Rotating magnetic field induction motor. Rotating magnetic field. The device of a three-phase asynchronous machine

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The rotating magnetic field of the stator, crossing the body of the armature, induces eddy currents in it. The interaction of eddy currents in the armature with the magnetic field of the stator creates a torque that causes the armature to rotate. If there is no load on the armature axis, then the armature rotation speed co will be equal to the rotation speed coc magnetic field.

At time 7, the current corresponds to instantaneous 1, when the field propagates down and to the right again. Thus, a complete rotation of the bipolar field was completed through one complete cycle of 360 electrical degrees of three-phase currents flowing through the stator windings.

The term "pole" should take into account the terms used in chapter 2 on magnetism. The following definition of a motor pole gives it practical application significance: A motor pole is a complete motor stator winding circuit that, when energized by current, will produce a magnetic field concentration or polarity.

The rotating stator magnetic field simultaneously crosses the stator and rotor windings and excites sinusoidal induction EMF in them.

How is the rotating magnetic field of the stator formed.

The magnetic lines of the rotating magnetic field of the stator tend to follow a path with less magnetic resistance.

Consider the characteristics of the rotating magnetic field of the stator, assuming that the rotor circuit is open.

The speed of the rotating stator field is called the synchronous speed. Synchronous speed depends on two factors. Number of poles. - Power supply frequency. The synchronous speed, in turn, determines the rotational speed of the motor rotor. As with the speed of the prime mover generator, RPM and rotor RPM are directly related. The number of poles in the motor determines how fast the rotating field will move along the inner periphery of the motor housing at a given frequency.

The more poles the motor has, the longer it takes to activate all sets of poles, and the slower the motor field will rotate at 60 hertz. The table shows the rotating field speed for a 60 hertz power supply. This can be checked from the switch.

The difference between the frequency of the rotating magnetic field of the stator o and the rotor speed n is called the lag or slip of the rotor.

With this inclusion of the machine, the rotating magnetic field of the stator, interacting with the field of the stationary rotor, creates a moment on the shaft, the sign of which changes with a frequency of 100 Hz. For example, when the south pole of the stator field approaches the north pole of the rotor, a moment is created that acts against the direction of rotation. When the positive pole of the stator field moves away from the north pole of the rotor, a positive moment is formed, giving a reverse impetus to the rotor. The rotor does not have time to turn, as its mechanical inertia is too great.

A set of indicators indicates the sequence of phases from the power supply. When the generator rotates, a current is generated in the armature. Each armature phase becomes electrically active. The order in which the phases become electrically active determines the order in which the motor stator receives current. This changes the direction of rotation of the rotating magnetic field in the stator.

When the rotating field in the motor stator changes direction, the motor rotates the direction of the rotor. Reversing the generator output will also drive the motor rotor in the opposite direction. By reversing any two-phase wires, both at the generator armature and at the motor terminals, the phase sequence will be reversed at that point. Handling any two leads at the same point will restore the normal phase sequence.

The difference between the number of revolutions of the rotating magnetic field of the stator and the number of revolutions of the rotor is characterized by slip.

The difference between the number of revolutions of the rotating magnetic field of the stator and the number of revolutions of the rotor depends on the slip.

The relative lag of the rotor from the rotating magnetic field of the stator is called slip. The power of electrical losses in the rotor of an induction motor is proportional to slip.

The rotor is supported by bearings at each end. The stator is released in position inside the motor frame. The frame covers all engine components. The frame of the engine, among other things, is a determining factor in the placement of the engine. Each engine frame housing has specific characteristics and special applications for ships. There are seven main body types.

In case open type end bells are open and provide maximum engine ventilation. This is the cheapest motor case. In a semi-restricted case, the end bells are open, but screens are provided to prevent objects from entering the engine. No protection against water or liquids.

When the rotor is in the rotating magnetic field of the stator, the process of rotational magnetization reversal of the active magnetic material takes place in it. The latter manifests itself in the lag of the magnetic induction vector Bp from the field strength vector H at any point in the material by some spatial angle ar. The magnitude of this angle and the hysteresis losses per cycle, as was noted, do not depend on the remagnetization frequency.

In a protected housing, screens and guards exist over any opening in the motor housing. Limited openings are provided in the motor housing to limit access to live and rotating components. In a hermetically sealed housing, the end bells are closed to prevent liquid from entering the housing at an angle of no more than 15 degrees from the vertical.

In the splash-proof housing, the motor openings are designed to prevent liquid or solid particles from entering the motor at any angle up to 100 degrees from vertical. The waterproof housing prevents moisture or water from entering the motor and hindering its successful operation.

When the rotor is in the rotating magnetic field of the stator, the process of rotational magnetization reversal of the active magnetic material takes place in it. The latter, however, manifests itself in the lag of the magnetic induction vector Bp from the field strength vector R at some point in the material by some spatial angle ar. The magnitude of this angle and the hysteresis loss per cycle, as was noted, does not depend on the remagnetization frequency.

The waterproof housing prevents water flow from the hose in any direction from the engine inlet for at least 15 minutes. Electrical equipment exposed to the weather or in a space where it is exposed to seas, splashes or similar conditions must be watertight or in a watertight enclosure. Electric motors, however, must be either waterproof or waterproof.

The motor stator is a fixed winding bolted to the inside of the motor housing. The stator windings have very low resistance. Each machine has a stationary wire winding insulated along its entire length to prevent shorts from turning to turn. The winding is also isolated from the frame. The motor stator winding is identical to the generator armature, which has the same number of poles. Each winding overlaps and is electrically and mechanically spaced 120 degrees apart.

With the help of these windings, a rotating magnetic field of the stator is created, which drags the rotor of the electric motor with it. If both windings are powered from the same network, then the excitation winding is connected through capacitor C.

The motor uses the principle of interaction between the rotating magnetic field of the stator and the multi-pole permanent magnet of the movable rotor. To determine the location of the axes of the rotor magnet relative to the stator windings and to control the commutator circuit, three photoelectric rotor position sensors are used. The light source of the sensors is the light bulb EL1, the light of which, through the slots of the rotor diaphragm, alternately enters the photodiodes of the commutator board.

The figure above shows the top view of the fixed windings. Each of the three-phase windings is divided into many additional coils evenly distributed throughout the stator. This uniform distribution makes better use of the magnetic fields that will develop inside the stator windings when current is present. It also provides more even torque to the rotor.

The rotor looks like a solid cylinder supported at each end by bearings. On closer inspection, you can see thin rods embedded in a laminated cylinder at an angle almost parallel to the rotor shaft. There are end rings at each end of the cylindrical core of the rotor. Each end of the rod is connected to the closing rings. These rotor windings are similar in design to shock absorbers or damper windings found in a generator.

A feature of multiphase systems is the ability to create a rotating magnetic field in a mechanically stationary device.
Coil connected to the source alternating current, forms a pulsating magnetic field, i.e. a magnetic field that changes in magnitude and direction.

Take a cylinder with an inner diameter D. On the surface of the cylinder we place three coils, spatially displaced relative to each other by 120 o . We connect the coils to a three-phase voltage source (Fig. 12.1). On fig. 12.2 shows a graph of instantaneous currents that form a three-phase system.

These short-circuited rotor rods become secondary transformers. The rotor bars and end rings complete the circuit, and current is established in these rotor bars. Remember, whenever a current is set, so there is a magnetic field. Since this magnetic field is a property of induction and induction opposes what creates it, the magnetic field pole in the rotor has the opposite polarity of the stator field pole that generated it. The principles of magnetism are applied and the polarity of the rotor is attracted to the opposite polarity of the stator.

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. 12.3 shows the magnetic induction vectors of each phase and the resulting vector constructed for three times t1, t2, t3. The positive directions of the axes of the coils are marked +1, +2, +3.

The rotating stator field, actually rotating magnetic polarity, pulls and pushes the initially established rotor field in the rotor. Pulling and pushing results in torque and the rotor of the motor turns. Words often used to describe the solid windings of rods found in the rotor of an induction motor are "short-circuited rods". A short circuit is a very low resistance situation that has very little limitation in reducing current.

Condition short circuit can have devastating effects on the entire electrical environment. Rotor bars are designed for very low drag to achieve specific engine performance. The rotors themselves are not entirely the cause of the short circuit. A large start of motor current is initiated due to the relative motion between the stationary rotor winding and the rotating stator field. This is a fraction of the maximum current that the motor will draw from the distribution system.

At the moment t \u003d t 1, the current and magnetic induction in the coil A-X are positive and maximum, at coils B-Y and C-Z 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 \u003d t 2 currents in coils A-X and C-Z are equal in magnitude and opposite in direction. The current in phase B is zero. The resulting magnetic induction vector rotated clockwise by 30 o . At the moment t \u003d t 3, the currents in the coils A-X and B-Y are the same in magnitude and positive, the current in phase C-Z is maximum and negative, the vector of the resulting magnetic field is placed in the negative direction of the C-Z coil axis. For a period of alternating current, the vector of the resulting magnetic field will turn 360 o.

As the rotor speed increases, the sharpness will be drastically reduced. Shortly after power is applied to the motor, the current drops to 10 percent. Once the motor is running at normal speed, the full load current indicated on the rating plate is maintained. Mechanical overload of the motor slows down the rotor and increases the current. This is an increase in current, no matter how small, that results in enough heat to destroy the motors.

If the rotor can rotate at synchronous speed, then there will be no relative motion between the stator magnetic field and the rotor conductor strips. This would end the induction process in the rotor and the rotor would lose its magnetic field. This is not possible with an asynchronous motor. If the rotor speed is equal to the synchronous speed, the rotor will stop. The rotor speed will be maintained somewhere below synchronous speed. Slip is the difference between synchronous speed and actual rotor speed.

Magnetic field speed or synchronous speed

(12.1)

where P is the number of pairs of poles.

The coils shown in fig. 12.1, 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 the cylinder, two for each phase. Then, according to formula (12.1), the magnetic field will rotate twice as slowly, with n 1 = 1500 rpm.
To obtain a rotating magnetic field, two conditions must be met.

Slip is often expressed as a percentage. An induction motor will always have a difference in speed between the rotor and the stator field. Without this distinction, there would be no relative motion between the field and the rotor and the inductive or magnetic field in the rotor.

The rotor, and therefore the speed of the motor, is determined by the number of poles, the frequency, and the percentage of slip. The resistance in the rotor determines the relative ease with which the magnetic field in the rotor is established. The starting current, slip and torque of the motor are changed by the resistance of the rotor. By developing a motor with a high rotor resistance, a larger slip develops because the rotor magnetic field cannot develop very quickly. The stepwise sequence of events depicts the actions between the stator and the rotor in a relatively high rotor resistance induction motor.

1. Have at least two spatially biased coils.

2. Connect out-of-phase currents to the coils.

12.2. asynchronous motors.
Design, principle of operation

The asynchronous motor has motionless the part called stator , and rotating the part called rotor . The stator contains a winding that creates a rotating magnetic field.
There are asynchronous motors with squirrel-cage and phase rotor.
In the slots of the rotor with a short-circuited winding, aluminum or copper rods are placed. 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 three-phase winding(for three-phase motor). The ends of the phases are connected into a common node, and the beginnings are brought out to three contact rings placed on the shaft. Fixed contact brushes are placed on 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 will be considered on the model shown in Figure 12.4.

High rotor resistance prevents quick creation magnetic field of the rotor. The inability of the rotor to quickly create a magnetic field does not allow you to quickly increase the speed of rotation of the rotor. Because the rotor does not increase rapidly with increasing speed, there is a higher relative motion between the rotating stator field and the slow rotor.

The increased current increases the magnetic field of the rotor. The increased magnetic field increases the magnetic attraction of the rotor to the rotating stator pole. The rotor develops more torque to handle heavier loads. However, additional torque does not occur without some complications. An increase in torque means an increased current demand in the distribution system. There is also an increase in slip at full load. Higher rotor resistances are unacceptable for all applications.

We represent the rotating magnetic field of the stator as a permanent magnet rotating at a synchronous speed n 1 .
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. 12.4.

This is the reason for many rotor designs. Rotor resistance is determined by the National Electrical Manufacturers Association and is designated by design. Already used in the oil and gas, energy, semiconductor and machine tool industries in many challenging applications, magnetic bearings hold significant potential for more efficient wastewater treatment solutions.

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Rice. 12.4

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 n 2 in the direction of rotation of the stator field.
The rotor rotates asynchronously, i.e. its rotational speed n 2 is less than the rotational speed of the stator field n 1.
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.
Rotating electromagnetic moment is balanced by the counteracting braking moment M em \u003d M 2.
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.
The slip of the stalled motor is equal to one. The motor is said to be in short circuit mode.
The rotational speed of an unloaded induction motor n 2 is approximately equal to the synchronous frequency n 1 . Slip of an unloaded engine S 0. The engine is said to be idling.
Slip asynchronous machine, operating in the engine 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 n 2 > n 1 . Slip asynchronous generator.
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 frequency of rotation of the field in an asynchronous motor is rigidly related to the frequency of the network f 1 and the number of pairs of stator poles. At a frequency f 1 = 50 Hz, there is the following series of rotation frequencies.

A locked-rotor asynchronous machine works like a transformer. The main magnetic flux induces in the stator and in the fixed rotor windings EMF E 1 and E 2k.

where Ф m - the maximum value of the main magnetic flux coupled with
stator and rotor windings;
W 1 and W 2 - the number of turns of the stator and rotor windings;
f 1 - voltage frequency in the network;
K 01 and K 02 - winding ratios 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