DC motor with parallel excitation - operating diagram. Independently excited DC motor (DPT NV)

A DC electric motor with mixed excitation (compound electric motor) to some extent has the properties of the electric motors with parallel and series excitation discussed above. This electric motor is equipped with two excitation windings: serial and parallel.

A schematic diagram of such an electric motor is shown in Fig. 31, where the series winding is designated OWL, and the parallelnaya- THE SEAM. Usually on the terminal boxes of electric motors they indicate: terminals from the series winding WITH 1 And WITH 2 , terminals from parallel winding - Sh 1 And Sh 2 , and the conclusions from the armature winding - I 1 And I 2 . In the diagrams, these windings can be designated differently: OWL And THE SEAM, WITH 1 - WITH 2 And Sh 1 -Sh 2 .

The series and parallel field windings can be connected in two ways. In some cases, they are turned on so that the ampere-turns they create, and therefore the magnetic fluxes, add up. This connection of windings is usually calledconsonant. It is obvious that with consistent switching on, the resulting magnetic flux of the electric motor

In other cases, the field windings are included in the circuit in such a way that the ampere-turns (and magnetic fluxes) they create are directed towards each other. This connection of the windings is calledcounter. With counter switching, the resulting magnetic fluxelectric motor

Counter-connection of field windings is used only in special-purpose machines. In ordinary crane electric motors with mixed excitation, the windings are always switched on accordingly, therefore, in further presentation of the material, we will assume that the ampere-turns of both windings (and magnetic fluxes) add up, i.e. the windings are switched on accordingly and equality (69) is valid for the electric motor.

The presence of two excitation windings allows you to design and manufacture electric motors with different properties and characteristics. With a natural switching scheme, the characteristics of the electric motor in question are tougher than those of electric motors with series excitation, and softer than those of electric motors with parallel excitation. However, depending on the ratio of ampere-turns created by the parallel and series windings, the characteristics of the electric motor are similar in nature to either those of a series-wound or a parallel-wound motor.

For hoisting and transport machines, electric motors are produced in which, at full load, half of the ampere-turns of excitation are created by parallel windings, and half by series windings.

If the load changes, the magnetic flux of an electric motor with mixed excitation does not remain constant, since the ampere-turns created by the series winding are determined by the armature current. The dependence of the resulting magnetic flux on the armature current is shown in Fig. 32, A, which shows that each value of the armature current corresponds to a certain magnetic flux and, therefore, torque M = To F I I when the load changes, it changes not only due to a change in the armature current, but also due to the excitation magnetic flux. Addiction M=f (I I ) for an electric motor with mixed excitation is shown in Fig. 32, b.

All performance characteristics of a DC motor, like a generator, depend on the method of connecting the excitation circuit in relation to the armature circuit. The connection of these circuits can be parallel, serial, mixed and, finally, they can be independent of each other.

Motors with parallel excitation.

Here the field winding and the armature winding are connected in parallel. The field winding has a larger number of turns than the armature winding, so the field winding current in most cases is a few percent of the armature current. An adjusting rheostat can be included in the excitation winding circuit. The PR starting rheostat is connected to the armature circuit.

Motor with independent excitation.

If the field winding is connected to another constant voltage source, we get a motor with independent excitation. Electric motors with permanent magnets have the same properties.

The speed characteristic of motors with independent and parallel excitation is the dependence n

n - speed

I - armature current

Iе - excitation current.

Fig.8.5.4. Speed ​​characteristic.

A change in rotation speed can occur due to changes in load and magnetic flux. Increasing the load current slightly changes the internal voltage drop due to the low armature circuit resistance and therefore only slightly reduces the motor speed. As for the magnetic flux, due to the reaction of the armature, when the load current increases, it decreases slightly, which leads to a slight increase in motor speed. Thus, the rotation speed of a parallel-excited motor changes very little. The engine rotation speed is determined by the formula:

n = (U – IаRя) / c∙Φ, where

c – coefficient depending on the device of the machine.

The rotation speed of a separately excited motor can be adjusted either by changing the resistance in the armature circuit or by changing the magnetic flux. It should be noted that an excessive decrease in the excitation current and, especially, an accidental break of this circuit are very dangerous for motors with parallel and independent excitation, because The armature current may increase to unacceptably high values. Under light load (or at idle), the speed can increase so much that it becomes dangerous to the integrity of the engine.

Motor with sequential excitation.

In such a motor, the armature current is also the excitation current, because the field winding is connected in series with the armature. For this reason, the magnetic flux of the motor changes with the load. Motor speed:

n =[ U – Iа (Rя + Rв)] / c∙Φ, where

Rya – armature resistance

Rв – resistance of the excitation winding.

Speed ​​characteristics of the engine last. excitement.

This graph shows the speed characteristic of a series-excited motor.

From this characteristic it is clear that the motor speed is highly dependent on the load. As the load increases, the drop in winding resistance increases while the magnetic flux increases, which leads to a significant decrease in the rotation speed. Therefore, such engines should not be run idle or at low load. Motors with series excitation are used in cases where a large starting torque or the ability to withstand short-term overloads is required. They are used as traction motors in trams, trolleybuses, subways and electric locomotives, as well as on cranes and for starting internal combustion engines (starters).

Motor with mixed excitation.

At each pole of such a motor there are two windings - parallel and series. They can be turned on so that the magnetic fluxes are added (consonant inclusion) or subtracted (counter inclusion). Formulas for rotation speed and torque for such an engine:

n = (U – Iа ∙ Rя) / c∙(Φparallel +/- Φsequence)

M = c ∙ Iа ∙ (Φparallel +/- Φsequence)

Depending on the ratio of magnetic fluxes, a motor with mixed excitation in its properties approaches either a motor with series excitation or a motor with parallel excitation. As a rule, in such motors the series winding is the main (working) winding, and the parallel winding is the auxiliary winding. Due to the presence of magnetic flux in the parallel winding, the speed of such a motor cannot increase significantly at low loads. Motors with consistent activation are used when high starting torque and speed control are required under variable loads. Motors with counter-connected windings are used in cases where constant speed is required with a changing load.

To change the direction of rotation of a DC motor, it is necessary to change the direction of the current either in the field winding or in the armature winding. By changing the polarity at the machine terminals, you can change the direction of rotation only in a motor with a permanent magnet or independent excitation. In other motors, it is necessary to change the direction of the current either in the armature winding or in the field winding. DC motor cannot be switched on by connecting full voltage. The starting current of DC machines is about 20 times higher than the rated current (the larger and faster the motor, the greater it is). In large machines, the starting current can be 50 times the rated current.

High current causes circular sparking in the collector and destroys the collector. To switch on, use a smooth increase in voltage or starting rheostats. Direct connection is allowed at low voltages in the case of small motors with high armature winding resistance.

Let us consider in more detail the characteristics of a parallel excitation motor, which determine its operating properties.

The speed and mechanical characteristics of the engine are determined by equalities (7) and (9) presented in the article "", with U= const and i in = const. In the absence of additional resistance in the armature circuit, these characteristics are called natural.

If the brushes are at geometric neutral, with increasing I and the flow Ф δ will decrease slightly due to the action of the transverse reaction of the armature. As a result, the speed n, according to expression (7) presented in the article “General information about DC motors”, will tend to increase. On the other hand, the voltage drop R a × I and causes a decrease in speed. Thus, three types of speed characteristics are possible, shown in Fig. 1: 1 – with predominant influence R a × I A; 2 – with mutual compensation of influence R a × I a and decrease Ф δ; 3 – when the influence of decreasing Ф δ predominates.

Due to the fact that the change in Ф δ is relatively small, the mechanical characteristics n = f(M) of a parallel-excitation motor, determined by equality (9), presented in the article "General information about DC motors", with U= const and iв = const coincide in appearance with the characteristics n = f(I a) (Figure 1). For the same reason, these characteristics are almost straightforward.

Characteristics of the species 3 (Figure 1) are unacceptable under the conditions of sustainable operation (see article ""). Therefore, parallel excitation motors are manufactured with slightly decreasing characteristics of the form 1 (picture 1). In modern highly used machines, due to the rather strong saturation of the armature teeth, the influence of the transverse reaction of the armature can be so large that a characteristic of the form 1 (Figure 1) is impossible. Then, to obtain such a characteristic, a weak series excitation winding of consonant inclusion is placed at the poles, the magnetizing force of which is up to 10% of the magnetizing force of the parallel excitation winding. In this case, the decrease in Ф δ under the influence of the transverse reaction of the armature is partially or completely compensated. Such a series field winding is called stabilizing, and a motor with such a winding is still called a parallel-excitation motor.

Rotation speed change Δ n(Figure 1) when moving from idle ( I a = I a0) to rated load ( I a = I an) the parallel excitation motor when operating on a natural characteristic is small and amounts to 2 - 8% of n n. Such weakly decreasing characteristics are called hard. Parallel excitation motors with rigid characteristics are used in installations in which it is required that the rotation speed remains approximately constant when the load changes (metal-cutting machines, etc.).

Figure 2. Mechanical and speed characteristics of a parallel-excitation motor at different excitation flows

Speed ​​control by weakening magnetic flux

Speed ​​control by weakening the magnetic flux is usually done using a rheostat in the excitation circuit R r.v (see Figure 1, b in the article "" and Figure 1 in the article "Starting DC motors"). In the absence of additional resistance in the armature circuit ( R pa = 0) and U= const characteristics n = f(I a) and n = f(M), defined by equalities (7) and (9), presented in the article "General information about DC motors", for different values R r.v., i in or Ф δ have the form shown in Figure 2. All characteristics n = f(I a) converge on the x-axis ( n= 0) at a common point with a very large current I a, which, according to expression (5) presented in the article "General information about DC motors", is equal to

I a = U / R A.

However, the mechanical characteristics n = f(M) intersect the x-axis at different points.

The lower characteristic in Figure 2 corresponds to the nominal flow. Values n in steady state operation correspond to the points of intersection of the characteristics under consideration with the curve M st = f(n) for a working machine connected to the engine (thick dashed line in Figure 2).

Engine idle point ( M = M 0 , I a = I a0) lies slightly to the right of the ordinate axis in Figure 2. With increasing rotation speed n due to increased mechanical losses M 0 and I a0 also increase (thin dashed line in Figure 2).

If in this mode, using an externally applied torque, you begin to increase the rotation speed n, That E and [see expression (6) in the article "General information about DC motors"] will increase, and I a and M will, according to equalities (5) and (8), presented in the article “General information about DC motors”, decrease. At I a = 0 and M= 0 mechanical and magnetic losses of the motor are covered by the mechanical power supplied to the shaft, and with a further increase in speed I a and M will change sign and the engine will switch to generator operating mode (characteristic sections in Figure 2 to the left of the ordinate axis).

Motors for general use allow, according to switching conditions, speed control by field weakening within the range of 1: 2. Motors with speed control in this way within the range of up to 1: 5 or even 1: 8 are also manufactured, but in this case, to limit the maximum voltage between the commutator plates, it is necessary to increase air gap, regulate the flow across individual groups of poles (see the article “Regulating the rotation speed and stability of DC motors”) or use a compensation winding. This increases the cost of the engine.

Speed ​​regulation by resistance in the armature circuit, artificial mechanical and speed characteristics

If you include an additional resistance in series with the armature circuit R ra (Figure 3, A), then instead of expressions (7) and (9) presented in the article "General information about DC motors", we get

(1)

(2)

Resistance R The ra may be adjustable and should be designed for long-term operation. The excitation circuit must be connected to mains voltage.

Figure 3. Scheme for regulating the rotation speed of a parallel-excitation motor using resistance in the armature circuit ( A) and corresponding mechanical and speed characteristics ( b)

Characteristics n = f(M) And n = f(I a) for different values R ra = const at U= const and iв = const are shown in Figure 3, b (R pa1< R ra2< R pa3). Upper characteristic ( R pa = 0) is natural. Each of the characteristics intersects the abscissa axis ( n= 0) at the point for which

The continuation of these characteristics under the x-axis in Figure 3 corresponds to engine braking by back-on. In this case n < 0, э. д. с. E a has the opposite sign and adds up to the network voltage U, as a result of which

and engine torque M acts against the direction of rotation and is therefore braking.

If in idle mode ( I a = I a0) with the help of an externally applied torque, begin to increase the rotation speed, then the mode is first achieved I a = 0 and then I a will change direction and the machine will switch to generator mode (characteristic sections in Figure 3, b to the left of the y-axis).

As can be seen from Figure 3, b, when turned on R ra characteristics become less stringent, and at higher values R ra - steeply falling, or soft.

If the torque curve M st = f(n) has the form shown in Figure 3, b thick dashed line, then the values n at steady state for each value R ra are determined by the intersection points of the corresponding curves. The more R ra, the less n and lower efficiency (efficiency).

Speed ​​control by changing armature voltage

Speed ​​control by changing the armature voltage can be carried out using a generator-motor unit (G-E), also called a Leonard unit (Figure 4). In this case the prime mover PD(alternating current, internal combustion and the like) rotates a direct current generator at a constant speed G. The generator armature is directly connected to the DC motor armature D which serves as a drive for the working machine RM. Generator field windings OVG and engine ATS powered from an independent source - a direct current network (Figure 4) or from exciters (small direct current generators) on the shaft of the prime mover PD. Regulation of generator excitation current i v.g should be produced practically from zero (in Figure 4 using a rheostat connected according to a potentiometric circuit). If it is necessary to reverse the engine, you can change the polarity of the generator (in Figure 4 using the switch P).

Figure 4. Diagram of the generator-motor unit for regulating the speed of an independent excitation motor

Starting the engine D and its speed is controlled as follows. At maximum i i.d. and i v.g = 0 start the prime mover PD. Then gradually increase i v.g, and at low generator voltage U engine D will come into rotation. Further regulating U within up to U = U n, you can get any engine rotation speeds up to n = n n. Further increase n perhaps by reducing i e.d. To reverse the engine, reduce i vg to zero, switch OVG and increase again i v.g from value i v.g = 0.

When the working machine produces a sharply pulsating load (for example, some rolling mills) and it is not desirable for the load peaks to be completely transferred to the prime mover or to the AC mains, the motor D can be equipped with a flywheel (G – D – M unit, or Leonard – Ilgner unit). In this case, when decreasing n during peak load, part of this load is covered by the kinetic energy of the flywheel. Flywheel efficiency will be greater with a softer engine characteristic. PD or D.

Recently, more and more often the engine PD and generator G replaced by a semiconductor rectifier with adjustable voltage. In this case, the unit in question is also called valve (thyristor) drive.

The considered units are used when it is necessary to regulate the rotation speed of an engine with high efficiency within a wide range - up to 1: 100 or more (large metal-cutting machines, rolling mills, and so on).

Note that the change U for the purpose of regulation n according to the diagram in Figure 1, b shown in the article "General information about DC generators" and Figure 3, A, does not give the desired results, since simultaneously with the change in the voltage of the armature circuit it changes proportionally U also excitation current. Since regulation U can only be derived from the value U = U n down, then soon the magnetic circuit will be saturated, as a result of which U And i in will change proportionally to each other. According to equality (7), presented in the article “General information about DC motors”), n however, it does not change significantly.

Recently, the so-called pulse regulation DC motors. In this case, the motor armature circuit is powered from a direct current source with constant voltage through thyristors, which are periodically turned on and off with a frequency of 1 - 3 kHz. To smooth out the armature current curve, capacitors are connected to its terminals. The voltage at the armature terminals in this case is almost constant and proportional to the ratio of the thyristor turn-on time to the duration of the entire cycle. Thus, the pulse method allows you to regulate the rotation speed of the engine when it is powered from a constant voltage source within a wide range without a rheostat in the armature circuit and with virtually no additional losses. In the same way, without a starting rheostat and without additional losses, the engine can be started.

The pulse control method is economically very beneficial for controlling engines operating in variable speed modes with frequent starts, for example in electrified transport.

Figure 5: Shunt motor performance P n = 10 kW, U n = 200 V, n n = 950 rpm

Performance characteristics

Performance characteristics are based on power consumption P 1 current consumption I, speed n, moment M, and efficiency η from useful power P 2 at U= const and unchanged positions of the regulating rheostats. The operating characteristics of a low-power parallel excitation motor in the absence of additional resistance in the armature circuit are presented in Figure 5.

Simultaneously with the increase in shaft power P 2 the torque on the shaft increases M. Since with increasing P 2 and M speed n decreases slightly, then M ∼ P 2 / n grows slightly faster P 2. Increase P 2 and M, naturally, is accompanied by an increase in motor current I. Proportional I The power consumed from the network also increases P 1 . At idle ( P 2 = 0) efficiency η = 0, then with increasing P 2, at first η increases rapidly, but at high loads, due to the large increase in losses in the armature circuit, η begins to decrease again.

There are several possible types of construction of electric motors operating from a constant voltage source. The principle of their operation is the same, but the differences lie in the features of connecting the field winding (OB) and the armature (I).

The DC electric motor with parallel excitation received its name because its I and OB windings are connected to each other in this way. An electric motor of this type provides the required modes, surpassing products of sequential and mixed types when an almost constant speed of its operation is required.

• Conclusion

Construction of the engine and its scope

The diagram of the electric motor of the type in question is shown below.

• the total current consumed by the electric motor from the source is I = I I + I V, where I I, I V are the currents through the armature and field winding, respectively;
• at the same time I B does not depend on I I, that is, it does not depend on the load.

The device is used when starting does not require high torque, that is, when the operating modes of the drive mechanisms do not involve the creation of large starting loads. This is typical for machine tools and fans.

For practice, such useful traction parameters of such electric mechanisms as

• stability of operation under load fluctuations;
• high efficiency due to the fact that I does not flow through the OB.

Starting in case of insufficient torque is ensured by switching to a mixed type circuit.

Behavior of the electric motor when loads change

The mechanical characteristic shows the stability of the electric motor over a wide range of load changes, describing the dependence of the torque created by the electric motor on the operating speed of the shaft.

The traction characteristics of the mechanism of this type make it possible to maintain the magnitude of the torque with significant changes in the number of revolutions. Typically, the traction parameters of the unit should ensure a decrease in this parameter by no more than 5%. A simple study demonstrates that the inhibitory parameters turn out to be similar due to the reversibility of the processes. These provisions also apply to the case of mixed excitation.

In other words, such an electric motor is characterized by a rigid characteristic. This nature of work is considered an important advantage of the unit of this type.

Varieties of approaches to speed control

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The operating principle of parallel connection of windings ensures a smooth start in combination with a large range of speed changes during operation using rheostats. They also ensure normal engine starting by limiting the current.

For parallel type units, methods are used to control the speed of operation by changing:

• magnetic flux of the main poles;
• armature circuit resistance;
• voltage supplied to it.

The objects of influence are the excitation winding, the armature winding, and its operating voltage.

The change in magnetic flux is carried out using a series rheostat R P. As its resistance increases, the OB passes less current, which is accompanied by a decrease in the magnetic flux. The external manifestation of this action is the increase in the speed of the ego at idle. The study shows that the slope of the characteristic increases.

The second principle is based on the inclusion of an additional serial control rheostat in the armature power circuit. As its resistance increases, the speed of rotation of the I decreases, while its natural mechanical characteristic acquires a greater inclination. Due to the series connection of an additional resistance with the main winding of the rheostat, on which significant power is dissipated, a noticeable drop in efficiency occurs.

The third principle is accompanied by a certain complication of circuit solutions and requires the use of a separate regulated power source while maintaining the possibility of separate regulation. If it is used in real conditions, only a reduction in the shaft rotation speed is possible.

Independently excited motor

An independently excited DC motor implements the third approach to regulation and is interesting in that OB and M are powered from different sources; its diagram is presented below.

For motors in this design, Iv is set unchanged, and only the voltage applied to M changes. This is accompanied by a change in the idle speed, but the rigidity of the characteristic does not change.

The operating principle of such a unit due to the independent functioning of two sources turns out to be more complex. However, its use provides such important advantages for practice as

• smooth, economical control of operating speed with great depth;
• starting the motor at reduced voltage without a rheostat.

If the start-up occurs at normal voltage, the rheostat limits the value of Iv.

The study shows that the maximum number of revolutions is limited only by the resistance M, and the minimum by the conditions for removing the generated heat during operation.

The characteristics in terms of energy consumption and response speed of the control system are improved in the case of sequential connection with M of various thyristor regulators. To set the number of shaft revolutions and stabilize them in the process of driving various mechanisms, various methods are used. Their common characteristic feature is the inclusion of a thyristor regulator in the frequency negative feedback circuit. Starting such a unit requires the implementation of special procedures.

Conclusion

The shunt motor is a very flexible drive mechanism and can be used in a very wide range of applications where high starting torques are not required. It has simple and reliable rotation speed control circuits and is easy to start.

Electric motors are machines that can convert electrical energy into mechanical energy. Depending on the type of current consumed, they are divided into AC and DC motors. This article will focus on the latter, which are abbreviated as DBT. DC motors surround us every day. They are equipped with battery-powered power tools, electric vehicles, some industrial machines and much more.

Design and principle of operation

The structure of a DFC is similar to an AC synchronous electric motor; the difference between them is only in the type of current consumed. The motor consists of a stationary part - a stator or 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 low-power, but more often it is equipped with an excitation winding having two or more poles. The armature consists of a set of conductors (windings) fixed in grooves. The simplest model of a DFC used only one magnet and a frame through which 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 operating principle of a DPT is based on Ampere'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 one 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 armature with a set of conductors. Current is applied to the conductors, charging them, resulting in a magnetic field around the armature, which begins to interact with the magnetic field of the field winding. As a result of this interaction, the anchor rotates at a certain angle. Next, the current flows to the next conductors, etc.
To alternately charge the armature conductors, special brushes made of graphite or a copper-graphite alloy are used. They play the role of contacts that close the electrical circuit to the terminals of a pair of conductors. All terminals are isolated from each other and combined into a collector unit - a ring of several lamellas located on the axis of the armature shaft. During engine operation, the contact brushes alternately close the lamellas, which allows the engine to rotate evenly. The more conductors the armature has, the more uniformly the DPT will operate.

DC motors are divided into:
— electric motors with independent excitation;
— electric motors with self-excitation (parallel, series or mixed).
The DPT circuit with independent excitation provides for connecting the excitation winding and the armature to different power sources, so that they are not electrically connected to each other.
Parallel excitation is realized by parallel connection of the inductor and armature windings to one power source. These two types of engines 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 loads, where it is important to regulate the shaft rotation speed
With series excitation, the armature and field winding are connected in series, so the value of the electric current is the same. Such motors are “softer” in operation, have a larger speed control range, but require a constant load on the shaft, otherwise the rotation speed may reach a critical point. They have a high starting torque, which makes starting easier, but the shaft rotation speed depends on the load. They are used in electric vehicles: 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

M. Faraday became a pioneer in the history of the creation of electric motors. He was unable to create a full-fledged working model, but it was he who made the discovery that made this possible. In 1821, he conducted an experiment using a charged wire placed in mercury in a bath containing a magnet. When interacting with a magnetic field, the metal conductor began to rotate, converting the energy of the electric current into mechanical work. Scientists of that time worked to create a machine whose operation would be based on this effect. They wanted to get an engine that worked on the piston principle, that is, so that the working shaft moved reciprocatingly.
In 1834, the first direct current electric motor was created, which was developed and created by the Russian scientist B. S. Jacobi. It was he who proposed replacing the reciprocating motion of the shaft with its rotation. In his model, two electromagnets interacted with each other, rotating a shaft. In 1839, he successfully tested a boat equipped with a DPT. The further history of this power unit is essentially an improvement of the Jacobi engine.

Features of DBT

Like other types of electric motors, DPT is reliable and environmentally friendly. Unlike AC motors, it can be adjusted in a wide range of shaft speed and frequency, and it is easy to start.
A DC motor can be used both as a motor and as a generator. It is also possible to 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 sequential excitation).
Rotation speed control is achieved by connecting a variable resistance to the circuit. With sequential excitation, it is located 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 the rheostat heats up significantly during operation. An increase in speed is ensured by connecting a rheostat to the excitation winding circuit.
For shunt-wound motors, 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, so in modern engine models they are replaced with electronic circuits that allow speed control without significant energy losses.
The efficiency of a DC motor depends on its power. Low-power models are low-efficiency, with an efficiency of around 40%, while 1000 kW motors can have an efficiency of up to 96%.

Advantages and disadvantages of DBT

The main advantages of DC motors include:
— simplicity of design;
— ease of operation;
— the ability to regulate the shaft rotation speed;
— easy starting (especially for engines with sequential excitation);
— possibility of use as generators;
- compact dimensions.
Flaws:
- have a “weak link” - graphite brushes that wear out quickly, which limits their service life;
— high cost;
— when connecting to the network, they require current 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. They can often be found on production machines, where it is necessary to regulate the speed of the working shaft over a wide range.