Drives on the turn07 June 2010

Variable speed drives have been a mainstay of plant engineering for many years, but the latest generation is changing the art of the possible. Brian Wall reports

Senior plant engineers probably remember when drives were not reliable enough to be used on critical plant. The technology has long since matured to the point where many now acknowledge drives as energy saving devices that can seriously impact greenhouse gas emissions – particularly on variable speed pump and/or fan control. However, that's fast becoming just the tip of drives' iceberg, as increasing intelligence enables a whole new generation of plant engineering possibilities.
That's certainly the view of Dr Dave Blood, who heads up Parker SSD Drives' R&D team. He says that drives engineering is "very real-world" – pointing to the fact that his engineering team is constantly either on site with clients, looking at issues affecting production, or troubleshooting with the applications teams. "The buzz words they come back with are efficiency, cost reduction and communications. Sometimes this relates to a grander scheme but, at plant floor level, it comes down to doing more for less."
That, he says, continues to force the pace of development, particularly in terms of functionality and communications. "There's a generational shift going on," says Blood. "Traditionally, drives were thought to be dumb devices. You could parameterise them to do clever things, but there was no intelligence that allowed the drive to change its working pattern in response to changing needs or conditions."
He argues that drives were always more capable than this depiction – with, for example, function block programming and the ability to link them with PLCs or other controllers. "But it is only now that plant engineers are exploiting this capability," he suggests.
However, there is more to it. Increasing acceptance of Ethernet communications at the plant level, for example, is making real-time communications more attractive. That means multiple drives can synchronised in software, so they all react at precisely the same time in exactly the same way.
"Previously this could only be done with a mechanical connection, such as a drive shaft. This involved serious engineering and put severe restrictions on plant layout. Now the same can be achieved by sending signals down a wire. So machines don't have to line up across the plant floor. They can be spread where they're need, giving greater flexibility in plant and machinery design," explains Blood.
Of the many different flavours of Ethernet, three are emerging for plant control – EtherCat, PowerLink and Profinet – all of which were developed for industrial automation, with fast cycle times, low communications jitter and low hardware costs. "Drives used to be programmed from a PC, via an RS232 port, but in recent years there has been a move to USBs. Now we are seeing the beginnings of a trend away from USB to Ethernet. Similarly, drives used to communicate with PLCs via an electrically isolated RS485 port, but over the last 10 years fieldbuses have taken on this duty. Soon these will give way to Ethernet-based fieldbuses."
And there have been similar developments in terms of remote monitoring of drives for troubleshooting. But the big deal now involves drives increasingly being built with on board soft PLCs, or similar capabilities based on the IEC 6-1131 standard. That makes it technically and economically viable to customise drives not only for whole market segments but also for particular application.
"This has been fairly common for general applications, such as pumps, fans, hoists and conveyors, for some years," agrees Blood. "But it is now moving to a whole new level. For instance, we can now customise our standard AC690+ drive for a single application, controlling banks of piston compressors for commercial refrigeration. The drive keeps each compressor within its safe working envelope, shares the load and duty around the bank, optimises energy consumption and improves temperature control."
Clearly, drives are now in a different league. And, as technology to protect against harmonic distortion becomes commonplace, even their occasional problems for plant electrical distribution systems will disappear. Add to that the machine safety standards – as in the EN ISO 13849-1 PL-e and EN 62061 SIL 3 (Safety of machinery – Functional safety of safety-related electrical, electronic and programmable electronic control systems) – demanding, for example, Torque Off functions, and intelligent drives will become part of plant engineers' standard armoury at all sorts of levels.

Brian Wall

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