Compressor Control System Basics: An Overview


The air and gas compressors manufacturing industry in the United States is worth about $12 billion in 2019. 90% of all manufacturing and processing industries use at least one compressor in their industrial processes.

Compressors are robust power tools vital to several industries. They are popular in the oil industry, chemical processing plants, pharmaceuticals industry, and heavy industries.

This article explains the uses, working, and need for compressor controls in industrial compressors.

But first, it is important to understand compressors before getting into their controls.

Understanding Compressors

Compressors draw low-pressure gas from auxiliary storage as raw input. Then, they output high-pressure gas either for storage or to feed other processes. A compressor system is fundamentally made up of three main components.

These are the compressor unit, the control system, and the driver.

The driver provides mechanical power to the compressor. In most modern compressors, the driver is usually an AC-driven electric motor.

Some compressors may have an internal combustion engine driver. Some are even driven by gas and steam turbines.

The choice of the driver depends on the power and torque requirements. The application and tolerances of the compressor unit are also considered.

The compressor unit consists of three parts. The first is a compressing mechanism enclosed in a tight metal casing. Then there is the inlet and outlet plumbing, and the cooling and lubrication systems.

Most compressors use water as a coolant, but for very low operating temperatures, they use better refrigerant fluids.

The lubricating system covers the contact surfaces of moving parts with oil. This reduces wear and overheating. The system works similarly to an engine’s oil system. It pumps, filters, cools, and recirculates oil within the machine.

Types of Compressors

There are basically two types of compressors. These are the positive displacement compressors and rotodynamic compressors. The image below shows the different types of compressors that comprise these two main categories.

c3controls Compressor Types

Rotodynamic compressors achieve compression by imparting momentum to gas particles. The kinetic energy is then converted into pressure. This type of compressor is popular in industries due to its small size and low vibrations. They are then subcategorized into centrifugal and axial compressors.

Positive displacement compressors are rotary or reciprocating compressors. They compress gas by mechanically reducing its volume. They only operate with a fixed amount of gas flow but are capable of achieving a wide range of pressure values.

Both types of compressors are often interchangeable in most industrial applications. One type or the other may be preferable depending on the application’s demands.

Uses of Compressor Control Systems

Compressor control systems are vital in maintaining the stable operation of a compressor. Their purpose is to guarantee safe working for both the compressor and its operators. Control systems improve the efficiency and durability of the machine.

The control system is composed of a collection of sensors and electrical components. All controls can be commanded from a central terminal. Innovations in sensor technology and microprocessors have increased the controllers’ functionality and versatility.

Large critical compressors typically have more computerized control systems. Such controllers are capable of several automatic functions.

Regardless of the technology used in compressor control systems, we can break their uses down into a list of seven critical operations.

  1. Safe Start and Stop Processes

Starting and shutting down an industrial compressor follows a series of careful steps. This ensures that the compressor starts and stops safely.

During startup, the operator does preliminary checks and preparations. That include valve checking, auxiliary checks, and purging if necessary. The operator must ensure that compressor stabilizers like lubricant and coolant systems are all in the green.

Sensors report on the status of the compressor and all the auxiliaries.

The compressor starts at a low speed to warm up while being carefully monitored. Gradually, the speed increases to ramp speed which is the lowest speed threshold for minimum compression. Eventually, the compressor reaches full speed and its peak performance.

Shutting down is an equally involving process. The compressor is gradually slowed down while its inlet supply is slowly constricted. After continued deceleration, the inlet supply is completely cut off. The compressor is eventually brought to a complete halt.

During these two processes, the compressor controls vary the compressor speed. This is to ensure safe and successful startups and shutdowns. Intelligent control systems can perform these tasks automatically or requiring little human intervention.

  1. Provide System Information

Real-time information from sensors is useful in determining the status of the compressor. For instance, low oil levels may indicate an oil leak. High temperatures may be indicative of worn out parts or insufficient lubrication.

Crucial sensors include:

  • Pressure sensors
  • Temperatures sensors
  • Level sensors
  • Flow sensors
  • Overload sensors

Real-time information from these sensors is useful in determining the status of the compressor. For instance, low oil levels may indicate an oil leak. High temperatures may be indicative of worn out parts or insufficient lubrication.

Sensor systems on auxiliary components are parts of the compressor control system. They monitor environmental variables outside the compressor. This information is still critical to the compressor’s operation.

Every compressor is rated for specific working conditions. Deviations of certain variables away from the optimum level may reduce the efficiency of the compressor. Inefficient machines wear out quicker and consume more energy.

This is why monitoring and reporting are important.

The data collected can help observe the rate of wear of the compressor’s parts. From there, maintenance procedures and schedules can be prepared.

Advance control systems keep large volumes of log data. Over time, the data presents as graphs and tabulated figures.

  1. Driver (Motor) Control

Most compressors use electric motors as their driver. They are efficient, clean, and deliver large figures of torque. Electric motors, however, need motor controls. They help protect the motor and manipulate its operational variables effectively.

Motor controls are normally operated by pilot devices. These are a family of components such as switches and indicators. Essentially, they provide control of the motor to the operator.

Various motor control devices include:

a. Pilot Devices

Pilot devices are mainly used in the commercial or industrial applications where human-to-machine interface is required. These comprise various types of selector switches, pushbuttons, pilot lights, signal beacons, as well as toggle switches. Based on their designs, pilot devices can be distinguished into two types: indication devices and actuation devices. And some devices provide both indication and actuation (ex: illuminated pushbuttons).

Typically used as a part of a control system, automated process, or a control panel, these devices provide information on condition and control monitoring of different types of processes, machinery, and equipment.

Types of Pilot Devices

Pushbuttons – These are the control devices used to manually close and open a set of contacts. Pushbuttons are available with a variety of operator designs such as flush, extended, or mushroom head, with or without illumination. These devices are usually provided with normally closed, normally open, or combination contact blocks.

Pilot Lights – As the name suggests, these devices provide the visual indication about the operating status of a circuit. They are mainly used for ON/OFF indication, changing conditions, and alarm signaling.

c3controls 22mm IEC Pilot Devices

b. Miniature Circuit Breakers

Circuit breakers offer electrical protection to people and equipment from sudden surges, overloads and short circuits.

Miniature Circuit Breakers (MCB) are used for handling current below 100 amps. They are a favorite for applications that don’t have high currents. There are two types of circuit breakers commonly called MCB’s, UL489 and UL1077.

Typically used as a part of a control system, automated process, or a control panel, these devices provide information on condition and control monitoring of different types of processes, machinery, and equipment.

UL 489 Circuit Breakers – UL 489 circuit breakers are “intended for installation in a circuit breaker enclosure or as parts of other devices, such as service entrance equipment and panelboards.” They are regularly required on panel designs, per the National Electrical Code.

UL 1077 Supplementary Protectors – UL 1077 defines supplementary protectors as devices intended for use as overcurrent, over-voltage or under-voltage protection within an appliance or other electrical equipment where branch-circuit overvoltage protection is already provided or is not required.

Important Note: While the term circuit breaker is used to describe both UL 489 and UL 1077 devices, UL 1077 devices are not considered Circuit Breakers by UL. They are defined as Supplementary Protectors.

UL 489 Circuit Breaker and UL 1077 Supplementary Protector

c. Motor Starters

Manual motor controls have a push button starter connected to the power panel. Starting and shutting down the motor is a matter of operating a switch on the starter or operating it remotely.

Large motors need more sophisticated start/stop controllers. These controllers mostly regulate the electrical power feed into the motor from the mains or power supply.

A motor starter describes the assembly of a contactor and overload relay. Additional controls, such as transformers, may vary the characteristics AC waveform going into the motor in terms of frequency, amplitude and voltage to ensure a safe start and shut down.

A relay is a controlled switch that works by responding to an external signal. It is mainly used to control high-powered circuits.

Both relays and contactors are electromagnetic switching components. Contactors usually operate at a higher control voltage and have an overload protection.

Below is a basic compressor control diagram.

Basic Compressor Control Diagram

d. Variable Drive and Speed Controllers

Variable drive and speed controllers enable the operator to adjust the direction of the drive and its speed. The controller is comprised of a series of speed controllers, power converters and regulators.

Many industrial motors use a Variable Frequency Drive (VFD) to control speed. A VFD varies the frequency of the AC input voltage supplied to a three-phase motor. Since the speed of a motor is controlled by the frequency of the supply voltage, increasing or decreasing the frequency changes the speed and torque of the motor. A VFD works by converting three phase AC into DC and then into simulated AC power. VFDs are used not only because they can save wear and tear on a motor, but also due to their energy efficiency. They are, however, much more expensive than the simple motor starter solution shown above.

e. Intelligent Controllers

Intelligent devices are used to monitor and adjust the power output of the motor. They automatically vary the torque and speed variables to match the motor load. This results in efficient power consumption, low noise, low vibration, and less radiated heat.

These controls use programmable logic controllers (PLC) to automate their processes. Linking the motor and control devices to the operator is the Human-Machine Interface (HMI).

HMI is an industrial computer interface. It enables the interaction between the operator and the motor though intelligent motor controls.

A combination PLC-HMI

The driver determines how much power goes into the compressor and how fast the compressor components spin. Most compressors have a variable speed range. Within that range, the compressor can achieve optimum compression.

In such a case, the speed of the driver can be used to vary the pressure or the gas output. In positive displacement rotary compressor, the rotation speed of the input shaft is directly proportional to the compressor’s displacement.

What this means is that varying the speed of the driver varies the output of the compressor. This is useful in applications where the output of the compressor needs to change frequently.

However, compressor efficiency falls with a decrease in drive speed. Varying the drive speed needs to be supplemented by changing other variables. This keeps operations within reasonable efficiency.

The compressor controls are capable of adjusting the driver’s output. They also adjust other variables to ensure that the motor is not overloaded or overheated. The motor controls balance the torque, power, and speed outputs of the motor to maintain efficient operation.

  1. Keep Compressor Operation Stable

With compressors, stability means running at optimum RPMs, optimum gas input, and steady output. Compressor controls have to deal with two common undesirable conditions – choke and surge. These conditions cause a compressor to be unstable.


Surge happens when the input gas supply falls below the optimum capacity. When this happens, the drive motor overloads. This is because the compressor tries to draw in more gas and push the output at the same time.

The result is fluctuations in output, irregular power usage by the motor, and increased vibration and noise. Most compressors are configured to automatically offload if inlet capacity drops below 40%.

Sometimes, adjusting the driver speed to match the reduced gas intake is not possible. Surge controls have to stabilize the compressor. Most compressors have a surge control system. This is a gas path controlled by an automatic valve linking the inlet system to the output system.

To prevent surge, once a drop in gas supply is noted, the valve linking the outlet pipe to the inlet pipe is opened. Some of the output gas is fed into the inlet to increase the volume of gas at the input. Once the original gas source restores regular gas supply, the valve closes. Normal operation resumes.

This controlled flow reversal solves the surge problem. But it also reduces the overall throughput of the compressor.


Choke is the opposite of surge. It is caused by a very high flow rate at the input of a compressor operating at low discharge pressure.

Choking reduces the compressor’s performance drastically. The compressor is unable to deliver optimum pressure and flow at the output.

Choke controls automatically constrict the inlet system by partially closing the inlet valve. Gas coming into the inlet can already be under pressure or accelerated. In such a case, choke controls may opt to dump the excess gas into low-pressure buffer storage to divert the gas from the inlet.

  1. Control the Desired Values of Various Process Variables

The compressor control system is responsible for maintaining the requirements of the compressor. To the operator, it’s only a matter of flipping switches or interacting with an HMI on the control panel, but a lot more goes into executing and adhering to those commands.

It is crucial that the compressor produces the expected output. It is the controller’s job to ensure that this is the case at all times.

Besides adjusting drive speed to control the flow rate and displacement of a compressor, the control system can also modulate the inlet valve to achieve the same results. Inlet valve modulation throttles the gas intake to keep pressure within a designated level.

Reducing the capacity of the incoming gas reduces the pressure and the amount of gas at the output. However, cutting off the inlet supply at full speed causes the compressor to draw a vacuum at the inlet. This may cause motor overload and overheat.

The modulating controls prevent this from happening by adjusting the motor controls. This matches the inlet reduction.

Most compressors operate with a partial load. This means displacement can be adjusted without engaging the driver controls.

  1. Alerts and Alarms

Compressor controls come equipped with alarm systems to alert and warn operators when something goes wrong.

Common alarms include alerts for leaks, overheating, oil pressure, and failure of vital components. These alarms may be visual lights on the control panel or accompanied by beeping sounds. They alert the operators or technical crew of dangers requiring immediate action.

These alerts are particularly helpful when the gas being compressed has dangerous physical or chemical properties such as being corrosive, flammable, or toxic.

  1. Automatic Shutdown in Unsafe Conditions

Most components inside compressors have very low fault tolerances. Sensors monitor the status of critical components. They can take drastic preemptive measure to prevent damage if something goes wrong.

Compressor control systems can initiate an automatic shutdown. This happens after a catastrophic failure of vital components or in unsafe working conditions. Hazardous conditions include uncontrollable surge and choke, or overload of the electrical systems.

Multiple Compressor Controls

In industries that need more than one compressor, the controls link together to form network controls. One compressor assumes the master role while the others become subordinates. All compressors are controlled from a master control system.

The sophisticated networked controls share trending data and commands. All devices are controlled through a central processing unit. This maintains the desired performance and output variables.

Using Compressor Control

Compressor control systems are vital in manipulating compressor variables. They are key to keeping the compressor in its optimum working condition. Their purpose is mainly centered around performance safety and efficiency.

There are several different types of compressors. They all come in different models, sizes, and performance ratings. However, none is complete without a reliable control system.

At c3controls, we are all about electrical control systems. We design and manufacture essential control components. Some of our products include circuit breakers, relays, cam switches, pilot device, and motor controls. Our aim is to safeguard valuable electrical installations.

Browse through our vast array of reasonably priced products. Feel free to inquire about our products and services if you have any questions.

The content provided is intended solely for general information purposes and is provided with the understanding that the authors and publishers are not herein engaged in rendering engineering or other professional advice or services. The practice of engineering is driven by site-specific circumstances unique to each project. Consequently, any use of this information should be done only in consultation with a qualified and licensed professional who can take into account all relevant factors and desired outcomes. The information was posted with reasonable care and attention. However, it is possible that some information is incomplete, incorrect, or inapplicable to particular circumstances or conditions. We do not accept liability for direct or indirect losses resulting from using, relying or acting upon information in this blog post.