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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Forced airflow for the refrigerator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric motors During the operation of refrigerators, their premature failure due to overheating of the compressor motor is often observed. Cramped operating conditions - insufficient distance from the cooler grate to the wall of the room and poor circulation of cooling air - lead to a long operation of the refrigerator compressor to reach the set cut-out temperature. Large refrigeration units use a fan to force cool the refrigerant to keep the temperature in the cooling chambers in line with food storage requirements. The absence of forced cooling simplifies the design of a household refrigerator, but reduces its service life. The proposed device for additional cooling of the radiator and refrigerator compressor consumes no more than 20 watts from the mains. The principle of its operation is based on the automatic inclusion of forced airflow to the radiator after the compressor starts. When the compressor is turned off, the device goes into standby mode with low power consumption. The device (Fig. 1) contains:
On LEDs HL1. HL2 indicates the compressor is on and the power is on. The power supply is made on a power transformer T2, followed by voltage stabilization by an analog chip DA1. At the moment of automatic start of the refrigerator from the internal temperature sensor (thermal relay) in the network, there is an almost fivefold current surge, which creates voltage on the I winding of the current transformer T1. Winding II T1 is energized by resistor R1, which reduces the voltage surge in the winding and protects it from interturn breakdown. The alternating voltage rectified by the diode bridge VD1 from the secondary winding T1 is limited by the diode VD4. which protects the VU1 optocoupler LED from breakdown. Capacitor C1 reduces the level of interference in the power supply circuit of the optocoupler LED when starting the refrigerator motor. Through the current-limiting resistor R2, the rectified voltage is supplied to the LED of the optocoupler VU1. The installation of an optocoupler at the input of the circuit provides, in addition to the current transformer, reliable galvanic isolation from the network (the isolation resistance of the optocoupler is about 10 MΩ). An amplified electrical signal appears at the output of the optocoupler. Optocoupler VU1 operates in the key mode of the photodiode with the base disabled (pin 3 of VU1 is not connected). The waiting multivibrator is made on the analog integrated timer DA2. In the initial state, output 3 of the microcircuit has a low voltage level (close to zero), since a voltage greater than 2/2 U is supplied to input 3 DA1 through resistor R3 (the optocoupler transistor is closed at this moment and has a high resistance). The appearance of voltage on the winding II T1 opens the VU1 optocoupler, the voltage at input 2 of DA2 drops to almost zero, the internal trigger of the DA2 timer switches, and a high voltage level is set at output 3 of DA2. Capacitor C3 of the timing circuit after the time M.1-(R4+R5)-C3 is charged to the level of 2/3Un, the internal bit transistor of the timer fires, and the capacitor C3 is discharged through the thermistor R6. Since pulses with a frequency of 1 Hz are received from the diode bridge VD100 at the input of the optocoupler, the next pulse starts the timer again, and a high level appears at the output of 3 microcircuits. The duration of the output pulse can be changed by a variable resistor R5. which causes the fan motor speed to change. To reduce the pause between high-level periods at output 3 DA2, the discharge of capacitor C3 is carried out bypassing R5 - through the diode VD5. Elevated room temperature affects the thermistor R6. as a result, the duration of the pause is further reduced, which leads to an increase in the speed of the fan motor. The VU2 optocoupler operates in the output power amplifier mode, which makes it possible to galvanically decouple the timer from the M1 electric motor. The input current of the optocoupler is limited by resistor R7 at 20 mA. This is quite enough to power the VU2 optocoupler LED. Capacitor C6 reduces the noise level when switching the motor windings by the internal control circuit. By the glow of the LED HL1. set at the output of the timer, one can judge the presence of a high level at output 3 and, accordingly, the operation of the refrigerator compressor. The power indicator is made on the HL2 LED. Resistors R6 and R10 are used to protect the LEDs from overcurrent. Capacitors C2, C5 smooth out the ripple of the rectified voltage and eliminate interference in the power circuits. An integral stabilizer on the DA1 chip is used as a supply voltage stabilizer. The M1 fan is a computer fan designed for blowing power supplies (type JA-1238S22H, dimensions 120x120x38 mm). The positive properties of such fans are high performance, low acoustic noise, long-term operation, and the absence of a collector. Current consumption at a capacity of 2.7 m3/min (2700 rpm) does not exceed 100 mA. The fan motor start voltage exceeds 5V due to the internal control circuit. At lower voltages, the fan will run erratically or not spin at all. This feature should be taken into account when setting the minimum engine speed. The current transformer T1 is made from a faulty transformer from the network adapter. The primary winding is removed, and one of the refrigerator power wires is wound with two turns on the frame. W-shaped iron plates are assembled into a pack, single plates are joined through a gasket made of newsprint (to eliminate saturation of the transformer) and are pulled together with a clamp. Checking the operation of the device should begin with a direct start of the fan motor from a voltage of 12 V. Further, by including the fan in the circuit, output 2 of the timer is short-circuited to a common wire. Lighting of the HL1 indicator and a short rotation of the fan indicate that the circuit is working. The voltage of 2 ... 3 V on the capacitor C1, when connecting a dummy load (150 W lamps) instead of a refrigerator, should periodically start the timer. If there is insufficient voltage on C1, you need to add 2-3 turns of wire to the IT1 network winding. The speed controller R5 sets the maximum fan speed with minimal noise. The device is assembled on a printed circuit board (Fig. 2), which, together with a power transformer, is installed in a plastic case of suitable dimensions.
LEDs and speed control are best located on the front panel of the device. Power can be supplied from a "tee"-extension cable, and a current transformer can be installed on it. The fan is mounted above the refrigerator compressor so that air is drawn upwards from the compressor along the radiator grille. It is desirable to fix the device next to the compressor at the bottom of the refrigerator. Literature
Author: V.Konovalov, Irkutsk
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