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 are made in order to more conveniently and economically regulate 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 of a large number of 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 motor can lead to a large starting current that exceeds the rated current of the armature 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 of the highest speed to the lowest can be 2:1 , and with a special version of the engine - 6:1 . The minimum rotational speed 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 engine can go "peddling".

Sequential excitation motors(serial) are switched on according to the scheme (Fig. 11.9, c). findings 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, the most widely used general-purpose DC machines of the series 2P and the newest 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 Machine 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 of the 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.

According to the power, DC electrical machines can conditionally be divided into the 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 with an electric 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 voltage should be applied 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 light bulb will light up brightly, and their resistances measured with a megohmmeter (or ohmmeter) will be almost 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 the design features of the parallel excitation winding compared to the series excitation winding?

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.

Electric motors are machines capable of converting electrical energy into mechanical energy. Depending on the type of current consumed, they are divided into AC and DC motors. In this article, we will focus on 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

DCT in its structure resembles a synchronous AC motor, 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. In the simplest DCT model, only one magnet and a frame were used, 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 its own magnetic field around itself, which, upon contact with an external magnetic field, will begin to 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 the 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 independently excited DCT circuit provides for connecting the field winding and the armature to different power sources, so that they are not electrically connected to each other.
Parallel excitation is implemented by connecting the inductor and armature windings in parallel 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 the excitation winding are connected in series, so they have the same electric current. 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 a high value of starting torque, which makes it easier to start, 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. He could not create a full-fledged working model, but it was he who owns the discovery that made this 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, the metal conductor began to rotate, turning the energy of the electric current into mechanical work. Scientists of that time were working on the creation of a machine whose operation 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 motors with parallel excitation, rheostats in the armature circuit are also used to reduce the speed to 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 engine models, they are replaced by electronic circuits that allow you to control the speed without significant energy losses.
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.
Disadvantages:
- have a "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.

Engine diagram. Sequential motor diagram excitation is shown in Fig. 1.31. The current consumed by the motor from the network flows through the armature and the field winding connected in series with the armature. Therefore, I \u003d I i \u003d I c.

Also, a starting rheostat R p is connected in series with the armature, which, like the parallel excitation motor, is output after release.

Mechanical equationcharacteristics. The mechanical characteristic equation can be obtained from formula (1.6). At load currents less than (0.8 - 0.9) Inom, we can assume that the motor magnetic circuit is not saturated and the magnetic flux Ф is proportional to the current I: Ф = kI, where k = const. (At high currents, the coefficient k decreases somewhat). Replacing Φ in (1.2), we obtain М = С m kI whence

We substitute Φ into (1.6):

n= (1.11)

The graph corresponding to (1.11) is shown in fig. 1.32 (curve 1). When the load torque changes, the engine speed changes dramatically - characteristics of this type are called "soft". When idling, when M » 0, the engine speed increases indefinitely and the engine "runs out".


The current consumed by the series excitation motor, with increasing load, increases to a lesser extent than that of the parallel excitation motor. This is explained by the fact that simultaneously with the increase in current, the excitation flux increases and the torque becomes equal to the load torque at a lower current. This feature of the sequential excitation engine is used where there are significant mechanical overloads of the engine: in electrified vehicles, in hoisting and transport mechanisms and other devices.

Frequency controlrotation. The speed control of DC motors, as mentioned above, is possible in three ways.

Changing the excitation can be done by turning on the rheostat R p1 in parallel with the excitation winding (see Fig. 1.31) or by turning on the rheostat R p2 in parallel with the armature. When the rheostat R p1 is turned on in parallel with the excitation winding, the magnetic flux Ф can be reduced from the nominal to the minimum Ф min. In this case, the engine speed will increase (in formula (1.11), the coefficient k decreases). The mechanical characteristics corresponding to this case are shown in fig. 1.32, curves 2, 3. When the rheostat is turned on in parallel with the armature, the current in the field winding, the magnetic flux and the coefficient k increase, and the engine speed decreases. The mechanical characteristics for this case are shown in fig. 1.32, curves 4, 5. However, the regulation of rotation by a rheostat connected in parallel with the armature is rarely used, since the power loss in the rheostat and the efficiency of the engine decreases.

Changing the speed by changing the resistance of the armature circuit is possible when the rheostat R p3 is connected in series to the armature circuit (Fig. 1.31). Rheostat R p3 increases the resistance of the armature circuit, which leads to a decrease in the rotational speed relative to the natural characteristic. (In (1.11) instead of R i it is necessary to substitute R i + R p3.) The mechanical characteristics for this method of regulation are shown in fig. 1.32, curves 6, 7. Such regulation is used relatively rarely due to large losses in the regulating rheostat.

Finally, regulation of the rotational speed by changing the mains voltage, as in parallel excitation motors, is only possible in the direction of reducing the rotational speed when the engine is powered from a separate generator or controlled rectifier. The mechanical characteristic for this method of regulation is shown in fig. 1.32, curve 8. If there are two motors operating on a common load, they can be switched from parallel to serial connection, the voltage U on each motor is halved, and the rotational speed decreases accordingly.

Braking modes of the enginesequential excitation. The regenerative braking mode with energy transfer to the network in a sequential excitation motor is impossible, since it is not possible to obtain a rotational speed n>n x (n x = ).

The reverse braking mode can be obtained, just as in a parallel excitation motor, by switching the terminals of the armature winding or the field winding.

In this motor, the field winding is connected in series to the armature circuit (Fig. 29.9, a), so magnetic fluxF it depends on the load current I = I a = I in . At low loads, the magnetic system of the machine is not saturated and the dependence of the magnetic flux on the load current is directly proportional, i.e. F = k f I a (k f- coefficient of proportionality). In this case, we find the electromagnetic moment:

The rotation frequency formula will take the form

On fig. 29.9, b performance data presented M = F(I) and n= (I) series excitation motor. At high loads, saturation of the magnetic system of the engine occurs. In this case, the magnetic flux practically does not change with increasing load, and the characteristics of the motor become almost rectilinear. The series excitation motor speed characteristic shows that the motor speed changes significantly with load changes. This characteristic is called soft.

Rice. 29.9. Sequential excitation motor:

a- schematic diagram; b- performance characteristics; c - mechanical characteristics; 1 - natural characteristic; 2 - artificial characteristic

With a decrease in the load of the sequential excitation motor, the rotational speed increases sharply and, at a load of less than 25% of the nominal value, it can reach values ​​\u200b\u200bthat are dangerous for the engine (“overshoot”). Therefore, the operation of a series excitation motor or its start-up with a shaft load of less than 25% of the nominal is unacceptable.

For more reliable operation, the shaft of the sequential excitation motor must be rigidly connected to the working mechanism by means of a coupling and a gear. The use of a belt drive is unacceptable, since if the belt is broken or reset, the engine may “run out”. Given the possibility of operating the engine at increased speeds, series excitation engines, according to GOST, are tested for 2 minutes to exceed the speed by 20% above the maximum indicated on the factory shield, but not less than 50% above the nominal.

Mechanical characteristics of a series excitation motor n=f(M) are presented in fig. 29.9, in. Sharply falling curves of mechanical characteristics ( natural 1 and artificial 2 ) provide the sequential excitation motor with stable operation under any mechanical load. The property of these motors to develop a large torque proportional to the square of the load current is important, especially under difficult starting conditions and during overloads, since with a gradual increase in the load of the motor, the power at its input increases more slowly than the torque. This feature of series excitation motors is one of the reasons for their widespread use as traction motors in transport, as well as crane motors in lifting installations, i.e. in all cases of electric drive with difficult starting conditions and a combination of significant loads on the motor shaft with low rotation frequency.

Rated speed change of series excitation motor

where n - rotational speed at an engine load of 25% of the nominal.

The rotational speed of series excitation motors can be controlled by changing either voltage U, or the magnetic flux of the excitation winding. In the first case, an adjusting rheostat R rg (Fig. 29.10, a). With an increase in the resistance of this rheostat, the voltage at the input of the engine and the frequency of its rotation decrease. This control method is mainly used in small power engines. In the case of a significant engine power, this method is uneconomical due to large energy losses in R rg . Besides, rheostat R rg , calculated on the operating current of the motor, it turns out to be cumbersome and expensive.

When several engines of the same type are working together, the rotational speed is regulated by changing the scheme of their inclusion relative to each other (Fig. 29.10, b). So, when the motors are connected in parallel, each of them is under full mains voltage, and when two motors are connected in series, each motor accounts for half the mains voltage. With the simultaneous operation of a larger number of engines, a greater number of switching options are possible. This method of speed control is used in electric locomotives, where several identical traction motors are installed.

It is possible to change the voltage supplied to the motor when the motor is powered from a DC source with regulated voltage (for example, according to a circuit similar to Fig. 29.6, a). With a decrease in the voltage supplied to the motor, its mechanical characteristics shift down, practically without changing their curvature (Fig. 29.11).

Rice. 29.11. Mechanical characteristics of a series excitation motor with a change in the input voltage

There are three ways to regulate the engine speed by changing the magnetic flux: by shunting the excitation winding with a rheostat r rg , sectioning the excitation winding and shunting the armature winding with a rheostat r w . Turning on the rheostat r rg , shunting the excitation winding (Fig. 29.10, in), as well as a decrease in the resistance of this rheostat leads to a decrease in the excitation current I in \u003d I a - I rg , and consequently, to an increase in the rotational speed. This method is more economical than the previous one (see Fig. 29.10, a), is used more often and is estimated by the regulation coefficient

Usually the resistance of the rheostat r rg taken so that krg >= 50% .

When sectioning the field winding (Fig. 29.10, G) turning off part of the turns of the winding is accompanied by an increase in the rotational speed. When shunting the armature winding with a rheostat r w (see fig. 29.10, in) excitation current increases I in \u003d I a + I rg , which causes a decrease in rotational speed. This method of regulation, although it provides deep regulation, is uneconomical and is used very rarely.

Rice. 29.10. Regulation of rotational speed of sequential excitation motors.