To study the various controls used in Refrigeration and Air conditioning system.
Apparatus used: – Schematic diagrams of Refrigeration and Air Conditioning controls.
Theory: –
The controls are very essential for satisfactory and economical working of a refrigerant. The electrical connection diagram of a domestic refrigerator is shown in fig. The refrigerant is fitted with following controls.
1. Starting Relay: –
The starting relay is used to provide the necessary starting torque required to start the motor. It also disconnects the starting winding of the motor when the motor speed increases. When the compressor motor is to be started, the thermostat is in closed position. When the electric supply is given, an electric current passes through the running winding of the motor and the starting relay. Due to the flow of electric current through relay coil & due to electromagnetism, its armature is pulled thereby closing the starting winding contacts. The current through starting winding provides the starting torque and the motor starts. As, the motor speed increase, the running winding current decrease. The current in the starting relay is no longer able to hold the relay and it gets released thereby opening the starting winding contacts. Thus, the starting winding gets disconnected.
2. Overload protector : –
The basic function is to protect the compressor motor winding from damage due to excessive current, in the event of overloading or due to some fault in the electric circuit. It consists of a bimetallic strip. During the normal working of the compressor, the contacts are closed. Whenever there is any abnormal behavior, the bimetallic strip gets heated and bands, thereby opening the motor contacts, and de- energizing it. The overload protector is fitted on the body of the compressor and operates due to the combined action of heat produced when current passes through the bimetallic strip and a heater element, and heat transferred from the compressor body. It may be noted that the abnormal behavior of compressor may be due to low voltage, high voltage, high load, low suction pressure, high suction & discharge pressure.
3. Thermostat: –
A thermostat is used to control the temperature in the refrigeration. The bulb of the thermostat is clamped to the evaporator or Freezer. The thermostat bulb is charged with few drops of refrigerant. The thermostat can be set to maintain different temperature at a time. When the desired temperature is obtained, the bulb of the thermostat senses it; the liquid in it compresses and operates the bellows of the thermostat and open compressor motor contacts. The temperature at which motor stops is called cut-out temperature. When the temperature increases, the liquid in the bulb expands thereby closing the bellow contact of the compressor motor. The temperature, at which compressor motor starts, is called cut-in temperature. A thermostat is very crucial in operation of refrigerator as the running time of compressor is reduced considerably thereby cutting the operation cost as well as enhancing the compressor life due to non-continuous working.
Viva Questions: –
1. Describe the functioning of various controls in a refrigerator?
2. What is Thermostat?
3. What is over load protector?
4. What is starting relay?
5. What is the Function of over load protector?