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Technology, Oil-Injected Air Compressors

Five Control Schemes for Rotary Screw Compressors


Adding the appropriate controls to your compressor system can help reduce energy costs by improving pressure control and operating efficiencies when you have multiple compressors.

Today, we’ll take a look at five control schemes for rotary screw compressors and how they work. 


The start/stop control is one of the simplest control schemes available. The controller sends a signal to the compressor’s main motor, letting it know when to start or stop based on preset pressure points. Start/stop controls are the most basic and used for compressors with low duty cycles at 25 hp or lower.


The load/unload control is also known as the constant speed control because the motor is constantly running. Instead of turning off the motor when the demand for air is reached, the motor will still run, but the inlet valve will close, which recirculates air inside the compressor and prevents compressed air from blowing out of the inlet. 


In a modulating inlet control, the output of a compressor is varied to meet flow requirements typically by using an inlet valve. First, a regulating valve senses the discharge pressure and sends proportional pressure to operate the inlet valve. As pressure rises in the compressor, the inlet valve starts closing, or throttling, which restricts inlet air and decreases pressure. This situation ultimately creates a vacuum on the compressor’s inlet side while its outlet pressure remains fairly constant.

Variable Displacement

With variable displacement, the compressor’s output capacity is varied using capacity control valves, also known as spiral, turn or poppet valves. This control scheme manages output pressure and the compressor’s power consumption by changing the length of the compression chamber.

Variable Speed Drive

Variable Speed Drive (VSD) uses a special drive to control the speed of the motor while maintaining a constant delivery pressure. It uses its inverter for a soft start while the motor ramps up to speed. By varying the electric motor’s frequency or voltage, the motor can be controlled by the demand for air.

Do you use a fixed or variable speed control with your rotary screw compressors? Let us know in the comments section below. If you have any questions, please reach out to an Atlas Copco expert on our Contact Us page.

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