The simplicity of the technical implementation of the circular motion for the rotation of the magnetic field is the basis for the operation of all 3-phase machines, including electric generators and motors.
Conditions for creating a rotating magnetic field. Its creation is achieved by the simultaneous fulfillment of two conditions:
1. By placing three windings with the same electrical parameters in the same plane of rotation with equal angular displacement (Δα=360°/3=120°);
2. By passing through these windings sinusoidal harmonics of currents equal in magnitude and shape, which are shifted in time by a third of the period (by 120 ° in angular frequency).
The formed circular magnetic field will begin to rotate. The constant induction of the created field has a maximum amplitude with the value Bmax directed along the field axis with a constant angular rotation speed ωp.
The location of the three coil windings in the same plane of rotation is shown in the figure and meets the requirements of the first condition.
By coil windings OH, B-Y, C-Z from their beginning (entrance) A, IN, WITH to the end (exit) X, Y, Z an electric symmetrical 3-phase current is passed, the value of which for any instant of time is calculated by the expressions:
iA=Im*sin(ωt+0);
iВ=Im∙sin(ωt-120°);
iС=Im∙sin(ωt+120°).
Each turn of the coil winding forms its own individual magnetic field, in which the induction is proportional to the current passing through the turn (B=k*i). The summation of the fields of all turns in each coil forms a system of three inductions, symmetrical with respect to the center of rotation (the origin of coordinates):
VA=Bm∙sin(ωt+0);
ВB=Вm∙sin(ωt+0);
ВC=Вm∙sin(ωt+0).
Magnetic fields as induction vectors VA, ВB, Sun have a strictly pronounced orientation in space, determined by the well-known gimlet rule with respect to the positive direction of the current in the coil winding.
The total (resulting) vector of magnetic induction B from the generated magnetic field in an electric machine is calculated by the geometric addition of phase vectors VA, ВB, Sun from all coils.
In a particular case, for the temporal estimation of the magnetic induction vector, several points of the period are selected, for example, those that correspond to 0, 30 and 60 degrees of its rotation relative to the initial ordinate.
The spatial arrangement of the induction vectors of each phase and the resulting vector obtained from their geometric addition for each case on the complex plane is shown by graphs.
It is convenient to analyze the results of graphical addition after they are presented in a separate table:
The results of the analysis indicate that the total induction vector B of all magnetic fields of the phases of the machine has one constant value at all points under consideration. Similar conclusions will be obtained by mathematically solving a similar problem for any other time moments.
Properties of the magnetic induction vector IN :
The direction of its rotation in space corresponds to the movement in the nearest direction from the coil A towards the coil IN;
It is known that a magnetic field is always formed around a conductor with current. Its direction is determined by the rule of a right-handed screw ("gimlet").
Let's draw a magnetic line of force around the conductors C and Y and, respectively, B and Z (see dashed lines in Fig. 5.2.2 a).
Consider now the time t 2 . During this time, there will be no current in phase B. In conductor A of phase A-X, it will have a sign (+), and in conductor C of phase C-Z, it will have a sign (·). Now let's put down the signs: in the conductor X - (·), and in the conductor Z - (+).
,
represents the total mechanical power developed by the engine.
5.8. SUBSTITUTION DIAGRAM OF ASYNCHRONOUS MOTOR
Equations of EMF and currents correspond to an equivalent equivalent circuit (Fig. 5.8.1.). Thus, the complex magnetic circuit of an electrical machine can be replaced by an electrical circuit. Resistance r 2 "(1 - S) / S can be considered as an external resistance included in the rotor winding. It is the only variable parameter of the circuit. A change in this resistance is equivalent to a change in the load on the motor shaft, and therefore a change in slip S.
5.9. LOSSES AND EFFICIENCY OF ASYNCHRONOUS MOTOR
Power P 1 is supplied to the stator winding from the network. Part of this power goes to losses in steel P sl, as well as losses in the stator winding Р e1:
The remaining power is transferred to the rotor by means of a magnetic flux and is called electromagnetic power:
Part of the electromagnetic power is spent to cover the electrical losses in the rotor winding:
The remaining power is converted into mechanical power, called the total mechanical power:
R 2 "= R em -R e2
Using the previously obtained formula
we write the expression for the total mechanical power:
P e2 \u003d SP em,
those. the power of the electrical losses is proportional to the slip.
The power on the motor shaft P 2 is less than the total mechanical power P 2 ’ by the value of mechanical P mech and additional P ext losses:
P 2 \u003d P 2 '- (P mech. + P ext.).
Thus:
SP \u003d P sl + R e1 + R e2 + R mech. +R ext.
The efficiency is the ratio of the shaft power P 2 to the power consumption P 1:
5.10. TORQUE EQUATION
The torque in an induction motor is created by the interaction of the rotor current with the magnetic field of the machine. The torque can be mathematically expressed in terms of the electromagnetic power of the machine:
,where w 1 =2pn 1 /60 is the angular frequency of rotation of the field.
In turn, n 1 \u003d f 1 60 / P, then
.
We substitute the expression R em \u003d R e2 / S into the formula M 1 and, dividing by 9.81, we get:
,
It follows that the motor torque is proportional to the electrical losses in the rotor. Substitute in the last formula the value of the current I 2 ’:
,
,
However, the wide development of technology, the technical creativity of students requires knowledge of a number of additional possibilities for using these materials. Let's consider just a few of them.
5.18.2 Induction regulators and phase regulators
Induction voltage regulators are a locked induction motor with a phase rotor. They can adjust the voltage over a wide range. The stator and rotor windings in the regulator are electrically connected, but in such a way that they can be displaced relative to each other by turning the rotor. When the induction regulator is connected to the network, the rotating magnetic flux induces EMF E 1 and E 2 in the stator and rotor windings. When the axes coincide in the windings, the EMF E 1 and E 2 are in phase, and the maximum voltage value is set at the output terminals of the regulator.
When the rotor rotates, the winding axes rotate through a certain angle a. The vector E 2 also shifts by the same angle. In this case, the output voltage decreases. By turning the rotor by 180°, we set the minimum voltage at the output.
The phase regulator is designed to change the phase of the secondary voltage relative to the primary. In this case, the value of the secondary voltage remains unchanged.
The phase regulator is an asynchronous machine, braked by a special rotary device. Voltage is supplied to the stator winding, and removed from the rotor. In contrast to the induction regulator, here the stator and rotor windings are not electrically connected. The change in the phase of the secondary voltage is carried out by turning the rotor relative to the stator.
It is used in automation and measuring technology.
5.18.3 Asynchronous frequency converter
As you know, the frequency of the current in the rotor circuit of an induction motor depends on the slip, i.e. is determined by the difference between the frequencies of rotation of the rotor and the stator field.
.
The specified property allows using the motor as a frequency converter (Fig. 5.18.3.1). If the stator winding is connected to a network of industrial frequency f 1, and the rotor is rotated against the stator field by an external motor, then the slip increases, and the frequency of the rotor current f 2 accordingly increases compared to the network frequency f 1 several times. If it is required to reduce the current frequency, then the converter rotor must be rotated in the direction of the rotating stator field.
5.18.4 Electromagnetic asynchronous clutch
The electromagnetic asynchronous clutch (Fig. 5.18.4.1) is arranged according to the principle of an asynchronous motor and serves to connect two parts of the shaft. On the leading part of the shaft 1 is placed the pole system 2, which is a system of pronounced poles with excitation coils. The direct current in the excitation coil is supplied through slip rings 4. The driven part of the clutch 3 is made according to the type of the rotor winding of the motor.
The principle of operation of the clutch is similar to the operation of an asynchronous motor, only the rotating magnetic flux here is created by the mechanical rotation of the pole system. The torque from the driving part of the shaft to the driven part is transmitted electromagnetically. The clutch is disconnected by turning off the excitation current.
A circular rotating magnetic field has the following characteristic properties:
a) the maxima of the resulting MMF and induction waves always coincide with the axis of the phase in which the current has a maximum. This position can be easily verified by setting the quantity ωt, corresponding to the maximum current in the phase, and determining by (3.15) the coordinate of the point X, in which MDS F" x maximum;
b) the magnetic field moves towards the axis of the phase in which the nearest maximum is expected. This property follows directly from the previous one;
c) to change the direction of rotation of the field, it is necessary to change the order of alternation of the current in the phases. In three-phase machines, for this, it is necessary to swap the wires that supply current from a three-phase network to any two phases of the winding. In two-phase machines, you need to switch the wires that connect the phases of the winding to the two-phase network.
Elliptical field. A circular rotating magnetic field occurs with the symmetry of the currents passing through the phases (symmetries of the MMF of coils of individual phases), the symmetrical arrangement of these phases in space, the time shift between phase currents equal to the spatial shift between the phases and the sinusoidal distribution of induction in the air gap of the machine along the circumference of the stator (rotor). If at least one of these conditions is not observed, not a circular, but an elliptical rotating field arises, in which the maximum value of the resulting MMF and induction for different moments of time does not remain constant, as with a circular field. In such a field, the spatial vector of the MDS describes an ellipse (see Fig. 3.12, V).
An elliptical field can be represented as two equivalent circular fields rotating in opposite directions. A field rotating in the direction of rotation of the resulting elliptical field is called direct; field rotating in the opposite direction reverse. The decomposition of the elliptical field into direct and reverse circular fields is carried out by the method of symmetrical components, with the help of which the MMF of the direct and reverse sequences is determined.
Consider, for example, a two-phase machine, in which two phase windings (phases) are located on the stator OH And BY, whose axes are displaced in space by some angle α (Fig. 3.16, A). The currents passing through these phases and the corresponding MMF vectors FxA And FxB shifted in time by some angle β. Phases OH And BY create pulsating magnetic fields sinusoidally distributed in space. MDS of these phases, acting at any point X air gap,
FxA = FmA sin ωt cos(πx/τ); FxB = FmB sin(ωt + β)cos(πx/τ + α).
The MMF of the phases AX and BY, similarly to (3.15), can be represented as the sum of two traveling waves of the MMF of opposite directions:
In expressions (3.21), temporal and spatial angles are added or subtracted, i.e., they become equivalent. This is explained by the fact that the spatial position of the MMF vector of the rotating field is determined by the time and frequency of the current supplying the phases - in one period the field moves to a pair of poles. The resulting magnetic field created by the combined action of the two windings can be obtained by adding the components of the positive sequence MMF vectors rotating clockwise (forming a direct field):
F "xA \u003d 0.5FmA sin (ωt - πx / τ) and F"xB \u003d 0.5FmB sin (ωt + β - πx / τ ± α),
As well as MDS vectors of the negative sequence, rotating counterclockwise (forming a reverse field)
| F "xA \u003d 0.5FmA sin (ωt + πx / τ) and F"xB \u003d 0.5FmB sin (ωt + β + πx / τ |
|
α). |
The total MMF of fields rotating in opposite directions, i.e. F"x \u003d F"xA + F"xB And F""x = F"xA + F"xB, are not equal in magnitude (Fig. 3.16.6), and therefore the resulting field of the machine is not pulsating, but rotating. In this field, the maximum value of the resulting MMF at different times does not remain constant, as in the case of a circular field, i.e., the field is elliptical. In a two-phase machine, a circular rotating field can also be obtained; while one of the components of the MDS F"x or F"x should be absent. The conditions for obtaining a circular field in such a machine are reduced to mutual compensation of one of the MMF pairs F"xA And F"xB or F"xA And F"xB. The latter can be if the specified MDS are equal in amplitude, but opposite in phase, i.e. if α ± β = π .
In inductive electrical machines, the stator and rotor windings are connected by a magnetic field. In order to communicate the rotating part of the machine with the machine that is stationary in the air gap through a system of stator windings, create rotating a magnetic field.
By rotating we will understand such a magnetic field, the induction vector of which moves in space (in a plane perpendicular to the axis of the rotor) with a certain angular velocity. If the amplitude of the induction vector is constant, then such a field is called circular. A rotating magnetic field can be created:
- alternating current in a two-phase system of windings shifted in space by 90 °;
- three-phase alternating current in a three-phase system of windings shifted in space by 120°;
- direct current switched in series along the windings distributed along the bore of the motor stator;
- direct current, switched by means of a commutator along winding branches located along the surface of the rotor (armature). Formation of a rotating magnetic field in a two-phase machine
- (rice. 1.2). IN such a machine, the winding axes are geometrically shifted by 90 ° (a machine with one pair of poles is considered, p n = 1). The stator windings are powered by a two-phase voltage, as shown in fig. 1.2, i. Assuming the machine is symmetrical and unsaturated, we assume that the currents in the windings are also shifted by 90 electrical degrees (90 ° el.) And the magnetomotive force of the windings is proportional to the current (Fig. 1 .2,6). IN moment of time, = 0 winding current A is zero, and the current in the winding b has the most negative value.
Rice. 1.2. Formation of a rotating magnetic field in a two-phase electric machine: a - winding switching circuit: b - system of two-phase currents in the stator windings: V- spatial vector diagram of magnetically moving forces generated by stator windings
Therefore, the total vector of magnetically moving forces (MMF) of the windings at the moment of time is equal to t and is located in space, as shown in Fig. 1.2 V. At the moment of time c 2 \u003d 7 s / currents in the windings will be Tl m / and, consequently, the total MDS vector will rotate through the angle To/ and_occupies in space the position indicated in Fig. 12, V, like 2 = 2 + 2 . In the moment
time w 2 \u003d i / 2, the total vector of the MDS will be equal. Similarly, one can trace how the position of the total MDS vector changes at time points, etc. It can be seen that the vector rotates in space with a speed ω = 2ts, keeping its amplitude constant. The direction of field rotation is clockwise. We propose to make sure that if you apply for the phase A voltage \u003d (co -), and per phase b voltage = co, then direction
rotation will be reversed.
Rice. 1.3. Schemes for switching on the windings of a three-phase motor: a - the location of the motor windings at p p \u003d 1; b - connection of windings in a star; V- diagrams of three-phase currents in motor windings
Thus, the combination of a spatial shift of the axes of the windings by 90 geometric degrees (90°) and a phase shift of the alternating current in the windings by (90° el.) electrical degrees makes it possible to form a magnetic field rotating along the circumference of the stator in the air gap of the machine.
The mechanism of formation of a rotating magnetic field in a three-phase AC machine. The windings of the machine are shifted in space by 120 ° (Fig. 1.3, a) and are powered by a three-phase voltage system. The currents in the winding of the machine are shifted by 120°el. (Fig. 1.3, V):
The resulting vector of the MMF of the stator windings is:
Where w- the number of turns of the windings.
Consider the position in space of the vector at the moment of time (Fig. 1.4, o). The winding MMF vector o t is directed along the o axis in the positive direction and is equal to 0, w, those. ABOUT, . Winding MDS vector With, directed along the axis With and is equal to 0, . The sum of the vectors j and j is directed along the axis b in the negative direction and with this sum the winding MMF vector is added b, equal to The sum of three vectors forms a vector X= 3 /2, occupying at the moment of time, the position shown in Fig. 1.4, about. After the lapse of time \u003d l / Zco (at a frequency of 50 Hz after 1/300 s), the moment of time 2 will come, at which the vector MMF of the winding o is equal, and the vectors of the MMF of the windings b And With are equal - 0.5 . The resulting MDS vector 2 at time 2 will take the position shown in Fig. 1.4.5, i.e. move relative to the previous position at 60° clockwise. It is easy to make sure that at time 3 the resulting vector of the MMF of the stator windings will take position 3, i.e. will continue to move clockwise. During the supply voltage period = 2l/co = 1/ the resulting MMF vector will make a complete revolution, i.e. the rotation speed of the stator field is directly proportional to the frequency of the current in its windings and inversely proportional to the number of pole pairs:
where n is the number of pole pairs of the machine.
If the number of motor pole pairs is greater than one, then the number of winding sections arranged around the stator circumference increases. So, if the number of pairs of poles n \u003d 2, then three phase windings will be located on one half of the stator circumference and three on the other. In this case, for one period of the supply voltage, the resulting MMF vector will make half a turn and the rotation speed of the stator magnetic field will be half that in machines with „=1-
Rice. 1.4.A- co \u003d 7s / b- co \u003d l / V- co \u003d 7s /
The operation of almost all AC motors: synchronous with electromagnetic excitation (SM), with excitation from permanent magnets (PMSM), synchronous reluctance motors (SRM), and asynchronous motors (IM) - lies the principle of creating a rotating magnetic field.
According to the principles of electrodynamics, in all electric motors (except reactive ones), the developed electromagnetic torque is the result of the interaction of magnetic fluxes (flux linkages) created in the moving and stationary parts of the electric motor. The moment is equal to the product of the vectors of these flows, which is shown in Fig. 1.5, and the value of the moment is equal to the product of the modules of the flux vectors and the sine of the spatial angle 0 between the flux vectors:
Where To - constructive factor.
Rice. 1.5.
Synchronous(SD, SDPM, SRD) and asynchronous motors have almost the same stator designs, and the rotors are different. The distributed stator windings of these electric motors fit into a relatively large number of half-closed stator slots. If the influence of tooth harmonics is not taken into account, then the stator windings form a magnetic flux constant in amplitude, rotating at a constant speed determined by the current frequency. In real structures, the presence of slots and teeth of the stator magnetic circuit leads to the appearance of higher harmonics of the magnetizing forces, which leads to pulsations of the electromagnetic torque.
An excitation winding is located on the SM rotor, which is powered by direct current from an independent voltage source - the exciter. The excitation current creates an electromagnetic field that is stationary relative to the rotor and rotates in the air gap together with the rotor at a speed co [see Fig. (1.7)]. For synchronous motors up to 100 kW, permanent magnet excitation is used, which is mounted on the rotor.
The magnetic lines of force of the rotor field, created by the field winding or permanent magnets, "engage" with the stator electromagnetic field rotating synchronously with it. Interaction of stator fields X and rotor 0 creates an electromagnetic moment on the shaft of the synchronous machine.
In the absence of a load on the shaft, the field vectors of the stator and rotor 0 coincide in space and rotate together at a speed of 0 (Fig. 1.6, i).
When a moment of resistance is applied to the motor shaft, the vectors [ and 0 diverge (stretch like a spring) at an angle of 0, and both vectors continue to rotate at the same speed from 0 (Fig. 1 .6,6). If the angle 0 is positive, then the synchronous machine operates in motor mode. A change in the load on the motor shaft corresponds to a change in the angle 0 Maximum torque M will be at 0 = l; / (0 - electrical degrees). If
load on the motor shaft exceeds M then the synchronous mode is violated, and the motor falls out of synchronism. With a negative angle of 0, the synchronous machine will operate as a generator.
Rice. 1.6.A- at ideal idling; b - with a load on the shaft
Reluctance synchronous motor - this is a motor with pronounced poles of the rotor without an excitation winding, where the torque is due to the desire of the rotor to take a position in which the magnetic resistance between the excited stator winding and the rotor takes on a minimum value.
In the SynRM, the rotor is salient-pole (Fig. 1.7). It has different magnetic conductivity along the axes. Along the longitudinal axis d, passing through the middle of the pole, the conductivity is maximum, and along the transverse axis q- minimum. If the axis of the magnetizing forces of the stator coincides with the longitudinal axis of the rotor, there is no curvature of the magnetic flux lines of force and the moment is zero. When the flow of the stator axis is offset relative to the longitudinal axis d when the magnetic field (MF) rotates, the flux lines of force are bent and an electromagnetic moment arises. The greatest moment at the same stator current is obtained at an angle 0 =45°el.
The main difference between an asynchronous motor and a synchronous motor is that the speed of rotation of the motor rotor is not equal to the speed of the magnetic field created by currents in the stator windings. The difference between the speeds of the stator and rotor fields is called sliding= co - co. Due to slip, the magnetic lines of force of the rotating stator field cross the conductors of the rotor winding and induce EMF and rotor current in it. The interaction of the stator field and the rotor current determines the electromagnetic torque of the induction motor.
Rice. 1.7.
Depending on the design of the rotor, asynchronous motors are distinguished with phase And short-circuited rotor. In motors with a phase rotor, a three-phase winding is located on the rotor, the ends of which are connected to slip rings, through which the rotor circuit is removed from the machine for connection to starting resistors, followed by shorting the windings.
In an asynchronous motor, in the absence of a load on the shaft, only magnetization currents flow through the stator windings, creating the main magnetic flux, and the amplitude of the flux is determined by the amplitude and frequency of the supply voltage. In this case, the rotor rotates at the same speed as the stator field. EMF is not induced in the rotor windings, there is no rotor current and, therefore, the moment is zero.
When a load is applied, the rotor rotates more slowly than the field, slip occurs, an EMF proportional to slip is induced in the rotor windings, and rotor currents arise. The stator current, as in a transformer, increases by an appropriate amount. The product of the active component of the rotor current and the stator flux modulus determines the motor torque.
What unites all motors [except for valve-reluctance motors (VID)] is that the main magnetic flux in the air gap rotates relative to the fixed stator with a given frequency of angular velocity co. This magnetic flux entrains the rotor, which rotates for synchronous machines with the same angular velocity co = co, or for asynchronous machines with some lag - slip 5. The field lines that form the main flux have a minimum length when the engine is idle (=). In this case, the vector axes of the magnetizing forces of the stator and rotor coincide. When a load appears on the motor shaft, the axes diverge, and the lines of force are bent and lengthened. Since the lines of force always tend to shorten in length, tangential forces appear that create a torque.
In recent years, they are beginning to be used switched reluctance motors. Such a motor has a salient pole stator with coil windings on each pole. The rotor is also salient pole, but with a different number of poles without windings. A unipolar current is alternately supplied to the stator windings from a special converter - a switch, and a nearby tooth of the rotor is attracted to these excited poles. Then the next stator pole is energized in turn. The stator pole windings are switched in accordance with the signals from the rotor position sensor. This, as well as the fact that the current in the stator windings is regulated depending on the load torque, is the main difference between a VID and a stepper motor.
In the VIEW (Fig. 1.8), the torque is proportional to the amplitude of the main flow and the degree of curvature of the magnetic field lines. At the beginning, when the rotor pole (tooth) begins to overlap the stator pole, the curvature of the field lines is maximum and the flux is minimum. When the overlap of the poles is maximum, the curvature of the field lines is minimal, and the amplitude of the flow increases, while the moment remains approximately constant. As the VID magnetic system is saturated, the increase in flux is limited, even with an increase in current in the VID windings. The change in torque when passing the poles of the rotor relative to the poles of the stator causes uneven rotation of the VID shaft.
Rice. 1.8.
In a DC motor, the excitation winding is located on the stator and the field created by this winding is stationary. A rotating magnetic field is created in the armature, the rotation speed of which is equal to the rotation speed of the armature, but directed oppositely. This is achieved by the fact that an alternating current flows through the turns of the armature winding, switched by a mechanical frequency converter - collector device.
The electromagnetic torque of a DC motor determines the interaction of the main flux created by the field winding and the current in the turns of the armature winding: M = k/ I
If we replace the brush-collector apparatus of a DC motor with a semiconductor switch, we get brushless DC motor. The practical implementation of such motors is a brushless motor. Structurally brushless motor is a three-phase synchronous machine with electromagnetic or permanent magnet excitation. The stator windings are switched using a semiconductor controlled converter - switch, depending on the position of the motor rotor.
A feature of multiphase systems is the ability to create a rotating magnetic field in a mechanically stationary device.
A coil connected to an alternating current source 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.
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.
At the moment t \u003d t 1, the current and magnetic induction in the A-X coil are positive and maximum, in the B-Y and C-Z 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 \u003d t 2, the currents in the coils A-X and C-Z 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 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 the C-Z phase is maximum and negative, the vector of the resulting magnetic field is located in the negative direction of the C-Z coil axis. For a period of alternating current, the vector of the resulting magnetic field will rotate 360 o.
Magnetic field speed or synchronous speed
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.
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 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 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.
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.
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.
The rotating electromagnetic moment is balanced by the counteracting braking moment M em = 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.
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 n 2 > n 1 . Sliding 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.
In the previous paragraph, it was shown that the speed of rotation of the magnetic field is constant and is determined by the frequency of the current. In particular, if the three-phase motor winding is placed in six slots on the inner surface of the stator (Fig. 5-7), then, as was shown (see Fig. 5-4), the magnetic flux axis will rotate
for half a period of alternating current by half a turn, and for a full period - by one turn. The speed of rotation of the magnetic flux can be represented as follows:
In this case, the stator winding creates a magnetic field with one pair of poles. This winding is called bipolar.
If the stator winding consists of six coils (two coils connected in series per phase) laid in twelve slots (Fig. 5-8), then as a result of constructions similar to those for a two-pole winding, it can be obtained that the axis of the magnetic flux in half a period will turn by a quarter of a turn, and for a full period - half a turn (Fig. 5-9). Instead of two poles with three
windings, the stator field now has four poles (two pairs of poles). The rotation speed of the stator magnetic field in this case is equal to
Increasing the number of slots and windings and making similar reasoning, we can conclude that the rotation speed of the magnetic field in the general case for pairs of poles is equal to
Since the number of pairs of poles can only be an integer (the number of coils in the stator winding is always a multiple of three), the speed of rotation of the magnetic field can have not arbitrary, but quite definite values (see Table 5.1).
Table 5.1
In practice, to obtain a constant value of the torque acting on the rotor during one revolution, the number of slots in the stator is significantly increased (Fig. 5-10) and each side of the coil is placed in several slots, with each winding consisting of several sections connected between itself sequentially. Windings, as a rule, are made in two layers. In each groove, two sides of sections of two different coils are laid one above the other, and if one active side lies at the bottom of one groove, then the other active side of this section lies at the top of the other groove, the sections and coils are interconnected so that in most of the conductors each slot the direction of the currents was the same.
Electrogravity is easy
Introduction. The article describes the simplest electrogravity generator capable of both reducing its weight and increasing it. To date, the working installation is able to change the weight in a very small range up to 50% of the original weight. Therefore, recommendations are given for its improvement. Experiments by Sergei Godin and Vasily Roshchin Two Russian physicists have created a very interesting generator. In fact, these are permanent magnets placed in a special disk with cavities for magnets. When the "disk with magnets" rotated clockwise, the weight of the generator decreased, and when rotated counterclockwise, it decreased.
And you can further enhance the anti-gravity effect by adding new magnets capable of rotating equipped with mini electric motors. The second step should
, replace permanent magnets in the "drum" with electromagnets.What is a permanent magnet? In fact, this is a set of ring currents of such small electromagnets "sewn" into the body of the magnet.
