17.Characteristics of the mixed excitation engine.

A schematic diagram of a mixed excitation motor is shown in fig. 1. This motor has two excitation windings - parallel (shunt, SHO), connected in parallel to the armature circuit, and serial (serial, CO), connected in series to the armature circuit. These magnetic flux windings can be connected in accordance with or counter.

Rice. 1 - Scheme of an electric motor of mixed excitation.

When the excitation windings are turned on consonantly, their MMFs are added and the resulting flux Ф is approximately equal to the sum of the fluxes created by both windings. With the opposite connection, the resulting flux is equal to the difference between the fluxes of the parallel and series windings. In accordance with this, the properties and characteristics of a mixed excitation electric motor depend on the method of switching on the windings and on the ratio of their MMF.

speed characteristic n=f (Ia) at U=Uн and Iв=const (here Iв is the current in the parallel winding).

With an increase in load, the resulting magnetic flux with the consonant inclusion of the windings increases, but to a lesser extent than that of a series excitation motor, therefore, the speed characteristic in this case turns out to be softer than that of a parallel excitation motor, but more rigid than that of a series excitation motor.

The ratio between the MMF of the windings can vary over a wide range. Motors with a weak series winding have a slightly decreasing speed characteristic (curve 1, Fig. 2).

Rice. 2 - Speed ​​characteristics of the mixed excitation engine.

The greater the proportion of series winding in the creation of MDS, the closer speed characteristic approaches the characteristic of a series excitation motor. In Fig. 2, line 3 depicts one of the intermediate characteristics of the mixed excitation motor, and for comparison, the characteristic of the sequential excitation motor is given (curve 2).

When the series winding is turned on in the opposite direction, the resulting magnetic flux decreases with increasing load, which leads to an increase in the motor speed (curve 4). With such a speed characteristic, the operation of the engine may turn out to be unstable, because. the flux of the series winding can greatly reduce the resulting magnetic flux. Therefore, motors with opposite windings are not used.

Mechanical characteristic n=f (M) with U=Un and Iv=const. mixed excitation motor is shown in Fig. 3 (line 2).

Rice. 3 - Mechanical characteristics of the mixed excitation engine.

It is located between the mechanical characteristics of motors of parallel (curve 1) and series (curve 3) excitation. By appropriately selecting the MMF of both windings, it is possible to obtain an electric motor with a characteristic close to that of a parallel or series excitation motor.

Scope of engines of sequential, parallel and mixed excitation.

Therefore, for series excitation motors, torque overloads are less dangerous. In this regard, series excitation motors have significant advantages in the case of difficult 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 railways) and in lifting and transport installations.

Natural high-speed and mechanical characteristics, scope in engines of parallel excitation.

Natural high-speed and mechanical characteristics, scope in engines of mixed excitation.

DC motors with sequential excitation are less common than other engines. They are used in installations with a load that does not allow the mode idle move. It will be shown later that running a series excitation motor in idle mode can lead to the destruction of the motor. The motor connection diagram is shown in fig. 3.8.

The armature current of the motor is also the excitation current, since the excitation winding of the OB is connected in series
with an anchor. The resistance of the excitation winding is quite small, since at high armature currents the magnetizing force sufficient to create a nominal magnetic flux and nominal induction in the gap is achieved by a small number of turns of a large-section wire. The excitation coils are located on the main poles of the machine. An additional rheostat can be connected in series with the armature, which can be used to limit the starting current of the motor.

speed characteristic

The natural speed characteristic of sequential excitation motors is expressed by the dependence at
U = U n = const. In the absence of an additional rheostat
in the armature circuit of the motor, the resistance of the circuit is determined by the sum of the resistance of the armature and the excitation winding , which are small enough. The speed characteristic is described by the same equation that describes the speed characteristic of a motor with independent excitation

The difference is that the magnetic flux of the machine Ф generated by armature current I according to the magnetization curve of the magnetic circuit of the machine. To simplify the analysis, we assume that the magnetic flux of the machine is proportional to the field winding current, that is, the armature current. Then , where k- coefficient of proportionality.

Replacing the magnetic flux in the velocity characteristic equation, we obtain the equation:

.

The graph of the speed characteristic is shown in fig. 3.9.

It follows from the characteristic obtained that in the idle mode, i.e., at armature currents close to zero, the armature speed is several times higher than the nominal value, and when the armature current tends to zero, the speed tends to infinity (the armature current in the first term the resulting expression is included in the denominator). If we consider the formula to be valid for very large armature currents, then we can make the assumption that . The resulting equation allows you to get the value of the current strength I, at which the armature rotation frequency will be equal to zero. For real series excitation motors, at certain current values, the magnetic circuit of the machine enters saturation, and the magnetic flux of the machine changes slightly with significant changes in current.

The characteristic shows that a change in the motor armature current in the region of small values ​​leads to significant changes in the speed.

Mechanical torque characteristic

Consider the torque characteristic of a DC motor with series excitation. , at U = U n = const .

As already shown, . If the magnetic circuit of the machine is not saturated, the magnetic flux is proportional to the armature current ,
and the electromagnetic moment M will be proportional to the square of the armature current .

The resulting formula from a mathematical point of view is a parabola (curve 1 in fig. 3.10). Real characteristic passes below the theoretical (curve 2 in fig. 3.10), since due to the saturation of the magnetic circuit of the machine, the magnetic flux is not proportional to the current of the field winding or the armature current in this case.

The torque characteristic of a DC motor with series excitation is shown in Figure 3.10.

Efficiency of series excitation motor

The formula that determines the dependence of the motor efficiency on the armature current is the same for all DC motors and does not depend on the method of excitation. For series excitation motors, when the armature current changes, the mechanical losses and losses in the steel of the machine are practically independent of the current I I. Losses in the field winding and in the armature circuit are proportional to the square of the armature current. The efficiency reaches its maximum value (Fig. 3.11) at such current values ​​when the sum of losses in steel and mechanical losses equal to the sum of the losses in the excitation winding and the armature circuit.

At rated current, the efficiency of the motor is slightly less than the maximum value.

Mechanical characteristics of the series excitation motor

The natural mechanical characteristic of a series excitation motor, i.e. the dependence of the rotational speed on the mechanical torque on the motor shaft , considered at a constant supply voltage equal to the rated voltage U = U n = const . If the magnetic circuit of the machine is not saturated, as already stated, the magnetic flux is proportional to the armature current, i.e. , and the mechanical moment is proportional to the square of the current . The armature current in this case is equal to

and the rotation frequency

Or .

Substituting instead of the current its expression through the mechanical moment, we obtain

.

Denote and ,

we get .

The resulting equation is a hyperbola intersecting the axis of moments at the point .

Because or .

The starting torque of such motors is ten times greater than the rated torque of the motor.

Rice. 3.12

A general view of the mechanical characteristic of a series-excited DC motor is shown in fig. 3.12.

In idle mode, the speed tends to infinity. This follows from the analytical expression for the mechanical characteristic at M → 0.

For real series excitation motors, the idle speed of the armature can be several times higher than the rated speed. Such an excess is dangerous and can lead to the destruction of the machine. For this reason, series excitation motors are operated under constant mechanical load conditions that do not allow idling. This type of mechanical characteristic is referred to as soft mechanical characteristics, i.e., to such mechanical characteristics that suggest a significant change in rotational speed with a change in torque on the motor shaft.

3.4.3. Characteristics of DC motors
mixed excitation

The connection diagram of the mixed excitation motor is shown in fig. 3.13.

D

The serial excitation winding OB2 can be switched on so that its magnetic flux may or may not coincide in direction with the magnetic flux of the parallel winding OB1. If the magnetizing forces of the windings coincide in direction, then the total magnetic flux of the machine will be equal to the sum of the magnetic fluxes of the individual windings. Armature speed n can be obtained from the expression

.

In the resulting equation, and are the magnetic fluxes of the parallel and series excitation windings.

Depending on the ratio of magnetic fluxes, the speed characteristic is represented by a curve that occupies an intermediate position between the characteristic of the same motor with a parallel excitation circuit and the characteristic of a motor with series excitation (Fig. 3.14). The torque characteristic will also take an intermediate position between the characteristics of a series and parallel excitation motor.

In general, with increasing torque, the armature speed decreases. With a certain number of turns of the series winding, a very rigid mechanical characteristic can be obtained, when the armature rotation frequency will practically not change when the mechanical moment on the shaft changes.

If the magnetic fluxes of the windings do not coincide in direction (when the windings are turned on in the opposite direction), then the dependence of the motor armature speed on the fluxes is described by the equation

.

As the load increases, the armature current will increase. With an increase in current, the magnetic flux will increase, and the rotational speed n decrease. Thus, the mechanical characteristic of mixed excitation motors with the consonant inclusion of windings is very soft (see Fig. 3.14).

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 equationspecifications. 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 the EP of hoisting machines, electric vehicles and a number of other working machines and mechanisms, DC motors of series excitation are used. The main feature of these motors is the inclusion of a winding 2 excitation in series with the winding / armature (Fig. 4.37, a), as a result, the armature current is also the excitation current.

According to equations (4.1) - (4.3), the electromechanical and mechanical characteristics of the engine are expressed by the formulas:

in which the dependence of the magnetic flux on the armature (excitation) current Ф(/), a R = L i + R OB+ /? d.

Magnetic flux and current are interconnected by a magnetization curve (line 5 rice. 4.37 a). The magnetization curve can be described using some approximate analytical expression, which in this case will make it possible to obtain formulas for the characteristics of the engine.

In the simplest case, the magnetization curve is represented by a straight line 4. Such a linear approximation, in essence, means neglecting the saturation of the motor magnetic system and allows you to express the dependence of flux on current as follows:

where a= tgcp (see Figure 4.37, b).

With the linear approximation adopted, the moment, as follows from (4.3), is a quadratic function of the current

Substitution (4.77) in (4.76) leads to the following expression for the electromechanical characteristic of the motor:

If now in (4.79) using expression (4.78) to express the current through the moment, then we get the following expression for the mechanical characteristic:

To display the characteristics of co (Y) and co (M) let us analyze the obtained formulas (4.79) and (4.80).

Let us first find the asymptotes of these characteristics, for which we direct the current and torque to their two limiting values ​​- zero and infinity. For / -> 0 and A/ -> 0, the speed, as follows from (4.79) and (4.80), takes infinitely great importance, i.e. co -> This

means that the velocity axis is the first desired asymptote of the characteristics.

Rice. 4.37. Scheme of inclusion (a) and characteristics (b) of a DC motor of series excitation:

7 - armature; 2 - excitation winding; 3 - resistor; 4.5 - magnetization curves

For / -> °o and M-> xu speed co -» -R/ka, those. straight line with ordinate co a \u003d - R/(ka) is the second, horizontal asymptote of the characteristics.

Co(7) and co dependencies (M) in accordance with (4.79) and (4.80) have a hyperbolic character, which allows, taking into account the analysis made, to represent them in the form of curves shown in Figs. 4.38.

The peculiarity of the characteristics obtained is that at low currents and torques, the motor speed takes on large values, while the characteristics do not cross the speed axis. Thus, for the series excitation motor in the main switching circuit of Fig. 4.37 a there are no idling and generator running modes in parallel with the network (regenerative braking), since there are no sections of characteristics in the second quadrant.

From the physical point of view, this is explained by the fact that at / -> 0 and M-> 0 the magnetic flux Ф -» 0 and the speed, in accordance with (4.7), increases sharply. Note that due to the presence of residual magnetization flux in the engine F ref, the idle speed practically exists and is equal to co 0 = U/(/sF ost).

Other modes of engine operation are similar to those of an engine with independent excitation. The motor mode takes place at 0

The resulting expressions (4.79) and (4.80) can be used for approximate engineering calculations, since the motors can also operate in the saturation region of the magnetic system. For accurate practical calculations, the so-called universal characteristics of the engine are used, shown in Fig. 4.39. They represent

Rice. 4.38.

excitation:

o - electromechanical; b- mechanical

Rice. 4.39. Serial Excited DC Motor Versatile Features:

7 - dependence of speed on current; 2 - dependences of the moment of outflow

are the dependences of the relative velocity co* = co / conom (curves 1) and moment M* = M / M(curve 2) on relative current /* = / / / . To obtain characteristics with greater accuracy, the dependence co*(/*) is represented by two curves: for motors up to 10 kW and above. Consider the use of these characteristics on a specific example.

Problem 4.18*. Calculate and plot the natural characteristics of a series-excited motor type D31 with the following data Р нш = 8 kW; pish = 800 rpm; U= 220 V; / nom = 46.5 A; L„ ohm \u003d °.78.

1. Determine the nominal speed co and moment M nom:

2. By first setting the relative values ​​of the current / *, according to the universal characteristics of the motor (Fig. 4.39) we find the relative values ​​of the moment M* and speed co*. Then, multiplying the obtained relative values ​​of the variables by their nominal values, we obtain points for constructing the desired engine characteristics (see Table 4.1).

Table 4.1

Calculation of engine characteristics

Based on the data obtained, we build the natural characteristics of the engine: electromechanical co(/) - curve 1 and mechanical (M)- curve 3 in fig. 4.40 a, b.

Rice. 4.40.

a- electromechanical: 7 - natural; 2 - rheostatic; b - mechanical: 3 - natural

The excitation winding is connected to an independent source. The characteristics of the motor are the same as those of a permanent magnet motor. The rotation speed is controlled by the resistance in the armature circuit. It is also regulated by a rheostat (regulating resistance) in the excitation winding circuit, but if its value is excessively reduced or if it breaks, the armature current increases to dangerous values. Motors with independent excitation must not be started at idle or with a small load on the shaft. The rotation speed will increase sharply and the motor will be damaged.

Independent excitation scheme

The remaining circuits are called circuits with self-excitation.

Parallel excitation

The rotor and excitation windings are connected in parallel to the same power source. With this inclusion, 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 machine tools, fans.

Adjustment of the rotation speed is provided by the inclusion of rheostats in the rotor circuit or in series with the excitation winding.

Parallel Excitation Circuit

sequential excitation

The excitation winding is connected in series with the anchor winding, the same current flows through them. The speed of such an engine depends on its load, it cannot be turned on at idle. But it has good starting characteristics, so the series excitation circuit is used in electrified vehicles.

Series excitation circuit

mixed excitement

This scheme uses two excitation windings located in pairs on each of the poles of the motor. They can be connected so that their flows either add up or subtract. As a result, the motor can have characteristics similar to series or parallel excitation.

Mixed excitation scheme

To change the direction of rotation change the polarity of one of the excitation windings. To control the start of the electric motor and the speed of its rotation, stepwise switching of resistances is used.

33. Characteristics of DPT with independent excitation.

DC motor of independent excitation (DPT NV) In this motor (Figure 1), the field winding is connected to a separate power source. An adjusting rheostat r reg is included in the excitation winding circuit, and an additional (starting) rheostat R p is included in the armature circuit. A characteristic feature of the NV DPT is its excitation current I in independent of armature current I am since the power supply of the excitation winding is independent.

Scheme of a DC motor of independent excitation (DPT NV)

Picture 1

Mechanical characteristic of a DC motor of independent excitation (dpt nv)

The equation for the mechanical characteristic of a DC motor of independent excitation has the form

where: n 0 - engine shaft speed at idle. Δn - change in engine speed under the action of a mechanical load.

It follows from this equation that the mechanical characteristics of a DC motor of independent excitation (DPT NV) are rectilinear and intersect the y-axis at the idle point n 0 (Fig. 13.13 a), while changing the engine speed Δn, due to a change in its mechanical load, is proportional to the resistance of the armature circuit R a =∑R + R ext. Therefore, at the lowest resistance of the armature circuit R a = ∑R, when Rext = 0 , corresponds to the smallest speed difference Δn. In this case, the mechanical characteristic becomes rigid (graph 1).

The mechanical characteristics of the motor, obtained at nominal voltages on the armature and excitation windings and in the absence of additional resistances in the armature circuit, are called natural(chart 7).

If at least one of the listed motor parameters is changed (the voltage on the armature or excitation windings differs from the nominal values, or the resistance in the armature circuit is changed by introducing Rext), then the mechanical characteristics are called artificial.

Artificial mechanical characteristics obtained by introducing additional resistance Rext into the armature circuit are also called rheostatic (graphs 7, 2 and 3).

When evaluating the adjusting properties of DC motors, the mechanical characteristics are of the greatest importance. n = f(M). With a constant load torque on the motor shaft with an increase in the resistance of the resistor Rext rotation speed decreases. Resistor resistance Rext to obtain an artificial mechanical characteristic corresponding to the required speed n at a given load (usually nominal) for motors of independent excitation:

where U is the supply voltage of the motor armature circuit, V; I i - armature current corresponding to a given engine load, A; n - required speed, rpm; n 0 - idle speed, rpm.

The idling speed n 0 is the limiting speed, above which the engine switches to generator mode. This speed exceeds the rated nnom as much as the rated voltage U nom supplied to the armature circuit exceeds the armature EMF Ei nom at rated motor load.

The shape of the mechanical characteristics of the engine is affected by the value of the main magnetic flux of excitation F. When decreasing F(when the resistance of the resistor r reg increases), the idle speed of the engine n 0 and the speed difference Δn increase. This leads to a significant change in the rigidity of the mechanical characteristics of the engine (Fig. 13.13, b). If you change the voltage on the armature winding U (with unchanged R ext and R reg), then n 0 changes, and Δn remains unchanged [see. (13.10)]. As a result, the mechanical characteristics are shifted along the y-axis, remaining parallel to each other (Fig. 13.13, c). This creates the most favorable conditions for regulating the speed of engines by changing the voltage U supplied to the armature circuit. This method of speed control has become most widespread also due to the development and widespread use of adjustable thyristor voltage converters.