Basic theoretical information

The control system of the generator consists of devices designed for the automatic switching on and off from its on-board network. Generator is plugged to the board network only when the voltage at its clams is equal to or is greater than voltage of the network, when the generator is able to provide the load. If the ЕРС of the generator is less than the voltage of the on-board network, the generator should be switched off, otherwise it becomes a consumer of electricity.

The devices, designed for protection of the generator and the power network from possible emergency operations, is called protective devices.

On modern aircraft to control the DC generators of high capacity, we use ДМР, which performs the following functions:

- generator is automatically switched on to the onboard network when its voltage exceeds the voltage on 0.2... 1 V;

- generator is automatically switched off from the network at reverse currents when the generator receives current 15... 50 A from the network;

- exclude the possibility of connecting the generator to the network with reverse polarity;

- The generator is automatically switched off from the network at the precipice of feeder of the generator (wire that connects the positive terminal of generator with clamp " Generator" ДМР).

In civil aviation aircraft two types of ДМР are mainly used: ДМР-400Д and ДМР-600Т. The numbers that stand after the letters, indicate the rated current's value, according to which the contacts of the main contactor are designed, the letters after numbers mean its line.

ДМР-400Д consists of such elements (fig. 3.1):

- auxiliary relay 1P that has two pairs of normally opened contacts 1R1 and 1R2;

- Differential relay 2Р with normally closed contacts;

- Polarized relay (PR) with normally opened contacts ПР1. Relay is polarized using a permanent magnet;

- Contactor K with double break of the circuit.

Fig. 3.1

To connect the generator to the network we need to enable two-way switch B1, that is located on the board power, and the circuit of relay 1Р and equilibration of the coil will be closed. When the voltage reaches 14V on clamps of the generator, relay 1P works and closes its normally opened contacts 1Р1 and 1Р2, which are plunged in a circuit of coil's contactor, relay's coil 2Р and differential winding ПР.

Winding of the relay 2P measures the difference of voltages of the network U _C and the generator U _Г (D U = U _C– U _Г). When this difference reaches 14 V, relays works and opens its normally closed contacts 2Р1, leading to rupture of circuit of the differential winding W _д ПР. With increasing of voltage at the clamps of the generator the difference is D U = U_C – U_Г diminished. When it is equal to or less than 4 V, the relay 2P switches off and its normally closed contacts 2P close the circuit of differential winding ПР. At this moment the voltage of the generator is lower than the network's one, so through the differential winding W _д ПР current will flow from the network to the generator.

In this case, the right-hand end of the anchor of relay has the south polarity and the left-hand the north one (Fig. 3.2). If at this moment contacts of ПР are closed, they opens and if they are open, the position of anchor will remain the same.

а б

Fig.3.2

In the fig.3.2 the scheme of the magnetic circuit is shown ПР (a -opened contacts; b -closed contacts): 1 - adjusting screw; 2 - upper board; 3 - bearing cramp; 4 - pole; 5 - contact screw; 6 - anchor; 7 - holder; 8 - lower board.

When the generator's voltage exceeds the voltage of network on 0.3... 0.7 V current in the coil W _д ПР remagnet anchor and contacts PR1 will be closed (Fig. 3.2). At the moment of contacts PR1 closure the contactor K will work, because its winding is applied to the voltage of the generator, and the generator is plunged to the onboard network. With the closure of the contacts K of the contactor of the relay's coil 2Р and differential W _д will be shunted, but anchor PR will remain in the previous position because of the influence of MRS permanent magnets and winding W _п ПР in series on it.

The generator is automatically disconnected from the onboard network if its voltage is below the voltage of the network. In this case, through the serial W_д ПР winding, the current flows in the reverse direction. When it reaches 15 A the anchor remagnets (Fig. 3.2) and returns clock wisely and opens the contacts of the polarized relay ПР1. As a result of contacts K of the contactor opens and the generator disconnects from on-board network. The magnitude of the reverse current I _зв is selected in such a way to not make the current emergent of the generator and the accumulator battery АБ (Fig.3.1) does not run down on the generator at their joint work.

If the generator during its switching to the onboard network has reverse polarity, the voltage on the winding of the relay 2Р will be equal to the sum of voltages of the generator and the network. This amount will be higher than voltage relay's 2Р (U _СР = 14 В) triggering, so its contacts 2Р1 opens and the differential winding W _д ПРwill be opened. The generator will not be switched on to the network. Thus, the relay 2Р eliminates the possibility of switching to the generator with reverse polarity at its clamps and protects the differential winding W _д ПР from overheating when the difference voltage D U = U _C – U _Г is equal to or greater than

14 V.

The clamp К (Fig. 3.1) is used for signaling the generator's operation or for switching on any consumer of electricity. The clamp A is designed to control the contactor K excepting relay 1Р, 2Р і ПР. In this case, you can use the electric generator as a starter to run the aircraft engine. Principle electrical scheme ДМР-600Т (Fig. 3.3) differs from the scheme ДМР-400Д with presence of two additional relays 3Р and 4Р. The relay 3Р is designed to enable the circuit with the contactor coil K of the current-limiting resistance R. Relay 4P has three pairs of contacts: 4P1, 4P2, 4P3.

Fig. 3.3

Two pairs of contacts 4Р1 and 4Р3 are normally closed. Contacts 4P3 run the signal lamp СЛ. If you switch off the generator from the network through the normally closed contacts 4P3 receiving power from an onboard network on the signal lamp СЛ, which is equipped with a red filter and has the inscription " Generator off."

During contactor K working (generator is switched on to the network) from its remote buss the power goes to the relay coil 4P. Relay 4P is triggered and at the same time opens the circuit СЛ and it goes out. At the same time 4P1 and 4P2 contacts are triggered, which connect the differential winding ПР on the " plus" of the generator. This allows you to switch off the generator from the network in case of breakage generator's feeder in the section " plus" of the generator to clamp " Ген." (ДМР). Thus on differential winding W _д the current will flow from the network to the generator.

To protect the electrical network from the overvoltage due to faulty of the control system we use various kinds of automatic devices.

In the aircraft there are two types of automatic protection:

1) With usage of winding of EM as measuring element;

2) Of the bridge circuit of measuring element which is manufactured from semiconductor elements.

The representative of the first type machines - АЗП-8М, electrical circuit of which is represented in Fig. 3.4. The automat switches the generator off the network while the excitation field at increasing of voltage at the clamps is more than 31... 32 V and has three relays and one contactor Fig. 3.4

Relay 1P and 2P have one pair of normally opened contacts 1P1 and 2P1, relay 3P has normally closed contacts 3P1. Contactor K has three pairs of normally closed contacts: 1K1, 1K3, 1K4 and one pair of normally opened contacts 1K2.

. Winding of the relay 1P through the adjusting of the resistance R _р is switched on in parallel to the excitation winding of the generator and is sensitive element of the machine. Relays 1P1 of the type РЗД-П has exposure of time for operation that eliminates false triggering of protection in short-term increasing of voltage. Time of relay's triggering in a sudden increasing of voltage from 15 to 31 V should not exceed 1.5 s, and at higher voltages - from 15 to 50 V should be not less than 0.06 sec. When the voltage of the generator reaches 31... 32 V the relay 1P operates and closes its contacts 1P1. At the moment the relay 2P operates and with its contacts 2R1 close the circuit of the contactor C.

Contactor triggers and contacts 1K1 turns off the excitation winding from ВС and contacts 1K2 turns it on into additional resistance Rd. The excitation winding is switched on to the on-board network because of its additional resistance that causes a sharp decreasing in excitation current, and hence the voltage at the clamps of the generator. With contacts 1K3 the generator is disconnected from the network, because the circuit is broken 1Р ДМР. With contacts 1K4 the circuit of relay 3P is broken, with its help the equilibration coil of the voltage regulator of emergency generator switches off.

K contactor is used with a latch. At the time of triggering the contactor comes on the latch and remains in this state as long as under the influence of mechanical force latch will be disconnected (to prevent faulty generator to switch on).