The mechanical characteristic of the engine is the dependence of the rotor speed on the torque on the shaft n = f (M2). Since the idle torque is small under load, then M2 ? M and the mechanical characteristic is represented by the dependence n = f (M). If we take into account the relationship s = (n1 - n) / n1, then the mechanical characteristic can be obtained by presenting its graphical dependence in the coordinates n and M (Fig. 1).

Fig.1.

The natural mechanical characteristic of an asynchronous motor corresponds to the main (passport) circuit of its inclusion and the nominal parameters of the supply voltage. Artificial characteristics are obtained if any additional elements are included: resistors, reactors, capacitors. When the motor is supplied with a non-rated voltage, the characteristics also differ from the natural mechanical characteristic.

Mechanical characteristics are a very convenient and useful tool in the analysis of static and dynamic modes of the electric drive.

Data for calculating the mechanical characteristics for a given drive and motor:

A three-phase asynchronous motor with a squirrel-cage rotor is powered by a network with a voltage of = 380 V at = 50 Hz.

Engine parameters 4AM160S4:

Pн= 12.5 kW,

nн= 1460 rpm,

coscn= 0.86, cn= 0.89, kn= 2.2

Determine: rated current in the stator winding phase, number of pole pairs, rated slip, rated shaft torque, critical torque, critical slip and build the mechanical characteristic of the motor. Solution.

(3.1) Rated power drawn from the network:

(3.2) Rated current drawn from the network:

(3.3) Number of pairs of poles

where n1 \u003d 1500 is the synchronous speed closest to the rated speed nн \u003d 1460 rpm.

(3.4) Rated slip:

(3.5) Rated torque on the motor shaft:

(3.6) Critical moment

Mk \u003d km x Mn \u003d 1.5 x 249.5 \u003d 374.25 Nm.

(3.7) We find the critical slip by substituting M = Mn, s = sn and Mk / Mn = km.

To build the mechanical characteristics of the engine using n = (n1 - s), we determine the characteristic points: the idle point s = 0, n = 1500 rpm, M = 0, the nominal mode point sn = 0.03, nn = 1500 rpm min, Mn = 249.5 Nm and the critical mode point sk = 0.078, Mk = 374.25 Nm.

For the start mode point sp = 1, n = 0 we find

Based on the data obtained, the mechanical characteristic of the engine is built. For a more accurate construction of a mechanical characteristic, it is necessary to increase the number of calculated points and determine the moments and rotational speed for given slips.

Building a natural mechanical characteristic of the engine

The mechanical characteristic of the engine is called the dependence of the rotational speed n on the moment M of the load on the shaft.

There are natural and artificial characteristics of electric motors.

Natural a mechanical characteristic is called - the dependence of the engine speed on the torque on the shaft under the nominal conditions of the engine in relation to its parameters (rated voltages, frequency, resistance, etc.). A change in one or more parameters causes a corresponding change in the mechanical characteristics of the engine. Such a mechanical characteristic is called artificial.

To construct an equation for the mechanical characteristic of an induction motor, we use the Klos formula (4.1):

where M k is the critical moment of the engine (4.1.1):;

S k is the critical slip of the engine (4.1.2);

Motor overload capacity (= 3);

S n - rated motor slip (4.1.3):

where n n - rotor speed;

n 1 - synchronous speed of the stator field (4.1.4);

where f is the industrial frequency of the mains current, (f = 50 Hz) (4.1.5);

Р - number of pairs of poles (for motor 4AM132S4 Р=2)

Rated motor slip 4AM132S4

Critical motor slip

Engine critical moment

To build a characteristic in coordinates, one passes from slip to the number of revolutions based on the equation

Slip is set from 0 to 1

S = 0 n = 1500 . (1 - 0) = 1500 rpm;

Lecture 3

Asynchronous motors have been widely used in industry due to a number of significant advantages over other types of motors. The asynchronous motor is simple and reliable in operation, as it does not have a collector; asynchronous motors are cheaper and much lighter than DC motors.

To derive the equation for the mechanical characteristic of an induction motor, you can use the simplified equivalent circuit shown in Fig. 3.1, where the following designations are accepted:

Uf - primary phase voltage; I 1 - phase current of the stator; I / 2 - reduced rotor current; X 1 and X" 2 - primary and secondary reduced scattering reactances; Ro and X 0 - active and reactive resistance of the magnetization circuit; s == (w 0 - w) / w 0 - engine slip; w 0 = 2pn 0 /60 - synchronous angular speed of the motor; w 0 = 2pf 1 /p; R1 and R/2 - primary and secondary reduced active resistances; f 1 - network frequency; R - number of pairs of poles.

Rice. 3.1 Simplified equivalent circuit of an asynchronous motor.

In accordance with the above equivalent circuit, it is possible to obtain an expression for the secondary current

(2.1)

The torque of an induction motor can be determined from the loss expression Mw 0 s = 3 (I / 2) 2 R / 2 , whence

(2.2)

Substituting the value of the current I / 2 in (2.1), we obtain:

(2.3)

Moment curve M = f(s) has two maxima: one - in the generator mode, the other - in the motor mode 1 .

Equating dM/ds= 0, we determine the value of the critical slip Sg, at which the engine develops the maximum (critical) torque

(2.4)

With significant resistance of the rotor circuit, the maximum torque may be in the mode of braking by counter-switching.

Substituting the value of Sk in (3.3), we find the expression for the maximum moment

(2.5)

The sign "+" in equalities (2.4) and (2.5) refers to the motor mode (or braking by counter-inclusion), the sign "-" - to the generator mode of operation in parallel with the network (w>w 0)

If the expression (2.3) is divided by (2.5) and the corresponding transformations are made,

Rice. 3.2 Mechanical characteristics of an asynchronous motor.

then you can get:

(2.6)

where Mk - maximum engine torque; S K - critical slip corresponding to the maximum moment; A= R 1 / R / 2 .

Here it is necessary to emphasize a circumstance that is very important for practice - the effect of changing the mains voltage on the mechanical characteristics of an induction motor. As can be seen from (3.3), for a given slip, the motor torque is proportional to the square of the voltage, so this type of motor is sensitive to mains voltage fluctuations.

The critical slip and the angular velocity of an ideal idle are independent of voltage.

On fig. 3.2 shows the mechanical characteristics of an asynchronous motor. Her characteristic points:

1) s = 0; M = 0, while the motor speed is equal to synchronous;

2) s = s NOM; M = M nom which corresponds to the rated speed and rated torque;

3) s == sk; M == M max - maximum torque in motor mode;

Initial starting torque;

5) s = - s K ; M=M K.G. - the maximum torque in the generator mode of operation in parallel with the network.

With s> 1.0, the motor operates in the anti-switching braking mode, with s< 0 имеет место генераторный режим работы параллельно с сетью.

It must be emphasized that the absolute values ​​of S k in the motor and generator modes in parallel with the network are the same

However, from (2.6) it follows that the maximum moments in the motor and generator modes are different. In the generator mode of operation in parallel with the network, the maximum torque is greater in absolute value, which follows from the relation

If in equation (2.6) we neglect the active resistance of the stator, then we get a formula that is more convenient for calculations:

(2.7)

Substituting in expression (2.7) instead of the current values ​​of M and s their nominal values ​​and denoting the multiplicity of the maximum moment M K / M NOM, through l, we get:

In the last expression, the “+” sign should be taken before the root.

An analysis of formula (2.6) shows that for s>s k (the non-working part of the characteristic), a hyperbola equation will be obtained if, in this case, the second terms of the denominator in equations (3.6) are neglected, i.e.

This part of the characteristic practically corresponds only to starting and braking modes.

For small values ​​of slip (s< s k) для M=f (s) we get the equation of a straight line if we neglect the first term in the denominator (3.6):

This linear part of the characteristic is its working part, on which the engine usually operates in steady state. On the same part of the characteristic there are points corresponding to the nominal data of the motor: M NOM, I NOM, n NOM, s NOM.

The static drop (difference) of speed in relative units on the natural mechanical characteristic of an asynchronous motor at a rated torque is determined by its rated slip.

The nominal slip depends on the resistance of the rotor. Motors with a squirrel-cage rotor of normal design usually have the smallest rated slip for the same power and number of poles. For these motors, due to their design features, the rotor resistance has a relatively small value, which leads to a decrease in the values ​​of critical slip s k (3.4) and nominal slip s NOM. For the same reasons, with an increase in engine power, its nominal slip decreases and the stiffness of the natural characteristic increases. The latter is illustrated by the curve in Fig. 11, built on average data for engines of different power.

The maximum moment, as can be seen from (3.5), does not depend on the active resistance of the rotor R 2 , the critical slip, according to (3.4), increases as the rotor resistance increases. As a result, for motors with a phase rotor, when resistors are introduced into the rotor circuit, the maximum of the torque curve is shifted towards large slips.

The resistance value R 2 , necessary to build the natural and rheostatic characteristics of a motor with a phase rotor, is determined from the expression

where E 2k, I 2NOM - linear voltage with a stationary rotor and the rated current of the rotor.

On fig. 12 shows the family of rheostatic characteristics in the motor mode in the coordinate axes M and with for different values ​​of the resistance of the rotor circuit. With a known approximation, the rheostatic characteristics in their working part can be taken as linear. This makes it possible, when calculating the resistance of resistors included in the rotor circuit of an asynchronous motor, to use methods similar to those used

Rice. 11. Curve of the nominal Fig. 12 Natural and rheostatic mechanical

slip for asynchronous characteristics of an induction motor with phase-

engines of different power. rotor

to calculate the armature circuit resistance of a DC motor of independent excitation. Some inaccuracy in the determination of the resistance of the resistor is introduced in this case due to the fact that the characteristic of the asynchronous motor in the section of the graph from M = 0 to the maximum torque at start-up is considered linear.

A more accurate method is when the characteristics are straightened over a smaller area. The multiplicity of the maximum moment l \u003d M K.D. /M nom should be at least 1.8 for motors of normal design with a phase rotor, and at least 1.7 for motors with a squirrel-cage rotor. Crane motors are characterized by a higher ratio of maximum torque. For example, for motors with a squirrel-cage rotor of the MTK series l=2.3¸3.4.

Motors with a phase rotor of the mentioned series have approximately the same values ​​of l .

For motors with a squirrel-cage rotor, the multiplicity of the initial starting torque and the initial starting current are essential from the point of view of the electric drive.

On fig. 13 shows the approximate natural characteristics of a motor with a normal squirrel-cage rotor having circular slots. These characteristics show that a squirrel-cage motor, consuming a very large current from the network, has a relatively

Rice. 13. Characteristics co = = f(M) and u == D (/) for an induction motor with a squirrel-cage rotor with round slots.

low starting torque. The multiplicity of the initial starting torque of the engines

and for crane engines

Starting current ratio

The lack of proportionality between the motor torque and the stator current during start-up (Fig. 13) is explained by a significant decrease in the motor magnetic flux, as well as a decrease in the power factor of the secondary circuit during start-up.

The moment of an induction motor, like any electrical machine, is proportional to the magnetic flux Ф and the active component of the secondary current

(2.8)

With increasing slip, the EMF of the rotor increases E 2 \u003d E 2K s , the rotor current I / 2 increases in accordance with (3.1), asymptotically tending to a certain limit value, and cos y 2 decreases with increasing s (very little in the working section of the characteristic), asymptotically tending to zero at s ®¥. The motor flux also does not remain constant, decreasing as the current increases due to the voltage drop across the stator winding resistances. All this causes the lack of proportionality between the current and the motor torque.

To increase the initial starting torque and reduce the starting current, motors with a squirrel-cage rotor of special designs are used. Electric motor rotors have two concentric cages or deep surfacings with tall and narrow shafts. The rotor resistance of these motors in the starting

Rice. 14. Mechanical characteristics of an asynchronous motor with a squirrel-cage rotor with a dip at low angular speeds.

the period is much longer than at rated speed, due to the skin effect due to the increased frequency of the current in the rotor at large slips. Therefore, when switching to motors with a deep groove or a double winding of the rotor, the multiplicity of the starting torque increases significantly (cos y 2 flux increases) and the multiplicity of the starting current decreases. True, in this case, the power factor and efficiency corresponding to the rated load are somewhat reduced.

It should be noted that for motors with a squirrel-cage rotor, the starting torque is practically not always the smallest value of the torque in the region of the motor mode. As can be seen from fig. 14, the mechanical characteristic of a motor with a squirrel-cage rotor sometimes has a dip at low angular speeds, caused by the influence of higher harmonics of the tooth fields. This circumstance should be taken into account when starting the engine under load.

For motors with a phase rotor, the initial starting torque increases as it increases to the known resistance limits of the resistor (Fig. 12), and the starting current decreases with increasing resistance. The initial starting torque can be adjusted to the maximum torque. With a further increase in the resistance of the rotor circuit, an increase in cos y 2 compensates for a decrease in the rotor current and the starting torque decreases.

Mechanical characteristics

asynchronous motor in braking modes

In § 3.7, the mechanical characteristics of an asynchronous machine operating in a motor mode were considered. However, an asynchronous motor can also operate in braking modes: during braking with energy transfer to the network, during anti-switching braking and during dynamic braking.

1. Braking with energy return to the network(generator operation mode

Rice. 15. Mechanical characteristics of an asynchronous motor for various operating modes.

in parallel with the mains) is possible at speeds higher than synchronous. The mechanical characteristics of an asynchronous motor in the coordinates M and w) are shown in fig. 15. In quadrant 1 there are sections of the characteristics of the motor mode for three different resistances of the rotor circuit. As the engine speed approaches ideal idle speed, or synchronous speed, the engine torque approaches zero.

With a further increase in the angular velocity under the influence of an external moment, when w>w 0 , the engine operates in the generator mode in parallel with the network, to which it can supply electrical energy, while consuming reactive power for excitation. Braking with energy transfer to the network corresponds to the sections of characteristics located in the upper part of quadrant 2. In this mode, as can be seen from (3.5), the maximum torque is greater than in the motor mode. The braking mode with energy transfer to the network is used practically for pole-changing motors, as well as for drives of hoisting machines (lifts, excavators, etc.) and in some other cases.

2. Reverse current braking has much more practical application. The reverse current braking mode can be obtained, in the same way as for a DC motor, with a load driving torque Ms > M P (Fig. 15). To limit the current and obtain the corresponding torque, it is necessary, when using a motor with a phase rotor, to include an additional resistor in its rotor circuit. The steady-state mode during braking by counter-wiring corresponds, for example, to the point - w SET, M C on the characteristic (Fig. 15).

The mechanical characteristic for Rp 1 in the anti-current braking mode and M C == const does not provide stable operation. Reverse braking can also be obtained by switching two phases of the stator winding on the go, which leads to a change in the direction of rotation of the magnetic field (transition from the point A exactly IN in fig. 16). The rotor then rotates against the direction of the field and gradually slows down. When the angular velocity drops to zero (point C in Fig. 16), the motor must be disconnected from the network, otherwise it may again switch to motor mode, and its rotor will rotate in the opposite direction to the previous one (point D).

3. Dynamic braking of asynchronous motor is usually carried out by turning on the stator winding on the DC network; the rotor winding is then closed to external resistors. To switch from motor mode to dynamic braking mode, contactor K1 (Fig. 17) disconnects the stator from the AC network, and contactor K2 connects the stator winding to the DC network. External resistors are provided in the rotor circuit to limit the current and obtain various braking characteristics.

Passing through the stator winding, the direct current forms a fixed field, the main wave of which gives a sinusoidal distribution of induction. An alternating current arises in a rotating rotor, which creates its own field, which

also stationary relative to the stator. As a result of the interaction of the total magnetic flux with the rotor current, a braking torque arises, which depends on the stator MMF, rotor resistance and the angular velocity of the motor. The mechanical characteristics for this mode are given in the lower part of quadrant 2 (see Fig. 15). They pass through the origin of coordinates, since at an angular velocity equal to zero, the braking torque in this mode is also equal to zero. The maximum torque is proportional to the square of the voltage applied to the stator 1 and increases with increasing voltage. The critical slip depends on

Fig. 16. Mechanical characteristics 17 Wiring diagram

Under the mechanical characteristic, it is customary to understand the dependence of the rotor speed as a function of the electromagnetic torque n = f(M). This characteristic (Fig. 2.15) can be obtained using the dependence M = f (S) and recalculating the rotor speed at different slip values.

Since S = (n0 - n) / n0, hence n = n0(1 - S). Recall that n0 = (60 f) / p is the rotation frequency of the magnetic field.

Section 1-3 corresponds to stable operation, section 3-4 - to unstable operation. Point 1 corresponds to the ideal idle speed of the engine when n = n0. Point 2 corresponds to the nominal mode of operation of the engine, its coordinates are Мн and нн. Point 3 corresponds to the critical moment Мcr and the critical speed ncr. Point 4 corresponds to the motor starting torque Mstart. The mechanical characteristic can be calculated and built according to the passport data. Point 1:

n0 = (60 f) / p,

where: p is the number of pairs of poles of the machine;
f is the network frequency.

Point 2 with coordinates nn and Mn. The rated speed nн is specified in the passport. The nominal moment is calculated by the formula:

here: Рн - rated power (power on the shaft).

Point 3 with coordinates Mcr ncr. The critical moment is calculated by the formula Мcr = Мн λ. Overload capacity λ is specified in the engine passport ncr = n0 (1 - Skr), , Sн = (n0 - nн) / n0 – nominal slip.

Point 4 has coordinates n=0 and M=Mstart. The starting torque is calculated by the formula

Mstart \u003d Mn λstart,

where: λstart - the multiplicity of the starting torque is specified in the passport.

Asynchronous motors have a rigid mechanical characteristic, because the rotor speed (section 1–3) does not depend much on the load on the shaft. This is one of the advantages of these engines.

The mechanical characteristics of induction motors can be expressed as n=f(M) or n=f(I). However, often the mechanical characteristics of induction motors are expressed as a dependence M = f (S), where S is the slip, S = (nc-n) / nc, where n s is the synchronous speed.

In practice, for the graphical construction of a mechanical characteristic, a simplified formula is used, called the Kloss formula:

here: Mk is the critical (maximum) value of the moment. This value of the moment corresponds to the critical slip

Where λm = Mk/Mn

The Kloss formula is used in solving issues related to the electric drive, carried out using an asynchronous motor. Using the Kloss formula, you can build a graph of the mechanical characteristic according to the passport data of an induction motor. For practical calculations in the formula, when determining the critical moment in front of the root, only the plus sign should be taken into account.

Rice. 1. Asynchronous motor: a - schematic diagram, b - mechanical characteristic M \u003d f (S) - natural in motor and generator modes, c - natural mechanical characteristic n \u003d f (M) in motor mode, d - artificial rheostatic mechanical characteristics, e - mechanical characteristics for various voltages and frequencies.

As can be seen from fig. 1, mechanical characteristic of an induction motor located in the I and III quadrants. Part of the curve in the I quadrant corresponds to a positive slip value and characterizes the motor mode of operation of the induction motor, and in the III quadrant - the generator mode. The motor mode is of the greatest practical interest.

The graph of the mechanical characteristics of the motor mode contains three characteristic points: A, B, C and can be conditionally divided into two sections: OB and BC (Fig. 1, c).

Point A corresponds rated motor torque and is determined by the formula Mn = 9.55 10 3 (P n/n n)

This moment corresponds to , which for engines of general industrial use has a value ranging from 1 to 7%, i.e., Sн=1 - 7%. At the same time, small engines have more slip, and large ones have less.

High slip motors, designed to work with shock loading, have S n ~ 15%. These include, for example, engines of a single AC series.

Point C on the characteristic corresponds to the value starting torque arising on the motor shaft during start-up. This moment Mn is called the initial, or starting. The slip in this case is equal to one, and the speed is equal to zero. it is easy to determine according to the reference table, which indicates the ratio of the starting torque to the nominal Mp / Mn.

The value of the starting torque at constant voltage and current frequency depends on the active resistance in the rotor circuit. In this case, at first, with an increase in active resistance, the starting torque increases, reaching its maximum when the active resistance of the rotor circuit is equal to the total inductive resistance of the motor. In the future, with an increase in the active resistance of the rotor, the value of the starting torque decreases, tending to zero in the limit.

Point B (Fig. 1, b and c) corresponds to maximum moment, which can develop the engine over the entire speed range from n = 0 to n = n s. This moment is called the critical (or overturning) moment Mk. critical moment corresponds to the critical slip Sc. The smaller the value of the critical slip Sk, as well as the value of the nominal slip S n, the greater the rigidity of the mechanical characteristic.

Both starting and critical moments are determined through the nominal. According to GOST for electrical machines for a squirrel-cage motor, the condition Mp / Mn \u003d 0.9 - 1.2, Mk / Mn \u003d 1.65 - 2.5 must be observed.

It should be borne in mind that the value of the critical moment does not depend on the active resistance of the rotor circuit, while the critical slip S k is directly proportional to this resistance. This means that with an increase in the active resistance of the rotor circuit, the value of the critical moment remains unchanged, however, the maximum of the torque curve shifts towards increasing slip values ​​(Fig. 1, d).

The magnitude of the critical moment is directly proportional to the square of the voltage supplied to the stator, and inversely proportional to the square of the voltage frequency and current frequency in the stator.

If, for example, the voltage supplied to the motor is equal to 85% of the nominal value, then the value of the critical moment will be 0.85 2 \u003d 0.7225 \u003d 72.25% of the critical moment at rated voltage.

The opposite phenomenon is observed when the frequency changes. If, for example, a motor designed to operate with a current frequency f = 60 Hz is supplied with a current with a frequency f = 50 Hz, then the critical moment will receive a value (60/50) 2 = 1.44 times greater than at its formal frequency (Fig. 1, e).

The critical moment characterizes the instantaneous overload capacity of the engine, i.e. it shows what instantaneous (for a few seconds) overload the engine is able to transfer without any harmful consequences.

The section of the mechanical characteristic from zero to the maximum (critical) value (see Fig. 1, biv) is called stable part of the characteristic, and the BC section (Fig. 1, c) - unstable part.

This division is explained by the fact that on the increasing part of the OF characteristic with increasing slip, i.e. as the speed decreases, the torque developed by the engine increases. This means that with an increase in load, i.e. with an increase in braking torque, the engine speed decreases, and the torque developed by it increases. When the load is reduced, on the contrary, the speed increases, and the torque decreases. When the load changes over the entire range of the stable part of the characteristic, the rotation speed and torque of the engine change.

The engine is not able to develop a moment greater than the critical one, and if the braking torque is greater, the engine must inevitably stop. It happens, as they say, engine rollover.

The mechanical characteristic at constant U and I and the absence of additional resistance in the rotor circuit is called natural characteristic(characteristic of a squirrel-cage asynchronous motor with a phase rotor without additional resistance in the rotor circuit). Artificial, or rheostatic, characteristics are called those that correspond to the additional resistance in the rotor circuit.

All values ​​of starting torques are different and depend on the active resistance of the rotor circuit. The same nominal moment Mn corresponds to slips of various sizes. With an increase in the resistance of the rotor circuit, slip increases and, consequently, the engine speed decreases.

Due to the inclusion of active resistance in the rotor circuit, the mechanical characteristic in the stable part is extended in the direction of increasing slip, in proportion to the resistance. This means that the motor speed begins to change strongly depending on the load on the shaft and the characteristic becomes soft from hard.

An asynchronous motor is an alternating current machine. The word "asynchronous" means non-simultaneous. This means that in asynchronous motors, the rotational speed of the magnetic field differs from the rotational speed of the rotor. The main parts of the machine are the stator and rotor, separated from each other by a uniform air gap.

Fig.1. The device of asynchronous motors

The stator is a fixed part of the machine (Fig. 1, A). Its core, in order to reduce eddy current losses, is recruited from stamped sheets of electrical steel with a thickness of 0.35 - 0.5 mm, isolated from each other by a layer of varnish. The winding is placed in the grooves of the stator magnetic circuit. In three-phase motors, the winding is three-phase. The phases of the winding can be connected in a star or in a triangle, depending on the magnitude of the mains voltage.

The rotor is the rotating part of the engine. The rotor magnetic circuit is a cylinder assembled from stamped sheets of electrical steel (Fig. 1, b, V). A winding is laid in the grooves of the rotor, depending on the type of winding, the rotors of asynchronous motors are divided into short-circuited and phase (with slip rings). The short-circuited winding consists of uninsulated copper or aluminum rods (Fig. 1, G), connected at the ends by rings of the same material (“squirrel cage”).

At the phase rotor (see Fig. 1, V) a three-phase winding is laid in the grooves of the magnetic circuit, the phases of which are connected by a star. The free ends of the winding phases are connected to three copper slip rings mounted on the motor shaft. The slip rings are insulated from each other and from the shaft. Carbon or copper-graphite brushes are pressed against the rings. Through slip rings and brushes, a three-phase ballast rheostat can be connected to the rotor winding.

The conversion of electrical energy into mechanical energy in an asynchronous motor is carried out by means of a rotating magnetic field. A rotating magnetic field is a constant flow rotating in space at a constant angular velocity.

The necessary conditions for excitation of a rotating magnetic field are:

Spatial shift of the axes of the stator coils,

Time shift of currents in stator coils.

The phase axes of the winding are displaced in space by an angle of 120º. The second condition is ensured by applying a three-phase voltage system to the stator coils.

When the motor is connected to a three-phase network, a system of currents of the same frequency and amplitude is established in the stator winding, the periodic changes of which relative to each other are made with a delay of 1/3 of the period.

Winding phase currents create a magnetic field rotating relative to the stator with a frequency n 1, rpm, which is called the synchronous speed of the motor:

Where f1– network current frequency, Hz;

R is the number of pairs of poles of the magnetic field.

At the standard network current frequency Hz, the field rotation frequency according to the formula (1) and depending on the number of pairs of poles has the following values:

R
n 1 , rpm

Rotating, the field crosses the conductors of the rotor winding, inducing an EMF in them. With a closed rotor winding, the EMF causes currents, the interaction of which with a rotating magnetic field produces a rotating electromagnetic moment. The rotor speed in the motor mode of an asynchronous machine is always less than the field speed, i.e. the rotor "lags behind" the rotating field. Only under this condition, an EMF is induced in the rotor conductors, a current flows and a torque is created. The phenomenon of the rotor lagging behind the magnetic field is called slip. The degree of rotor lagging behind the magnetic field is characterized by the value of the relative slip

Where n 2– rotor speed, rpm.

For asynchronous motors, the slip can vary from 1 (start) to a value close to 0 (idling).

For engines with squirrel-cage rotor direct start and reduced voltage start are used.

1. direct start - the stator winding is connected directly to the network at full voltage. Direct start is allowed only for asynchronous motors with a squirrel-cage rotor of low and medium power (up to 15-20 kW). However, with a significant power of the supply network, this method can be extended to motors of higher power (up to about 50 kW).

2. Reduced voltage start. The starting current of the motor is proportional to the voltage on the phases of the stator winding U 1, therefore, a decrease in voltage U 1 is accompanied by a corresponding decrease in the starting current. However, this method leads to a decrease in the initial starting torque, which is proportional to the square of the voltage on the phases of the stator winding. Due to a significant reduction in the starting torque, this starting method is applicable only for small loads on the shaft.

There are several ways to reduce voltage U 1 at the time of launch:

a) with an easy start of asynchronous motors of medium power, which normally operate when the phases of the stator winding are connected in a triangle, the voltage is reduced at the terminals of these phases by switching them to a star;

b) for any type of connection of the phases of the stator winding, the voltage can be reduced using a reactor (three-phase inductive coil) connected in series to the stator winding. It is less economical to reduce the voltage on the stator by connecting the rheostats in series, since they get very hot and additional losses of electrical energy occur;

c) for high power motors, it is advisable to reduce the voltage using a step-down three-phase autotransformer. This method is better than the previous one, but much more expensive. After the motor rotor accelerates and the current drops, the full mains voltage is applied to the stator winding.

Engine start with phase rotor is carried out by including a starting rheostat in the rotor circuit. The starting rheostat reduces the initial starting current and at the same time increases the initial starting torque, which can reach a value close to the maximum torque. As the engine accelerates, the starting rheostat is removed.

Regulation - forced change in speed at a constant load on the shaft. The disadvantage of induction motors is poor controllability. However, there are some possibilities for regulation.

From the slip formula (2), one can obtain the expression for the rotor speed of the induction motor

. (3)

From equality (3) it follows that the speed can be changed in the following ways: by changing the stator current frequency f1, the number of pairs of poles p and slip s. The rotor speed can also be adjusted by changing the supply voltage U 1. Let's consider these methods.

Regulation by changing the frequency of the stator currentf1. Frequency regulation of asynchronous motors is the most promising due to the availability of simple and reliable three-phase thyristor frequency converters, which are connected between the industrial network and the asynchronous motor. When frequency control f1 the engine speed can be smoothly changed so that its maximum value will be tens or hundreds of times higher than the minimum. p>

Regulation by changing the number of pole pairsR. Switching the number of pole pairs of asynchronous motors provides stepwise regulation of the rotor speed and is economical. It is used in machines with a special design of the stator winding, which allows switching its coils to a different number of pole pairs, and also when several alternately switched windings are placed in the grooves of the stator magnetic circuit, made for a different number of pole pairs, for example, R= 1 and R= 2. Motors with a change in the number of pole pairs are called multi-speed, the industry produces motors for two, three and four speeds.

Regulation by changing the input voltageU 1. Reducing the voltage causes a decrease in the speed of the rotor. Reduce voltage U 1 can be included in the stator circuit of rheostats, autotransformers or adjustable chokes. This method is used only for low power motors, since when the voltage decreases, the maximum torque of the motor decreases, which is proportional to the square of the voltage. Reducing the maximum torque reduces the margin for the stability of the engine. In addition, the speed control range is relatively small.

The control methods listed above are used for asynchronous motors with a squirrel-cage rotor.

For motors with a phase rotor, the speed is controlled by changing the slip. For this, an adjusting rheostat is included in the rotor winding. With an increase in the resistance of the adjusting rheostat, the slip increases, and the rotational speed decreases (Fig. 2).

This method provides a smooth change in speed.

Changing the direction of rotation of the rotor is called reversing. To reverse, it is necessary to swap two wires on the terminals of the stator winding of the motor.