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An air compressor with Variable Speed Drive (VSD) automatically adjusts the compressor’s operating speed to match production of compressed air to demand in real time. VSD compressors are designed for operations 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.

Compared to fixed speed compressors that operate only at full speed, VSD compressors offers several benefits:

• Because the VSD compressor only runs when needed, it reduces energy cost—typically enough to pay for the additional investment in VSD technology.
• A VSD compressor can start/stop under full system pressure. There is no need to unload, which saves both time and energy.
• No time is lost to idling.
• No blow-off losses occur in normal operations.
• Power company penalties for peak current at startup are avoided.
• Air system pressure is more consistent and also lower, minimizing leakage

The largest component (about 70%) in the total cost of ownership for an industrial air compressor is not the equipment itself. It is electricity. A VSD compressor costs more to purchase than an otherwise identical fixed speed compressor, but the extra initial investment is typically returned through energy savings. VSD technology can reduce energy cost for a compressor by 35% to 50%, depending on the application and the size of the compressor, so annual savings with VSD can range from hundreds of dollars to tens of thousands of dollars.

The savings can be so significant that governments and electric utilities may offer financial incentives, such as rebates, tax credits or interest free loans, to encourage companies to upgrade to more energy-efficient VSD technology that helps reduce overall consumption of electricity and minimize sudden spikes in demand.

The magic is in the drive train, the combination of motor and element. Variable Speed Drive technology works best with rotary screw compressors, as their flow rate and their power consumption are virtually proportional to their speed. So as the motor adapts its speed, so do the screw elements and, as a result, the amount of compressed air delivered. The electric motor is specifically designed for the job, with special attention to cooling requirements and efficiency across the entire speed range.

Some industrial processes are consistent in operation, so a properly sized, fixed speed compressor can meet the demand for compressed air reliably and efficiently. Where demand remains constant within 5% to 10% of the total free air delivery flow rate, a fixed speed compressor can provide higher efficiency than a Variable Speed Drive compressor. Importantly, a VSD compressor is not designed to operate continuously at full speed. When it does, the switching losses of the inverter result in lower energy efficiency than an otherwise identical fixed speed compressor.