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Basic theoretical information. In aircraft the primary power system of three-phase AC 200/115 V for supplying radio equipment, flight and navigation instruments






In aircraft the primary power system of three-phase AC 200/115 V for supplying radio equipment, flight and navigation instruments, electromagnetic relays, circuits of control and signalization the secondary power supply system with voltage DC 27 V is needed.

Capacity of DC consumers does not exceed 5... 10% of the total power consumption of AC. As sources of direct current energy in the secondary systems the dynamoelectric or static transducers may be used. Static converters have several advantages compared to dynamoelectric because they are widely used in aircraft. They have no moving parts, are characterized with the commutation of current without a spark, silent operation, relatively less weight per unit of power (kg / kW), high efficiency and high reliability, particularly in high-altitude conditions.

Transformer-rectifier blocks (can work independently or together (in parallel). During the parallel operation the loading characteristics ТВБ and their input loads must be identical for performance of uniform loading. Structurally ТВБ consist of two components: transformer and rectifier. As part of the rectifier the transformer includes silicon diodes. Scheme of switching on of rectifier diodes is shown in Fig. 4.1 (a, b, c, d).

 

 

Schemes of one half-periodical rectification (Fig. 4.1, a) pass the current only in case of positive potential at the anode and therefore the current in loading is intermittent. Efficiency of rectifiers depends on the quality of the rectifier, on the circuit connection, on size and nature of the load. Of course the efficiency of three-phase transformers taking into account the transformer is 0.8... 0.85.

Some schemes of transformer-rectifier blocks consist of structurally reducing transformer, rectifier transformers (silicon diodes) and auxiliary transformers for voltage regulation, cooling, overheating protection and alarm. Fig.4.1 б represents the single-phase rectifier with two half-periodical rectifier with the reducing transformer. ТВБ of three-phase alternating current, depending on the schemes of transformers and valves connection can be: three-phase one half-periodical (Fig.4.1 в) and two half-periodical (Fig.4.1, г). Number of secondary phases of transformer are selected a multiple number of initial phases.

If we get the same number of phases we receive three-phase rectification of the three-phase current, doubling the number of secondary phases we obtain six- phased rectification of three-phase current. While selecting secondary phases we tend to receive minimal collisions in the rectified voltage curve, weaken the high harmonics in AC circuit (to prevent excessive overheating of alternators and occurrence of resonance phenomena in alternating current), to ensure a high level of use of transformer windings in power output.

А
б
в
г

Fig. 4.1

In the scheme of the six-phase one halfperiodical ТВБ (Fig. 4.2) with the numbers 6 and 12 in the designation, the secondary voltage re latively to primary ones are shifted on 180° and 360°. However, the usual six-phase one halfperiodical rectifier circuit with zero output has low level of iron and transformer's windings as the time of current to flow through the coil and the diode is 1/6 period. The introduction of some elements in such scheme (eg, switching on the reactor between points 0 and 0 ') improves its work in terms of current commutation, increasing of loading ability of valves and coils. Fig. 4.3 represents the six-phased two halfperiodical scheme of ТВБ-3.

Fig. 4.2

Fig. 4.3

The transformer in the circuit has a primary three-phase winding and two secondary three-phase windings, connected star-star and star-triangle. It provides the shift of voltage vectors of the secondary windings to 30°. The frequency ripples of the rectified current is 4800 Hz. Due to increasing in frequency of ripples and reducing of their amplitude the quality of the rectified current increases, sizes and weights of compensating filters reduce. The scheme provides the most complete usage of iron and transformer's windings. The block provides heat protection of both rectifier and transformer. Thermocircuit breaker of the rectifier operates at 150° C and of the transformer - at 200° C. Thermocircuit breakers close the circuit of the signal lamp. Transformer consists of a three-phase Ш-shaped core and windings made from aluminum tape.

Silicon diodes (with 30 A each) are mounted on the cooling ribs, isolated from the housing block. For cooling ТВБ the fan is provided. Rated capacity of the block is 3 kW, rated current is 110 A. At the rated voltage of 200 V the circuit block provides an output voltage of 28.4 V at load of current of 110 A; 26.2 V at a current of 100 A.

Operation of ТВБ on an aircraft ТВБ in the aircraft receives power from the main network of AC. At a constant value of input voltage the value of the output voltage depends only on the magnetude of the load. If we consider the branch with the concentrated load at the end, which is powered by a rectifier, the voltage across the load will vary in proportion to current and to voltage deviation from the nominal value (Fig. 4.4). Thus, the voltage (of the consumer) is determined for a given load current, by internal resistance value of the rectifier and resistance of the branch. The lowest value of load Uмін meets the minimum value of voltage of AC network and nominal current of load.

 

 

Fig. 4.4

The initial area of load characteristics of the rectifier block has a complex law of change. In practical calculations it is excluded by the technique of linearization of the entire characteristics.

Because of a possible failure of individual ТВБ on aircraft there must be an reserve capacity of DC power. The magnitude of power reservation in this case can exceed 50% because of the very high reliability of static transformers.

Brief description of ТВБ. In modern aircrafts there are used transformer-rectifier blocks ТВБ-6Б. A block consists of the following parts (Fig. 4.5): three- phase step-down transformer, three-phase rectifier, capacitors, resistance of load, and electrical engine for rotation of fans.

All parts of the block, excepting the fans, are mounted inside the silumin casing, covered with a cap. Asynchronous motor is installed on the outside of the casing. Bolts K1 and K2 are for power wires connecting. At this time the positive bolt K1 is isolated from the casing of the block and the negative bold does not have such isolation.

Step-down transformer has three primary and three secondary windings. The primary windings are connected in a " triangle", and the secondary - in a " star". This winding connection eliminates the harmful effects of the third harmonics of ЕРС, so the curves of the resulting flux and therefore ЕРС tend to a sine wave. Rectifier part is made of silicon diodes 6 ПВКЛ-100-4. Capacitors C1, C2, C3 are the filter at idling and its value is 100 Ohm.

Technical data of ТВБ-6Б:

1. Consumption of current by the phase is no more than 25 A.

2. AC voltage is 204, 206 or 208 V.

3. Current of the load – 200 А.

4. Output voltage – 28 V.

5. Lifetime is 500 flight hours.

6. Weight is 15 kg.

On aircraft the block is installed horizontally to ensure favorable conditions for the engine fan.

 

Fig. 4.5






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