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What is variable speed drive?

Atlas Copco pioneered the variable speed drive (VSD) in the compressor industry back in 1994. Today, VSD compressors are recognized as the most energy-efficient compressors available and Atlas Copco offers the widest range on the market. But what is variable speed drive? How does it work and what are its advantages?

Atlas Copco’s most energy-efficient compressor, the GA VSD+


A variable speed drive (VSD), speed-controller or frequency-controlled, compressor automatically adjusts its motor speed to the air demand. Its counterpart, fixed speed or “idling compressor”, or “load/unload compressor, is either on full throttle or off. Compare it to a car: an “idling compressor” either drives 100 km per hour or not at all. That is fine if the application requires an either full load or no load compressed air supply. But most applications don’t, they have a fluctuating air demand and that is where a VSD compressor comes in the picture. A Variable Speed Drive compressor simply adjusts its motor and elements speed to match the demand.



The  advantage is obvious: a variable speed drive only runs at the required speed, which saves considerable amounts of energy. When compared to an idling compressor, a VSD allows for energy savings of 35% on average, a GA VSD+ even 50% on average. And those savings do matter:

70% of a compressor’s cost is energy
The initial investment or maintenance costs of a compressor do not compare to its energy costs. Producing compressed air can account for more than 40% of a plant’s total electricity bill. Studies show that many production facilities have variations in air demand, all of which could benefit from a VSD compressor. You can do the math for your own compressor installation, or calculate your energy savings with our online tool. Of course you can also leave the work up to Atlas Copco: every sales engineer is armed with the skills and tools to measure your installation and compressed air requirement.

Variable speed drive has more benefits
The energy savings are the most important advantages of VSD technology, but there are other reasons to choose VSD or VSD+:
  • A VSD compressor can start/stop under full system pressure. There is no need to unload. This saves time and energy.
  • No idling time lost, no blow-off losses in normal operations
  • With VSD, you avoid peak currents at start up and therefore also the penalties of most electricity companies.
  • Thanks to a lower system pressure, system leakages are minimized.

How does a VSD compressor work?

The heart of the GA VSD+: the drive train


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.

The motors in standard VSD units are induction motors, coupled to the screw elements with gears, but our VSD+ features an interior permanent magnet motor, a revolutionary in-house design. The motor shares a rotor with the screw elements, so it’s direct-driven, exceptionally quiet and most of all, very energy-efficient. Learn more on how the permanent magnet motor works. The whole drive train of the VSD+ is a closed circuit, where both motor and elements are cooled with oil, another great way to save energy. Watch the working principle of the VSD+.

The motor speed of a VSD is kept in check by a frequency inverter and the on-board controller, the Elektronikon®, electronics that measure the system pressure and adjust the motor speed to the actual air demand.

VSD technology for every application

Oil-injected screw compressors have always been the most obvious type of compressors to be outfitted with VSD. The rotary screw elements are particularly suitable to be VSD controlled. However, some applications demand oil-free air, so for those we also have a solution available.

These are the technologies for which we have a VSD variant:

1. Oil-injected screw compressors: GA VSD and GA VSD+
The solution for any general application that needs high-quality compressed air. From small workshops to large automotive companies, on board of ships and in all kinds of assembly plants. Available in sizes from 5 to 315 kW (7.5 – 200 hp)

2. Oil-free tooth compressors, ZT/ZR VSD
The ZR/ZT’s double tooth compression element requires no lubrication at all. These compressors are especially suitable for all oil-free, critical applications such as paint spraying, but also laboratories, medical and pharmaceutical applications, food and beverage plants and electronics prefer the benefits of absolutely oil-free technology. Available in sizes from 22 to 55 kW (30 – 75 hp)

3. Water-injected screw compressors, AQ VSD
Instead of oil, these compressors are cooled and lubricated with water. The AQ harbors polymer ceramic screw elements and even the bearings are water-lubricated, so the drive train is 100% free of oil… and so is the compressed air. These compressors can be used for any oil-free application, but they are usually chosen for very particular tasks, such as paint spraying, injection molding, to operate industrial robots and textile jet looms. Available in sizes from 15 to 55 kW (20 – 75 hp)

4. Oil-free screw compressors, Z VSD, ZS VSD
Available in low (ZE and ZA VSD) and medium (ZR and ZT VSD), the Z oil-free screw compressor offers the widest possible range of oil-free screw compressors with VSD technology. For any demand above 55 kW (75 hp), there is an oil-free screw compressor available. All those ranges combined, Atlas Copco offers solutions for food and beverage, pharmaceutical, medical and electronic industries of all sizes and shapes. Available in sizes from 22 to 900 kW (30 – 1253 hp)

5. Oil-free centrifugal and screw blowers, ZB VSD and ZS VSD
The ZS makes use of an oil-free rotary screw, the ZB is a centrifugal blower, both deliver low pressure, oil-free compressed air. They are popular in pneumatic conveying, mining and perfect for waste water treatment plants. Available in sizes from 22 to 355 kW (30 – 475 hp)