Electro-Pneumatic Control is used in air compressors to regulate the discharge air pressure.
Every compressor has to discharge some specific pressure so we can use this utility to operate pneumatic tools etc.
M-type unloader, W-type unloader, etc. are used in old days.
Hence the accuracy of such devices is not perfect, to overcome this problem” Electro-Pneumatic control” is introduced.
That is the need for compressor engineering in those days.
So, here are the details about Electro-Pneumatic Control Comes
This is an automatic type capacity control (electro-pneumatic) used to replace mechanical unloaders.
The discharge pressure may be controlled in 2-step, 3-step, or also 5-step, depending upon the model & capacity of the compressor.
Electro-pneumatic control consists of 2 way or 3-way solenoid valves operated by the pressure switches, actuating at receiver pressure.
Copper or Nylon tube is used for the flow of air from receiver to suction or inlet valve through pressure switch & solenoid valve.
The electro-pneumatic system also provides the compressor to operate at no-load, till the electric motor attains normal speed & lubricating oil builds up to the required pressure, at the time of initial starting.
How Electro-Pneumatic Control Works
At starting of Air Compressor, the air pressure inside the receiver is low so pressure switches energize the solenoid valves.
Hence this will stop the air supply to the inlet or suction valve & Compressor comes to full speed, which is known as 100% load.
As the receiver pressure increases than the set pressure of pressure switch-1, the contact of the switch opens & cuts the electric supply of the respective solenoid valve & de-energizes the same.
Due to this air supply flows to the inlet or suction valve and one set of inlet valves opens thus compressor unloads partially. I.e. 50% load.
If the consumption of compressed air is low than production then air pressure in the receiver further increases to the set pressure of another pressure (switch-2) again the contact of the switch opens & de-energizes the respective solenoid valve.
This unloads the other set of inlet or suction valves & compressor gets fully unloaded which is known as 0% load.
Similarly, a fall in receiver pressure causes the pressure switches to energize the solenoid valve & stop air supply to the inlet or suction valve & compressor comes on load in steps i.e. 50% load & 100% load.
The pressure switches may be acting direct at receiver pressure or at reduced pressure where the process controller is fitted.
As this is a modern type of capacity controller (electro-pneumatic), it is essential to take the following,
Precautions for a smooth operation of the electro-pneumatic control
01) Keep control filters clean, by draining the moisture collected in the filter bowl. Otherwise, this will mix up with dust, oil & other materials and form a paste and restrict the airflow of air or sometimes enter into the inlet valves.
02) Stop air leakage in the impulse line.
03) Do not disturb the setting of pressure switches.
04) Set a minimum of 0.5 kg/cm² as differential pressure to operate the switch.
05) Use only genuine quality pressure switch & solenoid valves.
Besides this information, you are suggested to read something more from below engineering books