Collection: Variable Speed Rotary Air Compressors

A variable speed drive compressor pays for itself when average demand sits somewhere between 50 and 70 percent of the machine's capacity and swings over the course of a shift. That is the whole test. A VSD turns down to roughly 20 to 25 percent of full flow and tracks your actual draw instead of loading and unloading against it.

The savings come out of what a fixed speed machine burns while it is unloaded. An unloaded rotary screw still pulls 20 to 35 percent of full load power just to spin the airend and keep the sump pressurized. A VSD slows down instead, so a plant averaging 40 percent load can take 25 to 35 percent off its compressed air bill.

There are two cases where a VSD is the wrong buy, and we will tell you so. If your plant runs flat out near full load most of the day, the drive does nothing for you and you have paid a premium for a VFD that will eventually want replacing. Buy fixed speed. And if the compressor runs two or three hours a day, the energy savings are too small to ever earn back the difference. The arithmetic is run hours times kilowatts times your power rate. In a two shift plant it works. In a one man shop it usually does not.

One more thing worth saying out loud. A VSD belongs on the trim duty, not the base load. When you add a second compressor, the VSD should cover the swing while the fixed speed machine carries the steady floor underneath it. Plants that wire it the other way around wonder why the new compressor never saved them anything.