Figure: eleven

In motors of series excitation, the excitation winding is connected in series with the armature winding (Fig. 11). The excitation current of the motor here is equal to the armature current, which gives these motors special properties.

For motors of sequential excitation, idling is unacceptable. In the absence of a load on the shaft, the current in the armature and the magnetic flux created by it will be small and, as can be seen from the equality

the armature speed reaches excessively high values, which leads to "runaway" of the engine. Therefore, starting and running the motor without load or with a load less than 25% of the rated load is unacceptable.

At low loads, when the magnetic circuit of the machine is not saturated (), the electromagnetic moment is proportional to the square of the armature current

Because of this, the series excitation motor has a large starting torque and copes well with difficult starting conditions.

With increasing load, the magnetic circuit of the machine saturates, and the proportionality between and is violated. When the magnetic circuit is saturated, the flux is almost constant, so the torque becomes directly proportional to the armature current.

With an increase in the load torque on the shaft, the motor current and magnetic flux increase, and the rotation frequency decreases according to a law close to hyperbolic, which can be seen from equation (6).

Under significant loads, when the magnetic circuit of the machine is saturated, the magnetic flux remains practically unchanged, and the natural mechanical characteristic becomes almost rectilinear (Fig. 12, curve 1). This mechanical characteristic is called soft.

When a starting and adjusting rheostat is introduced into the armature circuit, the mechanical characteristic shifts to the region of lower speeds (Fig. 12, curve 2) and is called an artificial rheostat characteristic.

Figure: 12

Regulation of the rotational speed of a sequential excitation engine is possible in three ways: by changing the voltage at the armature, the resistance of the armature circuit and the magnetic flux. In this case, the speed control by changing the resistance of the armature circuit is performed in the same way as in a parallel excitation motor. To regulate the speed by changing the magnetic flux, a rheostat is connected in parallel with the excitation winding (see Fig. 11),

from where. (8)

With a decrease in the resistance of the rheostat, its current increases, and the excitation current decreases according to formula (8). This leads to a decrease in the magnetic flux and an increase in the rotation frequency (see formula 6).

A decrease in the resistance of the rheostat is accompanied by a decrease in the excitation current, which means a decrease in the magnetic flux and an increase in the rotation frequency. The mechanical characteristic corresponding to the weakened magnetic flux is shown in Fig. 12, curve 3.

Figure: thirteen

In fig. 13 shows the performance characteristics of a series excitation motor.

The dotted parts of the characteristics refer to those loads at which the engine cannot be allowed to operate due to the high speed.

DC motors with series excitation are used as traction motors in railway transport (electric trains), in urban electric transport (trams, subway trains) and in lifting and transport mechanisms.

LABORATORY WORK 8

Engine diagram. Sequential motor circuit 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 in.

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

Mechanical equation specifications. The mechanical characteristic equation can be obtained from the formula (1.6). At load currents less than (0.8 - 0.9) I nom, it can be assumed that the magnetic circuit of the motor is not saturated and the magnetic flux Ф is proportional to the current I: Ф \u003d kI, where k \u003d const. (At high currents, the coefficient k decreases slightly). Replacing Ф in (1.2), we obtain М \u003d С м kI whence

Let us substitute Ф in (1.6):

n \u003d (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 sharply - characteristics of this type are called "soft". At idle, when M "0, the engine speed rises infinitely and the engine" runs out of gear. "


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

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

The change in excitation can be carried out by turning on the rheostat R p1 parallel to the excitation winding (see Fig. 1.31) or 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 the 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 motor speed decreases. The mechanical characteristics for this case are shown in Fig. 1.32, curves 4, 5. However, rotation control by a rheostat connected in parallel with the armature is rarely used, since the power loss in the rheostat and the motor efficiency decrease.

Changing the rotation speed by changing the resistance of the armature circuit is possible when the rheostat R p3 is connected in series in 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, R i + R p3 must be substituted.) Mechanical characteristics for this control method 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 possible only in the direction of decreasing the rotational speed when the engine is powered from a separate generator or controlled rectifier. The mechanical characteristic for this control method is shown in Fig. 1.32, curve 8. In the presence of two motors operating on a common load, they can be switched from parallel to serial, the voltage U on each motor is halved, and the speed also decreases accordingly.

Braking modes of the engine sequential 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 the speed n\u003e n x (n x \u003d).

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

DC motors with series excitation are less widespread than other motors. They are used in installations with a load that does not allow idling. It will be shown later that idling the sequential excitation engine can destroy the engine. The motor connection diagram is shown in fig. 3.8.

The armature current of the motor is simultaneously 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 wire with a large cross-section. The field coils are located at 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 \u200b\u200bcharacteristic

The natural speed characteristic of series excitation motors is expressed by the dependence at
U \u003d 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 field winding which are small enough. The speed characteristic is described by the same equation as the speed characteristic of an independent excitation motor

The difference lies in the fact that the magnetic flux of the machine F created by the 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 current of the field winding, that is, the armature current. Then , where k - coefficient of proportionality.

Replacing the magnetic flux in the equation of the speed characteristic, we get the equation:

.

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

From the obtained characteristic it follows that in the idle mode, i.e., with armature currents close to zero, the armature rotation frequency is several times higher than the nominal value, and when the armature current tends to zero, the rotation frequency tends to infinity (armature current in the first term the resulting expression is included in the denominator). If we consider the formula 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 I, at which the frequency of rotation of the armature will be equal to zero. In real motors of series excitation, 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 \u200b\u200bleads to significant changes in the speed.

Mechanical torque characteristic

Consider the torque characteristic of a DC motor with series excitation. , at U \u003d 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 Mwill be proportional to the square of the armature current .

The resulting formula from a mathematical point of view is a parabola (the curve 1 in fig. 3.10). The real characteristic is lower than the theoretical one (curve 2 in fig. 3.10), since due to saturation of the magnetic circuit of the machine, the magnetic flux is not proportional to the current of the excitation winding or the armature current in the case under consideration.

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

Serial excitation motor efficiency

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 excitation method. For motors of series excitation, when the armature current changes, the mechanical losses and losses in the machine steel are practically independent of the current I I. Losses in the excitation 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 \u200b\u200bwhen the sum of losses in steel and mechanical losses is equal to the sum of 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 characteristic of a series excitation motor

Natural mechanical characteristic of a sequential 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 \u003d 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

and the speed

Or .

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

.

We 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 tens of times greater than the rated motor torque.

Figure: 3.12

A general view of the mechanical characteristics of a DC motor of series excitation is shown in Fig. 3.12.

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

In real motors of sequential excitation, the idle speed of the armature can be several times higher than the rated speed. Such an excess is dangerous and can lead to destruction of the machine. For this reason, series excitation motors are operated under constant mechanical stress that does not allow idling. This type of mechanical characteristic is referred to as soft mechanical characteristics, that is, to those mechanical characteristics that involve a significant change in rotation speed when the torque on the motor shaft changes.

3.4.3. Characteristics of DC motors
mixed excitement

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

D

The series field winding OB2 can be switched on so that its magnetic flux may coincide in direction with the magnetic flux of the parallel winding OB1 or not. 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 obtained equation and are the magnetic fluxes of the parallel and series field windings.

Depending on the ratio of magnetic fluxes and 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 an increase in torque, the armature speed decreases. With a certain number of turns of the serial winding, a very rigid mechanical characteristic can be obtained, when the armature speed 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 oppositely), then the dependence of the motor armature speed on the fluxes will be described by the equation

.

As the load increases, the armature current will increase. With increasing current, the magnetic flux will increase, and the rotation frequency n decrease. Thus, the mechanical characteristic of mixed excitation motors with matching windings is very soft (see Fig. 3.14).

Natural speed and mechanical characteristics, field of application

In motors of series excitation, the armature current is simultaneously also the excitation current: i in \u003d I a \u003d 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. When k Ф \u003d const type of curve n = f(I) is shown by a dashed line. For small I engine speed becomes unacceptably high. Therefore, the operation of sequential excitation motors, with the exception of the smallest ones, 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 series excitation approximately M ∼ I ² and at start-up allowed I = (1,5 – 2,0) I n, then motors of series excitation develop a significantly higher starting torque compared to motors of parallel excitation. In addition, parallel excitation motors n ≈ const, and for motors of sequential excitation, according to expressions (2) and (3), approximately (at R a \u003d 0)

n ∼ U / I ∼ U / √M .

Therefore, in parallel excitation motors

P 2 \u003d Ω × M \u003d 2π × n × M ∼ M ,

and for motors of sequential excitation

P 2 \u003d 2π × n × M ∼ √ M .

Thus, for motors of series excitation, when the load torque changes M st \u003d M within wide limits, the power varies within smaller limits than in 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 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.

Figure 2. Schemes for regulating the speed of rotation of a series excitation motor by shunting the excitation winding ( and), shunting the anchor ( b) and the inclusion of resistance in the armature circuit ( at)

Note that when the rotational speed increases, the sequential excitation motor does not switch to 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, at 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 \u200b\u200bregulation 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, and), 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 field 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. Expressions for the speed and mechanical characteristics in this case, we obtain 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 \u003d w in.work / w c. full.

Figure 3 shows (curves 1 , 2 , 3 ) specifications n = f(M) for this case of speed regulation at several values k o.v (value k o.v \u003d 1 corresponds to the natural characteristic 1 , k o.v \u003d 0.6 - curve 2 , k o.v \u003d 0.3 - curve 3 ). The characteristics are given in relative units and correspond to the case when k Ф \u003d const and R a * \u003d 0.1.

Figure 3. Mechanical characteristics of a series excitation motor with different methods of speed control

Speed \u200b\u200bregulation by shunting the armature

When shunting an anchor (Figure 2, b) the current and 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 full mains voltage, 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, armature shunting is rarely used in practice.

Figure 3 shows the curve 4 n = f(M) at

I w.a ≈ U / R w.a \u003d 0.5 I n.

Speed \u200b\u200bregulation by including a resistance in the armature circuit

Speed \u200b\u200bregulation by including a resistance in the armature circuit (Figure 2, at). 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.

In this case, we obtain expressions for the speed and mechanical characteristics 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 * \u003d 0.5 is shown in Figure 3 as a curve 5 .

Figure 4. Parallel and series connection of series field motors to change the rotation speed

Speed \u200b\u200bregulation by voltage variation

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 sequential (Figure 4). Figure 3 shows the curve 6 is a characteristic n = f(M) for this case at U = 0,5U n.

A characteristic feature of the DC motor with PV is that its excitation winding (POV) with resistance is connected in series with the armature winding with resistance by means of a brush-collector unit, i.e. in such motors, only electromagnetic excitation is possible.

A schematic electrical diagram of switching on a DC motor with PV is shown in Fig. 3.1.

Figure: 3.1.

To start the DCT with PV, an additional rheostat is connected in series with its windings.

Equations of electromechanical characteristics of DC motor with PV

Due to the fact that in a DC motor with PV the current of the excitation winding is equal to the current in the armature winding, in such motors, in contrast to DC motors with NV, interesting features appear.

The excitation flux of the DCT with PV is associated with the armature current (it is also the excitation current) by a dependence called the magnetization curve shown in Fig. 3.2.

As you can see, the dependence for low currents is close to linear, and with an increase in the current, nonlinearity appears, associated with the saturation of the magnetic system of the DCT with PV. The equation for the electromechanical characteristics of DC motors with PV is also for DC motors with independent excitation:

Figure: 3.2.

Due to the lack of an accurate mathematical description of the magnetization curve, in a simplified analysis, one can neglect the saturation of the magnetic system of the DCT with PV, i.e., take the relationship between the flux and armature current to be linear, as shown in Fig. 3.2 with a dotted line. In this case, you can write:

where is the proportionality coefficient.

For the moment of DCT with PV, taking into account (3.17), we can write:

From expression (3.3) it can be seen that, in contrast to the DCC with NV, in the DCC with PV, the electromagnetic moment depends on the armature current not linearly, but quadratically.

For the armature current, in this case, you can write:

If we substitute expression (3.4) into the general equation of the electromechanical characteristic (3.1), then we can obtain an equation for the mechanical characteristic of DC motor with PV:

From this it follows that with an unsaturated magnetic system, the mechanical characteristic of a DCT with a PV is depicted (Fig. 3.3) by a curve for which the ordinate axis is an asymptote.

Figure: 3.3.

A significant increase in the speed of rotation of the engine in the region of low loads is caused by a corresponding decrease in the magnitude of the magnetic flux.

Equation (3.5) is estimated, since obtained under the assumption of unsaturation of the magnetic system of the engine. In practice, for economic reasons, electric motors are designed with a certain saturation factor and the operating points lie in the region of the knee of the magnetization curve.

In general, analyzing the equation of the mechanical characteristic (3.5), it is possible to draw an integral conclusion about the "softness" of the mechanical characteristic, which manifests itself in a sharp decrease in speed with an increase in the torque on the motor shaft.

If we consider the mechanical characteristics shown in Fig. 3.3 in the area of \u200b\u200blow shaft loads, we can conclude that the concept of ideal idle speed for DC motor with PV is absent, that is, when the moment of resistance is completely reset, the engine goes into "spacing". Moreover, its speed theoretically tends to infinity.

With an increase in the load, the rotation speed decreases and is equal to zero at the value of the short-circuit (starting) moment:

As can be seen from (3.21), for a DC motor with PV, the starting torque in the absence of saturation is proportional to the square of the short-circuit current. In specific calculations, it is impossible to use the estimated equation of the mechanical characteristic (3.5). In this case, the construction of characteristics has to be carried out by graph-analytical methods. As a rule, the construction of artificial characteristics is made on the basis of the catalog data, where the natural characteristics are given: and.

Real DPT with PV

In a real DCT with PV, due to saturation of the magnetic system, but as the load on the shaft (and, consequently, the armature current) increases in the region of large moments, there is a direct proportionality between the torque and the current, so the mechanical characteristic becomes almost linear there. This applies to both natural and artificial mechanical characteristics.

In addition, in a real DC motor with PV, even in the ideal idle mode, there is a residual magnetic flux, as a result of which the ideal idle speed will have a finite value and is determined by the expression:

But since the value is insignificant, it can reach significant values. Therefore, in DPT with PV, as a rule, it is forbidden to dump the load on the shaft by more than 80% of the nominal.

An exception is micromotors, which, even with full load shedding, have a residual frictional torque high enough to limit the idle speed. The tendency of DPT with PV to go into "runaway" leads to the fact that their rotors are mechanically reinforced.

Comparison of starting properties of motors with PV and NV

As follows from the theory of electrical machines, motors are designed for a specific rated current. In this case, the short-circuit current should not exceed the value

where is the overcurrent factor, which usually ranges from 2 to 5.

If there are two DC motors: one with independent excitation, and the second with series excitation, designed for the same current, then the permissible short-circuit current will also be the same for them, while the starting torque for DC motors with NV will be proportional to the current anchors in the first degree:

and for the idealized DC motor with PV, according to expression (3.6), the square of the armature current;

It follows from this that, with the same overload capacity, the starting torque of the DPT with PV exceeds the starting torque of the DPT with NV.

Value limitation

With direct start of the motor, shock current values, therefore, the motor windings can quickly overheat and fail, in addition, large currents also negatively affect the reliability of the brush-collector assembly.

(The above makes it necessary to limit to any acceptable value either by introducing additional resistance into the armature circuit, or by reducing the supply voltage.

The maximum allowable current is determined by the overload factor.

For micromotors, direct start-up is usually carried out without additional resistances, but with an increase in the dimensions of the DC motor, it is necessary to perform a rheostat start. especially if the drive with DC DC motor is used in loaded conditions with frequent starts and decelerations.

Methods for controlling the angular speed of rotation of the DC motor with PV

As follows from the equation of the electromechanical characteristic (3.1), the angular speed of rotation can be controlled, as in the case of a DC motor with NV, by changing, and.

Regulation of rotation speed by changing the supply voltage

As follows from the expression for the mechanical characteristic (3.1), when the supply voltage changes, one can obtain a family of mechanical characteristics shown in Fig. 3.4. In this case, the magnitude of the supply voltage is regulated, as a rule, using thyristor voltage converters or generator-motor systems.

Fig 3.4. Family of mechanical characteristics of DC motors with PV at different values \u200b\u200bof the supply voltage of the armature circuit< < .

The control range of the speed of open-loop systems does not exceed 4: 1, but with the introduction of feedbacks it can be several orders of magnitude higher. In this case, the control of the angular speed of rotation is carried out downward from the main (the main speed is called the speed corresponding to the natural mechanical characteristic). The advantage of this method is its high efficiency.

Regulation of the angular speed of rotation of the DPT with PV by introducing a series additional resistance into the armature circuit

As follows from expression (3.1), the sequential introduction of additional resistance changes the rigidity of the mechanical characteristics and also provides control of the angular speed of rotation of the ideal idle speed.

The family of mechanical characteristics of DC motors with PV for various values \u200b\u200bof additional resistance (Fig. 3.1) is shown in Fig. 3.5.

Figure: 3.5 Family of mechanical characteristics of DC motors with PV at various values \u200b\u200bof series additional resistance< < .

Regulation is downward from the main speed.

In this case, the control range usually does not exceed 2.5: 1 and depends on the load. In this case, it is advisable to regulate at a constant moment of resistance.

The advantage of this control method is its simplicity, and the disadvantage is large energy losses in the additional resistance.

This control method has found wide application in crane and traction electric drives.

Angular speed control

a change in the excitation flow

Since in DPT with PV the motor armature winding is connected in series with the excitation winding, in order to change the magnitude of the excitation flux, it is necessary to shunt the excitation winding with a rheostat (Fig. 3.6), the position change of which affects the excitation current. The excitation current in this case is defined as the difference between the armature current and the current in the shunt resistance. So in limiting cases at? and at.

Figure: 3.6.

In this case, regulation is carried out upward from the main angular velocity of rotation, due to a decrease in the magnitude of the magnetic flux. The family of mechanical characteristics of DCT with PV for different values \u200b\u200bof the shunt rheostat is shown in Fig. 3.7.

Figure: 3.7. Mechanical characteristics of DPV with PV at different values \u200b\u200bof shunt resistance

It increases with decreasing value. This method of regulation is quite economical, because the resistance value of the series field winding is small and, accordingly, the value is also chosen small.

The energy loss in this case is approximately the same as in the DCT with NV when the angular velocity is controlled by changing the excitation flux. In this case, the regulation range, as a rule, does not exceed 2: 1 at constant load.

The method finds application in electric drives requiring acceleration at low loads, for example, in flywheel blooming scissors.

All of the above control methods are characterized by the absence of the final angular speed of rotation of the ideal idle speed, but you need to know that there are circuit solutions that allow you to get the final values.

To do this, both motor windings or only the armature winding are shunted by rheostats. These methods are uneconomical in terms of energy, but allow for a fairly short time to obtain characteristics of increased rigidity with low final speeds of ideal idling. In this case, the control range does not exceed 3: 1, and the speed control is carried out downward from the main one. When switching to the generator mode, in this case, the DCT with PV does not give energy to the network, but works as a generator closed to resistance.

It should be noted that in automated electric drives, the resistance value is regulated, as a rule, by a pulse method by periodically shunting resistances with a semiconductor valve or with a certain duty cycle.