Mechanical characteristic of a series-excited DC motor. Types of excitation and switching circuits of DC motors
DC motors are not used as often as AC motors. Below are their advantages and disadvantages.
In everyday life, DC motors are used in children's toys, since batteries are used as sources for their power supply. They are used in transport: in the subway, trams and trolleybuses, cars. At industrial enterprises, DC electric motors are used in drives of units, for uninterrupted power supply of which rechargeable batteries are used.
DC motor design and maintenance
The main winding of the DC motor is anchor connecting to the power supply via brush apparatus... The armature rotates in a magnetic field created by stator poles (field windings)... The end parts of the stator are covered with shields with bearings, in which the motor armature shaft rotates. On the one hand, on the same shaft is installed fan cooling, which drives the air flow through the internal cavities of the engine during its operation.
The brush is a vulnerable element in the design of the engine. The brushes are rubbed against the collector in order to repeat its shape as accurately as possible, they are pressed against it with constant effort. In the process of operation, the brushes wear out, the conductive dust from them settles on stationary parts, it must be removed periodically. The brushes themselves sometimes need to be moved in the grooves, otherwise they get stuck in them under the influence of the same dust and "hang" over the collector. The characteristics of the motor also depend on the position of the brushes in space in the plane of rotation of the armature.
Over time, the brushes will wear out and be replaced. The collector at the points of contact with the brushes is also abraded. Periodically, the armature is dismantled and the collector is machined on a lathe. After piercing, the insulation between the collector lamellas is cut to a certain depth, since it is stronger than the collector material and will destroy the brushes with further development.
DC motor switching circuits
The presence of field windings is a distinctive feature of DC machines. The electrical and mechanical properties of the electric motor depend on the way they are connected to the network.
Independent excitement
The excitation winding is connected to an independent source. The performance of the motor is the same as that of a permanent magnet motor. The rotation speed is controlled by the resistance in the armature circuit. It is also regulated by a rheostat (control resistance) in the excitation winding circuit, but with an excessive decrease in its value or with a break, the armature current increases to dangerous values. Separately excited motors must not be started at idle speed or with light shaft load. The rotation speed will increase dramatically and the engine will be damaged.
The rest of the circuits are called self-excitation circuits.
Parallel excitation
The rotor and field windings are connected in parallel to the same power supply. With this connection, the current through the excitation winding is several times less than through the rotor. The characteristics of electric motors are tough, allowing them to be used to drive machines and fans.
Rotation speed control is provided by connecting rheostats to the rotor circuit or in series with the excitation winding.
Sequential excitement
The excitation winding is connected in series with the armature, the same current flows through them. The speed of such an engine depends on its load; it cannot be turned on at idle speed. But it has good starting characteristics, so the series-excited circuit is used in electrified vehicles.
Mixed excitement
In this scheme, two field windings are used, located in pairs at each of the poles of the electric motor. They can be connected so that their flows are either added or subtracted. As a result, the motor can have the characteristics of a series or parallel excitation circuit.
To change the direction of rotation change the polarity of one of the field windings. To control the start of the electric motor and the speed of its rotation, stepwise switching of resistances is used.
32. Mechanical characteristics of DC ED
DC motor of series excitation: The mechanical characteristic equation has the form:
, where ω is the rotation frequency, rad / s; Rob - resistance of the winding of series excitation, Ohm; α is the coefficient of linear dependence (in the first approximation) of the magnetic flux on the armature current.
From an examination of Fig. it follows that the mechanical characteristics of the engine under consideration (natural and rheostatic) are soft and hyperbolic. At low loads, the rotational speed and rises sharply and can exceed the maximum permissible value (the engine goes into "runaway"). Therefore, such motors cannot be used to drive mechanisms operating in idle mode or at low load (various machines, conveyors, etc.). Usually the minimum permissible load is (0.2 - 0.25) IN0M; only low-power motors (tens of watts) are used to operate in devices where idle is possible. To prevent the engine from running without load, it is rigidly connected to the drive mechanism (gear or blind coupling); the use of a belt drive or a friction clutch for engagement is unacceptable.
Despite this drawback, sequential excitation motors are widely used in various electric drives, especially where there is a wide variation in the load torque and severe starting conditions (lifting and slewing mechanisms, traction drive, etc.). This is because the soft characteristic of the motor under consideration is more favorable for the specified operating conditions than the hard characteristic of the parallel excitation motor.
DC motor of independent excitation: A characteristic feature of the motor is that its excitation current is independent of the armature current (load current), since the supply of the excitation winding is essentially independent. Therefore, neglecting the demagnetizing effect of the armature reaction, we can approximately assume that the motor flux does not depend on the load. Consequently, the mechanical characteristic will be linear.
The mechanical characteristic equation has the form: 
where ω - rotation frequency, rad / s; U is the voltage applied to the armature circuit, V; F - magnetic flux, Wb; Rя, Rд - resistance of the armature and additional resistance in its circuit, Ohm: α- constructive constant of the motor.
where p is the number of pairs of motor poles; N is the number of active conductors of the motor armature; α is the number of parallel branches of the armature winding. Engine torque, N * m.
- EMF of a DC motor, V. At a constant magnetic flux Ф = const, assuming c = to Ф, 
Then the expression for the torque, N * m: 
1. Mechanical characteristic e, obtained for the conditions Rd = O, Rw = 0, i.e. the armature voltage and the magnetic flux of the motor are equal to the nominal values, called natural (Fig. 17.6).
2, If Rd> O (Rw = 0), artificial rheostat characteristics 1 and 2 are obtained, passing through the point ω0 - the speed of the ideal idling of the machine. The more Poison, the steeper the characteristics.3, If you change the voltage at the armature terminals by means of a converter, provided that Rd = 0 and Rv = 0, then the artificial mechanical characteristics have the form 3 and 4 and run parallel to the natural one and the lower the lower the voltage value.
4, With a nominal armature voltage (Rd = 0) and a decrease in the magnetic flux (Rw> 0), the characteristics have the form 5 and the lower the magnetic flux, the higher the natural voltage and the steeper it is.
Mixed excitation DC motor: The characteristics of these motors are intermediate between the characteristics of parallel and series excitation motors.
When the serial and parallel field windings are connected in agreement, the mixed-field motor has a higher starting torque compared to the parallel-field motor. When the excitation windings are switched on oppositely, the motor acquires a rigid mechanical characteristic. With increasing load, the magnetic flux of the series winding increases and, subtracted from the flux of the parallel winding, reduces the total field flux. In this case, the speed of rotation of the engine not only does not decrease, but can even increase (Figure 6.19). In both cases, the presence of the magnetic flux of the parallel winding excludes the "runaway" mode of the motor when the load is removed.
The circuit of a series-excited DC motor is shown in Figure 6-15. The field winding of the motor is connected in series with the armature, so the magnetic flux of the motor changes along with the change. eat loads. Since the load current is large, the excitation winding has a small number of turns, this makes it possible to somewhat simplify the design of the starting
rheostat compared to a rheostat for a parallel excitation motor.
The speed characteristic (Fig. 6-16) can be obtained on the basis of the speed equation, which for a sequential excitation motor has the form:
where is the resistance of the excitation winding.
From the consideration of the characteristics, it can be seen that the engine speed is highly dependent on the load. With an increase in the load, the voltage drop across the resistance of the windings increases with a simultaneous increase in the magnetic flux, which leads to a significant decrease in the rotation speed. This is a characteristic feature of a sequential excitation motor. A significant reduction in load will result in an increase in speed, which is dangerous for the engine. At loads less than 25% of the nominal (and especially at idle), when the load current and magnetic flux, due to the small number of turns in the field winding, is so weak that the rotation speed rapidly increases to unacceptably high values (the motor can "spread"). For this reason, these motors are used only when they are connected to rotating machinery directly or via a gear train. The use of a belt drive is unacceptable, since the belt can break or come off, and the engine will be completely unloaded in this case.
The rotation speed of the sequential excitation motor can be controlled by changing the magnetic flux or by changing the supply voltage.
The dependence of the torque on the load current (mechanical characteristic) of a sequential excitation motor can be obtained if the magnetic flux is expressed in terms of the load current in the torque formula (6.13). In the absence of magnetic saturation, the flux is proportional to the excitation current, and the latter for a given motor is the load current, i.e.
On the graph (see Fig. 6-16), this characteristic has the shape of a parabola. The square-law dependence of the torque on the load current is the second characteristic feature of the series excitation motor, due to which these motors can easily withstand large short-term overloads and develop a large starting torque.
The engine performance is shown in Figure 6-17.
From consideration of all characteristics it follows that sequential excitation motors can be used in those cases
when a large starting torque or short-term overload is required; the possibility of their complete unloading is excluded. They turned out to be indispensable as traction motors in electric transport (electric locomotive, subway, tram, trolleybus), in lifting and transport installations (cranes, etc.) and for starting internal combustion engines (starters) in automobiles and aviation.
Economical regulation of the speed of rotation within a wide range is carried out in the case of simultaneous operation of several motors by means of various combinations of switching on the motors and rheostats. For example, at low speeds they turn on in series, and at high speeds - in parallel. The required switching is carried out by the operator (driver) by turning the switch knob.
Natural speed and mechanical characteristics, field of application
In motors of series excitation, the armature current is simultaneously also the excitation current: i in = I a = I... Therefore, the flux Ф δ varies within wide limits and it can be written that
 | (3) |
 | (4) |
The speed characteristic of the motor [see expression (2)] shown in Figure 1 is soft and hyperbolic. At kФ = const type of curve n = f(I) is shown by a dashed line. For small I the engine speed becomes unacceptably high. Therefore, the operation of sequential excitation motors, with the exception of the smallest, is not allowed at idle speed, and the use of a belt drive is unacceptable. Usually the minimum allowable load P 2 = (0,2 – 0,25) P n.
Natural characteristic of a series excitation motor n = f(M) in accordance with relation (3) is shown in Figure 3 (curve 1 ).
Since parallel excitation motors M ∼ I, and for motors of sequential excitation approximately M ∼ I² and at start-up allowed I = (1,5 – 2,0) I n, then sequential excitation motors develop a significantly higher starting torque compared to parallel excitation motors. In addition, parallel excitation motors n≈ const, and for motors of sequential excitation, according to expressions (2) and (3), approximately (at R a = 0)
n ∼ U / I ∼ U / √M .
Therefore, in parallel excitation motors
P 2 = Ω × M= 2π × n × M ∼ M ,
and for motors of sequential excitation
P 2 = 2π × n × M ∼ √ M .
Thus, for motors of series excitation, when the load torque changes M st = M within wide limits, the power varies within smaller limits than that of parallel excitation motors.
Therefore, torque overloads are less dangerous for series excitation motors. In this regard, series excitation motors have significant advantages in the case of severe starting conditions and changes in the load torque over a wide range. They are widely used for electric traction (trams, metro, trolleybuses, electric locomotives and diesel locomotives on the railways) and in hoisting and transport installations.
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| Figure 2. Schemes for regulating the speed of rotation of a series excitation motor by shunting the excitation winding ( but), shunting the anchor ( b) and the inclusion of resistance in the armature circuit ( in) |
Note that with an increase in the rotation speed, the sequential excitation motor does not go into the generator mode. In Figure 1, this is obvious from the fact that the characteristic n = f(I) does not intersect the ordinate axes. Physically, this is explained by the fact that when switching to the generator mode, for a given direction of rotation and a given voltage polarity, the direction of the current should change to the opposite, and the direction of the electromotive force (emf) E and the polarity of the poles must remain unchanged, however, the latter is impossible when the direction of the current in the field winding changes. Therefore, to transfer the series excitation motor to the generator mode, it is necessary to switch the ends of the excitation winding.
Speed regulation by field weakening
Regulation n by weakening the field, it is produced either by shunting the excitation winding with some resistance R sh.v (Figure 2, but), or a decrease in the number of turns of the excitation winding included in the work. In the latter case, appropriate outputs from the field winding must be provided.
Since the resistance of the excitation winding R in and the voltage drop across it is small, then R sh.v should also be small. Resistance losses R sh.v are therefore small, and the total excitation losses during shunting even decrease. As a result, the efficiency (efficiency) of the engine remains high, and this control method is widely used in practice.
When shunting the excitation winding, the excitation current from the value I decreases to
and speed n increases accordingly. In this case, we obtain expressions for the speed and mechanical characteristics if in equalities (2) and (3) we replace k F on k F k o.v, where
is the excitation attenuation factor. When regulating the speed, the change in the number of turns of the excitation winding
k o.v = w in.work / w in full.
Figure 3 shows (curves 1 , 2 , 3 ) characteristics n = f(M) for this case of speed regulation at several values k o.v (value k o.v = 1 corresponds to the natural characteristic 1 , k o.v = 0.6 - curve 2 , k o.v = 0.3 - curve 3 ). The characteristics are given in relative units and correspond to the case when kФ = const and R a * = 0.1.
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| Figure 3. Mechanical characteristics of a series excitation motor with different methods of speed control |
Speed regulation by shunting the armature
When shunting the anchor (Figure 2, b) the current and the excitation flux increase, and the speed decreases. Since the voltage drop R in × I small and therefore can be taken R at ≈ 0, then the resistance R sh. a is practically under the full voltage of the network, its value should be significant, the losses in it will be great and the efficiency will greatly decrease.
In addition, armature shunting is effective when the magnetic circuit is not saturated. In this regard, the shunting of the armature is rarely used in practice.
Figure 3 shows the curve 4 n = f(M) at
I w.a ≈ U / R w.a = 0.5 I n.
Speed regulation by including a resistance in the armature circuit
Speed regulation by including a resistance in the armature circuit (Figure 2, in). This method allows you to regulate n down from the nominal value. Since at the same time the efficiency decreases significantly, this method of regulation finds limited application.
Expressions for the speed and mechanical characteristics in this case will be obtained if in equalities (2) and (3) we replace R and on R a + R ra. Characteristic n = f(M) for this type of speed control at R pa * = 0.5 is shown in Figure 3 as a curve 5 .
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| Figure 4. Parallel and series connection of series field motors to change the speed of rotation |
Speed regulation by voltage variation
In this way, you can regulate n down from the nominal value while maintaining a high efficiency. The considered control method is widely used in transport installations, where a separate motor is installed on each drive axle and regulation is carried out by switching the motors from parallel connection to the network to serial (Figure 4). Figure 3 shows the curve 6 is a characteristic n = f(M) for this case at U = 0,5U n.
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 second, which are abbreviated as DPT. DC motors are around us every day. They are equipped with power tools powered by batteries or accumulators, electric vehicles, some industrial machine tools and much more.
Device and principle of operation
DPT in its structure resembles a synchronous AC motor, the difference between them is only in the type of current consumed. The motor consists of a fixed part - a stator or an inductor, a moving part - an armature and a brush-collector unit. The inductor can be made in the form of a permanent magnet if the motor is low-power, but more often it is supplied with an excitation winding having two or more poles. The armature consists of a set of conductors (windings) fixed in slots. The simplest DCT model used only one magnet and a frame through which the current passed. Such a design can be considered only as a simplified example, while a modern design is an improved version, having a more complex device and developing the required power. 
The principle of operation of 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 reaches a neutral position parallel to the external magnetic field. To set the system in motion, you need to add another frame. In modern DPT, the frames are replaced by an anchor with a set of conductors. A current is supplied 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. Then the current flows to the next conductors, etc.
For alternating 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 leads are isolated from each other and combined into a manifold assembly - a ring of several lamellas located on the axis of the armature shaft. During engine operation, brushes-contacts alternately close the lamellas, which allows the engine to rotate evenly. The more conductors the armature has, the more evenly the DPT 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 the armature to different power sources, so that they are not electrically connected to each other.
Parallel excitation is realized 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 regulate the speed of rotation of the shaft.
With sequential 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 wider 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 starting torque, which makes starting easier, 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 field winding is connected to the armature in parallel, and the second - in series, is rare.
A brief history of creation
M. Faraday became a pioneer in the history of the creation of electric motors. 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 bathroom with a magnet. When interacting with a magnetic field, the metal conductor began to rotate, converting the energy of an electric current into mechanical work. Scientists of the time were working to create a machine that would work based on this effect. They wanted to get an engine that works on the principle of a piston, that is, so that the working shaft moves back and forth.
In 1834, the first DC electric motor was created, which was developed and created by the Russian scientist B.S. Jacobi. It was he who suggested 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 also successfully tested a boat equipped with 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 speed of rotation of the shaft in a wide range, frequency, and besides, it is distinguished by easy start-up. 
The DC motor can be used both as a motor and as a generator. Also, it can change the direction of rotation of the shaft by changing the direction of the current in the armature (for all types) or in the field winding (for motors with series excitation).
Speed control of rotation is achieved by connecting a variable resistance in the circuit. With sequential excitation, it is in the armature circuit and makes it possible to reduce the speed in ratios of 2: 1 and 3: 1. This option is suitable for equipment that has long periods of downtime because the rheostat heats up significantly during operation. The increase in speed is provided by connecting the rheostat to the field winding circuit.
For motors 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 field 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 of motors, they are replaced by electronic circuits that allow speed control 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 construction;
- ease of management;
- the ability to control the frequency of rotation of the shaft;
- easy starting (especially for motors with series excitation);
- the ability to use as generators;
- compact size.
Flaws:
- have a "weak link" - graphite brushes, which wear out quickly, which limits the service life;
- high cost;
- when connected 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 machinery. They can often be found on production machines, where it is required to regulate the rotational speed of the working shaft in a wide range.
