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.
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.
But evaporator temperature slowly increases due to heater operation. When evaporator temperature is above the frost melting temperature, the defrost thermostat contacts will open. By this moment, the evaporator will be fully defrosted. Duration of operation in this mode depends on evaporator frost amount and is usually 10...20 minutes.
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 powered, therefore it will switch over its contacts after a while and set to the rapid cooling mode.
This operation mode usually lasts for 15..25 minutes.