The second wind of the refrigerator. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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The second wind of the refrigerator. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / Electrician's Handbook

Compression and adsorption refrigerators often fail due to the fact that their electromechanical relays or circuit breakers based on bimetallic plates fail. The first of them serve to start synchronous electric motors serving the refrigerator compression system, and the second ones are the basis of systems for tracking and maintaining the temperature of freezers at a given level [1]. The reason for the failure of both is burnout or some kind of mechanical damage to the spring contacts in these devices. Especially the ego applies to outdated models of refrigerators. And one can often see how, due to an insignificant, but difficult to repair breakdown (due to the lack of spare devices), apparatuses that are still quite suitable for use are thrown away. The material attempts to eliminate this kind of malfunctions in household appliances.

It is known that today it is quite possible to replace the outdated scheme for starting asynchronous electric motors using a starting relay with a capacitor circuit. There are no mechanical contacts in it [2]. The following can be said about the temperature control device in the freezer. Since the refrigerator works in an apartment where the temperature is maintained throughout the year within fairly stable comfortable limits (using air conditioning systems, centralized heating, etc.), then under these conditions the temperature difference between the environment of the apartment and the freezer (a properly functioning refrigerator) remains almost unchanged .

The temperature control system of such a refrigerator only "feeds" the freezer with stable portions of cold, equal to the outflow into the air of the room. Therefore, to maintain the temperature at the desired level, it is not at all necessary to monitor temperature changes, but it is enough to determine the size of the portions of cold. This can be done indirectly by calculating the time intervals between turning on and stopping the compressor of the refrigerator, in which the temperature control system is working properly. Then, in a refrigerator with a failed thermal control, having fallen certain and constant intervals of operation and downtime of the compressor, we will obtain a fairly stable temperature in the freezer and its internal volume. This opens up the possibility of building a timer circuit that forms the intervals for switching on and off the compressor motor without electromechanical contacts. On these principles, the electrical circuit shown in the figure was built, according to which the ZIL-Moscow refrigerator was modernized - manufactured in 1956 and today it functions perfectly, although it failed before the alteration for the reasons mentioned above.

The scheme works as follows.

The master oscillator on the microcircuits DD2.2, DD2.3 generates clock pulses close to the "meander" shape in two modes of formation (analog MOS key DD3 is used to switch from one mode to another). In the first mode, pulses are generated with a constant repetition period of about 0,6 s (in the closed state of the MOS key as part of DD3), and in the second - with a tunable repetition period from 0,6 to 0,8 s (in the open state of the same key) . The adjustment is provided by the potentiometer R5. In both cases, pulses are generated at levels close to the supply voltage level (from 0V to 10V). In this case, the level of log.1 at the control input DD3 (vyv. 15) corresponds to the first mode of formation, and the level of the log. 0 - second. Why these two regimes are needed will be clear from the following discussion.

From one of the outputs of the master oscillator (pin 2 DD2.2), the generated pulses are fed to the input of a binary counter on the DD1 chip, and it divides these pulses with coefficients from 2 to 16 in the range of 384 bits. Moreover, each digit has its own separate output (except for the 14nd and 2rd), from which impulses can be taken at intervals from 3 s (at output 1.2 of the lower digit) to 9 hours (at output 3,6 of the highest digit). Each subsequent discharge (in ascending order) doubles the pulse repetition period. Of practical importance for controlling the period of operation of the cold unit, modernized according to the proposed principle, are pulses only from the 3th and 11th digits (vyv.12, 1), the frequency of which is close to the rhythm of the operation of the refrigerator with a serviceable thermal relay in the steady state (from 15 to 20 minutes). The reason for this choice was the observation of the operation of the refrigerator even before it deteriorated. Then it was noticed that the thermal relay turned on the compressor for 40 minutes and turned it off for about the same time.

From pin 1 DD1 through the buffer inverter DD2.1, the pulses selected in this way are fed to the electronic switch of the asynchronous motor-compressor. This switch consists of a transistor VT1 operating in the key mode, and two optothyristors - U1 and U2. When the level of the logical state on the pin. 1 DD1 (as a result of the counter) will reach the log. 0, then through the buffer inverter DD2.1 and the limiting resistor R1, it enters the base of the transistor VT1 and opens it. In this state, the transistor has a very low resistance between the collector and emitter (less than 1 ohm) and, thus, the lower output of the resistor R2 in the circuit is connected to zero potential. A current (about 1 mA) will begin to flow through the series-connected LEDs in the optothyristors U2 and U60 - and they light up, and their light exposure leads to switching pnpn thyristor structures in these devices to the open state.

Due to the fact that these thyristor structures are connected in anti-parallel, as increasing. so the decreasing half-cycles of the mains voltage get access to the windings of the electric motor as part of the compressor - and it starts to work. Its working winding - directly, and the starting one - through the capacitor C1, are connected to the 220 V network. At the same time, due to the optocoupler pairs in the composition of U1 and U2, the separation of the power circuit and the control circuit is achieved, which is very favorable for electrical safety and reliability of the refrigerator.

Capacitor C1 is used to start the asynchronous electric motor of the cold unit in single-phase mode. Such electric motors usually contain two windings - working and starting, shifted relative to each other at a certain angle.

The capacitance of the capacitor required for starting can be calculated using the formula given in the book by I. Aliyev for this kind of winding configuration [2]:

C (μF) \u1600d XNUMX In / Un where:

In - motor phase current, Un - rated phase voltage. Even before the refrigerator failed, it was possible to measure its phase current (it is also the current consumed by the refrigerator in the mode when the compressor is running). The measurement gave 1,6A. The nominal phase voltage is known - 220 V. Substituting these values into the formula, we obtain a value from the capacitance of about 12 microfarads. To ensure reliability and safety in the operation of the device, it is necessary that a capacitor of such a capacitance has a margin for operating voltage. We stop the choice on the capacitor K42-19-12 uF ± 10% 500 V, which provides a current shift in the starting winding relative to the working winding at an angle of about 90 °. In this case, the shift of the windings leads to the appearance in the magnetic field of the stator of power lines containing torque. When they act on the rotor, the electric motor starts.

At the same time, the presence of these lines of force creates some obstacle for the working winding to perform its function by acting on the rotor with pulsating shocks, maintaining the stability of its revolutions. As a result, the magnetic field acting on the rotor, with this inclusion, begins to contain a reactive component, leading to the return of some of the power consumed by the motor to the supply network [2]. However, due to the moderation and invariance of the load on the shaft, these losses are insignificant and the remaining part of the electric motor power is quite enough to ensure the operation of the compressor. This, moreover, saves electricity - the refrigerator consumes less power from the network. The phase current, which was measured after the modernization of the refrigerator, will be 1.1 A. Thus, the need to use a starting relay is eliminated.

Schematic diagram of a timer that ensures the operation of a home refrigerator when the temperature control system fails (click to enlarge)

Observations of the operation of the unit even before its breakdown, as already noted, indicate that the steady-state mode of refrigeration in it occurs at approximately equal 20-way intervals when the compressor is on and when it is off. However, during the modernization it was found that this mode provides a sufficient influx of cold, but the outflow of cold is too small. As a result, the freezer quite quickly (within 2 weeks) becomes covered with heavy frost, requiring defrosting. Therefore, while maintaining the specified interval of compressor operation, it became obvious that the 20-minute interval should be extended when the compressor is stopped. while allowing the degree of this increase to be adjusted. For this purpose, a master oscillator with two modes of pulse formation was built.

Log.0 level with vyv.1 DD1, as mentioned above, includes the compressor. It is also fed through the inverter DD2.1 to the pin. 15 DD3, which switches the analog key included in this microcircuit into a closed state. And the master oscillator begins to generate pulses of minimum duration. providing a 20-minute interval of compressor operation. Upon its completion, the level of the logical state on pin 1 DD1 changes to the opposite. As a result of this, the compressor stops, and the master oscillator switches to the mode of generating pulses of variable duration. By changing the position of the potentiometer R5 slider, this duration is adjusted, and the compressor stop interval is adjusted accordingly from 20 to. approximately 33 minutes. By setting this interval, it is possible to adjust the average temperature level in the refrigerator.

The VD1 LED as part of the circuit serves to indicate the state of the electronic switch that controls the operation of the electric motor. This LED lights up when the motor is turned off and goes out when the motor is turned on.

Thermal relay K1 type RT-10 serves to protect against possible overloads on the motor shaft, which, in principle, is not excluded in the event of an emergency in the kinematics of the compressor. The presence of this relay violates the general concept of the proposed modernization, which seeks to free the refrigerator from all mechanical and springy contacts. However, since this relay is a stably present element in all obsolete units, and extremely rarely comes into action (which keeps its service life very high), it was decided to keep it. This element is absent in adsorption refrigerators, and therefore it can be omitted from the upgraded circuit.

All details of domestic production. Capacitor C2 type KM-6. The rated power of the resistors is from 0,125 W, except for the resistor R2, whose power is 0,25 W. A special adapter serves as a source of constant voltage necessary to power the electronics in the circuit (about 10 V). It is used as an adapter for charging MOTOROLA mobile phone batteries, which consumes about 20 watts of power from the network. When the electronic switch of the compressor induction motor is on, the current load on the adapter will increase and the voltage that it generates will decrease to about 6,5 V.

Structurally, the circuit is assembled on a textolite board with dimensions of 60x60 mm, containing the wiring of printed conductors for mounting electronic components in the layout design of circuit diagrams. All elements of the circuit are installed on it, with the exception of the capacitor C1 and the thermal relay K1, which, due to their considerable size, are installed under the bottom of the refrigerator near the compressor unit. The board is, as it were, the second link of the MOTOROLA adapter and is connected to it with small pieces of wires (about 10 cm), which serve to supply the voltage generated by the adapter and the mains voltage to the board. The elements placed on the board are covered from above with a plastic cover, which is attached to the board on racks with M3 screws. The lid also has a hole for the VD1 LED so that it protrudes above the surface of the lid and is visible from the outside. On the reverse side of the board (opposite the side on which the elements of the circuit diagram are mounted), in addition to the mounting conductors that carry out its wiring, there is also a conventional electrical socket XT1, which is connected to the output of the electronic switch that controls the operation of the electric motor, and is a cover for the reverse side of the board. A file is inserted into the socket from the power cable of the refrigerator connected to the capacitor C1 and the terminals of the compressor electric motor, which connects the elements of the circuit into a single whole. The scheme does not require settings. If all components of the circuit are in good order and the connections are correct, the device and the refrigerator work immediately after being turned on.

Literature

Lepaev D.A. All about the repair of electrical appliances. - M: 1985
Aliyev I. Asynchronous motors in three-phase and single-phase modes. M: Radio Soft, 2004.

Author: O.Cherevan, St. Petersburg

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