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February 19, 2025How Variable-Frequency-Drive Air Compressors Work
Kaishan USA | February 5, 2025| Uncategorized
Technology changes have made variable-frequency and variable-speed drives more attractive in industries like metalworking.
Variable-frequency drives have become popular additions to rotary screw air compressors for many reasons.
First, a word about terminology.
VFD or VSD?
There’s a lot of confusion about the terminology. Is it a variable-speed or variable-frequency drive? There is a difference.
However, it is relatively easy to distinguish between the two terms when you have one key piece of information: there are several ways to vary speed. The mining industry, for instance, uses variable voltage variable frequency drives, which change both the voltage and the frequency to control motor speed.
So, variable-frequency and VVVF drives are different ways to achieve the same result: operating a motor at variable speeds. Because VFDs and VVVF drives both do that, they are both VSDs. But, as we’ve seen, not all VSDs are VFDs.
Since the compressed air industry primarily uses VFDs and uses the two terms interchangeably, we will use both in this post.
VSDs and VFDs can save energy and money and boost performance. Here’s how.
How VFDs Work
A variable frequency drive is an electronic component that adjusts the frequency of the power supplied to your air compressor’s motor.
Based on input from sensors throughout your system, the VFD speeds up or slows down the motor to match the compressed air demand, delivering several significant benefits.
Facilities may benefit from using VFD/VSD air compressors if they have significant differences in compressed air demand between shifts.
Benefits of VSDs/VFDs
First, by reducing motor speed when demand is low, you save energy. The Compressed Air & Gas Institute estimates a VSD will reduce energy use by about 33%.
CAGI says you can keep the pressure band to +/- 1.5 PSIG with a variable-speed compressor.
Second, you’ll save money. A study by the American Society of Mechanical Engineers placed the average annual cost savings of VSD compressors at $17,208 with a two- to five-year payback. Plus, you may be able to qualify for rebates from state and local governments and utilities.
You can ramp up to full speed more gradually, avoiding the high in-rush current (six times as high as standard operating current) that fixed-speed compressors draw during startup. Thus, they reduce peak amps, avoiding utility company penalties for increased demand on the power grid. In general, you can match the demand profile as efficiently as possible.
They also help address rapid cycling, where your compressor is damaged by turning on and off too frequently. (For more information on this problem, see our blog post, “My Compressor is Rapid Cycling. Now What?“
We provide more detail on the benefits of VSDs and VFDs in our blog post, “The Benefits of Installing Variable-Speed Compressors.”
The good news is that a few technical advances have extended the benefits of VSDs to more applications. Let’s start with liquid-cooled VSDs.
Liquid-cooled VSDs
For many years, Kaishan did not recommend using VSDs in industrial environments where the air contained dust and dirt.
In a traditional VSD, airborne contaminants clog the VSD’s heat sink, causing it to overheat. That’s why we did not recommend installing VSDs where the unit would be near a welding operation, for example, which adds conductive material to the ambient air.
That has changed, however. Newer, liquid-cooled VSD drives can alleviate some of the problems associated with dust and dirt in the air.
Kaishan’s new liquid-cooled VFD/VSD can dissipate heat using water, glycol or other appropriate coolant. Because of the superior heat-dissipation capabilities of liquids, the liquid-cooled VSD is smaller than an air-cooled model.
So, while a traditional VSD-driven air compressor can overheat when dust, dirt and airborne contaminants clog the drive’s heat sink and fans, newer VSDs will quickly remove the heat with a liquid-cooling system.
Other advantages of the liquid-cooled unit include:
- Temperature. A liquid-cooled VSD runs cooler due to the efficiency of heat removal. Operating electronics at lower temperatures is always preferred.
- Efficiency. Because a liquid-cooled VSD does not add as much heat to the environment, the compressor does not have to remove as much as it would with an air-cooled VSD, allowing the unit to run more efficiently.
- Smaller footprint. Depending on the manufacturer, the VSD may not require as much space in your machine.
Remote VSDs
Another option for extremely dirty environments is changing the VSD’s location. You could, for instance, locate a variable-speed drive unit in a separate control room with cleaner air and cooler temperatures than adjacent manufacturing and production areas. More and more companies are creating these rooms to protect sensitive controls.
The distance between the VFD/VSD and the compressor should not exceed 50 feet because the various harmonic frequencies in the factory create too much interference at greater distances.
Variable-frequency and variable-speed drives, like those newly offered by Kaishan USA, allow rotary screw air compressors to operate more efficiently at part-load conditions. But they’re not for everyone.
Best Applications
VSDs and VFDs deliver the greatest benefits when you have significant variation in demand.
They work best for uses between 30-70% of capacity in facilities with changes in demand between shifts or at different times of the year. VSDs are not recommended at capacities below 20% or above 80%.
Oil-Free Units
Because they have more significant heat-dissipation challenges, oil-free air rotary screw compressors have a more limited range for VSDs than oil-flooded units. Keeping an oil-free unit in the 60% range is preferable.
And while VSDs offer a wide range of benefits, there are some situations in which they are not the best solution.
When a VSD/VFD Is Not Recommended
You should probably avoid using a VFD/VSD where you have:
- Little to no variation in demand
- An inconsistent power supply
- Excessive dirt, dust, moisture, heat or cold (unless you are using a liquid-cooled VSD instead of a traditional air-cooled VFD)
There are alternatives to VSDs, including modulation. You may also find a multiple-compressor strategy is a more effective solution, especially in providing backup during a compressor shutdown. For more on these VSD alternatives, see our blog post, “My Compressor is Rapid Cycling. Now What?"
If your company would benefit from adding a VSD or VFD air compressor, we recommend working with a compressed air professional.
Not a DIY Project
Unless you’re a controls engineer, you probably don’t want to take on a VSD installation as a do-it-yourself project. You will save yourself a lot of potential headaches by working with a qualified compressed air professional. And avoid some of the pitfalls that occur when VSDs or VFDs are not appropriate for your application.
We recommend you work with your local compressed air expert to specify, select and install a VSD air compressor.
Kaishan USA works with a nationwide network of independent distributors, who can provide on-site help and consultation as needed. These factory-trained air compression experts can audit your air compressor system and help you decide whether a VSD or VFD compressor would be an asset in your application.
Key Takeaways
- Variable-frequency drives have become popular additions to rotary screw air compressors for many reasons.
- A variable frequency drive is an electronic component that adjusts the frequency of the power supplied to your air compressor’s motor.
- First, by reducing motor speed when demand is low, VFDs/VSDs save energy.
- VSDs also save energy and qualify for state, local and utility company rebates.
- Newer, liquid-cooled VSD drives can alleviate some of the problems associated with dust and dirt in the air.
- Remote VSDs are another option for extremely dirty environments.
- VSDs and VFDs deliver the greatest benefits when you have 20-80% variation in demand or significant variation between shifts and seasons.
- You should avoid using a VFD/VSD where you have little to no variation in demand, an inconsistent power supply or dirt, dust, moisture, heat or cold more than what a liquid-cooled VSD could handle.
Let Us Help
Specifying and installing an air compressor with a VFD or VSD is critical to operating your compressed air system and all the processes that rely on that system. If you need help deciding whether a VSD is right for your application, get in touch with the experts at Kaishan. Contact us today.
