Control Systems: Load-unload-stop

Compressed Air Wiki Compressor Control and Data Monitoring Compressor Regulation and Control Compressors

Regulation systems are becoming more advanced and fast-paced development offers a variety of new solutions. Relay systems have been replaced by programmable equipment (PLC). They, in turn, are currently being replaced by product-adapted systems based on microcomputers. These articles present a few of the control and monitoring systems for the most common types of compressors. This one covers load-unload-stop control.

What is load-unload-stop control?

The most common regulation principles for displacement compressors are "produce air" / "don't produce air" (unload control). When air is required, a signal is sent to a solenoid valve that guides the compressor's inlet valve to the fully open position. The valve is either fully opened (loaded) or fully closed (unloaded); there is no intermediate position. Traditional control, now common on smaller compressors, uses a pressure switch placed in the compressed air system that has two selectable values: one for the minimum pressure (= loaded) and one for maximum pressure (unloaded).

The compressor will then work within the limits of the set values, for example, within a range of 0.5 bar. If the air requirement is very small, the compressor runs predominantly in off-loaded (idling) mode. The length of the idling period is limited by a timer (set, for example, to 20 minutes). When the set time period elapses, the compressor stops and does not start again until the pressure has dropped to the minimum value. The disadvantage of this method is that it offers slow regulation.

A further development for this traditional system is to replace the pressure switch with an analogue pressure transducer and a fast electronic regulation system. Together with the regulation system, the analogue transducer can sense how quickly the pressure in the system changes. The system then starts the motor and controls the opening and closing of the damper at the right time. This method offers quick and accurate regulation within ± 0.2 bar.

If no air is used, the pressure will remain constant and the compressor will run in off-loaded (idling) mode. The length of the idling period is controlled by the maximum number of starts that the electric motor can withstand without running too hot, and by the overall operating cost strategy, as the system can analyse trends in air consumption and thereby decide whether to stop the motor or continue to idle.

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