Unwinding the spin — Part II

A critical look at variable-speed-drive air compressors

By Chris E. Beals

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Part 1 of this two-part article appeared in the October 2006 issue. It discussed different way to quantify compressor efficiency and operation schemes that can lead to energy savings. View Part 1.

Much of the VSD compressor’s popularity is a result of marketing spin. Often, free audits exaggerate paybacks by including savings the end user can achieve without purchasing a VSD compressor. The marketers also fail to provide the necessary engineering details required to maximize the purported benefits. It’s time to eliminate the spin and point out both the limitations and engineering required if you are to derive the benefits of a VSD compressor.

Power factor

The spinners claim there are no penalties for spikes or low power factor. In the limited context of the compressor, this statement is true and refers to the fact that the VFD provides a soft start similar to a Wye-Delta or electronic starter and that the power factor of a motor on a VSD compressor is close to unity over the complete speed range. A unity power factor is a benefit and it can reduce energy costs if the VSD motor constitutes a significant portion of the plant’s motor load. On the other hand, if the local utility doesn’t have a power factor penalty or the VSD compressor constitutes only an insignificant portion of the motor load, then a unity power factor offers little cost benefit.

Free energy analysis

The spinners offer free energy analyses or audits. The old adage, you get what you pay for applies here. Many compressor manufacturers gear these free audits to get their sales personnel into a plant to sell equipment. Free air audits don’t address pressure drop, piping, air quality, risk of an outage or demand-side issues. Nor do they offer any non-equipment solutions.

The whole process is automated so it produces an equipment quote for the sales personnel regardless of expertise or experience in the industry. When clients ask us to review these free audit reports, we generally find that the salesperson has misinterpreted the data, the proposals are incomplete, and plant personnel can achieve the majority of the projected savings without purchasing any compressor.


At a 100-psi operating pressure, many lubricated VSD compressors can regulate capacity from 100% down to 15% before they must operate in either start-stop or load-unload mode. Spinner literature suggests that all VSD compressors have the same turndown capability, but the fact is that at 100 psi, some can only regulate output capacity from 100% down to 55%.

In addition, turndown range decreases as discharge pressure increases. For example, at 125 psi, the turndown can be as little as 40%, from 100% capacity to 60% capacity.

Turndown range is important to VSD compressor selection because a smaller turndown decreases the purported savings, increases the size and frequency of pressure swings, and can result in compressors short cycling. Or, it can increase the cost to integrate the VSD compressor into the system properly. In summary, turndown varies significantly among compressor manufacturers, models and pressure settings.

Nameplate horsepower

The actual horsepower size of a VSD compressor can be confusing because spinners often play games with the motor nameplate data. For example, a motor might be a 125-hp unit with a 1.15 service factor or a 100-hp motor with a 1.4 service factor. The motor manufacturer can nameplate the motor either way. This practice has resulted in end users installing undersized electrical circuit breakers. In summary, it’s important to determine the actual power applied at full load at the operating pressure to understand efficiency and installation requirements.

Constant pressure

The spinners claim that VSD compressors hold pressure within 1.5 psi. The truth is that VSD compressors are only capable of maintaining the pressure at their discharge within 1.5 psi over their turndown range. You must properly integrate the VSD compressor into your system if you want to prevent larger pressure swings.

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