ON AND OFF CONTROLS-
REFRIGERATION CLASS NOTES
On and off controls makes the operation
automatic. These save electricity. They
also act as safety for the machine.
There are number of automatic off and on controls for the motor compressor of a refrigeration system.
These are pressure, voltage and temperature controls.
(i) Suction Pressure Motor Controls
(a) Low Pressure Cut out OR suction pressure control
This cut out is only an OFF control. If the pressure in the evaporator becomes lower than the preset low pressure limit, the control makes the motor off. It happens when enough cooling load is not available in evaporator.
(b) Low Pressure Cut in Or suction pressure control
Suction pressure control is only an ON control. If the pressure in the evaporator becomes higher than the preset low pressure, the motor starts automatically. It happens when enough cooling load is available in evaporator.
(ii) Discharge Pressure Motor Control
(a) High Pressure Cut out OR discharge pressure control— It is only an OFF control. If the pressure in the condenser increases beyond a preset high pressure, the control will make the motor off. It happens when enough cooling is not available in the condenser.
(b) High Pressure Cut in OR discharge pressure control
This is only an ON control. If the pressure in the condenser falls within the preset discharge pressure limit, the motor will start automatically. It happens when enough cooling is available in condenser.
(iii) Voltage controls
(a) High voltage cut out
When voltage exceeds a certain value, it makes the operation off. As soon as voltage comes to the required value, the unit automatically restarts.
(b) Low voltage cut out
If voltage falls below a certain value, it makes the operation off. As soon as voltage comes to the required value, the unit automatically restarts.
(iv) Solenoid valve
This control is in between the condenser and expansion valve. It operates electrically. It has only fully open and fully closed positions. Connection to electric motor is via the solenoid valve. If electricity fails, solenoid valve closes. There is no flow of refrigerant to the evaporator. Now It no longer faces high starting torque.
Temperature control is thermostat. It is an ON and OFF control. Set a temperature in a refrigerating machine. As soon as that temperature reaches, this control makes the motor off. After sometime the temperature inside increases, then this control makes the motor/compressor on automatically. This is common in a fridge.