The principle of operation of the generator is based on the phenomenon of electromagnetic induction. If a frame with active conductors ab and cd (Fig. 3.1, a) rotates in the field of permanent magnets NS, then according to the law of electromagnetic induction, an EMF occurs in the conductors ab and cd:

where B is induction magnetic field;

1 - the length of the active conductor;

V - circumferential speed of the conductor;

sin α - the angle between the direction of the magnetic lines of force and the direction of movement of the conductor at the considered moment of time.

Rice. 3.1. The principle of the generator direct current

If the ends of the conductors are connected to the rings and from them through the brushes 1 and 2 power the load circuit of the lamp Rn, then when the knife switch P is closed, the current I H will flow through the circuit, also changing according to a sinusoidal law, i.e. alternating current. To rectify this variable EMF, we connect the conductors ab and cd not to the rings, but to the half rings (Fig. 3.1, b). Brushes 1 and 2 are installed in such a way that they move from one half-ring to another at the moment when there is no EMF in the conductors of the frame (the frame is rotated 90 ° relative to the longitudinal axis of the poles, i.e. located along transverse axis poles). In this case, the EMF of one direction is applied to brushes 1 and 2 during a complete revolution of the frame, although in the conductors ab and cd the EMF is still variable.

Under the action of an EMF in one direction, a current of 1 V will flow through the load circuit, in one direction, but pulsating. Brush 2, from which the current flows into the external circuit (load), is considered positive ("positive"), and brush 1, to which the current flows, is considered negative ("minus").

Thus, the use of half rings instead of rings made it possible to obtain a current in one direction in the load circuit, although a variable EMF occurs in the conductors of the frame, i.e. half rings are a mechanical rectifier. To reduce the ripple of the rectified current and obtain great importance EMF on brushes 1 and 2 of the DC generator, a large number of plates are used, located on the collector, and a large number of active armature conductors.

In real DC generators, the magnetic field is created not by permanent magnets, but by excitation windings located on the cores of the poles. A magnetic field with a flux F (Fig. 3.2) is created due to the flow of current Ib, in the excitation winding W B. In undercar generators, the winding is connected in parallel with the armature winding I - to brushes 1 and 2.

Fig.3.2. Wiring diagram DC generator

with parallel excitation

Due to the residual magnetization of the pole cores, the generator always has a small magnetic field (magnetic flux). As the car moves, the armature rotates in this weak magnetic field. Under its action, an EMF arises in the conductors of the armature winding, so that a small EMF rectified by the collector appears on the brushes, under the action of which an excitation current will flow through the excitation winding. The excitation current will cause the appearance of a magnetic flux, which is of greater importance than the flux of residual magnetism, therefore, an EMF of a larger magnitude occurs on the brushes: E=C E nF, where C E is the design coefficient of the generator; n - armature speed, rpm; Ф - magnetic flux created by the excitation windings.

A large EMF will cause an increase in the excitation current (according to Ohm's law I B \u003d E / r B, where r B is the resistance of the excitation winding, which will lead to a further increase in EMF, etc. The generator self-excites. When the switch R is closed under the action of EMF through a resistor Rн, the load current will flow, which will cause a voltage drop across the resistance r V of the armature winding, equal to I r I. This means that the voltage on brushes 1 and 2 will be less than the EMF by the value of this voltage drop, i.e.

U \u003d E - I r I, or U \u003d C E nФ - I r I.

It follows from the last formula that the voltage depends on the generator speed, i.e. wagon speed; from the magnetic flux created by the excitation windings, which in turn depends on the excitation current; from the load current of the generator (the greater the load current, the lower the voltage).

1. General information

DC generators are used in power plants as sources electrical energy. When the generator is running, its armature is driven by a drive motor, and a direct current is supplied to the excitation winding to create the main magnetic flux. As a result, an EMF is induced in the armature winding of the generator E=CwF and a consumer of electrical energy (load) can be connected to its outputs.

Depending on the method of supplying the excitation windings, generators with independent excitation and self-excitation are separated.

In a generator with independent excitation, the excitation winding located on the main poles is fed with current 1 AT from an external source of direct current that does not have an electrical connection with the armature winding. Low power generators can be independently excited by permanent magnets. In a self-excited generator, the excitation winding is powered from the terminals of the generator armature circuit. Depending on the connection scheme of the excitation winding, generators are distinguished with parallel, serial and mixed excitation. For generators with parallel excitation, the excitation winding is connected in parallel with the armature winding and the load; with serial excitation - in series with the armature winding and the load. Generators with mixed excitation have two excitation windings on the main poles, through which excitation currents flow 1 AT and I v2. One of them is connected in parallel with the armature winding, and the other in series with it.

For electromagnetic excitation of generators, 0.3 ... 5% of their rated power is consumed. Independent excitation is used in high power generators, as well as in low voltage generators. The sequential excitation scheme in generators is practically not used. Schematic diagrams of DC generators with different excitation systems are shown in Figure 4.1. Designations of the beginning and end of the windings according to GOST: armature winding - I1, I2; winding of additional poles -D1, D2; compensation winding - K1, K2; excitation winding independent - M1, M2; excitation winding parallel (shunt) - SH1, SH2; excitation winding serial (serial) - CI, C2.

In the idling mode of the generator, an insignificant moment of the primary engine is applied to its shaft M 1 overcoming moment of the generator M 0 , due to braking torques arising during its operation from friction forces, eddy currents in

armature and other electromagnetic phenomena. When connected to the terminals of the load resistance armature circuit RH current I will flow in the armature winding, from the interaction of which with the magnetic field of excitation a braking electromagnetic torque is created M=SFI, also overcome by the prime mover. The total energy balance in a self-excited generator can be represented as

where - ventilation and mechanical power losses due to friction; - magnetic losses (for hysteresis and eddy currents); - additional losses; - power loss for excitation.

Generator efficiency is the ratio of useful power R 2 , given by the generator to the load, to the mechanical power R 1 , connected to the generator,

where - sum of power losses .

§ 111. METHODS OF EXCITATION OF DC GENERATORS

DC generators can be made with magnetic and electromagnetic excitation. To create a magnetic flux in generators of the first type, permanent magnets are used,

and in generators of the second type - electromagnets. Permanent, magnets are used only in machines of very low power. Thus, electromagnetic excitation is the most widely used method for creating magnetic flux. With this method of excitation, the magnetic flux is created by the current flowing through the excitation winding.

Depending on the method of supplying the excitation winding, DC generators can be independently excited and self-excited.

With independent excitation (Fig. 143, a), the excitation winding is connected to the network of an auxiliary DC energy source. To regulate the excitation current Iv, resistance r p is included in the winding circuit. With such excitation, the current Iv does not depend on the current in the armature Ia.

The disadvantage of generators independent excitation is the need for an additional source of energy. Despite the fact that this source usually has a low power (a few percent of the power of generators), the need for it is a great inconvenience, so independent excitation generators find very limited use only in machines. high voltage, in which the supply of the excitation winding from the armature circuit is unacceptable for design reasons.

Self-excited generators, depending on the inclusion of the excitation winding, can be parallel (Fig. 143, b), series (Fig. 143, c) and mixed (Fig. 143, d) excitation.

For parallel excitation generators, the current is small (a few percent rated current armature), and the excitation winding has a large number of turns. With series excitation, the excitation current is equal to the armature current and the excitation winding has a small number of turns.

With mixed excitation, two excitation windings are placed on the poles of the generator - parallel and series.

The process of self-excitation of DC generators proceeds in the same way for any excitation scheme. For example, in generators parallel excitation, which have received the widest application, the process of self-excitation proceeds as follows.

Any prime mover rotates the armature of the generator, the magnetic circuit (yoke and cores of the poles) which has a small residual magnetic flux F 0 . This magnetic flux in the winding of the rotating armature is induced e. d.s. E 0 , which is a few percent of the rated voltage of the machine.

Under the influence of e. d.s. E 0 in a closed circuit consisting of an armature and an excitation winding, a current Iv flows. The magnetizing force of the excitation winding Ivw (w is the number of turns) is directed in accordance with the flow of residual magnetism, increasing the magnetic flux of the machine F, which causes an increase in both e. d.s. in the armature winding E, and the current in the excitation winding Iv. An increase in the latter causes a further increase in F, which in turn increases E and Iv.

Due to the saturation of the steel of the magnetic circuit of the machine, self-excitation does not occur indefinitely, but up to a certain voltage, depending on the speed of rotation of the armature of the machine and the resistance in the excitation winding circuit. When the steel of the magnetic circuit is saturated, the increase in the magnetic flux slows down and the process of self-excitation ends. Increasing the resistance in the excitation winding circuit reduces both the current in it and the magnetic flux excited by this current. Therefore, the emf decreases. With. and the voltage to which the generator is excited.

Changing the rotation speed of the generator armature causes a change in emf. s, which is proportional to the speed, as a result of which the voltage to which the generator is excited also changes.

Self-excitation of the generator will occur only under certain conditions, which are as follows:

1. >Presence of residual magnetism flow. In the absence of this flow, e will not be created. d.s. E 0, under the influence of which a current begins to flow in the excitation winding, so that the excitation of the generator will be impossible. If the machine is demagnetized and has no residual magnetization, then a direct current must be passed through the excitation winding from some extraneous source of electrical energy. After turning off the excitation winding, the machine will again have a residual magnetic flux.

2. The excitation winding must be connected in accordance with the flow of residual magnetism, i.e. so that the magnetizing force of this winding increases the flow of residual magnetism.

When the excitation winding is turned on in the opposite direction, its magnetizing force will reduce the residual magnetic flux and, during prolonged operation, can completely demagnetize the machine. If the excitation winding turned out to be turned on in the opposite direction, then it is necessary to change the direction of the current in it, i.e., swap the wires suitable for the terminals of this winding.

3. The resistance of the excitation winding circuit must be excessively large; with a very high resistance of the excitation circuit, self-excitation of the generator is impossible.

4. The resistance of the external load must be large, since with a low resistance, the excitation current will also be small and self-excitation will not occur.

11. DC generator with parallel excitation: operating principle, self-excitation conditions, characteristics.

Shunt excitation generator. In this generator (Fig. 8.47, a) the excitation winding is connected through an adjusting rheostat in parallel with the load. Consequently, in this In this case, the self-excitation principle is used, in which the excitation winding is powered directly from the generator armature winding. Self-excitation of the generator is possible only under certain conditions. To establish them, consider the process of changing the current in the circuit "field winding - armature winding" in the idle mode. For the circuit under consideration, we obtain the equation

where e and i c - instantaneous values ​​of EMF in the armature winding and excitation current; Σ R in = R in + R r.v - total resistance of the generator excitation circuit (resistance Σ R and can be neglected, since it is much less than Σ R in); L c is the total inductance of the excitation and armature windings. All terms included in (8.59) can be depicted graphically (Fig. 8.47, b). EMF e at some value i in the excitation current can be determined by the characteristic OA idling of the generator, and the voltage drop i in Σ R c - according to the current-voltage characteristic OV its excitation circuits. Characteristic OV is a straight line passing through the origin at an angle y to the x-axis; wherein tg γ= Σ R in. From (8.59) we have

Therefore, if the difference ( e - i in Σ R c) > 0, then the derivative di in / dt> 0, and there is a process of increasing the excitation current i in.

The steady state in the excitation winding circuit is observed when di in / dt= 0, i.e. at the point of intersection FROM idle characteristics OA with a straight line OV. In this case, the machine operates with a certain steady excitation current I v0 and emf E 0 = U 0 .

From equation (8.60) it follows that for the self-excitation of the generator, certain conditions must be met:

1) the self-excitation process can start only if at the initial moment ( i c \u003d 0) some initial EMF is induced in the armature winding. Such an EMF can be created by a flow of residual magnetism, therefore, to start the process of self-excitation, it is necessary that the generator has a flow of residual magnetism, which, when the armature rotates, induces an EMF in its winding E rest. Usually there is a flow of residual magnetism in the machine due to the presence of hysteresis in its magnetic system. If there is no such flow, then it is created by passing a current from an external source through the excitation winding;

2) during the passage of current i in the winding of its excitation MDS F in must be directed according to the MMF of residual magnetism F Oct. In this case, under the action of the difference e - i in Σ R in the process of increasing current i c, excitation magnetic flux F c and EMF e. If these MMFs are directed oppositely, then the MMF of the excitation winding creates a flow directed against the flow of residual magnetism, the machine is demagnetized and the self-excitation process will not be able to start;

3) positive difference e - i in Σ R c, necessary to increase the excitation current i from zero to steady state I v0, can occur only if in the specified range of current change i in a straight line OB located below the idle speed characteristic OA. With an increase in the resistance of the excitation circuit Σ R the angle of inclination increases γ straight OB to the current axis I in and at some critical value of the angle γ cr (corresponding to the critical resistance value Σ R c.cr) straight OV" practically coincides with the rectilinear part of the idling characteristic. In this case e≈ i in Σ R in and the process of self-excitation becomes impossible. Consequently, for self-excitation of the generator, it is necessary that the resistance of the excitation circuit be less than the critical value.

If the parameters of the excitation circuit are chosen so that Σ R in< ΣR v.cr, then at the point FROM stability of the self-excitation mode is ensured. With an accidental decrease in current i at below steady state I in 0 or increase it over I in0, a positive or negative difference arises, respectively ( e - i in Σ R c), seeking to change the current i in so that it becomes equal again I in0 . However, for Σ R c > Σ R c.cr stability of the self-excitation mode is violated. If, during the operation of the generator, the resistance of the excitation circuit is increased Σ R in up to a value greater than Σ R v.cr, then its magnetic system is demagnetized and the EMF decreases to E rest. If the generator started to work at Σ R c > Σ R v.kr, then he will not be able to self-excite. Consequently, conditionΣ R in< ΣR c.cr limits the possible range of regulation of the generator excitation current and its voltage. It is usually possible to decrease the generator voltage by increasing the resistance Σ R c, only up to (0.6-0.7) U nom. External characteristic of the generator is a dependency U=f(I m) at n= const and R in = const (curve 1, rice. 8.48). It is located below the external characteristic of the generator with independent excitation (curve 2). This is due to the fact that in the considered generator except for two reasons that cause a decrease in voltage with increasing

load (voltage drop in the armature and the demagnetizing effect of the armature reaction), there is a third reason - a decrease in the excitation current I in = U/Σ R in, which depends on the voltage U, i.e. on the current I n.

The generator can only be loaded up to a certain maximum current I cr. With a further decrease in load resistance R n current I n = U/R n begins to decrease, as the voltage U falling faster than decreasing R n. Work on the site ab external characteristics are unstable; in this case, the machine switches to the operating mode corresponding to the point b, i.e. into the mode short circuit.

The action of the causes that cause a decrease in the generator voltage with increasing load is especially clearly seen from the consideration of Fig. 8.49, which shows the construction of an external characteristic according to the idling characteristic and the characteristic triangle.

The construction is carried out in the following order. Through the dot D on the ordinate axis corresponding to the rated voltage, a straight line is drawn parallel to the abscissa axis. The vertex is located on this line. BUT characteristic triangle corresponding to the rated load; leg AB must be parallel to the y-axis, and the vertex FROM must lie on the idling characteristic 1. Through origin and vertex BUT direct 2 to the intersection with the idling characteristic; this straight line is the current-voltage characteristic of the resistance of the excitation winding circuit. On the ordinate of the point of intersection E characteristics 1 and 2 get generator voltage U 0 = E 0 at idle.

Excitation current I in.nom at nominal mode corresponds to the abscissa of the point BUT, and generator EMF E nom at rated load - the ordinate of the point AT. It can be determined from the idling characteristic if the excitation current is reduced I v.nom by the length of the segment sun, taking into account the demagnetizing effect of the armature reaction. When constructing an external characteristic 3 her points a and b, corresponding to no-load and rated load, are determined by voltages U 0 and U nom. intermediate points With, d,... receive by spending

straight A"C", A"C", A""C"",..., parallel to the hypotenuse AC, before crossing with the current-voltage characteristic 2 at points A", A", A"",..., and also with the characteristic of idling 1 at points C", C", C"",.... Ordinates of points A "A" A "",... correspond to voltages at load currents I a1 , I a2 , I a3 ,..., whose values ​​are determined from the relation

I a nom: I a 1:I a 2 ,Ia 3… = AC: A"C": A"C":A""C"" ...

When switching from the rated load mode to the idle mode, the generator voltage changes by 10 - 20%, i.e. more than in a generator with independent excitation.

With a steady short circuit of the armature, the current I to the generator with parallel excitation is relatively small (see Fig. 8.48), since in this mode the voltage and excitation current are zero. Therefore, the current to. only EMF is created from residual magnetism and is (0.4 - 0.8) I nom. The control and load characteristics of a generator with parallel excitation are of the same nature as those of a generator with independent excitation.

Most of the DC generators produced by the domestic industry have parallel excitation. To improve external performance, they usually have a small series winding (one to three turns per pole). If necessary, such generators can also be switched on according to a scheme with independent excitation.

The excitation of the generator is the creation of a working magnetic flux, due to which an EMF is created in the rotating armature. DC generators, depending on the method of connecting the excitation windings, are distinguished, independent, parallel, series and mixed excitation. The independent excitation generator has an excitation winding OB, connected to an external current source through an adjusting rheostat (Figure 6-10, a) The voltage at the terminals of such a generator ( curve 1 in Fig. 6-11) with increasing load current decreases slightly as a result of the voltage drop across internal resistance anchors, and the voltages are always stable. This property turns out to be very valuable in electrochemistry (powering electrolytic baths)

The parallel excitation generator is a self-excited generator; the excitation winding of the OB is connected through an adjusting rheostat to the terminals of the same generator (Fig. 6-10, b). Such an inclusion leads to the fact that with an increase in the load current, the voltage at the generator terminals decreases due to the voltage drop across the armature winding. This, in turn,

causes a decrease in the excitation current and EMF in the armature. Therefore, the voltage at the terminals of the UH generator decreases somewhat faster (curve 2 in Fig. 6-11) than that of an independent excitation generator.

A further increase in load leads to such a strong decrease in the excitation current that when the load circuit is short-circuited, the voltage drops to zero (a small short-circuit current is due only to residual induction in the machine). Therefore, it is believed that the parallel excitation generator is not afraid of a short circuit.

The sequential excitation generator has an OB excitation winding connected in series with the armature (Fig. 6-10, e). In the absence of a load in the armature, a small EMF is nevertheless excited due to the residual induction in the machine (curve 3 in Fig. 6-11). With an increase in load, the voltage at the generator terminals first increases, and after reaching the magnetic saturation of the magnetic system of the machine, it begins to decrease rapidly due to the voltage drop across the armature resistance and due to the demagnetizing effect of the armature reaction.

Due to the large variability of the voltage with a change in load, series-excited generators are not currently used.

The mixed excitation generator has two windings: OB - connected in parallel to the armature, (additional) - in series (Fig. 6-10, d). The windings are switched on so that they create magnetic fluxes in one direction, and the number of turns in the windings is chosen so that the voltage drop on the internal resistance of the generator and the EMF of the armature reaction would be compensated by the EMF from the parallel winding flux.