It used to be that automotive, woodworking, fabrication and other smaller businesses focused on choosing between a piston and a rotary screw compressor. In recent years, many have shifted to newer rotary compressors and have been keeping a keen eye on the latest technology. Specifically, variable speed drive —also commonly referred to as variable frequency drive (VFD).
Variable frequency drive units present many advantages and we sell a lot of them, but sometimes the latest isn’t always the greatest. Here are some points to consider if you’re thinking about purchasing a variable speed drive compressor for your shop.
What is a variable speed drive compressor?
A variable speed machine varies the speed of the motor to vary the compressed air flow rate (CFM) in response to air demand. This is most commonly done using sophisticated electronics to modify incoming power to vary the frequency of the electricity in response to a pressure signal at the compressor discharge (sometimes further downstream in larger, multi-compressor systems).
What are the benefits?
Energy savings. Applied correctly, variable frequency can very efficiently maintain steady discharge pressure in widely varying loads. They will be more efficient than compressors that are idling a lot or that use modulation control. We generally recommend a variable unit when it will operate between 40% – 85% of its capacity range.
Consistent discharge pressure. As mentioned above, variable speed units don’t operate on pressure bands and some can maintain discharge pressure within +/-1.5 psig of the set pressure.
Soft motor starts. Variable frequency drives prevent inrush current that can overheat windings. This can extend motor life.
What are the down sides?
First and foremost, they cost more due to the complex drive electronics.
Secondly, they have a higher cost to repair. Some brands use special motors or all in one motor-airend assemblies. If there is a problem with these or the drive electronics, the repair costs are higher than standard fixed speed compressors.
The electronics that control the motor are expensive to fix/replace. See points below about environment and power.
They are finicky about their environment and incoming power. Piston compressors are well known to run and run in less than optimal conditions with minimal maintenance. Rotary compressors can be very robust but need more service (filter and fluid changes, etc.). They pay back with more flow for less power and higher quality air. Variable speed rotary compressors can deliver even better energy performance but like your laptop or desktop computer, the electronics in the drive are intolerant of moist, hot, or dusty environments. They must be kept inside and in cooler, cleaner spaces than standard units.
Also, they require clean, stable power. If your voltage is weak or fluctuates, or you suffer from frequent power outages, your VFD won’t have a long healthy life.
Other considerations:
VFDs must be sized properly to have benefit. This is important to understand. Quite often, VFDs are described as offering very large energy savings—and while there is potential for savings, it will only happen when properly applied. There’s a reason they are called variable. They save money when the demand for air varies.
If you need a fairly steady supply of the same amount of air, then a VFD is not going to save you any money. If the unit often runs in the higher end of its capacity, the energy benefit is minimal and you would be better off with a less expensive fixed speed machine. If it routinely operates on the low end, you’ve oversized (and over spent). Also, most variable units cannot run below 20-25% of their capacity, simply because the motor will be running too slow to cool itself. We’ve studied it and found that many non-industrial businesses greatly oversize their systems, believing “bigger is better”. So before you invest in a VFD compressor, have a solid understanding of how much air you are really using.
Resist the “one size fits all” approach. While one variable speed unit may be able to cover your varying load, having only one compressor presents a very practical question. Do you have a back-up? Consider the cost of downtime when deciding between one compressor (whether variable or fixed speed) and two smaller units. In many cases, two smaller fixed speed units can deliver nearly the same energy savings as one variable unit.
Steady discharge pressure does not guaranty stable system pressure at the points of use. If system storage and piping are not properly sized, users can still experience low pressure or pressure swings.
When does it make sense to get a VFD?
There are definitely situations when a VFD is the right solution. If you have high power costs, your compressor runs many hours per day and your demand varies widely, then a VFD makes sense.
Additionally, many local utility companies will offer an energy rebate for buying a VFD which can also help offset the higher purchase price (compared to a fixed speed compressor).
Finally, VFDs need a clean, dry installation area and a good power supply. Talk to a compressed air expert and be sure to voice any concerns about your power and shop environment. They can help you come up with a long-lasting plan that will meet your needs and provide an energy-efficient and reliable source of compressed air for your shop.
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