Characteristics of DPT with parallel excitation. DC motor excitation

Electric motors are machines capable of converting electrical energy into mechanical. Depending on the type of current consumed, they are divided into AC motors and direct current. In this article we will talk about the second, which are abbreviated as DPT. DC motors surround us every day. They are equipped with power tools powered by batteries or accumulators, electric vehicles, some industrial machines and much more.

Device and principle of operation

DPT in its structure resembles synchronous motor alternating current, the difference between them is only in the type of current consumed. The engine consists of a fixed part - a stator or an inductor, a moving part - an armature and a brush-collector assembly. The inductor can be made in the form of a permanent magnet if the motor is small, but more often it is provided with an excitation winding having two or more poles. The armature consists of a set of conductors (windings) fixed in grooves. V the simplest model DPT used only one magnet and a frame through which the current passed. This design can only be considered as a simplified example, while the modern design is an improved version that has a more complex structure and develops the necessary power.

The principle of operation of a DPT is based on Ampère's law: if a charged wire frame is placed in a magnetic field, it will begin to rotate. The current, passing through it, forms around itself its own magnetic field, which, upon contact with an external magnetic field will rotate the frame. In the case of a single frame, the rotation will continue until it takes a neutral position parallel to the external magnetic field. To set the system in motion, you need to add another frame. In modern DPTs, the frames are replaced by an anchor with a set of conductors. Current is applied to the conductors, charging them, as a result of which a magnetic field arises around the armature, which begins to interact with the magnetic field of the excitation winding. As a result of this interaction, the anchor rotates through a certain angle. Next, the current flows to the next conductors, etc.
For alternate charging of the armature conductors, special brushes are used, made of graphite or an alloy of copper with graphite. They play the role of contacts that close electrical circuit to the terminals of a pair of conductors. All conclusions are isolated from each other and combined into a collector assembly - a ring of several lamellas located on the axis of the armature shaft. While the engine is running, the brush contacts alternately close the lamellas, which allows the engine to rotate evenly. The more conductors the armature has, the more evenly the DCT will work.

DC motors are divided into:
— electric motors with independent excitation;
- electric motors with self-excitation (parallel, series or mixed).
The DCT circuit with independent excitation provides for the connection of the excitation winding and armature to different sources power supply, so that they are not electrically connected to each other.
Parallel excitation is realized by parallel connection windings of the inductor and armature to the same power source. These two types of motors have tough performance characteristics. Their rotational speed of the working shaft does not depend on the load, and it can be adjusted. Such motors have found application in machines with variable load, where it is important to control the speed of rotation of the shaft.
With serial excitation, the armature and field winding are connected in series, so the value electric current they have the same. Such motors are “softer” in operation, have a larger range of speed control, but require a constant load on the shaft, otherwise the rotation speed may reach a critical level. They have high value starting torque, which facilitates starting, but the speed of rotation of the shaft depends on the load. They are used in electric transport: in cranes, electric trains and city trams.
The mixed type, in which one excitation winding is connected to the armature in parallel, and the second in series, is rare.

Brief history of creation

The pioneer in the history of the creation of electric motors was M. Faraday. Create a complete working model he could not, but it is he who owns the discovery that made it possible. In 1821, he conducted an experiment using a charged wire placed in mercury in a bath with a magnet. When interacting with a magnetic field metal conductor began to rotate, I turn the energy of the electric current into mechanical work. Scientists of that time were working on the creation of a machine whose work would be based on this effect. They wanted to get an engine that works on the principle of a piston, that is, that the working shaft moves back and forth.
In 1834, the first electric DC motor was created, which was developed and created by the Russian scientist B.S. Yakobi. It was he who proposed to replace the reciprocating motion of the shaft with its rotation. In his model, two electromagnets interacted with each other, rotating the shaft. In 1839, he also successfully tested a boat equipped with a DPT. The further history of this power unit, in fact, is the improvement of the Jacobi engine.

Features of DPT

Like other types of electric motors, DPT is reliable and environmentally friendly. Unlike AC motors, it can adjust the shaft rotation speed in a wide range, frequency, and besides, it is easy to start.
The DC motor can be used both as a motor and as a generator. It can also change the direction of shaft rotation by changing the direction of the current in the armature (for all types) or in the field winding (for motors with series excitation).
Rotation speed control is achieved by connecting a variable resistance to the circuit. With sequential excitation, it is in the armature circuit and makes it possible to reduce speed in ratios of 2:1 and 3:1. This option is suitable for equipment that has long periods of inactivity, because during operation there is a significant heating of the rheostat. The increase in speed is provided by connecting a rheostat to the excitation winding circuit.
For engines with parallel excitation rheostats are also used in the armature circuit to reduce the speed within 50% of the nominal values. Setting the resistance in the excitation winding circuit allows you to increase the speed up to 4 times.
The use of rheostats is always associated with significant heat losses, therefore, in modern models engines they are replaced by electronic circuits, allowing you to control the speed without significant loss of energy.
The efficiency of a DC motor depends on its power. Low power models are characterized by low efficiency with an efficiency of about 40%, while motors with a power of 1000 kW can have an efficiency of up to 96%.

Advantages and disadvantages of DPT

The main advantages of DC motors are:
- simplicity of design;
— ease of management;
- the ability to control the frequency of rotation of the shaft;
- easy start (especially for engines with sequential excitation);
— possibility of use as generators;
- compact dimensions.
Flaws:
- have " weak link"- graphite brushes that wear out quickly, which limits the service life;
- high cost;
- when connected to the network require the presence of rectifiers.

Scope of application

DC motors are widely used in transport. They are installed in trams, electric trains, electric locomotives, steam locomotives, motor ships, dump trucks, cranes, etc. in addition, they are used in tools, computers, toys and moving mechanisms. Often they can also be found on production machines, where it is necessary to control the speed of the working shaft in a wide range.

DC motors, depending on the methods of their excitation, as already noted, are divided into motors with an independent, parallel(by shunt), consistent(serial) and mixed (compound) excitation.

Motors of independent excitation, require two power sources (Fig. 11.9, a). One of them is needed to power the armature winding (conclusions Z1 and Z2), and the other - to create a current in the excitation winding (winding terminals Ш1 and SH2). Additional resistance Rd in the armature winding circuit is necessary to reduce the starting current of the motor at the moment it is turned on.

With independent excitation, mainly powerful electric motors for the purpose of more convenient and economical control of the excitation current. The cross section of the excitation winding wire is determined depending on the voltage of its power source. A feature of these machines is the independence of the excitation current, and, accordingly, the main magnetic flux, from the load on the motor shaft.

Motors with independent excitation are practically identical in their characteristics to motors of parallel excitation.

Parallel excitation motors are switched on in accordance with the scheme shown in Fig. 11.9, b. clamps Z1 and Z2 refer to the armature winding, and the clamps Ш1 and SH2- to the excitation winding (to the shunt winding). Variable resistance Rd and Rv designed respectively to change the current in the armature winding and in the excitation winding. The excitation winding of this motor is made from a large number turns of copper wire of relatively small cross section and has a significant resistance. This allows you to connect it to the full mains voltage specified in the passport data.

A feature of this type of motors is that during their operation it is forbidden to disconnect the excitation winding from the anchor chain. Otherwise, when the excitation winding opens, an unacceptable EMF value will appear in it, which can lead to engine failure and damage to the operating personnel. For the same reason, it is impossible to open the excitation winding when the engine is turned off, when its rotation has not yet stopped.

With an increase in the speed of rotation, the additional (additional) resistance Rd in the armature circuit should be reduced, and when the steady speed is reached, it should be removed completely.

Fig.11.9. Types of excitation of DC machines,

a - independent excitation, b - parallel excitation,

c - sequential excitation, d - mixed excitation.

OVSH - shunt excitation winding, OVS - serial excitation winding, "OVN - independent excitation winding, Rd - additional resistance in the armature winding circuit, Rv - additional resistance in the excitation winding circuit.

The absence of additional resistance in the armature winding at the time of starting the engine can lead to a large starting current exceeding rated current anchors in 10...40 times .

An important property of the parallel excitation motor is its almost constant rotational speed when the load on the armature shaft changes. So when the load changes from idling to the nominal value, the speed decreases by only (2.. 8)% .

The second feature of these engines is economical speed control, in which the ratio top speed to the smallest can be 2:1 , and with a special version of the engine - 6:1 . Minimum frequency rotation is limited by the saturation of the magnetic circuit, which does not allow to increase the magnetic flux of the machine, and the upper limit of the rotational speed is determined by the stability of the machine - with a significant weakening of the magnetic flux, the motor can go "peddling".

Sequential excitation motors(serial) are switched on according to the scheme (Fig. 11.9, c). conclusions C1 and C2 correspond to the serial (serial) excitation winding. It is made from a relatively small number of turns of mainly large-section copper wire. The field winding is connected in series with the armature winding.. Additional resistance Rd in the circuit of the armature and excitation windings, it allows to reduce the starting current and regulate the engine speed. At the moment the engine is turned on, it should have such a value at which the starting current will be (1.5...2.5)In. After the engine reaches a steady speed, additional resistance Rd output, i.e. set to zero.

These motors develop large starting torques at start-up and must be started at a load of at least 25% of its rated value. Turning on the engine with less power on its shaft, and even more so in idle mode, is not allowed. Otherwise, the engine may develop unacceptably high speed, which will cause it to fail. Engines of this type are widely used in transport and lifting mechanisms, in which it is necessary to change the rotational speed over a wide range.

Mixed excitation motors(compound), occupy an intermediate position between parallel and series excitation motors (Fig. 11.9, d). Their greater belonging to one or another type depends on the ratio of parts of the main excitation flow created by parallel or series excitation windings. At the moment the engine is turned on, to reduce the starting current, additional resistance is included in the armature winding circuit Rd. This engine has good traction characteristics and can idle.

Direct (non-rheostatic) switching on of DC motors of all types of excitation is allowed with a power of not more than one kilowatt.

Designation of DC machines

At present, DC machines are the most widely used. general purpose series 2P and most new series 4P. In addition to these series, engines are produced for crane, excavator, metallurgical and other drives of the series D. Engines and specialized series are manufactured.

Series engines 2P and 4P subdivided along the axis of rotation, as is customary for asynchronous AC motors of the series 4A. Machine series 2P have 11 dimensions, differing in the height of rotation of the axis from 90 to 315 mm. The power range of the machines in this series is from 0.13 to 200 kW for electric motors and from 0.37 to 180 kW for generators. Motors of the 2P and 4P series are designed for voltages of 110, 220, 340 and 440 V. Their nominal speeds are 750, 1000, 1500,2200 and 3000 rpm.

Each of the 11 machine dimensions of the series 2P has two lengths (M and L).

Electric cars series 4P have some better technical and economic indicators in comparison with the series 2P. the complexity of manufacturing a series 4P compared with 2P reduced by 2.5...3 times. At the same time, copper consumption is reduced by 25...30%. According to a number of design features, including the method of cooling, protection from atmospheric influences, the use of individual parts and assemblies of the machine of the series 4P unified with asynchronous motors series 4A and AI .

The designation of DC machines (both generators and motors) is presented as follows:

ПХ1Х2ХЗХ4,

where 2P- a series of DC machines;

XI- execution according to the type of protection: N - protected with self-ventilation, F - protected with independent ventilation, B - closed with natural cooling, O - closed with airflow from an external fan;

X2- height of the axis of rotation (two-digit or three-digit number) in mm;

HZ- conditional length of the stator: M - first, L - second, G - with tachogenerator;

An example is the designation of the engine 2PN112MGU- DC motor series 2P, protected version with self-ventilation H,112 height of the axis of rotation in mm, the first dimension of the stator M, equipped with a tachogenerator G, used for temperate climates At.

In terms of power, DC electrical machines can conditionally be divided into following groups :

Micromachines ………………………...less than 100 W,

Small machines ……………………… from 100 to 1000 W,

Low power machines…………..from 1 to 10 kW,

Medium power machines………..from 10 to 100 kW,

Large machines……………………..from 100 to 1000 kW,

High power machines……….more than 1000 kW.

According to the rated voltages, electrical machines are conventionally divided as follows:

Low voltage…………….less than 100 V,

Medium voltage ………….from 100 to 1000 V,

High voltage……………above 1000V.

According to the rotational speed of a DC machine, it can be represented as:

Low-speed…………….less than 250 rpm.,

Medium speed………from 250 to 1000 rpm,

High-speed………….from 1000 to 3000 rpm.

Super high speed…..above 3000 rpm.

Task and method of work performance.

1. To study the device and the purpose of individual parts of DC electrical machines.

2. Determine the conclusions of the DC machine related to the armature winding and to the excitation winding.

The conclusions corresponding to one or another winding can be determined with a megohmmeter, an ohmmeter, or using light bulb. When using a megohmmeter, one of its ends is connected to one of the terminals of the windings, and the other is touched in turn to the rest. The measured resistance, equal to zero, will indicate the correspondence of the two terminals of one winding.

3. Recognize the armature winding and the excitation winding by the conclusions. Determine the type of excitation winding (parallel excitation or series).

This experiment can be carried out using an electric light bulb connected in series with the windings. Constant pressure should be fed smoothly, gradually increasing it to the specified nominal value in the machine's passport.

Given the low resistance of the armature winding and the series excitation winding, the bulb will light up brightly, and their resistances, measured with a megohmmeter (or ohmmeter), will be practically equal to zero.

A light bulb connected in series with a parallel excitation winding will burn dimly. The resistance value of the parallel excitation winding must be within 0.3...0.5 kOhm .

The armature winding leads can be recognized by attaching one end of the megohmmeter to the brushes while touching the other end to the winding leads on the electrical machine panel.

The conclusions of the windings of the electrical machine should be marked on the conditional label of the conclusions shown in the report.

Measure winding resistance and insulation resistance. Winding resistance can be measured using an ammeter and voltmeter circuit. The insulation resistance between windings and windings relative to the housing is checked with a megohmmeter rated for 1 kV. The insulation resistance between the armature winding and the excitation winding and between them and the housing must be at least 0.5 MΩ. Display the measurement data in the report.

Depict conditionally in a cross section the main poles with the excitation winding and the armature with the turns of the winding under the poles (similar to Fig. 11.10). Independently take the direction of the current in the field and armature windings. Specify the direction of rotation of the motor under these conditions.

Rice. 11.10. Double Pole DC Machine:

1 - bed; 2 - anchor; 3 - main poles; 4 - excitation winding; 5 - pole pieces; 6 - armature winding; 7 - collector; Ф - main magnetic flux; F is the force acting on the conductors of the armature winding.

Control questions and tasks for self-study

1: Explain the structure and principle of operation of the motor and DC generator.

2. Explain the purpose of the collector of DC machines.

3. Give the concept of pole division and give an expression for its definition.

4. Name the main types of windings used in DC machines and know how to implement them.

5. Indicate the main advantages of parallel excitation motors.

6.What are design features parallel excitation windings compared to series excitation windings?

7. What is the peculiarity of starting DC motors of series excitation?

8. How many parallel branches do simple wave and simple loop windings of DC machines have?

9. How are DC machines designated? Give an example of a notation.

10. What is the allowed insulation resistance between the windings of DC machines and between the windings and the housing?

11. What value can the current reach at the moment of starting the engine in the absence of additional resistance in the armature winding circuit?

12. What is the allowed motor starting current?

13. In what cases is it allowed to start a DC motor without additional resistance in the armature winding circuit?

14. Due to what can the EMF of an independent excitation generator be changed?

15. What is the purpose of the additional poles of the DC machine?

16. At what loads is it allowed to turn on the series excitation motor?

17. What determines the value of the main magnetic flux?

18. Write expressions for the EMF of the generator and the engine torque. Give an idea of their components.

LABORATORY WORK 12.

The shunt motor is the best choice among DC motors for driving applications that require almost constant speed and at the same time economical speed control. The diagram of this engine is shown in fig. 4-25.

Rice. 4-25. Motor of parallel excitation.

The clamps of the starting rheostat are designated: L - connected to the line (mains supply); M - to the excitation winding terminals and I - to the armature terminals. The black circles (Fig. 4-25) indicate the working contacts, and the gaps between them correspond to the resistance sections of the rheostat. When the engine is running, a metal arc 3 constantly connects the L terminal to the terminals of the shunt rheostat that regulates the excitation current. extreme left position, at which the resistance of the rheostat is minimal.

When the switch is closed and the lever of the starting rheostat is moved to the first of the working contacts, the motor current branches into the armature current and the excitation winding current

Thus, the current in the supply circuit

The first current surge depending on the value of the starting resistance Under the influence of the initial torque, the armature begins to rotate and with increasing speed, the armature current decreases. Then the lever of the starting rheostat can be transferred to the second contact. In this case, the armature current, having increased by a throw, will cause an increase in torque and a further increase in speed, and then again begins to decrease. Then the rheostat lever is transferred to next contact etc. The start ends when all the resistance is removed and full voltage is applied to the armature. The resistance of the starting rheostat is usually designed for short-term start operation and it is impossible to leave the rheostat handle on the intermediate contacts for a long time.

Rice. 4-26. Speed characteristics of the motor of parallel excitation.

The faster counter-e. d.s. armature, the sooner the current decreases and the less heating of the armature winding. Therefore, the start is always carried out at highest current excitation, short-circuiting the resistance of the regulating rheostat (Fig. 4-25). Then the magnetic flux of the machine F and counter-e. d.s. will be maximum. In addition, the electric motor at start-up must develop an increased torque, and this can also be with the highest magnetic flux formula (4-8)].

Before turning off the engine, the starting rheostat lever is switched to the zero contact, and then the knife switch is opened. This prevents burning of the switch contacts.

The speed characteristic of the engine at is shown in fig. 4-26 Curve 1. When there is no mechanical load, no-load current and speed are the highest:

With an increase in the load (resistance torque) on the motor shaft, the rotational speed drops slightly, since the automatic increase in torque occurs due to an increase in the current in the armature circuit, which, according to equation (4-14a), increases sharply with a slight decrease in counter-e. d.s. due to small size armature circuit resistance This characteristic is called rigid.

Rice. 4-27. Operating characteristics of the parallel excitation motor.

With a constant excitation current, the magnetic flux F can be considered approximately constant, since the influence of the armature reaction is insignificant.

Then the motor torque

approximately proportional to the current Therefore, if we plot M along the x-axis in Fig. 4-26, then the mechanical characteristic of the engine will be obtained, i.e.

Very easy to use performance data (Fig. 4-27) given in the catalogs and descriptions of the electric motor. This

at , where is the efficiency of the engine, and is the net power on the shaft.

Motor power developed on the shaft

and the torque

At a constant rotation frequency, the dependence would be a straight line passing through the origin. However, the speed decreases with increasing and the torque is not proportional. The current at a constant U is proportional to the power in the power circuit. Since the losses of the motor are small, the current is approximately proportional to .

Speed control of a shunt motor is usually done by varying the field current. This method gives an economical smooth control within 1: 1.5, and in a special version - up to 1:8. The regulation is as follows. The motor torque at Ф = const is proportional to the current and the current

Due to the small value, the voltage drop in the armature circuit is small. Therefore, at constant values of U and armature, it can increase significantly with a slight decrease in counter-e. d.s.

For example, at and at armature current counter-e. d.s. . If counter-e. d.s. decreases by only 10 V (approximately 5%) and will, then the armature current, i.e., will increase by 3 times.

Thus, if, at a certain constant load and speed, the excitation current is reduced, for example, by 5%, then. the magnetic flux Ф and counter-e will immediately decrease by the same amount. d.s. E. It will cause sharp increase armature current and torque, and the excess torque will go to accelerate the rotation of the armature. However, as the speed of the anchor increases, the counter-e. d.s. will increase again, the armature current will decrease to a value at which the torque will take its previous value. Thus, if equal, a new constant speed will be established, greater than the previous one.

With this method of regulation, the energy losses in the regulating rheostat (loss power Gvgv) are very small, since it is only

This method allows you to change the engine speed in the direction of its increase above the nominal.

If, with a constant load on the motor shaft, an additional resistance ch is connected in series with the armature winding, then at the first moment the armature current will decrease, which will reduce the torque and, since the resistance moment will be greater, the speed will decrease. However, due to the decrease in speed and counter-e. d.s. the armature current will increase, the torque will increase, and if the torques are equal, the further decrease in speed will stop.

The engine will continue to run at a constant, but reduced frequency rotation. This method of regulation is uneconomical due to significant energy losses in the resistance of the rheostat.

A DC motor with parallel excitation is an electric motor in which the armature and excitation windings are connected to each other in parallel. Often, in terms of its functionality, it surpasses the aggregates of mixed and sequential types in cases where it is necessary to set a constant speed of operation.

Parallel Excited DC Motor Characteristics

Formula total current, coming from the source, is derived according to the first Kirchhoff law and has the form: I \u003d I i + I in, where I i is the armature current, I in is the excitation current, and I is the current that the motor consumes from the network. It should be noted that in this case I in does not depend on I I, i.e. the excitation current is independent of the load. The value of the current in the excitation winding is less than the armature current and is approximately 2-5% of the mains current.

In general, these electric motors are distinguished by the following very useful traction parameters:

High efficiency (since the armature current does not pass through the field winding).
Stability and continuity of the working cycle with load fluctuations over a wide range (since the magnitude of the torque is maintained even if the number of shaft revolutions changes).

In case of insufficient torque, starting is carried out by switching to mixed type arousal.

Engine Applications

Since the speed of such motors remains almost constant even when the load changes, and can also be changed using an adjusting rheostat, they are widely used in work with:

fans;
pumps;
mine lifts;
overhead electric roads;
machine tools (turning, metal-cutting, weaving, printing, straightening, etc.).

Thus, this type of motor is mainly used with mechanisms that require a constant speed of rotation or its wide adjustment.

Speed control

Speed control is a purposeful change in the speed of an electric motor forcibly using special devices or fixtures. It allows you to provide optimal mode the operation of the mechanism, rational use and also reduce energy consumption.

There are three main ways to control the speed of a motor:

Change in the magnetic flux of the main poles. It is carried out using an adjusting rheostat: with an increase in its resistance, the magnetic flux of the main poles and the excitation current Iv decrease. This increases the number of revolutions of the armature at idle, as well as the angle of inclination of the mechanical characteristic. Rigidity mechanical characteristics is saved. However, increasing the speed can lead to mechanical damage unit and worsen commutation, therefore it is not recommended to increase the speed by this method more than twice.
Changing the resistance of the armature circuit. An adjusting rheostat is connected in series to the armature. The rotation speed of the armature decreases with increasing resistance of the rheostat, and the slope of the mechanical characteristics increases. Adjusting the speed in the above way:

helps to reduce the rotational speed relative to the natural characteristic;
associated with a large amount of losses in the regulating rheostat, therefore, uneconomical.

Rheostatic change in the voltage applied to the armature. In this case, a separate power supply with adjustable voltage, for example, a generator or a controlled valve.

Motor with independent excitation

The independent excitation DC motor implements the third principle of speed control. Its difference is that the excitation winding and the magnetic field of the main poles are connected to different sources. The excitation current is a constant characteristic, and the magnetic field changes. In this case, the number of revolutions of the shaft at idle changes, the rigidity of the characteristic remains the same.

Thus, the principle of operation of a DPT with independent excitation is rather complicated due to the independent operation of two sources, however, its main advantage is its high efficiency.

Lecture #9

DC motors

By dimensions;

According to the method of protection;

By power;

By rotation speed;

The excitation circuits of DC motors are shown in the figure.

Rice. 9.1 Excitation circuits for DC motors: a - independent, b - parallel, c - series, d - mixed

Basic formulas and equations

If we accept the armature rotation speed in the SI system (rad / s), then formula 4.13 from lecture No. 4 will take the form

M -electromagnetic moment DC machines, N/m (newton divided by meter)

k is a constant value for a given machine;

Ф - main magnetic flux, Wb (weber)

R - number of pairs of poles of the armature winding

N is the number of slot sides of the armature winding

a - number of pairs of parallel branches of the armature winding

I a or just I- armature current, A;

For a motor running at a constant speed, one can obtain stress equation(EDS) for the generator armature circuit:

This equation is obtained on the basis of Kirchhoff's second law

. (9.3)

The sum of the resistances of all sections of the armature circuit:

Armature windings r a or, r i

Windings of additional poles r d,

Compensation winding r ko,

Series excitation winding r

Transitional brush contact r sh.

In the absence of any of the indicated windings in the machine, (9.4) does not include the corresponding terms.

From (9.3) it follows that the voltage supplied to the motor is balanced by the back EMF of the armature winding and the voltage drop in the armature circuit.

Based on (9.3), we obtain the armature current formula

. (9.5)

Multiplying both parts of equation (9.3) by the armature current I a, we obtain the power equation for the armature circuit:

, (9.6)

, (9.7)

(9.8)

ω- angular frequency of rotation of the armature;

The electromagnetic power of the motor.

Therefore, the expression is the electromagnetic power of the motor.

Operating characteristics

The performance characteristics of the engine are shown in Figure 9.2b

The engine speed decreases with increasing load P 2, and graph ω \u003d f (P 2) acquires falling view. In order to give the speed characteristic a falling curve, some parallel-excited motors use a light (with a small number of turns) series excitation winding, which is called a stabilizing winding. When this winding is turned on in coordination with the parallel excitation winding, its MMF compensates for the demagnetizing effect of the armature reaction so that the flux Ф remains practically unchanged over the entire load range.

Speed change engine during the transition from rated load to x.x., expressed as a percentage, is called the nominal change in speed:

, (9.12)

∆ω nom = 100

where 0 (n 0) is the engine speed in the cold mode.

Usually for parallel excitation motors ∆ω nom = 2-8%, therefore, the characteristic of the rotation frequency of the parallel excitation motor is called tough.

The dependence of the useful moment on the load is established by the formula . At schedule would look like a straight line. However, with increasing load, the engine speed decreases, and therefore the dependence curvilinear.

Dependence graph M el = f(P 2) runs parallel to the curve M 2 = f(P 2).

Engine start

Motor armature current is given by

At the initial moment of starting, the motor armature is stationary and no EMF is induced in its winding E a = 0. Therefore, when the motor is directly connected to the network, an inrush current occurs in the winding of its armature

I p \u003d (9.13)

Usually the resistance is small, so the value of the starting current reaches unacceptably high values, 10-20 times the rated current of the motor.

Such a large starting current is very dangerous for the motor. Firstly, it can cause a circular fire in the machine, and secondly, with such a current, an excessively large starting torque develops in the motor, which has a shock effect on the rotating parts of the motor and can mechanically destroy them. And finally, this current causes a sharp drop in voltage in the network, which adversely affects the operation of other consumers included in this network. Therefore, starting the engine by direct connection to the network (non-rheostatic start) is usually used for engines with a power of not more than 0.7-1.0 kW. In these motors, due to the increased resistance of the armature winding and small rotating masses, the starting current value is only 3-5 times higher than the rated current, which does not pose a danger to the motor.

As for motors of greater power, when they are started, starting rheostats (PR) are used to limit the starting current, which are connected in series to the armature circuit (rheostat start).

Before starting the engine, it is necessary to introduce a rheostat, i.e. put greatest resistance. Then turn on the switch and gradually reduce the resistance of the rheostat.

Rice. 9.4. Scheme of switching on the starting rheostat

Armature starting current at full resistance starting rheostat

. (9.14)

The resistance of the starting rheostat is usually chosen so that the largest starting current exceeds the rated current by no more than 2-3 times.

It is not advisable to use starting rheostats to start motors of greater power, as this would cause significant energy losses. Also, starting rheostats would be bulky. Therefore, in engines high power use rheostatless start of the engine by lowering the voltage.

Examples of this are starting the traction motors of an electric locomotive by switching them from serial connection when starting in parallel with normal operation or starting the engine in the "generator-motor" scheme.

Reversing engines

Motor reversal is a change in the direction of rotation of the armature.

Reversing the motor is carried out either by changing the polarity of the voltage on the armature winding, or on the excitation winding. In both cases, the sign of the electromagnetic torque of the engine M em changes and, accordingly, the direction of rotation of the armature.

Efficiency of DC machines

η = P 2 /P 1 , (9.20)

P 2 - useful power of the machine (for a generator - this is the electrical power given to the receiver, for the engine - mechanical power on the shaft);

P 1 is the power supplied to the machine (for the generator, this is the mechanical power supplied to it by the prime mover, for the engine, the power consumed by it from a direct current source; if the generator has independent excitation, then P 1 also includes the power required to power excitation winding circuit).

Obviously, the power P 1 can be expressed as follows: P 1 \u003d P 2 + ΣΔP,

where ΔP is the sum of the power losses listed above.

With considering last expression

η \u003d P 2 / (P 2 + ΣΔP). (9.21)

When the machine is idling, the useful power Р 2 is equal to zero and η = 0. The nature of the change in efficiency with an increase in useful power depends on the value and nature of the change in power losses. An approximate graph of the dependence η=f(Р 2) is shown in fig. 9.5.

With an increase in useful power, the efficiency first increases at a certain value of P 2, reaches the highest value, and then decreases. The latter is explained by a significant increase in variable losses proportional to the square of the current. Machines are usually calculated in such a way that highest value The efficiency was in the area close to the rated power Р 2nom. The nominal value of the efficiency of machines with a power of 1 to 100 kW lies approximately in the range from 0.74 to 0.92, respectively.

Literature: Katsman M.M. Electric cars. Chapter 29

§29.1, 29.2, 29.3, 29.4, 29.5, 29.6, 29.8, 29.10

Lecture #9

DC motors

Ways to excite DC motors

DC motors are used in industry if it is necessary to control the speed of the ED (electric drive). Mainly used are HC-D (controlled rectifier-motor) systems, which provide high quality speed control.

According to the method of excitation, DC electric motors are divided into four groups:

1. With independent excitation, in which the excitation winding of the NOV is powered by an external DC source.

2. With parallel excitation (shunt), in which the excitation winding SHOV is connected in parallel with the power source of the armature winding.

3. With serial excitation (series), in which the excitation winding of the SOW is connected in series with the armature winding.

4. Engines with mixed excitation (compound), which have a serial WTS and a parallel WOW of the excitation winding.

Motors with independent excitation and parallel excitation have the same properties, therefore, these groups are combined and referred to one group: motors with independent excitation designed to operate in controlled EA.

The industry produces DC motors of the main general industrial series 2P and 4P, they are divided according to the following features: