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A VSD Compressor Doesn’t Save You Money

VSD (Variable Speed / Frequency Drive) Compressors

The energy savings were huge… of course. The pay-back time would be a few months, or days, or less.. for sure!

Stop here!

Yes, a VSD compressor can save huge amounts of energy (and thus money), but only if a propoer homework is one, make some calculations and use your common sense.

[For those in the dark: a VSD or variable-speed compressor uses a frequency drive to match the compressor speed (and thus capacity) to the amount of compressed air needed. This as opposed to 'load-unload' machines that runs at a fixed speed but 'loads' and 'unloads' between a minimum and maximum pressure).

And IF a VSD compressor is a good idea in your situation, buy the right size of VSD compressor.

Here's when a VSD compressor will save you money: when it runs at less than 80% speed for most of the time and when it runs continuously (daily).

In these cases, the cost-savings can be huge. Tens of thousands of dollars per year in energy costs can be saved.

But: A VSD compressor that runs at 100%, fully loaded, uses 5 to 10% more energy than a fixed-speed compressor! This is because of the 'overhead' of the electronic frequency drive.

The optimum frequency range for a VSD is 50 – 80% speed for maximum savings.

Don't just believe the salesman with his fancy brochures and his pitch about all the wonderful things a VSD compressor does to your budget.

I hate to say this, but does he know what he's talking about? As an engineer I have had many talks with salespeople and their pitches and brochures and claims… and 90% is either hyped up, not applicate to your situation, or simply not true.

The sad truth is that in my years as a compressor mechanic, I have seen many, many VSD compressors that were just not needed, were only costing money, or were simply slowly dying.

Here are a few examples:

1) A VSD compressor that was way too big. It ran for only 5 to 10 minutes every hour, at maximum 30% speed.

This machine was slowly dying from water and rust because it never really heated up. A smaller fixed-speed machine, matched to the actual needed output would be way more logical. (this was a 90 kW machine!)

2) A VSD compressor that ran between 30 and 50% all the time, every day.

Sure, it was saving money over a load-unload machine of the same size. But a VSD compressor that was half the size and ran at 80% would be more cost-effective (both in investment and running costs).

3) A VSD compressor that was needed for one specific machine and was only used intermittently

This compressor was like one unit with the machine it supplied air to, and was way too big. The air demand was basically stable, and was only needed for 10 or 20 minutes at a time, a few times per day.

A basic compressor of correct size with a large air receiver would be far more cost-effective in this case.

4) A VSD compressor that ran at 100% each and every day, all day.

That customer invested extra money for a variable speed compressor, and ended up paying 5 to 10% MORE in energy costs each and every year! Yeah, great!

Again, a VSD compressor CAN be a great choice, and can save you a lot of money. And this is true in many cases.

But, beware of the salesman with a hyped-up story about energy savings, who doesn't know when NOT to try to sell the a VSD compressor.

Or you end up paying MORE in energy costs, MORE in maintenance costs and MORE in up-front capital investment.

And the compressor mechanics can try and clean up the mess afterwards ;)


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