electromechanical defrost control system of household refrigerators
Standard feedback circuit
This circuit differs from the system without feedback only in the method of timer motor connection. As for the rest, the two circuits are identical. However, such a minor change significantly improved the system operation logic.
Feedback, provided in the system due to the change in the timer connection circuit, ensures switch-off of the timer motor during defrost heater operation. The timer motor will be de-energized while the defrost thermostat contacts are closed during defrost. In this case, duration of the pause between defrost heater switch-off and compressor and fan switch-on depends only on one timer characteristic - the defrost time. That is, as distinct from the circuit without feedback, pause duration can be strictly set by selecting a suitable timer model. In practice, timers with defrost time from 1 to 7 minutes are usually used in such feedback circuits.
Designations
M - timer motor
The electromechanical timer operating principle is based on rotation of the shaped drum sequencer by the micromotor. The drum periodically switches over the timer contacts. Thus, time is counted down only when the micromotor is powered.C - compressor
The refrigeration unit operating principle is based on physical processes in its pipelines during coolant circulation in them. Circulation is provided by refrigeration unit compressor. Its operation causes intense cooling of the refrigeration unit evaporator.F - fan
Heat exchange between the refrigeration unit evaporator surface and food products inside the refrigerator is provided by the air flow created by the No Frost system fan.R - defrost heater
The moisture in the air, blowing down the intensely cooled evaporator of the No Frost system, gradually settles on its surface as frost. The heater, attached on the evaporator, assures its heating for frost melting and surface cleaning. It is needed to prevent formation of an excessively thick frost layer, impairing effectiveness of heat exchange between the evaporator and air flow.DT - defrost thermostat
Temperature of the No Frost system evaporator must be monitored during automatic defrost, so as to switch off the heater in time. The defrost thermostat is used for temperature monitoring. Its contacts open at a temperature slightly above the frost melting temperature.Interactive Circuit Diagram
The system is in the mode of initial rapid cooling, the timer supplies power to the compressor and fan. However, the evaporator hasn’t yet cooled down sufficiently, that’s why the defrost thermostat contacts are open.
Note that the timer motor is powered not directly from the power source but via the defrost heater. Motor resistance is significantly higher than that of the heater, that’s why almost all power at their series connection is released on the motor, leaving the heater cold.
The system operates in this mode not long, about 10 minutes. Usually this time is enough for sufficient cooling of the evaporator and closing of the defrost thermostat contacts.
The system is in the cooling mode, the timer supplies power to the compressor and fan. The evaporator has already cooled down sufficiently, that’s why the defrost thermostat contacts are closed.
The system operates for most of the time in this mode. Its duration is from 6 to 12 hours. Air and food products inside the refrigerator are cooled in this mode, while the evaporator is gradually covered with a frost layer.
The system has set to the defrost mode. The timer de-energized the compressor and fan and powered up the defrost heater. Evaporator temperature is low as yet, that’s why the defrost thermostat contacts are closed.
Note that the timer motor is de-energized while the defrost thermometer contacts are closed. This is feedback in action.
Duration of operation in this mode depends on evaporator frost amount and is usually 10...15 minutes. In this time, the heater melts all the frost on the evaporator surface.
The heater warmed up the evaporator to above the frost melting temperature and the defrost thermostat contacts opened, de-energizing the heater. The timer is still in the defrost mode, but its motor is now powered, therefore it will switch over its contacts after a while and set to the rapid cooling mode.
Duration of operation in this mode depends on timer model and is usually from 1 to 7 minutes.
Conclusion
As compared with the circuit without feedback, system shown on this page has a substantial advantage.The component cost was not increased, whereas the minor change in the circuit allowed a significant reduction and a constant duration of pause between the defrost heater switch-off and compressor and fan switch-on. It remarkably reduced the negative impact of evaporator defrost process on temperature conditions inside the refrigerator.
We’ll continue studying No Frost systems with another feedback circuit.