electric motor is a mechanism that converts electrical energy into mechanical energy. The principle of operation of any electric motor is based on the law of electromagnetic induction. Typically, an electric motor consists of a stationary part (stator) and a rotor (or armature), in which stationary or rotating magnetic fields are created. Electric motors are the most various types and modifications, are widely used in many branches of human activity, and are one of the main components in the mechanisms and drives of the for various purposes. The efficiency of production directly depends on the characteristics of the electric motor.

Classification of electric motors

The main parts that make up Electric motors , are the stator and rotor. The rotor is the part of the engine that rotates, and the stator is the part that remains stationary. The principle of operation of the electric motor lies in the interaction of the rotating magnetic field, created by the stator winding and electric current, which is in the closed rotor winding. This process initiates the rotation of the rotor in the direction of the field.

The main types of electric motors:

• Engine alternating current;
• Engine direct current;
• Polyphase motor;
• single phase motor;
• valve motor;
• Stepper motor;
• Universal collector motor.

When it comes to motors like asynchronous electric motors, then they refer to the type of AC motors. Such engines are single-phase electric motors , as well as two- and three-phase. In asynchronous motors, the frequency of the alternating current in the winding does not match the speed of the rotor. The process of operation of an asynchronous electric motor is provided by the difference in the time of generation of the magnetic fields of the stator and rotor. Because of this, the rotation of the rotor is delayed relative to the stator field. Buy an electric motor asynchronous type possible for machines that do not require special conditions trigger mechanism operation.

Types of electric motors according to the degree of protection from the external environment:

• Explosion-proof;
• protected;
• Closed.

Explosion-proof electric motors have a strong case, which, if an engine explosion occurs, will prevent damage to all other parts of the mechanism and prevent a fire.

Protected motors during operation they are closed with special dampers and nets that protect the mechanism from foreign objects. They are used in an environment where there is no high humidity and impurities of gases, dust, smoke and chemicals.

Enclosed motors have a special shell that prevents the penetration of dust, gases, moisture and other substances and elements that can harm the engine mechanism. Such electric motors are hermetic and non-hermetic.

Application area frequency converters quite extensive. They are in demand in machine tools and electric drives of industrial mechanisms, conveyors, exhaust ventilation systems, and so on. The principle of operation of the chastotnik lies in the rule for calculating the angular velocity of rotation of the shaft, which includes such a factor as the frequency of the power supply. Thus, by changing the power frequency of the motor winding, it is possible to regulate the speed of rotation of the motor rotor in direct proportion, thus reducing the motor speed or increasing it. These devices are also called "inverters", due to the method by which the problem of simultaneously regulating the frequency and voltage at the output of the converter is solved. All frequency converters are necessarily marked with plates, which indicate their characteristics:

• The maximum possible power of the electric motor;
• Supply voltage;
• Number of phases (single-phase, three-phase).

Most industrial frequency converters are designed to operate in three-phase networks alternating current, however, there are other models, such as frequency converters for single-phase motors.

Application of electric motor

Life modern man hard to imagine without such a mechanism as an electric motor. Take a look around - they have become almost ubiquitous. Today they are used not only in all industries, but also in transport, objects and devices that surround Everyday life, at work and at home. Hair dryers, fans, sewing machines, construction tools - these are far from complete list devices that use electric motors.

It is asynchronous electric motors that are particularly reliable, due to which they are widely used in drives of metalworking, woodworking machines and other industrial machines, in forging presses, hoisting machines, elevators, weaving, sewing and earthmoving machines, industrial fans, compressors, pumps, centrifuges, concrete mixers . Crane motors are used in capital, industrial and civil construction, mining, metallurgical industries, energy, transport.

Metro, tram, trolleybus - all these modes of transport owe their existence to the electric motor. Any office or residential building today cannot be imagined without an air conditioner or an air purification system - they also use electric motors. Functioning of the majority modern equipment is impossible without an electric motor, and therefore a lot depends on the quality and reliability of this mechanism. Its breakdown can lead to very sad results, up to a stop in production and huge financial losses. Therefore, you can only purchase electric motors from a reliable and trusted supplier that guarantees product quality.

The principle of operation of the electric motor

The principle of operation of the electric motor lies in the effect of magnetism, which allows you to effectively convert electrical energy into mechanical. The principle of converting energy into different types electric motors is the same for all types of electric motors, but the design of the motors and how the speed of the torque can be controlled may differ. Everyone from the school bench is known the simplest example electric motor - when the frame rotates between the poles of a permanent magnet. Of course, the device of an electric motor, which is used in industrial mechanisms or household appliances, is much more complicated. Let's look at how an asynchronous electric motor works, which is most widely used in industry.

The principle of operation of an asynchronous motor.

Operating principle induction motor, like others, is based on the use of a rotating magnetic field. The speed of rotation of the magnetic field is called synchronous, since it corresponds to the speed of rotation of the magnet. In this case, the speed of rotation of the cylinder is usually called asynchronous, that is, not coinciding with the speed of rotation of the magnet. The speed of rotation of the cylinder (rotor) differs from the synchronous speed of rotation of the magnetic field by a small amount, called slip. To force to force electricity to create a rotating magnetic field and use it to rotate the rotor, a three-phase current is usually used.

Motor device

Three windings, networks three-phase current located one relative to the other at an angle of 120 °. Inside the core, a metal cylinder is fixed on the axis, called the rotor of the electric motor. If the windings are connected to each other and connected to a three-phase current network, then the total magnetic flux created by the three poles will turn out to be rotating. The total magnetic flux at the same time will change its direction with a change in the direction of the current in the stator windings (poles). In this case, in one period of current change in the windings, the magnetic flux will make a complete revolution. The rotating magnetic flux will drag the cylinder along with it, and we will thus obtain an asynchronous electric motor.

The stator windings can be connected in a "star", however, a rotating magnetic field is also formed when they are connected in a "delta". If you swap the windings of the second and third phases, then the magnetic flux will change the direction of its rotation to the opposite. The same result can be achieved without swapping the stator windings, but by directing the current of the second phase of the network into the third phase of the stator, and the third phase of the network into the second phase of the stator. Thus, the direction of rotation of the magnetic field can be changed by switching any two phases.

Motor connection

The stator of a modern asynchronous electric motor has unexpressed poles, that is, the inner surface of the stator is made completely smooth. To reduce eddy current losses, the stator core is made from thin stamped steel sheets. The assembled stator core is fixed in a steel case. A copper wire winding is laid in the stator slots. The phase windings of the stator of the electric motor are connected by a "star" or "triangle", for which all the beginnings and ends of the windings are brought to the housing - to a special insulating shield. Such a stator device is very convenient, as it allows you to turn on its windings for different standard voltages.

The rotor of an induction motor, like the stator, is assembled from stamped steel sheets. The winding is laid in the grooves of the rotor. Depending on the design of the rotor, asynchronous electric motors are divided into motors with squirrel-cage rotor and phase rotor. The winding of the squirrel-cage rotor is made of copper rods placed in the grooves of the rotor. The ends of the rods are connected with a copper ring. Such a winding is called a "squirrel cage" winding. Note that the copper rods in the grooves are not insulated.

4-6. CALCULATION OF A DC ELECTRIC MOTOR

We start the calculation with a DC motor, since its calculation is simpler and clearer than AC motors. Here is a detailed explanation of all the calculated values, which will then be found in AC motors. The calculation is given for two-pole electric motors with series excitation.

Given the power, speed, voltage of the motor, you can determine all the dimensions and winding data of the electric motor. The calculation of the electric motor begins with the determination of two main dimensions, which are the diameter and length of the armature. These dimensions are included in the formula:

where D is the anchor diameter, m; l is the length of the anchor, m; P I - design power, W; A—linear load of the armature, A/m; B is the magnetic induction in the air gap, T; n - rated speed, rpm.

The length and diameter of the armature of the electric motor are expressed in meters, since in this case the calculation formulas relating the dimensions of the motor with induction and flow are more convenient and simpler. Calculation results obtained in meters can be easily converted into centimeters or millimeters for practical purposes in the manufacture of various parts.

The left side of the formula is proportional to the volume of the anchor. Indeed, if it is multiplied by π and divided by 4, then the volume of the cylinder will be obtained, which is the armature of the electric motor. As can be seen from the right side of the formula, the volume of the armature is proportional to the power of the electric motor P i and inversely proportional to the rotation frequency n. From this we can conclude that the higher the rotation frequency of the motor armature, the smaller its dimensions will be at the same power. And the dimensions of the remaining parts of the electric motor also depend on the size of the armature.

Estimated power of the electric motor, W,

where E is e. d.s., induced in the armature winding when it rotates in a magnetic field, V; I is the current consumed by the electric motor from the source, A; P is the rated power of the electric motor, W; η is the efficiency of the electric motor, the value of which can be taken from the curve in fig. 4-2; as can be seen from the curve, the efficiency values ​​decrease as the motor power decreases.

The numerical value of the design power is obtained by solving (4-2), where the values ​​of all quantities are known. The rated power is always greater than the rated power of the electric motor, since part of the supplied energy is lost in the electric motor itself.

The current consumed by the electric motor, A,

where P is the rated power, W; U - nominal voltage, V; η is the efficiency according to the curve in Fig. 4-2.

Now we can define e. d.s. E, which will be needed in the future:

where N is the number of armature winding conductors; the factor 2 in the denominator shows that total current armature I from the collector plate branches between two winding conductors and only half of the current passes through each conductor; the product πD expresses the circumference of the armature.

Thus, the linear load shows how many amperes fall on 1 m of the circumference of the armature. Linear load A and magnetic induction in the air gap B are called electromagnetic loads. They show how heavily loaded the electric motor is in electrical and magnetic terms. From (4-1) it can be seen that the larger the product AB, the smaller the anchor dimensions will be. But the values ​​​​of A and B should not exceed a certain limit, since otherwise the electric motor will become very hot during operation.

However, the heating of the electric motor depends not only on electromagnetic loads, but also on the time of its operation. Some motors run for a long time without stopping, such as fan motors. Other electric motors run intermittently, during which they have time to cool down, for example, electric motors of crane models, electric players, vacuum cleaners. The operation of electric motors with interruptions is called intermittent operation. This means that the motor is switched on for a short time, then there is a break and the motor is switched on again.

The duration of the inclusion of such an electric motor is expressed as a percentage of a certain period, which is taken as 10 minutes. For example, if the motor runs for one period of 2.5 minutes, and the rest of the time is idle, then the duty cycle is 25%. If the electric motor runs for 4 minutes, then the duty cycle is 40%.

The choice of linear load and magnetic induction is made according to the curves of fig. 4-3, where the ratio of rated power to rated speed is plotted along the horizontal axis. On fig. Table 4-3 gives A and B values ​​for continuous duty motors. For example, if an electric motor with a power of 80 W at a speed of 4000 rpm works for a long time at full load, then we set aside the value 80/4000=20 10 -3 on the horizontal axis. On the vertical line, we count the value of the linear load A \u003d 9,000 A / m and induction in the air gap B \u003d 0.35 T.

With intermittent operation with a duty cycle of 25%, the values ​​of electromagnetic loads can be increased by 30%, i.e., they can be taken 1.3 times more. Then

A \u003d 9000 1.3 \u003d 11 700 A / m,

and the magnetic induction

B \u003d 0.35 1.3 \u003d 0.455 T.

Denote l/D=e. The value of e for small electric motors ranges from 0.4 to 1.6. If you need to get an electric motor with a shorter length, but with a larger diameter, then we take e = 0.4. On the contrary, if the electric motor must fit into a pipe of small diameter, then we choose e = 1.6. If the dimensions of the electric motor are not bound by any conditions, then e = 1 is usually taken. Introducing the ratio l/D = e to the left side of (4-1), we get rid of one unknown l and (4-1) looks like:

Having defined D, we find l=De. Thus, the main dimensions of the electric motor are determined.

Now let's move on to the calculation of the armature winding. To do this, you need to determine the magnetic flux of the electric motor.

If the magnetic induction in the air gap is multiplied by the area through which the lines of force enter the armature, then we get the magnetic flux of the electric motor, which we denote by the Greek letter F (phi):

Magnetic flux is measured in webers. The Greek letter τ (tau) denotes the pole division, that is, the part of the armature circle that falls on one pole. In a two-pole electric motor, the pole division is τ=πD/2. The Greek letter a (alpha) indicates which part of the pole division is occupied by the arc of the pole b t (Fig. 4-5). Usually take a = 0.65. Thus, the product aτl gives the area of ​​the pole facing the armature.

The number of anchor slots is determined from the ratio Z≈3D, in which the diameter of the anchor is expressed in centimeters. It is recommended to take the odd number closest to the received one. The number of armature conductors is determined by the formula

The number of conductors in one groove N z =N/Z. The number N z obtained during the calculation must be rounded up to the nearest even integer number so that the winding can be wound in two layers. The choice of the number of slots and the number of conductors will be clear from the numerical example of the calculation of the electric motor.

The cross section of the wire for the armature winding can be determined by dividing the current in the conductor by the current density. Current density indicates how many amperes pass through each square millimeter wire section, and is denoted by the Greek letter A (delta). Thus, the cross section of the wire, mm 2,

The current density for homemade DC motors should be selected in the range from 6 to 12 A / mm 2. For small motors with high speeds, the current density is taken closer to the upper recommended value. For larger engines with lower speeds, closer to the lower value.

This wire cross-section s is preliminary. In the second column of Table. 4-1 you need to find the cross section of the standard wire, which is closest to the calculated one. In the first column of this table we find the wire diameter d. The absence of a wire of the required diameter cannot interfere with the manufacture of an electric motor, since there are great opportunities for replacing the wire. First of all, one wire can be replaced by two wires, if the section of these wires is the same as that of the replaced wire. The cross section of the wire depends on the square of its diameter, which means that a wire with a cross section 2 times smaller will have a diameter √2 times smaller. For example, instead of a wire with a diameter of 0.29 mm, you can take two wires with a diameter of 0.2 mm. In this case, the current density will hardly change, but the number of wires in the groove will increase by 2 times. The density of filling the groove with wires will also increase, since each wire has a two-layer insulation. Winding such a winding will be more difficult. You can replace one wire with two with different diameters. For example, instead of a wire with a diameter of 0.29 mm, you can take two wires: one with a diameter of 0.31 mm and the other with a diameter of 0.27 mm. As can be seen from Table. 4 1, the sum of the cross sections of two replacement wires is equal to the cross section of the wire being replaced:

0.075 + 0.057 \u003d 0.132 mm 2.

Having finally chosen the diameter of the wire d, it is necessary according to the table. 4-2 determine the diameter insulated wire d from, adding the double-sided thickness δ from the insulation:

Determine the dimensions of the groove. The groove cross section S, mm 2, required to accommodate the winding conductors, can be calculated by the formula:

where k s is the slot filling factor, showing how tightly the conductors fill the slot.

The smaller the fill factor, the larger the groove area should be. The larger the fill factor and the thicker the slot insulation, the more difficult it is to wind the winding. In self-made electric motors, it is recommended to insulate with a groove sleeve 2 made of 0.2 mm thick electric cardboard. On top of the winding, a wedge 3 made of cardboard 0.3 mm thick is installed in the groove (Fig. 4-4). In the calculations, you can take the fill factor k 3 =0.4.

In factory-made motors, the slots are intricately pear-shaped (see Figure 2-10) to accommodate more conductors without weakening the thickness of the teeth between the grooves. In homemade electric motors, it is easiest to drill round grooves in the compressed armature core (Fig. 4-5).

The groove diameter is determined by its cross section:

Distance between centers of adjacent grooves, mm,

and tooth thickness, mm,

The thickness of the tooth at the narrow point must be at least 2 mm. If, according to the calculation, the thickness of the tooth is less than 2 mm, it is necessary to increase the diameter of the anchor. The slot of the groove a must be 1 mm larger than the diameter of the insulated wire.

The number of collector plates in electric motors for low voltage (12 V and below) is taken equal to the number of armature slots. Laying the armature winding in the grooves and connecting them to the collector plates are described in Ch. 5. The cross section of the carbon-graphite brush S sh, cm 2, is selected by the formula:

where? u is the current density under the brush, ? y \u003d 5÷8 A / cm 2.

This concludes the calculation of the anchor.

We proceed to the calculation of the magnetic system and the excitation winding. For a homemade electric motor, the easiest way is to use a magnetic system open type(Fig. 4-5). When calculating, first of all, the air gap δ between the armature and the poles is determined. In DC machines, the air gap is determined by the formula

The angle of the pole arc can be found from the value a = 0.65. Half of the circle is 180°; therefore, a=180° 0.65= 117°, round up to 120°.

The dimensions of the magnetic circuit are calculated according to the recommended magnetic inductions in its sections. When calculating the cross section of the poles and the frame, the magnetic flux is increased by 10%, since part of the lines closes between the sides of the frame, bypassing the anchor. Therefore, the magnetic flux of the poles and the frame F st \u003d 1.1 F.

Induction in the frame is taken B st \u003d 0.5 T. Length field line in the frame L st is determined according to the sketch (Fig. 4-5). Here, the dotted line shows the path of the magnetic flux. It consists of the following sections: two air gaps, two teeth, an anchor and a frame. To find out which With. should create an excitation coil, it is necessary to calculate n. With. (Iw) for each of these sections and then add them all up. Let's start the calculation. With. from the air gap.

Magnetizing force of two air gaps:

where δ is the air gap on one side of the anchor, m; k δ is the air gap coefficient, which takes into account how much the magnetic resistance of the air gap increases due to the presence of groove slots on the anchor; can be considered k δ =1,1; B - induction in the air gap, T.

To determine n. With. armature teeth, you need to know the induction in the tooth. The thickness of the tooth is determined by (4-12). The magnetic flux enters the tooth through part of the armature circumference, which is occupied by one crown of the tooth and one slot of the groove. It is called tooth division t 1 and and is determined by the formula

The induction in the tooth will be as many times greater than the induction in the air gap, how many times the thickness of the tooth is less than the tooth division. In addition, it should be taken into account that part of the length of the armature is occupied by insulating layers between the steel sheets of the armature, which make up about 10%. Therefore, the induction in the tooth can be determined by the formula

This induction according to Table. 4-3 corresponds to the field strength H z . To calculate n. With. by two tooth heights, H z must be multiplied by twice the height of the tooth. However, given that with round grooves, the induction in the upper and lower parts of the tooth decreases, we multiply H z by the height of one tooth lw z \u003d H z h z.

When calculating the induction in the armature core, it should be taken into account that the magnetic flux in it branches, and therefore only half of the flux falls on one section. The cross section of the anchor core according to fig. 4-5 is equal to the distance from the base of the groove to the shaft, multiplied by the length of the armature l:

It is also necessary to take into account the insulating layers between the sheets. Thus, the induction in the armature core

This induction according to Table. 4-3 corresponds to H i. Magnetizing force of the armature core:

where L i is the length of the power line in the core, m, according to fig. 4-5:

As seen in fig. 4-5, this motor does not have protruding poles as they are fused with the frame. Therefore, the calculation of the fixed part of the magnetic circuit is reduced to the calculation of the frame. The bed width is determined by the given induction B=0.5 T, m,

The field strength H st for an induction of 0.5 T is found in Table. 4-3. When determining the length of the field line in the frame, we encounter difficulty, since the length of the side of the frame depends on the thickness of the coil, and we do not yet know it. Therefore, we take the thickness of the coil b k \u003d 30 δ, where δ is the air gap. The dependence between the thickness of the coil and the gap is explained by the fact that n depends mainly on the size of the gap. With. coils, and hence the dimensions of the coil. Having determined the length of the power line in the frame L st from the sketch, it is possible to calculate n. With. beds:

Now let's add n. With. all areas:

Such n. With. should create a coil when the motor is idling. But under load, when the current in the armature will increase, the demagnetizing effect of the magnetic field of the armature will appear. Therefore n. With. coils must have some margin, which is calculated by the formula

Thus, n. With. coils under motor load

The armature current will pass through the excitation coil, and therefore the number of turns of the coil will be w \u003d Iw / I.

To determine the cross section of the wire, the current must be divided by the current density. It is taken less than for the armature winding, since the turns of the coil are stationary and therefore cool worse.

Coil wire cross section, mm 2, s = I/?.

According to the table 4-1 find the nearest standard section and wire diameter. Selecting the brand of wire, according to the table. 4-2 we find the diameter of the insulated wire d pz. To find out the thickness of the coil, you need to know the area, mm 2, occupied by the turns of the coil, which can be determined by the formula

Dividing the area by the length of the coil, which is indicated on the sketch l k, we get the thickness of the coil, mm,

So, according to the nominal data of the electric motor, which are expressed in just three numbers, using formulas and tables, we determined all the dimensions of the electric motor necessary for its manufacture. The calculated electric motor will work reliably, and its heating will not go beyond the permissible norms. This is the value of calculating the electric motor. Would it be possible to "guess" all these dimensions without calculations? Probably, the electric motor would have to be redone several times in order to obtain a satisfactory result, spending several times more time on these alterations than on the calculation, not to mention damaged materials. In addition, during the calculation process, you will gain skills in technical calculations and knowledge in the theory of electrical machines.

N.V. Vinogradov, Yu.N. Vinogradov
How to calculate and make an electric motor yourself
Moscow 1974

Conditions for choosing an electric motor

The choice of one of the catalog types of electric motors is considered correct if the following conditions are met:

a) the most complete correspondence of the electric motor to the working machine (drive) in terms of mechanical properties. This means that the electric motor must have such a mechanical characteristic that it could provide the drive with the necessary speed and acceleration values ​​both during operation and during start-up;

b) maximum use of the electric motor power during operation. The temperature of all active parts of the electric motor in the most severe operating modes should be as close as possible to the heating temperature stipulated by the standards, but not exceed it;

c) compliance of the electric motor with the drive and conditions environment by design;

d) compliance of the electric motor with the parameters of its supply network.

To select an electric motor, the following initial data are required:

a) name and type of mechanism;

b) maximum power on the drive shaft of the mechanism, if the operating mode is continuous and the load is constant, and in other cases - graphs of changes in power or moment of resistance as a function of time;

c) speed of rotation of the drive shaft of the mechanism;

d) the method of articulation of the mechanism with the motor shaft (in the presence of gears, the type of gear and gear ratio are indicated);

e) the amount of torque at start-up, which the electric motor must provide on the drive shaft of the mechanism;

f) speed control limits of the driven mechanism, indicating the upper and lower speed values ​​and the corresponding power and torque values;

g) the nature and quality (smoothness, stepping) of the necessary speed adjustment;

h) frequency of starts or switching on of the drive within an hour; i) characteristics of the environment.

The choice of an electric motor based on all conditions is made according to the catalog data.

For mechanisms wide application the choice of an electric motor is greatly simplified due to the data contained in the relevant information of manufacturers, and is reduced to clarifying the type of electric motor in relation to the parameters of the network and the nature of the environment.

The choice of electric motors by power

The choice of power of the electric motor must be made in accordance with the nature of the loads of the working machine. This character is evaluated on two grounds:

a) according to the nominal mode of operation;

b) by changes in the magnitude of power consumption.

There are the following modes of operation:

a) long (long), when the working period is so long that motor heating reaches its steady state value (for example, pumps, belt conveyors, fans, etc.);

b) short-term, when the duration of the working period is insufficient for the electric motor to reach temperature - heating corresponding to a given load, and the periods of stop, on the contrary, are sufficient to cool the motor down to ambient temperature. In this mode, electric motors of a wide variety of mechanisms can operate;

c) intermittent - with a relative duty cycle of 15, 25, 40 and 60% with a duration of one cycle of not more than 10 minutes (for example, for cranes, some machine tools, single-station welding motor-generators, etc.).

The following cases are distinguished by changes in the magnitude of power consumption:

a) constant load, when the amount of power consumed during operation is constant or has slight deviations from the average value, such as centrifugal pumps, fans, compressors with constant air flow, etc.;

b) variable load when the amount of power consumed changes periodically, as, for example, with excavators, cranes, some machine tools, etc.;

c) pulsating load, when the amount of power consumed changes continuously, as, for example, with piston pumps, jaw crushers, screens, etc.

The power of the electric motor must satisfy three conditions:

a) normal heating during operation;

b) sufficient overload capacity;

c) sufficient starting torque.

All electric motors are divided into two main groups:

a) for long-term operation (without limiting the duration of the inclusion);

b) for intermittent operation with duty cycles of 15, 25, 40 and 60%.

For the first group, the catalogs and passports indicate the long-term power that the electric motor can develop indefinitely, for the second group - the power that the electric motor can develop, working intermittently for an arbitrarily long time at a certain duty cycle.

In all cases, such an electric motor is considered to be correctly selected, which, working with a load according to the schedule set by the working machine, reaches the full allowable heating of all its parts. The choice of electric motors with the so-called "power margin", based on the highest possible load according to the schedule, leads to underutilization of the electric motor, and, consequently, to an increase in capital costs and operating costs due to a decrease in power and efficiency factors.

Excessive increase in motor power can also lead to jerks during acceleration.

If the electric motor must operate for a long time with a constant or slightly changing load, then its power determination is not difficult and is carried out according to formulas that usually include empirical coefficients.

It is much more difficult to choose the power of electric motors of other modes of operation.

Short-term load is characterized by the fact that the switching periods are short and the pauses are sufficient for the complete cooling of the motor. It is assumed that the load of the electric motor during the switching periods remains constant or almost constant.

In order for the electric motor to be used correctly in this mode for heating, it is necessary to select it so that its continuous power (indicated in the catalogs) is less than the power corresponding to short-term load, i.e. so that the electric motor during periods of its short-term operation has a thermal overload.

If the periods of operation of the electric motor are significantly less than the time required for its full heating, but the pauses between the switching periods are significantly shorter than the time for complete cooling, then there is an intermittent load.

In practice, two types of such work should be distinguished:

a) the load during the period of work is constant in magnitude and, therefore, its graph is depicted by rectangles alternating with pauses;

b) the load during the period of work changes according to a more or less complex law.

In both cases, the problem of choosing an electric motor by power can be solved both analytically and graphically. Both of these methods are quite complex, so a simplified method of equivalent greatness is practically recommended, which includes three methods:

a) RMS current;

b) RMS power;

c) mean square moment.

Checking the mechanical overload capacity of the electric motor

After selecting the power of the electric motor according to the heating conditions, it is necessary to check the mechanical overload capacity of the electric motor, i.e. make sure that the maximum load torque according to the schedule during operation and the torque at start-up will not exceed the values maximum torque by catalogue.

For asynchronous and synchronous electric motors, the amount of permissible mechanical overload is determined by their overturning electromagnetic moment, upon reaching which these electric motors stop.

The multiplicity of the maximum moments in relation to the nominal ones should be 1.8 for three-phase asynchronous electric motors with slip rings, at least 1.65 for the same short-circuited electric motors. The multiplicity of the maximum torque of the synchronous motor must also be at least 1.65 at rated voltages, frequency and excitation current, with a power factor of 0.9 (with leading current).

Practically asynchronous and synchronous electric motors have a mechanical overload capacity of up to 2-2.5, and for some special electric motors this value rises to 3-3.5.

The permissible overload of DC motors is determined by the operating conditions and, according to GOST, is from 2 to 4 in terms of torque, and the lower limit applies to electric motors with parallel, and the upper limit - to electric motors with series excitation.

If the supply and distribution networks are sensitive to load, then the mechanical overload capacity must be checked taking into account voltage losses in the networks.

For asynchronous squirrel-cage and synchronous electric motors, the multiplicity of the initial torque must be at least 0.9 (in relation to the nominal one).

In reality, the multiplicity of the initial torque for electric motors with a double squirrel cage and with a deep groove is much higher and reaches 2-2.4.

When choosing the power of an electric motor, it should be borne in mind that the heating of electric motors is affected by the frequency of switching on. The permissible switching frequency depends on the normal slip, the flywheel moment of the rotor and the multiplicity of the starting current.

Asynchronous electric motors of normal types allow without load from 400 to 1000, and electric motors with increased slip - from 1100 to 2700 starts per hour. When starting under load, the permissible number of starts is significantly reduced.

The starting current of electric motors with a squirrel-cage rotor is large, and this circumstance is important under conditions of frequent starts, and especially with increased acceleration time.

In contrast to electric motors with a phase rotor, in which part of the heat generated during start-up is released in the rheostat, i.e. outside the machine, in short-circuited electric motors all the heat is released in the machine itself, which causes its increased heating. Therefore, the choice of power of these electric motors must be made taking into account the heating during multiple starts.

Conditions for choosing an electric motor

The choice of one of the catalog types of electric motors is considered correct if the following conditions are met:

a) the most complete correspondence of the electric motor to the working machine (drive) in terms of mechanical properties. This means that the electric motor must have such a mechanical characteristic that it could provide the drive with the necessary speed and acceleration values ​​both during operation and during start-up;

b) maximum use of the electric motor power during operation. The temperature of all active parts of the electric motor in the most severe operating modes should be as close as possible to the heating temperature stipulated by the standards, but not exceed it;

c) compliance of the electric motor with the drive and environmental conditions according to the design;

d) compliance of the electric motor with the parameters of its supply network.

To select an electric motor, the following initial data are required:

a) name and type of mechanism;

b) the maximum power on the drive shaft of the mechanism, if the operating mode is continuous and the load is constant, and in other cases - graphs of changes in power or moment of resistance as a function of time;

c) speed of rotation of the drive shaft of the mechanism;

d) the method of articulation of the mechanism with the motor shaft (in the presence of gears, the type of gear and gear ratio are indicated);

e) the amount of torque at start-up, which the electric motor must provide on the drive shaft of the mechanism;

f) speed control limits of the driven mechanism, indicating the upper and lower speed values ​​and the corresponding power and torque values;

g) the nature and quality (smoothness, stepping) of the necessary speed adjustment;

h) frequency of starts or switching on of the drive within an hour; i) characteristics of the environment.

The choice of an electric motor based on all conditions is made according to the catalog data.

For mechanisms of wide application, the choice of an electric motor is greatly simplified due to the data contained in the relevant information of manufacturers, and comes down to clarifying the type of electric motor in relation to the network parameters and the nature of the environment.

The choice of electric motors by power

The choice of power of the electric motor must be made in accordance with the nature of the loads of the working machine. This character is evaluated on two grounds:

a) according to the nominal mode of operation;

b) by changes in the magnitude of power consumption.

There are the following modes of operation:

a) long (long), when the working period is so long that motor heating reaches its steady state value (for example, pumps, belt conveyors, fans, etc.);

b) short-term, when the duration of the operating period is insufficient for the electric motor to reach the temperature-heating corresponding to the given load, and the shutdown periods, on the contrary, are sufficient to cool the electric motor to the ambient temperature. In this mode, electric motors of a wide variety of mechanisms can operate;

c) intermittent - with a relative duty cycle of 15, 25, 40 and 60% with a duration of one cycle of not more than 10 minutes (for example, for cranes, some machine tools, single-station welding motor-generators, etc.).

The following cases are distinguished by changes in the magnitude of power consumption:

a) constant load, when the amount of power consumed during operation is constant or has slight deviations from the average value, such as centrifugal pumps, fans, compressors with constant air flow, etc.;

b) variable load, when the amount of power consumed changes periodically, as, for example, with excavators, cranes, some machine tools, etc.;

c) pulsating load, when the amount of power consumed changes continuously, as, for example, with piston pumps, jaw crushers, screens, etc.

The power of the electric motor must satisfy three conditions:

a) normal heating during operation;

b) sufficient overload capacity;

c) sufficient starting torque.

All electric motors are divided into two main groups:

a) for long-term operation (without limiting the duration of the inclusion);

b) for intermittent operation with duty cycles of 15, 25, 40 and 60%.

For the first group, the catalogs and passports indicate the long-term power that the electric motor can develop indefinitely, for the second group - the power that the electric motor can develop, working intermittently for an arbitrarily long time at a certain duty cycle.

In all cases, such an electric motor is considered to be correctly selected, which, working with a load according to the schedule set by the working machine, reaches the full allowable heating of all its parts. The choice of electric motors with the so-called "power margin", based on the highest possible load according to the schedule, leads to underutilization of the electric motor, and, consequently, to an increase in capital costs and operating costs due to a decrease in power and efficiency factors.

Excessive increase in motor power can also lead to jerks during acceleration.

If the electric motor must operate for a long time with a constant or slightly changing load, then its power determination is not difficult and is carried out according to formulas that usually include empirical coefficients.

It is much more difficult to choose the power of electric motors of other modes of operation.

Short-term load is characterized by the fact that the switching periods are short and the pauses are sufficient for the complete cooling of the motor. It is assumed that the load of the electric motor during the switching periods remains constant or almost constant.

In order for the electric motor to be used correctly in this mode for heating, it is necessary to select it so that its continuous power (indicated in the catalogs) is less than the power corresponding to short-term load, i.e. so that the electric motor during periods of its short-term operation has a thermal overload.

If the periods of operation of the electric motor are significantly less than the time required for its full heating, but the pauses between the switching periods are significantly shorter than the time for complete cooling, then there is an intermittent load.

In practice, two types of such work should be distinguished:

a) the load during the period of work is constant in magnitude and, therefore, its graph is depicted by rectangles alternating with pauses;

b) the load during the period of work changes according to a more or less complex law.

In both cases, the problem of choosing an electric motor by power can be solved both analytically and graphically. Both of these methods are quite complex, so a simplified method of equivalent greatness is practically recommended, which includes three methods:

a) RMS current;

b) RMS power;

c) mean square moment.

Checking the mechanical overload capacity of the electric motor

After selecting the power of the electric motor according to the heating conditions, it is necessary to check the mechanical overload capacity of the electric motor, i.e., make sure that the maximum load torque according to the schedule during operation and the torque during start-up will not exceed the values ​​of the maximum torque according to the catalog.

For asynchronous and synchronous electric motors, the amount of permissible mechanical overload is determined by their overturning electromagnetic moment, upon reaching which these electric motors stop.

The multiplicity of the maximum moments in relation to the nominal ones should be 1.8 for three-phase asynchronous electric motors with slip rings, at least 1.65 for the same short-circuited electric motors. The multiplicity of the maximum torque of the synchronous motor must also be at least 1.65 at rated voltages, frequency and excitation current, with a power factor of 0.9 (with leading current).

Practically asynchronous and synchronous electric motors have a mechanical overload capacity of up to 2-2.5, and for some special electric motors this value rises to 3-3.5.

The permissible overload of DC motors is determined by the operating conditions and, according to GOST, is from 2 to 4 in terms of torque, and the lower limit applies to electric motors with parallel, and the upper limit - to electric motors with series excitation.

If the supply and distribution networks are sensitive to load, then the mechanical overload capacity must be checked taking into account voltage losses in the networks.

For asynchronous squirrel-cage and synchronous electric motors, the multiplicity of the initial torque must be at least 0.9 (in relation to the nominal one).

In reality, the multiplicity of the initial torque for electric motors with a double squirrel cage and with a deep groove is much higher and reaches 2-2.4.

When choosing the power of an electric motor, it should be borne in mind that the heating of electric motors is affected by the frequency of switching on. The permissible switching frequency depends on the normal slip, the flywheel moment of the rotor and the multiplicity of the starting current.

Asynchronous electric motors of normal types allow without load from 400 to 1000, and electric motors with increased slip - from 1100 to 2700 starts per hour. When starting under load, the permissible number of starts is significantly reduced.

The starting current of electric motors with a squirrel-cage rotor is large, and this circumstance is important under conditions of frequent starts, and especially with increased acceleration time.

In contrast to electric motors with a phase rotor, in which part of the heat generated during start-up is released in the rheostat, i.e. outside the machine, in short-circuited electric motors all the heat is released in the machine itself, which causes its increased heating. Therefore, the choice of power of these electric motors must be made taking into account the heating during multiple starts.