Control Types

Start/Stop

Start/stop controls are generally the easiest to comprehend. This type of control either turns the motor on or off in response to a pressure signal to meet demand. So, this means either the compressor is working, or it is not, there is no in-between or partially loaded options. This is generally controlled by a simple pressure switch and because of the simplicity of this control, it should not be used in applications where frequent cycling occurs. Look at the following example to better understand how this works:

The compressor has a start pressure of 120 psig and a stop pressure of 150 psig (the difference between the two is considered a pressure differential). Once the compressor reaches 150 psig, the compressor switch will flip to stop the compressor. Once the pressure drops to 120 psig, the pressure switch will switch back on to start the compressor.

Start/stop is best fit for applications utilizing smaller compressors at lower duty cycles. This control type is available for rotary screw, reciprocating, and centrifugal compressors.

Load/Unload (On line/off line or constant speed)

In Load/Unload, the motor is running continuously but unloads the compressor once it has reached an adequate discharge pressure.  Once the discharge pressure lowers to a specified inadequate level, the compressor will reload to bring it back up and perform its designated duties. Here is how it achieves this:

1. The compressor reaches the upper pressure set point.
2. The compressor inlet valve then closes.
3. The blow down solenoid valve opens which allows the air/oil separator pressure to drop to the minimum pressure setting.
4. The blow down process is a controlled blow down through an orifice in order to minimize oil foaming.

This type of control is best fit for large, industrial compressors and applications. This control type is available for rotary screw, reciprocating, and centrifugal compressors.

Modulating (Inlet throttling)

Modulating inlet controls progressively reduce the air output to match flow requirements.  This is done by throttling the inlet valve which reduces the amount of air drawn into the compressor. See the below scenarios to better understand modulation:

1. At full load, the inlet valve is fully open, allowing atmospheric pressure to be factored into the compression ratio.
2. When the inlet valve is throttled, the capacity goes down and the inlet valve creates a larger pressure drop, ultimately causing the compression ratio to go up. The greater the compression ratio, the more work it takes to compress the air. As the inlet valve is closed, the compression ratio will increase along with the required horsepower, however, as the inlet valve is closed or throttles, the mass flow of air through the compressor decreases and reduces the horsepower.

This type of control is also best fit for large, industrial applications. You can use this control type in both rotary screw and centrifugal air compressors.

Variable Displacement

Variable displacement controls allow for the compressor to operate in a few partially-loaded conditions, without the compressor having to be fully loaded or fully unloaded. This control does this by varying the length of the airend without affecting the inlet pressure.  The inlet valve will stay wide open when the variable displacement control is operating between 50-100%. These types of controls can deliver more accurate and efficient pressure control due to their ability to respond better to changing output pressure demand without having to fully load or unload. This type of control is available for rotary screw and reciprocating compressors.

Variable Speed Drives (Variable Frequency Control)

Variable speed drive controls (VFD) can adjust the air compressor output by changing the speed of the motor to match the demand exactly. This type of control is considered the most efficient and can yield energy savings as well. However, these controls are very sensitive to environmental factors like heat, contaminants, or moisture. VFD control is very effective for applications where there is constant varying demand. VFDs are only available for rotary screw compressors.

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