Using variable speed drives (VSDs) to control fans and pumps saves energy and cuts running costs, but, according to Stephen Takhar, managing director at Vacon UK, if the VSDs have been in service for over  five years, it may be time to assess their performance

For the last decade or more, the technical press has featured a vast amount of stories about the energy wasted through the use of fixed speed drives with pumps and fans, and the savings that can be made by fitting VSDs. This has encouraged many companies to install VSDs, and when engineers in these companies see the VSD energy saving stories, they must be very tempted to take the line that they’ve been there and done that!

However, are they right to feel that they’ve done everything they could and should do to cut their company’s energy usage and reduce its carbon footprint? Are they justified in sitting back and enjoying the warm glow of a job well done? The answer is that it rather depends how long ago their VSDs were installed. If they’re just a few years old and were supplied by a reputable manufacturer, it’s unlikely that any further action is needed. But if the VSDs are more than five years old, it’s a different story.

This is because the intrinsic efficiency of VSDs has improved considerably over recent years, and drives that were considered very efficient five or so years ago would now be classified as relatively poor. The situation is not, however, quite as simple as saying that VSD efficiency has improved by 1 or 2% over the full operating range. In fact, the biggest  improvements in efficiency delivered by the latest drives relate to operation at reduced loads and speeds.

The efficiency of older drives running at anything less than 50% load can be surprisingly low – many small drives over five years old struggle to rise above 60% efficiency when running at 40% full load. The larger drives of the era fare somewhat better, but even with them, 80% efficiency at 40% full load would have been considered satisfactory.

Improved efficiency

In contrast, the latest AC VSDs from leading manufacturers can be expected to deliver efficiencies of at least 90% right down to 20% of full load. At higher loads, modern VSDs will deliver even greater efficiency – 98% is by no means unachievable for, say, a 37kW drive running at 70% load or greater.

At first sight, it may be tempting to wonder how much the extra efficiency offered by the latest drives really matters. After all, the gains are biggest when the drives are running lightly loaded, and how often does that happen? In many fan and pump applications that have been converted from fixed speed operation, the answer is that the drives run lightly loaded much of the time.

It’s not hard to see why.  Designers have little choice but to size motors to cope with worst-case conditions – in the case of an air-conditioning installation, for example, this would be the hottest day of the year. Then they invariably increase the motor size even further, “just to be on the safe side”. This means that typical fan or pump motors are rarely, if ever, called upon to deliver full output. Instead, they spend their lives running well below this level, which corresponds to the least efficient area of operation for older VSDs.

Taken together, these arguments offer a convincing case for reviewing the efficiency of older VSDs and, if necessary, replacing them with the latest products. It is, however, worth bearing in mind that it’s not only efficiency that will be improved by installing new drives. The latest units will be more reliable in operation than their predecessors and, given the problems associated with fixing breakdowns in today’s de-skilled business world, this can be a real and valuable advantage.

In addition, modern VSDs are often smaller, so it may be possible to recover valuable space by installing them. They also have superior harmonic and EMC performance, which makes it easier for users to satisfy increasingly strict demands made by the supply authorities for harmonic mitigation. Finally, modern VSDs are more versatile in terms of the control functions and communications options they offer. This means it is often possible to enhance overall plant control to further reduce energy usage.

There’s no doubt that, in appropriate applications, using VSDs can lead to big energy savings. Older VSDs in particular cannot, however, be considered as a “fit-and-forget” solution. They almost certainly did a great job in their day, but it’s very likely that in terms of efficiency and in other ways, they are now markedly inferior to the latest products.

In short, if your VSDs have passed their fifth birthday, it’s time to talk to a   company like Vacon about reviewing their performance. You could be very surprised at the substantial savings you could make in return for a modest investment!