Over the past ten years, drives have become smaller and ever more
capable. But there is more to come, as Geoff Brown explains
The modern variable speed drive is very different to its ancestors of ten
years ago. For one thing they are much smaller, some drives being just
one-tenth of the volume of a drive ten years ago. The benefits to users
are many. Smaller drives are easier to install - which in turn can lead
to smaller and less costly control rooms, a boon to the process engineer
looking to keep expenditure to a minimum.
But just how small can ac drives get? ABB believes that there are few
restrictions, particularly in the lower power range, and that over the
next ten years, drives in this range will shrink by another 60 to 70%.
Smaller and cheaper drives are already finding new applications as
diverse as treadmills and small centrifuges used in honey production.
These applications would simply not have been feasible or cost-effective
a few years ago.
But drives are not only getting smaller. They are also becoming more
intelligent, have better communications and are easier to install and
control than drives of ten years ago. Regenerative drives have also come
of age over the past ten years. Active rectifiers help suppress
harmonics, an area that iss receiving increasing attention.
The last ten years has seen an advance in the way drives communicate with
users, making them far easier and quicker to set up. Modern drives
provide a lot more information compared with their predecessors, most
notably in the form of start-up macros that offer plug-and-play type
operation to get the drive up and running. Although end users may still
need the presence of a helpline, the time needed to get a drive up and
running has dropped very dramatically - often to less than an hour or so.
One of the biggest drives developments in recent years has been the
advent of Direct Torque Control, or DTC. Widely recognised as a major
leap in drive control, DTC is an optimised ac drives control principle
where inverter switching directly controls motor variables such as flux
and torque. The measured motor current and voltage are used as inputs to
an adaptive motor model which produces the actual value of flux and
torque every 25us. Since its launch in 1994, DTC has proved to be the
technology of choice for demanding applications, and ensures the fastest
torque and speed response for any drive. It also achieves a high level of
motor control, eliminating the need for costly encoders, often required
to feed back signals of speed and rotor position in demanding
applications.
The last ten years has seen drive software developing rapidly, with
drives starting to become a more integral part of control, maintenance
and monitoring systems. Over the next ten years, experts foresee the
emergence of self-healing control systems, in which multiple drives will
share control functions. If a fault or error occurs in one drive, another
drive could take over instantly. This would dramatically reduce the
reliance on costly PLCs and improve the reliability of automation.
Ethernet communications will also enable drives to handle and communicate
a huge amount of monitoring information. By monitoring the connected
load, an intelligent drive can provide valuable process data, vital for a
successful preventative maintenance programme. The last ten years has
seen a shake out of communication protocols, with a reduction in numbers
to leave the more successful and capable protocols such as Profibus and
LonWorks. This process will no doubt continue, perhaps eventually
resulting in a single overall communication standard for drives. With the
increasing power of the Internet, it has also become possible to
interrogate drives over long distances, allowing corporate managers to
keep track of drive and process performance from anywhere in the world.
Drives are much more effi