Energy Savings of 30% can typically be gained, but energy savings as high as 50% are not uncommon on an oversized screw air compressor. A Variable Speed Drive (VSD) has the ability to vary the output speed of the compressor’s airend depending on the actual pressure in the system. The net result of this means that when air demand is low, the speed of the compressor is reduced, but when the air demand is high the compressor runs at full speed. One of the benefits of a Variable Speed screw compressor is that it stops the compressor ‘unloading’ when the air demand is low, unlike a fixed speed machine which will vent unused air to atmosphere when not required. Venting unused air to atmosphere is a particularly wasteful method of flow control when compared to a VSD screw compressor which backs off the speed when demand is low, therefore saving energy. On a constant torque application such as a screw compressor, power is proportional to speed. For example if the screw compressor is loaded but the air demand is low and running at 50% speed, the energy savings will be 50%.

Because a VSD compressor offers dynamic pressure control, it is capable of maintaining  a smooth and stable pressure when compared to the ‘on-off’ load control of a fixed speed machine. A VSD screw compressor can typically maintain pressure to within +/- 0.1 Bar while fixed speed compressors can typically hold pressure to within 0.5 bar. As the air pressure is controlled much more tightly on a VSD screw compressor, a small reduction of the  pressure set point is often possible, which again offers additional savings over a fixed speed machine. In addition a 0.3 bar reduction in pressure will also reduce air leakages by around 4%.

Typically a Variable Speed Screw Air Compressor will maintain Full Load Current (FLC) on start up within 1-2 times of the nominal rated motor current. This method of starting a motor offers the purest form of soft start. Even a dedicated soft start unit can only maintain FLC on start up within 3-4 times of the motors nominal current. Compare this to a standard Star Delta starter which maintains FLC on start up between 7-8 times and you can see the benefit an SCR Variable Speed screw compressor can offer. This pure soft start means reduced mechanical stress on belts, couplings and gearboxes, as well as increasing the reliability of the screw air compressor. It can also help keep the sites electrical peak demand  to a minimum when starting the compressor

A Variable Speed Drive offers improved protection of the compressors main motor over a traditional thermal overload. A modern VSD can sense a number of faults within the motor such as Overcurrent Faults, Earth Faults and Short Circuit faults. This enhanced level of motor protection can help reduce costly motor repairs. On board drive diagnostics help reduce down time and fault finding.

Installing a VSD Compressor can help lift the sites Power Factor due to the internal capacitance within the VSD itself.