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Hysteresis
Hysteresis is when a system’s output depends on the current input and past history. If you increase the input and then decrease it, the output doesn’t retrace the same path but forms a loop.
What Is Hysteresis in Air Compressors?
Hysteresis is the intentional pressure difference between the point where the compressor motor turns off (cut-out pressure) and turns back on (cut-in pressure). Think of hysteresis as the programmed lag in the control system.
The compressor’s on and off state depends on the current pressure and its direction. When the tank pressure reaches the upper pressure limit or cut-out, the motor stops. When the tank pressure drops to the low-pressure limit or cut-in, the motor starts again. For instance, if the cut-out pressure is 125 pounds per square inch (psi) and the cut-in is 95 psi, the hysteresis is 30 psi.
FAQs
Hysteresis prevents rapid cycling or short-cycling in air compressors, a phenomenon where the motor constantly switches on and off with minor pressure drops. Short-cycling causes:
- Massive and quick component wear and tear.
- Extreme energy consumption, resulting in enormous energy bills.
- Motor and compressor pump overheating and burnout.
Hysteresis solves the rapid cycling problem by creating a pressure band that allows the system to “breathe.” This ensures the motor runs for a reasonable period to fill the tank and then stays off for a reasonable period while you use the stored air.
Yes. A pressure regulator does experience a different kind of hysteresis. While the hysteresis in a compressor’s control system is deliberate and essential, hysteresis in a pressure regulator is a small but undesirable mechanical issue resulting from friction in the internal components. Spring behavior and diaphragm material can also cause pressure regulator hysteresis.
This type of hysteresis is defined as a phenomenon where the outlet pressure is higher when approaching a set point with decreasing flow than when approaching the same set point with increasing flow. To maintain accuracy, choose pressure regulators with low hysteresis for high-precision applications.
Hysteresis is the technical term for the lag in the system’s response. Pressure band or differential are the more common terms referring to the pressure range between the cut-in and cut-out points.
The ideal pressure band varies across different applications. For example, the automotive and general manufacturing industries may prefer a wide differential, while food and beverage manufacturing companies may use a moderately wide differential, carefully managed from a centralized control system.
You control hysteresis using the pressure switch, which features two adjustment screws. One sets the cut-out pressure and the other sets the cut-in pressure.
Many industrial models feature an adjustable pressure switch, allowing you to match the hysteresis to the demands of your air tools and processes for optimal performance and efficiency.
The pressure switch monitors the air pressure in the tank. When the pressure drops to the set cut-in pressure, the switch activates the motor to start compression. The motor continues to run until the pressure reaches the cut-out pressure. At this point, the pressure switch deactivates the motor.
A considerably wide pressure differential means the system pressure will drop greatly before the compressor turns back on. While this lowers the number of motor starts, it causes a noticeable decline in the performance of air tools that require consistent pressure.
A significantly narrow hysteresis causes the compressor to cycle on and off frequently, even with small air demands. This constant cycling increases energy consumption, heat generation and wear and tear.
No. Fixed-speed air compressors, such as rotary screw and reciprocating piston units, require hysteresis. Variable speed drive (VSD) compressors adjust the motor speed to match real-time air demand, eliminating the need for hysteresis.