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

Published:  13 October, 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 energy wasted. So how is a VSD compressor different from a conventional fixedspeed unit? PWE reports

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 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. Where 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.

Where 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% 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 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 less expensive to own and operate over their lifetime. That is because they consume far less energy and because 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 line, it also helps to protect the environment. 

New technology

New technology, such as Atlas Copco’s latest compact GA VSD+ compressors, eliminate 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 8% improvement in Specific Energy Requirement (SER) over that of any comparable compressors. The principal 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 inhouse with special attention to cooling requirements and efficiency.

With no coupling or gears between the motor and screw element the compressors are 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 a 6% increase in free air delivery for improved efficiency and performance, plus a reduction in the compressors’ total lifecycle costs in the order of 37%.

Smart communication

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. As such, manufacturers are increasingly using OPC UA communication technology, which 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 for example, has therefore 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.

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