You are designing the PLC logic for a walk-in freezer at a commercial restaurant. Most of the refrigeration equipment is located on the roof (controller, compressor, condenser) and the rest is located inside the freezer (expansion valve, evaporator). A pair of thermostats inside the freezer send a request for cooling signal and a cooling satisfied signal, respectively. When the controller receives the request for cooling, the compressor is turned on. When it receives the cooling satisfied signal, the compressor is turned off.
Freezer evaporators are prone to developing frost buildup on the cooling coils. Ice insulates the evaporator, preventing the rejection of heat into the refrigerant. This causes the freezer to warm up and will trip the thermostat to request cooling, but little can be delivered under this condition. Running the compressor excessively while the evaporator is jammed with insulating ice can also damage the compressor over time as this causes it to ingest liquid. To combat this issue, the controller includes an electromechanical defrost clock that periodically cuts out the compressor circuit. This evaporator is also equipped with electric defrost, which is an electrical heater that should activate for the duration of the defrost cycle.
This fan motors on the condenser should run whenever the compressor is on. The evaporator fan motors should run whenever the compressor is on or when the defrost cycle is activated.
Write a ladder logic program that satisfies these requirements. The ladder diagram can be prepared by hand or with the software of your choice.
