The purpose of any compressor control panels is to match compressed air supply to the compressed air demand as efficiently as possible. Although old style electro-pneumatic controls have been supplanted by microprocessor-based controls and software updates that optimize system and control parameters, any control type can be put into one of two categories-fixed speed controls and variable speed controls.

Fixed speed Compressor Controls
As the name implies, the idea is to hold compressor speed constant. The exact speed is a function of the speed of the drive motor and the ratio of the gearing or V-Belt system in use, if any. The capacity output control is based on either modulation or a poppet valve.

Modulation Control
This form of control modulates the compressor's inlet valve according to the compressor's outlet pressure. When the controller detects rising outlet pressure, it causes the inlet valve to start closing. While this control is effective, it's the least efficient. The reason is that compressor efficiency is inversely proportional to the compression ratio, which is the ratio of inlet pressure to outlet pressure. The closing inlet valve produces a vacuum at the compressor's inlet side while the outlet pressure remains relatively constant. This has the same effect as raising the compression ratio.

A compromise is to restrict the modulation control range to something like 40 percent and automatically convert the scheme to a load/no load control any time output drops below 60 percent. Unfortunately, this form of control doesn't lend itself to use with multiple machines.

Variable Displacement Control
Controlling the effective rotor length varies the compressor's output. This is accomplished with internal bypasses machined into the airend housing and controlled by either poppet valves or a turn (or spiral) valve. While the efficiency is better than with modulation control, poppet-style controls are effective only above 50 percent to 60 percent of capacity. Also, they can be complex and difficult to troubleshoot.