What is a Variable Speed Drive Compressor and Do You Need One?

Compressed air is critical in the manufacturing industry; however, if your compressor runs all day when you only need full capacity for a fraction of the time, you may be wasting a staggering amount of energy every year. In facilities with fluctuating air demand, a properly sized variable speed drive (VSD) air compressor may help eliminate energy loss and minimize electricity costs.

What is a VSD Compressor and How Does it Work?

While fixed-speed compressors operate at the same speed regardless of air demand, variable-speed drive compressors adjust motor speed to match air demand. VSD compressor designs offer comparatively low in-rush currents as well, providing additional benefits in systems where a compressor may need to start & stop often. 

VSD vs. VFD: Understanding the Terminology

The terms VSD and VFD (variable frequency drive) are often used interchangeably in the compressor industry, as VFD is the technology inside the VSD compressor that controls the motor’s frequency (and, thus, the speed). Variable Speed Drive is a term that can be used to describe a broader range of variable speed drive technologies, not just VFD.

Fixed-Speed vs. VSD: Unlock Cost Savings with a VSD Compressor

When you understand the difference between fixed-speed and VSD compressors, it’s easy to see how a VSD unit can save facilities with partial-load demand big bucks in energy cost reductions.

Fixed-Speed Compressors when not properly applied are Costly for Partial-Load Demand

For manufacturers with consistent partial-load demand, fixed-speed compressors (15 hp or larger) may be wasteful because their operational technology doesn’t support the demand associated with variable loads efficiently. Fixed-speed units rely on “load/no-load” control schemes ( either a fully open or closed inlet for load and unload), modulation (cutting back the inlet throttle plate) to manage fluctuating demand or variable displacement control. 

And, while fixed-speed compressors may be a good fit when running base loaded (100% capacity) for facilities with constant demand or when using multiple compressors to handle variable demand 24/7. Storage required for an oil-flooded rotary screw load/no-load type compressor may exceed 10 gallons/cfm to approach the efficiency of a VSD compressor when it comes to handling partial loads.  As a result, constant speed may not be the most efficient solution for partial-load runtime. Think of a fixed-speed compressor running unloaded for hours every week, like leaving your car idling while you run into a store – it unnecessarily wastes energy.  Constant speed compressors utilizing variable displacement control can be a good alternative to VSD compressors in certain applications.  Modulation control is low cost, low maintenance and maintains a steady output pressure at varying demands, but significantly less efficient than the other technologies in most applications.

Key Benefits of VSD Technology

Due to its operational technology, VSD units deliver these key benefits:

• A reduction in energy consumption between 30 and 70% in variable applications.
• Soft start/stop functionality that reduces component wear and lowers starting currents.
• Constant pressure that delivers exact pressure and flow when needed.
• Reduced maintenance because there is less wear and tear.
• Optimized performance due to lower delivery pressures and tighter tolerances.

VSD Technology Delivers Power When Needed

VSD compressors feature a control system that supplies the proper flow and pressure when it’s needed.

VSD/VFD vs. Fixed-Speed Control

Stops/Starts: VSD motor designs have comparatively low in-rush currents, allowing VSD compressors to stop and start at lower air demands without issue.

Pressure Control: The VSD measures system pressure and maintains a constant delivery pressure with a narrow band (for example, within a 1.5 psi range). For comparison, fixed-speed load/ no-load units typically have a 10 to 15 psi pressure fluctuation and require a plant demand regulator to utilize storage effectively and maintain a steady pressure output. The accurate delivery pressure offered by VSD technology maintains the minimum working pressure. And, for every 2 psi reduction in pressure, energy consumption is lowered by 1%, which results in a potential 6% energy savings.

Built in Soft Start Function

The VSD compressor performs a “soft” start by ramping up motor speed, which provides two main benefits:

• Benefit 1: This eliminates amperage draw peaks, which typically incur power company penalties and higher rates.
• Benefit 2: It also protects electrical and mechanical components from the mechanical stress of start-up, lengthening the life of the compressor.

The Business Case for VSD Compressors

Use of a VSD compressor is likely justifiable when you look at the potential savings and return on investment (ROI).

Power Savings

A properly sized VSD compressor can yield significant power savings compared to a fixed-speed unit. In some cases, based on the demand profile, this can reduce compressed air costs by one-third.

Oversizing Problem

Due to economic challenges, many facilities are operating oversized air compressors, highlighting the need to review compressed air requirements when demand profiles change.

Cost Savings

Consider that to run a 200 HP fixed-speed air compressor, the annual cost at 3 cents/kWh was $41,273. At today’s 8 cents/kWh, the cost is $110,062 annually. After assessment, a VSD compressor saved a facility $38,521 annually (or $192,000 over five years) due to a 35% average fluctuation with the VSD’s control range.

ROI

Combining savings often results in a return on investment of less than two years. Additionally, many local municipalities and state utilities offer rebate incentives for energy-saving solutions like VSD technology, further offsetting the cost of upgrading to VSD technology.

While not every installation will yield this payback, a professional air demand assessment and assistance with proper compressor selection from JHFOSTER, your partner in compressed air solutions, will determine the level of savings you can unlock with a VSD compressor. Contact us to learn more.

Take a Step Toward Savings with VSD Compressors and JHFOSTER

Manufacturers who choose a properly sized VSD compressor experience immediate energy savings that will compound over time due to the reliability, efficiency, and cost-cutting potential of VSD units.

A compressed air audit will determine if a VSD compressor is a good fit for your facility. Contact the experts at JHFOSTER to achieve better air compression efficiency and lower your operating costs.