Air compressor load/unload/stop control systems

Compressor loading and unloading was once managed by relay-based logic. This was later replaced by programmable logic controllers (PLCs). Today, those PLCs are being phased out in favor of faster, more integrated microcomputer-based control systems capable of real-time monitoring and data-driven decision-making.

The most common regulation principles for displacement compressors dictate whether air is produced or not. When air is required, a signal is sent to the solenoid valve (also known as the unloader valve), which guides the compressor's inlet valve to the fully open position.

The valve then either fully opens (loaded) or becomes closed (unloaded). There is no intermediate position. This binary on/off operation is the basis of minimum compressor pressure regulation

Traditional control, now common on smaller compressors, uses a pressure switch placed in the compressed air system. This has two selectable valves

• Minimum pressure: triggers the compressor to load
• Maximum pressure: triggers it to unload

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 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. For more advanced strategies that offer faster regulation and energy efficiency, see our overview of other air compressor control systems.

A load/unload/stop control system relies on a few key mechanical and electrical components that work together to regulate air supply based on system demand:

• Inlet valve
  Opens to allow air into the compression chamber during loading and closes during unloading. It acts as the primary gate for managing air intake and determines whether the compressor is actively producing compressed air.

• Solenoid valve (unloader valve)
  Electrically actuated valve that receives control signals and directs the operation of the inlet valve. When energized, it switches the inlet valve to the loaded (open) position; when de-energized, it unloads the compressor by closing the inlet valve.

• Pressure switch
  Measures system pressure and sends a signal when the pressure hits predefined thresholds. It controls whether the compressor should load, unload, or stop entirely, based on minimum and maximum pressure settings.

An upgrade to traditional load/unload control is the use of an analog pressure transducer in place of the pressure switch—paired with a fast electronic regulation system.

This setup allows for more responsive and efficient control:

• The analog transducer continuously measures how quickly system pressure is changing.
• The control system uses this data to start the motor and open or close the inlet valve (or damper) precisely when needed.
• This method enables tight pressure regulation—typically within ± 0.2 bar—compared to the broader range of a pressure switch.
• If air demand is low, the system keeps the compressor running in unloaded (idling) mode until conditions change.

To protect the electric motor, the maximum allowable number of starts per hour is limited by controlling the idling period.

The system also analyzes air consumption trends over time to decide whether it's more efficient to stop the motor or let it continue idling—helping your overall operating cost strategy..

• Increased energy consumption:
  Compressors draw significant power even during unloaded (no-load) operation. When short cycling occurs, the compressor spends more time in energy-intensive transition phases without delivering usable air—driving up your overall power costs.
• Component wear
  Repeated start/stop cycles accelerate wear on key components such as:
  • Electric motors
  • Motor starters
  • Inlet and solenoid valves
    These parts are not designed for continuous, rapid cycling and may fail prematurely under such conditions.
• Reduced compressor lifespan
  The combined mechanical stress and thermal cycling lead to a shorter service life, increasing the total cost of ownership over time.
• Pressure fluctuations
  Short cycling creates instability in the compressed air supply. Pressure swings can negatively impact process performance, especially in sensitive or precision applications.

• Oversized compressor
  A compressor that exceeds actual air demand will fill the system quickly, triggering frequent unload events. Since the demand doesn’t justify continuous operation, the unit cycles more often than necessary.

• Insufficient storage receiver capacity
  Without adequate storage, even minor demand changes cause pressure swings. A small air receiver can't buffer these fluctuations, forcing the compressor to cycle rapidly to maintain pressure.

• System air leaks
  Leaks generate artificial demand, which forces the compressor to start more often than true consumption requires. Even small leaks can significantly disrupt system stability.

• Incorrect pressure settings
  A narrow pressure band or poorly configured cut-in/cut-out points increases the frequency of switching between load and unload states.

• Blocked filters
  Restrictions from clogged inlet or inline filters can cause false pressure readings, leading to improper load/unload timing and excessive cycling.

• Right-sizing the compressor
  Conduct a professional air audit to align compressor size with actual consumption patterns. A properly sized compressor avoids excessive cycling and supports long-term system efficiency.

• Adequate air storage receivers
  Air receivers serve as buffers, absorbing demand spikes and reducing the frequency of load/unload cycles. They improve system stability and energy efficiency.
  → Use our air receiver sizing calculator to evaluate your needs.

• Leak detection and repair
  Implement a scheduled leak detection program using ultrasonic tools or pressure decay testing. Timely repair of leaks significantly reduces artificial demand and improves compressor performance.

• Optimizing pressure settings
  Review and adjust control settings to allow the widest acceptable pressure band without compromising operations. This helps reduce unnecessary cycling while maintaining supply reliability.

• Regular maintenance
  Routine inspection and service of filters, valves, and sensors ensure proper operation and prevent control errors that lead to short cycling.

• Considering VSD upgrades
  Variable Speed Drive (VSD) compressors adapt motor speed to actual demand. In applications with fluctuating usage, VSD technology reduces cycling, lowers energy costs, and improves overall system responsiveness.

Is your compressor constantly loading and unloading? Frequent cycling and inefficient control systems can lead to accelerated wear and tear and significantly higher energy bills. Connect with our air compressor experts today to get a professional assessment of your system.