September 2021

A n 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 does not just improve a company’s bottom Process, Controls & Plant Focus on: Compressed Air 26 | Plant & Works Engineering September 2021 Variable Speed Drive: why is it important for compressor users? 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 fixed- speed unit? PWE reports.