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Variable Speed Drive: why is it important for compressor users?

01 June 2021

Typically, a fifth of UK industry’s electricity bill is attributed to the production of compressed air, but it is a sobering fact that a substantial portion of the generated power dedicated to its production is actually energy wasted.

That is why, over 25 years ago, Atlas Copco addressed the problem and was the first to introduce VFD/VSD technology in its screw compressors, which is recognised as the greatest energy-saving innovation in the history of compressed air developments.

Now, in the form of the latest advanced solutions, compressors with VSD+ specifications are not only capable of achieving outstanding savings of up to 50 percent of their energy requirement, but they offer the attributes of total functional reliability and the essential benefit of smart communication. 

As we will see later, Atlas Copco was once again at the forefront with essential connectivity for the progressive factories of today and tomorrow. Furthermore, these innovative concepts offer low operating costs, reduce payback time, and increase the return on investment over the compressor's lifetime. 

So, how is a VSD compressor different from a conventional fixed-speed unit?

VSD vs fixed-speed compressors

An air compressor with VSD automatically adjusts the compressor’s operating speed to match the production of compressed air to the demand in real time. By varying the speed of its drive motor as air demand decreases, the VSD lowers the delivered airflow as well as the electrical power consumption. 

VSD compressors are specifically designed for applications where demand for compressed air fluctuates, such as facilities that operate different processes or multiple shifts, so flow demand increases and decreases throughout the day.

As the name suggests, a fixed-speed air compressor operates at the same speed, whenever it is running, and can only be stopped and started a certain number of times within a given time frame. By design, it always draws the amount of power needed to meet the maximum compressed air demand, even if the demand is less than the maximum. Because the speed of the motor cannot change, whenever the demand for air fluctuates, instead of stopping the compressor, it will be unloaded, throttled, and an air inlet valve will adjust to allow either more or less air to flow out.

In contrast, a VSD air compressor can operate anywhere in the range between its minimum and maximum speed. When demand increases, the motor speeds up. When demand decreases, the motor slows down. This capability saves energy because the compressor does not have to run flat out whenever it is operating.

A VSD makes sense when demand fluctuates significantly between jobs, when demand varies by shift – on weekends or seasonally – or when manufacturing processes require tight tolerances for operating pressure.

Fixed speed makes sense when the demand for compressed air fluctuates very little; it is worth noting that variable speed drive compressors are not necessarily appropriate for all industrial applications. If a variable speed drive compressor operates continuously at full speed, the switching losses of the frequency converter can result in lower energy efficiency than an otherwise identically sized fixed speed compressor.

How does VSD work?

Automatically adjusting the speed of the motor to match the air demand allows the pressure band of the system to be maintained within narrow tolerances. The motor speed of the compressor is regulated by frequency conversion, which results in the variation of airflow. 

With the addition of advanced electronic controls, the delivery pressure is kept within a 0.10 bar pressure band, rather than the conventional 0.68 -1.03 bar pressure band used on inlet controls for non-VSD equipment. Therefore, a lower air compressor delivery pressure can be used to maintain the system's required minimum working pressure, resulting in increased energy savings and lower operating costs.

Additionally, the converter in the VSD system performs a “soft” start/stop operation by automatically controlling both acceleration and deceleration levels. This eliminates the start-up amperage spikes, which can result in hefty peak current cost penalties from utility companies. Eliminating these peaks also protects electrical and mechanical components from the stresses that can shorten the life of an air compressor.

The primary benefits of VSD

Of the three primary benefits that VSD technology brings to compressor users - energy efficiency, reliability, and connectivity – it is energy efficiency that is the key to affordable air, with up to 50 percent achievable savings and lower cost of ownership. 

It is a fact that most operators who decide to purchase a variable speed drive compressor do so primarily because of the prospect of a low total cost of ownership. Even though the compressor’s initial price tag is likely to be higher than that of equivalent, fixed-speed models - because of its greater sophistication and advanced components - VSD compressors are actually less expensive to own and operate over their lifetime. That is because they consume far less energy and the cost of that energy makes up the bulk of a compressor’s total cost of ownership.

An additional benefit of this increased efficiency is that it also lowers emission levels and allows for a more sustainable production. As a result, a VSD compressor does not just improve a company’s bottom line, it also helps to protect the environment.

Stating the obvious, plant operators can only enjoy improved energy efficiency if the compressor keeps on running reliably. That is why Atlas Copco’s VSD compressors are built to function unfailingly, even in the toughest conditions. That means being capable of operating at ambient temperatures of up to 46°C and making certain that the electrical cubicle of the inverter, which is a crucial piece of technology for the efficiency of the compressor, can withstand temperatures of up to 55°C.

It also includes components such as the IP66-rated drive train, which is specifically designed to keep out dust and moisture. The net results of these dependable features are smooth operation, productivity gains, increased uptime, and lower maintenance costs.   

New technology 

More air, smaller footprint, less energy cost - Atlas Copco’s latest compact GA VSD+ compressors bring industry-leading savings to medium and large-scale operations. The new technology eliminates most challenges associated with the original VSD designs, starting with the performance of the integral, in-house designed Neos compact inverter unit, which contributes to an overall eight percent improvement in Specific Energy Requirement (SER) over that of any comparable compressors.

The principal game-changing innovation of the VSD+ design is in the close-coupled drive train - the combination of a high-efficiency motor and technically advanced, asymmetric compression element. The IP55 motor was designed in-house, with special attention to cooling requirements and efficiency.

With no coupling or gears between the motor and screw element, the compressors are exceptionally quiet, 73dB (A) in operation and extremely energy efficient in performance. The vertical aspect drive train of the VSD+ machines is a closed circuit in which both the motor and the element is cooled with oil, thus contributing additional energy savings. Added to this is an impressive six percent increase in free air delivery for improved efficiency and performance, plus a reduction in the compressors’ total lifecycle costs in the order of 37 percent.

Smart communication

Atlas Copco is not just ahead in terms of efficiency and reliability but also connectivity. The importance of connectivity with advanced monitoring and control systems is an area that is becoming increasingly important because it not only makes the compressor easier to operate, but also increases both efficiency and reliability.

For example, Atlas Copco’s VSD+ compressors come with a high-tech Elektronikon® Touch controller and the optional SMARTLINK monitoring system. In combination, these features make it possible to optimise a compressor remotely from just about anywhere on the planet, using only a computer or even a mobile device. That makes compressed air production even more efficient (and greener). In addition, the system anticipates potential issues and alerts the operator before they become problems, which further increases uptime.

Technology of the future

The entire manufacturing sector is in the midst of a major technological revolution. The move toward connectivity and digitalisation in the era of Industry 4.0 is now a reality rather than a distant future. 

Atlas Copco is at the forefront in the use of OPC UA communication technology in compressors. OPC UA is the common language that allows equipment to “speak” with each other. It is technology that ensures that an operation runs smoothly by making sure that every item of equipment can effortlessly communicate with every other device.

Because a compressor is too important not to be part of a networked production operation, Atlas Copco has released its first series of compressors that speak this universal language and has assured compressor users that, in the future, it will be possible to install this technology retroactively and enjoy all its benefits.

The upsides to being at the forefront of this technology are obvious: the possibilities that the new connectivity offers will completely change manufacturing as we know it. In the world of compressed air, it is the biggest thing since VSD compressors redefined energy efficiency and changed the industry forever.

It should come as no surprise that Atlas Copco, which invented VSD compressors, is once again at the cutting edge of this new technology, determined to make its products, as well as industry’s production, even more efficient and reliable.

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