DC motor with series excitation. DPT of sequential excitation. Video on the topic
DC motors, depending on the methods of their excitation, as already noted, are divided into motors with an independent, parallel(shunt), consistent(series) and mixed (compound) excitation.
Independent excitation motors, require two power supplies (Figure 11.9, a). One of them is needed to power the armature winding (conclusions I1 and I2), and the other - to create a current in the excitation winding (winding leads Ш1 and W2). Additional resistance Rd in the armature winding circuit is necessary to reduce the starting current of the motor at the moment it is turned on.
With independent excitation, mainly powerful electric motors are made in order to more convenient and economical regulation of the excitation current. The cross-section of the field winding wire is determined depending on the voltage of its power source. A feature of these machines is the independence of the excitation current, and accordingly the main magnetic flux, from the load on the motor shaft.
Motors with independent excitation in their characteristics practically coincide with motors of parallel excitation.
Parallel excitation motors are switched on in accordance with the scheme shown in Figure 11.9, b. Clamps I1 and I2 refer to the armature winding, and the clamps Ш1 and W2- to the excitation winding (to the shunt winding). Variable resistances Rd and Rv are intended, respectively, to change the current in the armature winding and in the excitation winding. The field winding of this motor is made of a large number of turns of copper wire of a relatively small cross-section and has significant resistance. This allows it to be connected to the full mains voltage specified in the passport data.
A feature of this type of motors is that during their operation, it is forbidden to disconnect the excitation winding from the armature circuit. Otherwise, when the excitation winding is opened, an unacceptable EMF value will appear in it, which can lead to failure of the motor and damage to the maintenance personnel. For the same reason, the excitation winding must not be opened when the engine is turned off, when its rotation has not yet stopped.
With an increase in the speed of rotation, the additional (additional) resistance Rd in the armature circuit should be reduced, and when the established speed is reached, it should be completely withdrawn.
Figure 11.9. Types of excitation of DC machines,
a - independent excitation, b - parallel excitation,
c - sequential excitation, d - mixed excitation.
OVSh - shunt excitation winding, OVS - serial excitation winding, "OVN - independent excitation winding, Rd - additional resistance in the armature winding circuit, Rv - additional resistance in the excitation winding circuit.
The absence of additional resistance in the armature winding at the time of starting the motor can lead to the appearance of a large starting current exceeding the rated armature current in 10 ... 40 times .
An important property of a parallel excitation motor is its almost constant rotation frequency when the load on the armature shaft changes. So when the load changes from idle move to the nominal value, the speed is reduced by only (2.. 8)% .
The second feature of these motors is economical speed control, in which the ratio of the highest speed to the lowest speed can be 2:1 , and with a special version of the engine - 6:1 ... The minimum rotational speed is limited by saturation of the magnetic circuit, which does not already allow an increase in the magnetic flux of the machine, and the upper limit of the rotational speed is determined by the stability of the machine - with a significant weakening of the magnetic flux, the engine can "run wild".
Series excitation motors(series) are switched on according to the scheme (Figure 11.9, c). conclusions C1 and C2 correspond to a serial (series) field winding. It is made of a relatively small number of turns of mainly copper wire of large cross-section. The excitation winding is connected in series with the armature winding... Additional resistance Rd in the armature and excitation winding circuit allows to reduce the starting current and to regulate the engine speed. At the moment the engine is turned on, it must have such a value at which the starting current will be (1.5 ... 2.5) In... After the engine reaches a steady speed, additional resistance Rd is displayed, that is, set to zero.
These motors, when starting, develop high starting torques and must be started at a load of at least 25% of its rated value. Turning on the engine with less power on its shaft, and even more so in idle mode, is not allowed. Otherwise, the engine may develop unacceptably high revs, which will cause its failure. Engines of this type are widely used in transport and lifting mechanisms, in which it is necessary to change the speed over a wide range.
Mixed excitation motors(compound), occupy an intermediate position between the motors of parallel and series excitation (Figure 11.9, d). Their large belonging to one or another type depends on the ratio of parts of the main field of excitation, created by parallel or series windings of excitation. At the moment the engine is turned on to reduce the starting current, an additional resistance is included in the armature winding circuit Rd... This engine has good traction characteristics and can work in idle mode.
Direct (rheostatic) switching on of DC motors of all types of excitation is allowed with a power of no more than one kilowatt.
DC machine designation
Currently, the most widespread are the general-purpose DC machines of the series 2P and the most recent series 4P. In addition to these series, engines are produced for crane, excavator, metallurgical and other drives of the series D. Engines are also manufactured in specialized series.
Series motors 2P and 4P subdivided along the axis of rotation, as is customary for AC induction motors of the series 4A... Machine series 2P have 11 dimensions, differing in axle rotation height from 90 to 315 mm. The power range of the machines in this series is from 0.13 to 200 kW for electric motors and from 0.37 to 180 kW for generators. Motors of the 2P and 4P series are designed for voltages of 110, 220, 340 and 440 V. Their rated speeds are 750, 1000, 1500,2200 and 3000 rpm.
Each of the 11 dimensions of the series machines 2P has two bed lengths (M and L).
Electric Machine Series 4P have some better technical and economic indicators in comparison with the series 2P... labor intensity of production of a series 4P compared with 2P reduced by 2.5 ... 3 times. At the same time, copper consumption is reduced by 25 ... 30%. For a number of design features, including the cooling method, weather protection, the use of individual parts and assemblies of the series machine 4P unified with asynchronous motors series 4A and AI .
DC machines (both generators and motors) are designated as follows:
PH1X2XZX4,
where 2P- DC machine series;
XI- execution according to the type of protection: H - protected with self-ventilation, F - protected with independent ventilation, B - closed with natural cooling, O - closed with blowing from an external fan;
X2- the height of the axis of rotation (two-digit or three-digit number) in mm;
HZ- conditional stator length: M - first, L - second, G - with tachogenerator;
An example is the designation of the engine 2PN112MGU- DC motor series 2P, protected design with self-ventilation N,112 height of the axis of rotation in mm, the first dimension of the stator M, equipped with a tachogenerator G, used for temperate climates Have.
By capacity, DC electric machines can be conditionally subdivided into the following groups:
Micromachines ……………………… ... less than 100 W,
Small machines ……………………… from 100 to 1000 W,
Low-power machines ………… ..from 1 to 10 kW,
Medium power machines ……… ..from 10 to 100 kW,
Large machines …………………… ..from 100 to 1000 kW,
High power machines ………. More than 1000 kW.
According to rated voltages, electrical machines are conditionally subdivided as follows:
Low voltage ……………. Less than 100 V,
Medium voltage …………. From 100 to 1000 V,
High voltage …………… above 1000V.
In terms of rotation frequency, DC machines can be represented as:
Slow-speed ……………. Less than 250 rpm.,
Average speed ……… from 250 to 1000 rpm.,
High-speed …………. From 1000 to 3000 rpm.
Ultra-fast… ..more than 3000 rpm.
The task and the method of performing the work.
1. To study the device and the purpose of individual parts of DC electric machines.
2. Determine the conclusions of the DC machine related to the armature winding and the field winding.
The conclusions corresponding to a particular winding can be determined with a megohmmeter, ohmmeter or using an electric light bulb. When using a megohmmeter, one end of it is connected to one of the terminals of the windings, and the other ends alternately touch the rest. A measured resistance of zero will indicate that the two terminals of the same winding match.
3. To recognize the armature winding and the excitation winding by the conclusions. Determine the type of excitation winding (parallel excitation or series).
This experiment can be carried out using an electric light bulb connected in series with the windings.The constant voltage should be applied smoothly, gradually increasing it to the specified nominal value in the machine's passport.
Taking into account the low resistance of the armature winding and the winding of series excitation, the light bulb will light up brightly, and their resistances measured with a megohmmeter (or ohmmeter) will be practically zero.
A bulb connected in series with the parallel field winding will be dim. The resistance value of the parallel field winding must be within the range 0.3 ... 0.5 kOhm .
The terminals of the armature winding can be recognized by attaching one end of the megohmmeter to the brushes, while touching the other end to the terminals of the windings on the shield of the electric machine.
The terminals of the windings of an electrical machine should be indicated on the conditional terminal label shown in the report.
Measure winding resistance and insulation resistance. The resistance of the windings can be measured using the ammeter and voltmeter circuit. The insulation resistance between windings and windings relative to the case is checked with a megohmmeter designed for a voltage of 1 kV. The insulation resistance between the armature winding and the excitation winding and between them and the case must be at least 0.5 MOhm... Display the measured data in the report.
Draw conditionally in cross-section the main poles with an excitation winding and an armature with winding turns located under the poles (similar to Figure 11.10). To independently take the direction of the current in the field windings and armature. Indicate the direction of rotation of the motor under these conditions.
Rice. 11.10. DC bipolar machine:
1 - bed; 2 - anchor; 3 - main poles; 4 - excitation winding; 5 - pole pieces; 6 - armature winding; 7 - collector; F is the main magnetic flux; F is the force acting on the conductors of the armature winding.
Test questions and assignments for self-preparation
1: Explain the design and operation of the motor and DC generator.
2. Explain the purpose of the collector of DC machines.
3. Give the concept of pole division and give an expression for its definition.
4. Name the main types of windings used in DC machines, and know how to make them.
5. Indicate the main advantages of parallel excitation motors.
6.What are design features parallel winding versus series winding?
7.What is the peculiarity of starting DC motors of series excitation?
8.How many parallel branches have a simple wave and simple loop winding of DC machines?
9.How are DC machines labeled? Give an example of designation.
10. What value is the insulation resistance between the windings of DC machines and between the windings and the frame allowed?
11. What value can the current reach at the moment of starting the engine in the absence of additional resistance in the armature winding circuit?
12. What is the allowed starting current of the motor?
13. In what cases is it allowed to start a DC motor without additional resistance in the armature winding circuit?
14. How can you change the EMF of an independent excitation generator?
15. What is the purpose of the additional poles of a DC machine?
16. Under what loads is it allowed to turn on the sequential excitation motor?
17. What determines the value of the main magnetic flux?
18.Write the expressions for the EMF of the generator and the torque of the engine. Give the concept of their constituents.
LABORATORY WORK 12.
In a motor of series excitation, which is sometimes called a series motor, the excitation winding is connected in series with the armature winding (Fig. 1). For such a motor, the equality I in = I a = I is true, therefore, its magnetic flux Ф depends on the load Ф = f (I a). This is the main feature of the sequential excitation motor and it determines its properties.
Rice. 1 - Diagram of an electric motor of sequential excitation
Speed characteristic represents the dependence n = f (I a) at U = U n. It cannot be accurately expressed analytically over the entire range of load change from idle to nominal due to the absence of a direct proportional relationship between I a and F.
With an increase in the load current, the hyperbolic character of the speed characteristic is violated and approaches linear, since when the magnetic circuit of the machine is saturated with an increase in the current I a, the magnetic flux remains practically constant (Fig. 2). The slope of the characteristic depends on the value of? R.
Rice. 2 - Speed characteristics of the sequential excitation motor
Thus, the speed of a serial motor changes sharply with a change in load, and this characteristic is called "soft".
At low loads (up to 0.25 I n), the speed of the sequential excitation motor can increase to dangerous limits (the engine is "running out of gear"), so the idling of such motors is not allowed.
Moment characteristic- this is the dependence M = f (I a) at U = U n. If we assume that the magnetic circuit is not saturated, then Ф = кI a and, therefore, we have
М = с м I a Ф = с м кI a 2
This is the equation of a quadratic parabola.
The torque characteristic curve is shown in figure 3.8. As the current I a increases, the magnetic system of the motor saturates, and the characteristic gradually approaches a straight line.
Rice. 3 - Moment characteristic of the sequential excitation motor
Thus, an electric motor of sequential excitation develops a torque proportional to I a 2, which determines its main advantage. Since at start-up I a = (1.5..2) I n, the sequential excitation motor develops a significantly higher starting torque compared to parallel excitation motors, therefore it is widely used in conditions of heavy starts and with possible overloads.
Mechanical characteristic represents the dependence n = f (M) at U = U n. An analytical expression of this characteristic can be obtained only in the special case when the magnetic circuit of the machine is unsaturated and the flux Ф is proportional to the armature current I a. Then we can write
Solving the equations together, we obtain
those. the mechanical characteristic of the sequential excitation motor, as well as the high-speed one, has a hyperbolic character (Fig. 4).
Rice. 4 - Mechanical characteristics of the series excitation motor
Efficiency characteristic of the sequential excitation motor has the form (), usual for electric motors.
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, at idle is not allowed, 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 ( a), shunting the anchor ( b) and the inclusion of resistance in the armature circuit ( v) |
Note that with an increase in the rotation speed, 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, 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, a), or by a decrease in the number of turns of the excitation winding included in the operation. 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 ) specifications 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 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, v). 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 rotation speed |
Speed regulation 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 each drive axle is equipped with separate engine and regulation is carried out by switching 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.
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.
Independent excitation circuit
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 field 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.
Parallel excitation circuit
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 excitation circuit is used in electrified vehicles.
Sequential excitation circuit
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.
Mixed excitation scheme
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
33. Characteristic dpt with independent excitation.
DC motor of independent excitation (DC motor NV) In this motor (Figure 1), the excitation winding is connected to a separate power source. An adjustment 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 DCP NV is its excitation current I in independent of armature current I i since the power supply of the excitation winding is independent.
Diagram of a DC motor of independent excitation (DPT NV)
Picture 1
Mechanical characteristic of a DC motor of independent excitation (dpt NV)
The equation of the mechanical characteristics of a DC motor of independent excitation has the form
where: n 0 - engine 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 direct current motor of independent excitation (DCM NV) are rectilinear and intersect the ordinate at the idle point n 0 (Figure 13.13 a), while the change in engine speed Δn, due to a change in its mechanical load, in proportion 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 the nominal voltage values on the armature and field windings and in the absence of additional resistances in the armature circuit, are called natural(graph 7).
If at least one of the listed engine parameters is changed (the voltage on the armature or excitation windings differ 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 R add into the armature circuit are also called rheostat (graphs 7, 2 and 3).
When assessing the control properties of DC motors, the mechanical characteristics are of greatest importance. n = f (M)... At a constant moment of load on the motor shaft with an increase in the resistance of the resistor Rext the speed decreases. Resistor resistance Rext to obtain an artificial mechanical characteristic corresponding to the required rotational speed n at a given load (usually nominal) for independent excitation motors:
where U is the supply voltage of the motor armature circuit, V; I I - armature current corresponding to a given load of the motor, A; n is the required speed, rpm; n 0 - idle speed, rpm.
The idle speed n 0 is the boundary speed, when exceeded, the engine goes into the generator mode. This speed exceeds the nominal nnom as much as the nominal voltage U nom supplied to the armature circuit exceeds the armature EMF Ei'm nom at rated motor load.
The shape of the mechanical characteristics of the engine is influenced by the magnitude of the main magnetic field of excitation. F... When decreasing F(with an increase in the resistance of the resistor r peg), the engine idle speed 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 we change the voltage on the armature winding U (with constant R ext and R reg), then n 0 changes, and Δn remains unchanged [see. (13.10)]. As a result, the mechanical characteristics shift along the ordinate, remaining parallel to each other (Fig. 13.13, c). This creates the most favorable conditions for regulating the speed of motors by changing the voltage. U supplied to the anchor chain. This method of speed control is most widely used due to the development and widespread use of adjustable thyristor voltage converters.
In the considered motors, the field winding is made with a small number of turns, but is designed for high currents. All the features of these motors are associated with the fact that the excitation winding turns on (see Fig.5.2, v) in series with the armature winding, as a result of which the excitation current is equal to the armature current and the generated flux Ф is proportional to the armature current:
where a= / (/ i) - non-linear coefficient (Fig.5.12).
Non-linearity a associated with the shape of the motor magnetization curve and the demagnetizing effect of the armature reaction. These factors appear when / i>, / yang (/ yang - rated armature current). At lower currents a can be considered a constant value, and when / I> 2 / I n the motor is saturated and the flux depends little on the armature current.
Rice. 5.12.
The basic equations of the sequential excitation motor, in contrast to the equations of independent excitation motors, are nonlinear, which is associated, first of all, with the product of variables:
When the current in the armature circuit changes, the magnetic flux Ф changes, inducing eddy currents in the massive parts of the magnetic circuit of the machine. The effect of eddy currents can be taken into account in the motor model in the form of an equivalent short-circuited loop described by the equation
and the equation for the armature chain is:
where w B, w B t - the number of turns of the excitation winding and the equivalent number of turns of eddy currents.
In steady state
From (5.22) and (5.26) we obtain expressions for the mechanical and electromechanical characteristics of a DC motor of sequential excitation:
As a first approximation, the mechanical characteristic of a sequential excitation motor, without taking into account the saturation of the magnetic circuit, can be represented as a hyperbola that does not cross the ordinate axis. If we put L i q = /? i + /? в = 0, then the characteristic will not cross the abscissa axis either. This characteristic is called perfect. The real natural characteristic of the engine crosses the abscissa axis and, due to saturation of the magnetic circuit, at torques greater M n straightens (fig.5.13).
Rice. 5.13.
A characteristic feature of the characteristics of a sequential excitation motor is the absence of a perfect idle point. When the load decreases, the speed increases, which can lead to uncontrolled acceleration of the engine. You cannot leave such an engine without load.
An important advantage of series excitation motors is their high overload capacity at low speeds. With an overcurrent of 2-2.5 times, the motor develops a torque of 3.0 ... 3.5 M n. This circumstance determined the widespread use of sequential excitation motors as a drive for electric Vehicle, for which maximum moments are required when starting off.
Reversing the direction of rotation of series field motors cannot be achieved by reversing the polarity of the armature circuit supply. In motors of sequential excitation, when reversing, it is necessary to change the direction of the current in one part of the armature circuit: either in the armature winding or in the excitation winding (Fig.5.14).
Rice. 5.14.
Artificial mechanical characteristics for speed and torque control can be obtained in three ways:
- the introduction of additional resistance in the armature circuit of the engine;
- a change in the voltage supplying the motor;
- by shunting the armature winding with additional resistance. When additional resistance is introduced into the armature circuit, the rigidity of the mechanical characteristics decreases and the starting torque decreases. This method is used when starting motors of sequential excitation, receiving power from sources with unregulated voltage (from contact wires, etc.) In this case (Fig.5.15), the required starting torque is achieved by sequentially short-circuiting the starting resistor sections by means of K1-KZ contactors.
Rice. 5.15. Rheostat mechanical characteristics of the sequential excitation motor: /? 1do - R iao- resistance of steps of the additional resistor in the armature circuit
The most economical way to control the speed of a series-excited motor is to change the supply voltage. The mechanical characteristics of the engine are shifted down parallel to the natural characteristic (Fig. 5.16). In form, these characteristics are similar to rheostat mechanical characteristics (see Fig. 5.15), however, there is a fundamental difference - when regulating by changing the voltage, there are no losses in the additional resistors and the regulation is carried out smoothly.
Rice. 5.1
Series field motors, when used as a drive for mobile units, are in many cases powered by an overhead catenary or other power source with a constant voltage value supplied to the motor, in which case the regulation is carried out by means of a pulse-width voltage regulator (see § 3.4). Such a scheme is shown in Fig. 5.17.
Rice. 5.17.
Independent regulation of the excitation flux of a series excitation motor is possible if the armature winding is shunted with a resistance (Fig. 5.18, a). In this case, the excitation current w = i + / w, i.e. contains a constant component that does not depend on the motor load. In this case, the engine acquires the properties of a mixed excitation engine. Mechanical characteristics (Fig. 5.18.6) acquire greater rigidity and intersect the ordinate axis, which makes it possible to obtain a stable reduced speed at low loads on the motor shaft. A significant drawback of the circuit is the large energy loss in the shunt resistance.
Rice. 5.18.
DC motors with series excitation are characterized by two braking modes: dynamic braking and opposition.
Dynamic braking is possible in two cases. In the first, the armature winding is closed to a resistance, and the excitation winding is powered from the network or another source through an additional resistance. The characteristics of the motor in this case are similar to the characteristics of an independent excitation motor in the dynamic braking mode, (see Fig. 5.9).
In the second case, the diagram of which is shown in Fig. 5.19, when the KM contacts are disconnected and the KV contacts are closed, the engine works as a self-excited generator. When switching from the motor mode to the brake mode, it is necessary to maintain the direction of the current in the excitation winding in order to avoid demagnetization of the machine, since in this case the machine goes into self-excitation mode. The mechanical characteristics of this mode are shown in Fig. 5.20. There is a boundary velocity ω φ, below which self-excitation of the machine does not occur.
Figure 5.19.
Rice. 5.20.
In the opposing mode, an additional resistance is included in the armature circuit. In fig. 5.21 shows the mechanical characteristics of the motor for two opposing options. Characteristic 1 is obtained if when the engine is running in the forward direction B (point With) change the direction of the current in the field winding and introduce additional resistance into the armature circuit. The engine goes into the opposite mode (point a) with braking torque M brake.
Figure 5.21.
If the drive is running in mode of lowering the load, when the task of the drive is to slow down the lifting mechanism when working in the "backward" direction H, then the engine is turned on in the "forward" direction B, but with a large additional resistance in the armature circuit. The work of the drive corresponds to the point b on the mechanical characteristic 2. Operation in the counter-switching mode is associated with large energy losses.
The dynamic characteristics of a DC motor of sequential excitation are described by the system of equations arising from (5.22), (5.23), (5.25) in the transition to the operator form of writing:
In the structural diagram (Fig.5.22), the coefficient a= D / i) reflects the saturation curve of the machine (see Fig. 5.12). We neglect the influence of eddy currents.
Rice. 5.22.
It is rather difficult to determine the transfer functions of a sequential excitation motor analytically, therefore, the analysis of transient processes is carried out by the method of computer simulation based on the circuit shown in Fig. 5.22.
Mixed-field DC motors have two field windings: independent and consistent. As a result, their static and dynamic characteristics combine the characteristic properties of the two previously considered types of DC motors. To which of the types a particular motor of mixed excitation belongs more depends on the ratio of the magnetizing forces created by each of the windings: ...
The initial equations of the mixed excitation motor:
where in, R B,w b - current, resistance and the number of turns of the independent excitation winding; L m - mutual inductance of the field windings.
Steady State Equations:
Whence the equation of the electromechanical characteristic can be written in the form:
In most cases, the sequential excitation winding is performed at 30 ... 40% of the MD C, then the ideal idle speed exceeds the rated motor speed by about 1.5 times.