Basic theoretical information. Operation of generators on AC is impossible without regulation of voltage since the flight can vary their load and rotation speed of driving devices

Operation of generators on AC is impossible without regulation of voltage since the flight can vary their load and rotation speed of driving devices. In power systems of DC current the coal regulators are used to stabilize the voltage.

In Alternating Current systems, thyristor and transistor regulators are used. To control the voltage of aviation generators the coal regulators (ВР) of type РН-180, РН-400Б, РН-600Б and so on are applied. Any BP consists of two basic nodes (Fig. 2.1): electromagnet (EM) and coal column (ВС). From the elektrokinetic scheme (fig.2.1, a) it is seen that the node of electromagnet consists of a core 1, the working winding WP, an anchor 2 and a spring 3. а б

Fig. 2.1

The node BC consists of coal column 4 with the coal contacts 5. Coal column, consisting of thin washers from refined coal that is contained in the conductive tube is pressed into the ribbed casing. BC connected in series with the winding of excitation of the generator Г, is the executive element of the system of voltage regulation. Working winding W _P of the regulator controls the voltage of the generator and is a sensitive element of the system. Fig. 2.1, б shows the curves of electrical resistance of carbon column (R _y) and the magnitude of its deformation (D x) from the compressive force (F).

In the area of small pressure (F) a slight change of force of pressure leads to the significant change in the resistance of ВС and unstable operation of the regulator. In the area of great pressure the regulator sensitivity is very low. So for operation of BP the medium range pressure changes in BC are selected.

In the steady state of the generator's operation at its clams the given voltage is maintained and the pulling power EM is balanced with the mechanical force (F_ел = F_м). However, under the notion " mechanical force" we mean resultant force of the spring F_пр and the forces of reaction of ВС: F_м = F_пр – F_c.

Changing the mode operation of the generator may increase or decrease the voltage at its clams. If the voltage is increased, the current increases in the working winding of the regulator and it increases the pulling power of ЕМ F_ел. Because of this difference of forces

F_ел – F_м the anchor moves towards the core EM and the gap between them is diminished.Moving the anchor tends the reduction of pressure on BC, increasing of its resistance, reducing the current of excitation, therefore, the generator's voltage return to the original setting takes place.

In the steady state mode of the generator's operation the power are balanced. The anchor of the regulator takes a new position at a smaller gap and the balance of powers is characterized with the increased electromagnetic and mechanical forces.

The basic condition that affects the accuracy of voltage regulation is coordination of mechanical and electromagnetic characteristics of BP.

Electromagnetic characteristic is called the dependence of pulling power EM F_ел from the size of the gap between the core EM and the anchor at a constant value of the current i_е in the working winding, ie F_ел = f (d) at

i_е = const.

Mechanical characteristics is the dependence of the mechanical force F_м from the size of the gap: F_м = f (d).

Fig. 2.2

BP is made with a greater inclination of the mechanical characteristics than of the electromagnetic ones (Fig. 2.2).

In this case, the regulatory process takes place with positive statics. At the same time, it increases the stability of parallel operation of generators and even distribution of load between them.

Fig. 2.3 shows the external characteristics 1 and 2 that meet the regulatory processes with positive and negative statics. In case of coincidence of mechanical and electromagnetic characteristics the unstable regulatory process occurs (curve 3 (Fig. 2.3).

Fig. 2.3

Improving the accuracy of operation of voltage regulators In terms of operation this rate is affected with the temperature regulator, mechanical hysteresis of the magnetic material, some differences of mechanical and electromagnetic characteristics of the regulator from the calculated ones. To reduce the influence of mechanical and magnetic hysteresis under control the materials with the most stable characteristics are used in the designing.

To reduce errors in regulation due to the influence of temperature and differences in the mechanical and electromagnetic characteristics they apply special compensating elements.

Regulators of voltage work in a wide range of changes of the ambient temperature. Then the heating of their parts can be up to 200 ° C. Especially in severe temperature conditions the coal column works. The total error of voltage regulation under influence of temperature conditions without the special actions is 5... 8 V. This is explained with at the fluctuations in temperature of heating of the regulator the resistance of the working winding, the stiffness of the spring of the anchor are changed, the primary power of compression of the coal column due to differences in expansion and contraction of individual parts of the regulator.

To reduce the effect of temperature on accuracy of voltage adjustment the following methods of temperature compensation are used:

– structural;

– Including of the temperature compensation in the circuit of regulators resistance and winding;

- Blowing of the regulator with the outboard air.

To the structural techniques of temperature compensation belong: usage of the ribbed casing for heat dissipation from the ВС; Insulating screen between the case and EM; usage of special mounting of casing of the regulator.

Casing of the regulator is attached with steel pins to the shaped ring. In this case, the temperature increasing mainly entails the regulator's casing expansion in the direction of EM as moving in the opposite direction is limited by steel pins (temperature coefficient of expansion of steel is small).

The resistance of temperature compensation R _T is included in series with the working winding of the regulator W _P (Fig. 2.4), and the winding of temperature compensation W _TK is connected to a voltage generator. Resistance R _T is made from constantan, which has a small temperature coefficient of resistance. The value of this resistance exceeds the value of the resistance of the working winding in several times. Fig. 2.4

Consequently the change in resistance of the working winding under the influence of temperature changes the effect on the total resistance of the whole range of working winding less. The dependence of the resistance of metals from the temperature is determined by the expression:

where – initial resistance of the conductor; – temperature coefficient of the resistance of the conductor; – increment, of temperature of heating.

If there is resistance the resistance of the entire working winding varies less at the same changing of the temperature of heating :

Switching on of the thermos-compensational resistance is equivalent to the reduction of temperature coefficient of circuit's resistance in several times.

Winding of temperature compensation is made from copper and is wounded on a core with a working winding .

At that time the magnetomotive force (МРС) of the winding of temperature compensation is directed towards МРС of the working winding , and consists of 0... 20% of its value. When the temperature of the windings EM is changed, their resistance is changed too, and, consequently, the value of МРС is change as well, because the resulting МРС will change slightly. The most fully compensation is in compliance with the condition:

,

where and are the number of spring coils of working winding and temperature compensation winding respectively; and is the resistance of working winding and temperature compensation winding at 20°C; is the resistance of temperature compensation.

In regulators which are designed to operate with generators of high capacity (i.e. in РУГ-82) at high loads and low speeds there is a " blockage" of external characteristics (Fig. 2.5, б). This phenomenon is explained due to a large mismatching between the mechanical and electromagnetic characteristics at the presence of large gaps between the core and the anchor. To maintain the voltage at a given level at high loads and low speeds of rotation of the generator further increasing of the force of contraction ВС should be done. This task performs automatic winding correction.

а б

Fig. 2.5

Winding of automatic correction is included to the diagonal of bridge, whose arms are resistance of correction , winding of the generator's and BC excitation (Fig. 2.5, a). In the circuit of winding of the automatic correction rectifier В is included. The winding of automatic correction is wounded on the same core with a working winding of the regulator.

When setting scheme of automatic correction the slider of impedance at rated load on the generator is set so that the correction was operated at a speed of anchor rotation, beginning with 5000 rev / min. And the current through the winding is directed from point A to point C, because of increasing load of the generator and a decreasing in its speed of rotation, the ВС is compressed and the potential of point A is increased relatively to the potential of the point C. МРС of the winding of automatic correction is directed towards МРС of the working winding, so pulling power of EM is reduced, preventing the generator voltage reduction.

For large values of resistance BC, when the rotational speed of the generator's anchors is more than 5, 000 rev / min and the load does not exceed the nominal, the potential of point A appears to be below the potential of the point C, but the valve B prevents the leakage of current of correction from point C to A and unnecessary lowering voltage is not going to occur.

Stableness of voltage regulators operation and techniques of its increasing Voltage regulation systems using BP have insufficient stability in case of abrupt change in mode of operation of generators.

Setting of BP to operate with a positive statics improves the quality of transient process at voltage adjustment. But to improve the stability of the system we must use special stabilizing devices, which are: stabilizing resistances and transformers. Stabilizing effect of these devices is that they create additional current in the coil EM by which the forces occur that slow down the movement of EM anchor at its approximation to equilibrium. Fig. 2.4 shows the scheme of the coal regulator Р-25А with stabilizing resistance and stabilizing transformer. The stabilizing resistance is involved in diagonal bridge, which arms are resistances of BC, windings of excitation, windings EM and the resistance of temperature compensation and adjusting resistance .

In transient modes of the generator the current flows through the resistance , the magnitude and direction of which depends on the potential difference of points A and B. As resistances of all elements of the bridge (excepting ВС) are constant, so the potential of point B does not change, and the potential of point A varies with the resistance ВС, since it depends on the mode of the generator.

In the steady state of generator's operation the stabilizing resistance as a harsh negative reverse connection, gives inaccuracy in the voltage adjustment (fig. 2.6). Thus, at the maximum rotational speed of the generator and with the minimum load of ВС regulator it has a great resistance and current that is directed from point B to point A through resistance (fig.2.4), which helps to increase the voltage of the generator. Fig. 2.6

Adjustment of ВР. In the process of adjustment the voltage regulators reach the best possible coordination of electromagnetic and mechanical characteristics in range of operation of air gaps. Regulators are adjusted in the process of repairing or designing at the factory. Tuning up is provided by changing the initial pressing the ВС (with adjusting screw) and changing the magnitude of the air gap (with the core EM). Before setting of the regulator it is a must:

- To put the slider of the deferent resistance in the middle position;

- To unscrew the adjusting screw with a coal contact completely;

- To set the core of electromagnet to make it come forward the cap of the electromagnet in one-two strands of thread. At the same time graduations on the butt end of the core and ones on the cap must match. If the regulator was not taken apart, it is inappropriate to pre rotate the electromagnet’s core.

In the process of adjustment is changed according to the curve (fig. 2.7). Fig. 2.7

To the point A the voltage increases slowly, since at the time screwing the adjusting screw gap between coal washers is selected and the voltage value is conditioned mainly with residual magnetism of the generator.

On the area AB the resistance ВС decreases sharply. On the area AC the axial movement of the adjusting screw corresponds approximately to the same movement of anchor to the direction of the core under the effect of electromagnetic forces. On the area CD the movement of anchor forestalls the velocity of the movement of the adjusting screw and voltage decreases. The area DE is characterized by the vibrational mode, which occurs through a breaking of electrical connection between coal washers. This area during adjustment must be done as soon as possible, as coal washers can be broken or burned.

After the point E the zone of stable regulator's operation begins. On this area we select the operating point of the regulator, for what we twist in the adjusting screw on 180 ± 45 ° from the position which corresponds to the point E. If we twist in the adjusting screw further from the point M on the curve, the voltage will grow because of ВС deformation. This can lead breaking of its coal washers.

After selecting the operating point of the section ЕМ we set rated voltage of 28.5V with rotation of the core ЕМ.

Setting is correct, if during changing the rotation speed from minimum to maximum one the voltage does not vary more than 0.5 V. Otherwise we unscrew the adjusting screw, while approximation to the point E on the curve, which corresponds to the astatic regulator adjustment.