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Variable speed drive air compressors

VSD Variable Speed Drive VSD+ Oil-lubricated compressors GA VSD

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

Watch this video to learn more about VSD technology

Energy savings

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.

How a VSD compressor works

VSD compressor
Variable Speed Drive saves energy by adjusting the speed (RPM) of the compressor by means of an inverter.

Instead of all the power going directly into a traditional AC fixed-speed compressor running at full capacity, the inverter provides the VSD compressor with the specific voltage required.

Electronics measure system pressure and an on-board controller adjusts motor speed to match actual air demand. It’s a simple idea although the technology that makes it possible is sophisticated.

VSD drivetrain

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.

VSD motor

iPM motor
Standard VSD units are equipped with induction motors, coupled to the screw elements with gears, but the latest VSD technology features an interior Permanent Magnet motor. In the VSD+ design, the iPM motor shares a rotor with the screw elements.

That means it’s direct drive, very energy efficient and quiet. The whole drive train is a closed circuit, where both motor and elements are cooled with oil, another great way to save energy. Compressors with the iPM technology reduce energy consumption by 50% on average compared to an idling compressor.

The new technology also features a vertical design concept which reduces the footprint by up to 55 percent compared to traditional air compressors.

When a VSD compressor makes sense

VSD versus fixed
For operations with fluctuating demand for compressed air, a VSD compressor is worth looking into. A traditional fixed-speed air compressor is either on or off, and when on it only operates at full capacity.

If less air is needed, a lot of energy is consumed and wasted. A VSD air compressor automatically adjusts motor speed to match air demand, providing significant energy savings.

When a VSD compressor may not make sense

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. 

Determining if a VSD compressor makes sense

determining VSD performance
A professional air audit is the best way to identify if a VSD compressor is most appropriate for any given compressed air application.

These audits are available from companies that specialize in the implementation of compressed air equipment.

An air audit can help to determine the most efficient controls for a compressed air system, including Variable Speed Drive.