Is variable speed compressor better?

Fixed vs Variable Speed Drive Compressors

Traditional air compressors are fixed speed, meaning they run at a constant and consistent speed. This produces a fixed amount of compressed air per minute. There are many benefits to fixed speed compressor technology if your compressed air demand is constant and unchanging. However, this isn't always the case. As fixed speed compressors are always operating at full-throttle, if all of the output is not required then energy is being wasted.

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Furthermore, fixed speed compressors run unloaded as the stress of an engine start-up would put pressure on the motor. This can be a waste of energy as the machine is running without producing any compressed air. Variable speed compressors avoid this issue by matching the output with the demand created. By simply producing the exact amount of air being used by the downstream equipment, variable speed compressors help to improve plant efficiency.

Watch this video to see how a fixed speed compressor can be sequenced with a variable speed machine to precisely match output with network demand to save energy.

First, a little background on fixed and variable-speed drive.

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A fixed-speed compressor operates at the same speed continuously, cranking along at the same RPM, no matter what the system's demand. And, unless you change the pressure, the compressor's output of CFM will also remain the same.

A VSD air compressor, on the other hand, is equipped with a variable frequency drive or inverter that allows it to adjust its motor speed or RPM (and thus its CFM) to match the real-time demand for compressed air.  When the demand drops, the inverter dials back the motor operation to a lower speed (and lower CFM). It thus consumes proportionately less energy (and money), creating a relatively linear output-to-input efficiency curve.

VSDs are an extremely energy-efficient way to address rapid cycling, which occurs when your air compressor switches back into the loaded state before the transition to the no-load state is complete. If that happens too frequently, it can cause increased wear and reduce compressor reliability and lifespan.

VSDs are recommended when the compressor operates at a load between 30% and 80% of its total capacity most of the time. They are not recommended when your compressor is running at 100% load most of the time as the VSD drive itself has some losses (that can be overcome by optimizing tip speeds). But they are very valuable as a trim compressor complementing a base load compressor (see 'System Reconfiguration' section below).

Variable-speed drives complement rotary screw air compressor technology exceptionally well.

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