator and a rotating part called the rotor. The stator contains a winding that creates a rotating magnetic field. There are asynchronous motors with squirrel-cage and phase rotor. Aluminum or copper rods are placed in the slots of the rotor with a short-circuited winding. At the ends, the rods are closed with aluminum or copper rings. The stator and rotor are made from electrical steel sheets to reduce eddy current losses. The phase rotor has a three-phase winding (for a three-phase motor). The ends of the phases are connected into a common node, and the beginnings are brought out to three slip rings placed on the shaft. Fixed contact brushes are applied to the rings. A starting rheostat is connected to the brushes. After starting the engine, the resistance of the starting rheostat is gradually reduced to zero. The principle of operation of an induction motor The principle of operation of an asynchronous motor will be considered on the model shown in Fig. 16.9. We represent the rotating magnetic field of the stator as a permanent magnet rotating with a synchronous rotation frequency u. Currents are induced in the conductors of the closed winding of the rotor. The poles of the magnet move clockwise. To an observer placed on a rotating magnet, it seems that the magnet is stationary, and the conductors of the rotor winding move counterclockwise. The directions of the rotor currents, determined by the right hand rule, are shown in Fig. 16.9.
Using the left hand rule, we find the direction of the electromagnetic forces acting on the rotor and causing it to rotate. The motor rotor will rotate at a speed of n1 in the direction of rotation of the stator field. The rotor rotates asynchronously, i.e. its rotation frequency n2 less than the frequency of rotation of the stator field w. The relative difference between the velocities of the stator and rotor fields is called slip:
The slip cannot be equal to zero, since at the same speeds of the field and the rotor, the induction of currents in the rotor would stop and, consequently, there would be no electromagnetic torque. The electromagnetic torque is balanced by the opposing braking torque If the stalled motor slip is equal to one, then the motor is said to be in short circuit mode. Unloaded asynchronous motor speed n2 approximately equal to the synchronous frequency n1. If the slip of an unloaded engine is S = 0, then the engine is said to be idling. The slip of an asynchronous machine operating in motor mode varies from zero to one. An asynchronous machine can operate in generator mode. To do this, its rotor must be rotated by a third-party motor in the direction of rotation of the stator magnetic field with a frequency n2 > n1. Asynchronous generator slip S < 0. An asynchronous machine can operate in the mode of an electric machine brake. To do this, it is necessary to rotate its rotor in the direction opposite to the direction of rotation of the stator magnetic field. In this mode, S > 1. As a rule, asynchronous machines are used in motor mode. The induction motor is the most common type of motor in the industry. The rotation frequency of the field in an asynchronous motor is rigidly related to the network frequency f1 and the number of pairs of stator poles. At frequency f1 = 50 Hz there is the following speed range (P - n1, rpm): 1 - 3000; 2 - 1500; 3 -1000; 4 - 750. From formula (16.7) we obtain
The speed of the stator field relative to the rotor is called the slip speed
Current frequency and EMF in the rotor winding
A locked-rotor asynchronous machine works like a transformer. The main magnetic flux induces in the stator and in the fixed rotor windings EMF E1 and E2K:
where fm - the maximum value of the main magnetic flux coupled to the stator and rotor windings; W1 and W2 - the number of turns of the stator and rotor windings; In order to obtain a more favorable distribution of magnetic induction in the air gap between the stator and rotor, the stator and rotor windings are not concentrated within one pole, but distributed along the circumferences of the stator and rotor. The EMF of the distributed winding is less than the EMF of the lumped winding. This fact is taken into account by introducing winding coefficients into the formulas that determine the magnitude of the electromotive forces of the windings. The values of the winding coefficients are slightly less than unity. EMF in the winding of a rotating rotor
Rotor current of running machine
where R2 - active resistance of the rotor winding; X2 - inductive resistance of the rotor winding,
A single-phase motor has one winding located on the stator. A single-phase winding powered by alternating current will create a pulsating magnetic field. Let us place a rotor with a short-circuited winding in this field. The rotor will not rotate. If you spin the rotor with a third-party mechanical force in any direction, the engine will work stably. This can be explained as follows. The pulsating magnetic field can be replaced by two magnetic fields rotating in opposite directions with a synchronous frequency n1 and having magnetic flux amplitudes equal to half the amplitude of the pulsating field magnetic flux. One of the magnetic fields is called forward-rotating, the other is called reverse-rotating. Each of the magnetic fields induces eddy currents in the rotor winding. When eddy currents interact with magnetic fields, torques are formed that are directed opposite to each other. On fig. 16.10 shows the dependencies of the moment on the forward field M ', the moment on the reverse field M "and the resulting moment M in the slip function M \uXNUMXd M ' - M ".
The sliding axes are directed opposite to each other. In the starting mode, the rotor is subjected to torques that are equal in magnitude and opposite in direction. Let us spin the rotor by a third-party force in the direction of a reciprocal magnetic field. An excess (resulting) torque will appear, accelerating the rotor to a speed close to synchronous. In this case, the slip of the motor relative to the recto-rotating magnetic field
Motor slip relative to a reverse-rotating magnetic field
Considering the resulting characteristic, we can draw the following conclusions. 1 output. A single-phase motor has no starting torque. It will rotate in the direction in which it is spun by an external force. 2 output. Due to the braking action of the reverse rotating field, the performance of a single-phase motor is worse than that of a three-phase motor. To create a starting torque, single-phase motors are supplied with a starting winding that is spatially displaced relative to the main, working winding by 90 °. The starting winding is connected to the network through phase-shifting elements: a capacitor or active resistance. Figure 16.11 shows the motor winding switching circuit, where P is the working winding, P is the starting winding. The capacitance of the phase-shifting element C is selected so that the currents in the working and starting windings differ in phase by 90 °. A three-phase asynchronous motor can operate from a single-phase network if its windings are connected according to the following diagrams (Fig. 16.12). In the diagram shown in fig. 16.12, and the stator windings are connected by a star, and in the diagram in fig. 16.12, b - a triangle. Capacitance value C ~ 60 uF per 1 kW of power.
Author: Koryakin-Chernyak S.L.
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, constructed for three moments of time t1, T2, T3. The positive directions of the axes of the coils are marked +1, +2, +3.
- AC voltage frequency; T is the period of the sinusoidal current; pg - frequency of rotation of the magnetic field or synchronous frequency of rotation. For a period T, the magnetic field moves a distance 
where
- pole division or distance between the poles of a magnetic
whence
With an increase in the load on the motor shaft, the braking torque becomes greater than the torque, and the slip increases. As a result, the EMF and currents induced in the rotor winding increase. The torque increases and becomes equal to the braking torque. The torque can increase with increasing slip up to a certain maximum value, after which, with a further increase in the braking torque, the torque decreases sharply and the motor stops.
- voltage frequency in the network; TO01 and K02 - winding coefficients of the stator and rotor windings.
, where x2K - inductive resistance of the braked rotor. Then
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