Figure 1. This approach to compressor control converts from modulation control to a load/no load any time output drops below 60 percent.
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% to 60% of capacity. Also, they can be complex and difficult to troubleshoot.
Online - idle/offline (dual-control)
This simple, effective control uses a pressure switch at the compressor discharge to close the inlet valve completely at a high limit (cut-out) pressure and open it completely at a low limit (cut-in) pressure. The difference between this approach and the modulation control is that, in this case, the compressor is internally unloaded. The reduced compression ratio reduces idle power consumption. This type of control can connect multiple compressor installations easily using a sequencer (master) controller. On the other hand, the air system will need a properly sized air receiver tank.
Start/stop control
This is the most efficient control scheme. The compressor runs either fully loaded or off, depending on the signal from the pressure switch. Unfortunately, motors larger than five to 10 hp can't be started and stopped as often as this control may require without overheating. This control is used on small piston compressors, which typically are mounted on a storage receiver. They use a relatively wide pressure differential of 15 psig to 25 psig.
Variable speed control
Advanced and affordable technology brought variable frequency drives into compressor applications. The principle seems simpleadjust the compressor/motor speed and output to match system demand exactly. Designed correctly, variable frequency control is the most advanced and energy-efficient trim compressor control. However, the following issues must be considered:
The airend must be designed to be efficient over the complete speed range. Airend efficiency is a function of rotor tip speed and may be greatly reduced at lower or higher rpm (see Figure 2).
The variable speed drive controller needs to be efficient, as it represents an additional link between the power supply and the drive motor. The drive system and motor cables need to be free of power distortions and electromagnetic emissions to prevent electromagnetic interference to computers or other sensitive electronic equipment.
The drive motor must be able to handle the higher speeds as well as the lower speeds, where bearing design and cooling issues may be problematic.
An intelligent controller makes efficient and reliable connections among air pressure, the drive and the compressor inlet valve. A well-engineered device can control compressor outlet pressure to within one psi, despite wildly fluctuating air demands.
Variable speed control is suited for trimming in multi-compressor applications. It allows a sequencer/controller to operate multiple compressors efficiently. Needed machines are loaded fully. Unneeded compressors are in stand-by mode. And the variable speed control efficiently compensates for fluctuating air demand (see Figure 3).
