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.