28 n ENERGY EFFICIENCY September 2025 www.drivesncontrols.com VSD compressors vary more than you might think Variable-speed drive (VSD) compressors are a more energyefficient alternative to fixed-speed machines in applications with fluctuating demand. As industry looks to cut energy use and reduce emissions, attention has turned to these technologies. But while the core principle behind VSD compressors is consistent, their real-world performance differs markedly depending on how they are engineered. Rather than running at full capacity regardless of demand, VSD compressors adjust the motor speed to match airflow requirements. Most VSD compressors use an inverter to regulate motor speed, with sensors and control logic determining air needs. When well-implemented, this can yield significant electricity savings. However, the effectiveness of VSD compressors depends heavily on their design. Some manufacturers treat the inverter as an add-on to a fixed-speed system. Others develop the motor, element, inverter and controls as an integrated package. These differences, often invisible on datasheets, can affect performance significantly– especially in harsh environments. Differences in design are especially evident during periods of idling or low demand. In theory, a VSD compressor will slow down and consume less power. In practice, many cannot operate efficiently at very low loads. When demand drops below the minimum stable speed, some compressors must shut down or unload, continuing to consume energy while producing no air. Others need to vent system pressure before restarting, introducing energy-intensive “blow-off” losses. Some systems can close the inlet valve and pause while maintaining pressure, avoiding full depressurisation. Others cannot, leading to frequent energy-consuming cycles. Over time, these differences can affect operational costs significantly. Energy savings come not from the VSD alone, but from how components are matched and controlled. While some systems use standard motors and off-theshelf inverters, others rely on high-efficiency motors (such as IE4 or IE5), paired with advanced controllers that adjust dynamically. Thermal management also plays a critical role. In environments with frequent load changes or high ambient temperatures, oversized coolers, efficient oil separators and smart regulation algorithms can make the difference between stability and shutdown. Real-world resilience Rated performance rarely reflects factoryfloor conditions. High temperatures, dust and humidity are common in industry, but not all VSD compressors are built to cope. Heat, for instance, can degrade oil and motor performance. While most compressors are rated up to 46°C, some allow operation up to 55°C using measures such as speed-reducing algorithms, oversized cooling systems and separate aftercoolers. Humidity introduces the risk of internal condensation during low-load operation. If not managed correctly, this can cause oil contamination, corrosion or failure – especially when compressors are oversized and run below optimal speed. Proper sizing and smart temperature control can help to mitigate this risk. Dust presents further challenges. In industries such as woodworking and metal processing, inverter protection is crucial. Some compressors use inverters with basic IP20 ratings, while others offer IP54 or above, with sealed drivetrains and outdoor-ready enclosures. While VSD compressors promise energy savings, actual results depend on correct specification and control. Oversizing – often done to allow for future expansion – can reduce efficiency, increase cycling and accelerate wear. Proper sizing requires an understanding of air demand across shifts, seasons and production cycles. Even limited data can inform better decisions, especially if specifiers consider turn-down ratios and unload losses. A wider operating range and low cycling losses are key to success in variable-demand applications. Control systems can also influence outcomes. Intelligent logic that coordinates multiple machines and adjusts them dynamically will outperform isolated systems. Integration with wider automation platforms can further enhance visibility and responsiveness. Although VSD compressors often cost more upfront, their electricity use – typically 80% of lifetime operating costs – means payback periods can be short if performance is optimised. That depends not just on the principle, but on engineering and integration. VSD compressors can offer efficiency, flexibility and control, but these benefits are not guaranteed. Differences in motor design, start/stop logic, environmental protection and control integration all influence performance. As pressure grows to reduce energy use and emissions, VSDs will play a key role. But specifiers must look beyond headline figures and ask how a compressor performs under real conditions. Simply choosing a variable-speed model is not enough. Understanding how that performance is delivered is essential. n Industrial compressors controlled by VSDs can save significant amounts of energy. But not all are the same. Jason Pollard, business line manager for industrial air at Atlas Copco, suggests some points to consider when choosing VSD-based compressors. Not all VSD-controlled compressors are equal: there can be significant differences behind their panels
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