Getting ready to address tomorrow's issues
AC drives technology has surprised many industry pundits by continuing to
develop steadily over a period of 25 years or more, and it looks as if
the development is going to be continuing on several fronts for some
years yet, predicts Guy Kennett
The most important development is that drives will become even more
intelligent, continuing a trend that started back at the beginning of the
1990s, some ten years on from the earliest industrial uses of ac
inverters. Back in the 1980s it typically took two or three days to
commission a drive. The cost of this in terms of man-hours and also
possibly lost production could be huge and restricted the take-up of the
then new technology. By the early-mid 1990s technology had advanced and
commissioning time was down to a single day. A big breakthrough came with
the development of auto-tuning which, at a stroke, reduced commissioning
time to an hour or two.
The trend has continued so that it now rarely takes more than ten minutes
to get a drive up and running. In the foreseeable future, commissioning
time will reduce to zero; the function will become one of simply fitting
a drive. This future saving of ten minutes will not be significant in
terms of cost saving, but it will have a massive effect in changing the
culture surrounding drives engineering.
For some applications drives will become commodities, in that they will
be completely accessible to users. Where previously users have had to
rely on drives experts, soon they will be able to do everything
themselves with ease, confidence and (most importantly) guaranteed
outcomes. It can be compared to the rise of word processors and decline
of the need for secretaries, or the development of cook-chilled meals,
intelligent cameras and many other consumer conveniences.
This is great news for users, but specialist drives companies are going
to have to reinvent themselves! Those that supply drives as part of a
broad automation portfolio have crossed this Rubicon before with other
technologies; but the truly specialist companies are likely to find
themselves focused more and more on the high performance technologies of
vector and servo solutions.
Drives' increasing intelligence also opens up the possibility of local
process control. The effect is that drives will adopt some PLC
functionality, and indeed several manufacturers already offer such
things. Cynics may argue that automation companies won't want to go down
this line because it will undermine their PLC sales, while drives-only
companies will find it attractive but impossible to source the most
up-to-date PLC technologies: in fact the development will be user-driven.
In most cases it will make very little technical difference whether one
uses a drive-and- separate-PLC or an intelligent drive, the decision will
be made by personal preference and custom and practise. Intelligent
drives offer advantages where the installation is remote or inaccessible,
where space is limited, where local technical capabilities are very low,
or where technical capabilities are expensive to implement.
Within a few years many drives will have IEC61131 programming
capabilities, just like a PLC. However they will tend to have their
intelligence capped at being equal to micro and small PLCs; drives could
theoretically be fitted with the capabilities of a large PLC or PAC
(programmable automation controller) but this would be unattractive to
users who would consider the resultant architecture to be restrictive
when it came to later systems reconfiguration. Another significant
difference is that intelligent drives are likely to use expansion racks
rather than have on board I/O. This is to maintain configuration
flexibility in the field, and in fact follows a trend that is already
evident in PLCs.
However, the question arises: how many machine and mechanical engineers
are sufficiently familiar with IEC61131 that they will be able to set
drives parameters and enable process improvement? The final result of
this development strand is likely to be that integrated controllers will
offer many benefits in terms of ease of installation and assured
communications, but for those (relatively few) situations where 'best in
class' performance is required, standalone equipment will win out.
While great things can be expected of the control circuits, developments
in the power circuits are a little harder to forecast. In recent years
the size, performance and reliability of IGBTs, the backbone of a drive's
switching capabilities, have all advanced significantly. However they all
now appear to be at a final plateau. Any further reduction in size is
likely to adversely affect switching performance and may intensify local
heating effects, which will in turn impact upon reliability and long term
performance. Therefore to improve switching and waveform manipulation,
either better cooling has to be achieved, a fundamental development in
power electronic technology used or drives must be redesigned to reduce
their reliance on power circuitry.
However by using the control electronics in conjunction with the power
electronics, we are already seeing improvements in drives' ability to
'map' the motor with which they are used. Some drives are already able to
dynamically map their motors in real time, which leads to instant
optimisation of performance and therefore a significant reduction in the
voltage vector. The effect of this is a substantial improvement in a
drive's ability to save energy when compared with a fixed speed drive.
This is of particular significance because energy saving is fast becoming
the Number One reason for installing drives. So just as energy efficiency
is becoming a key driver for industry, drives are stepping up to the
plate ready and able to address the issues of the day
Guy Kennett is with Mitsubishi Electric
http://www.meuk.mee.com
Back in November 1995, Stephen Darnell attempted to answer the question:
is all this technological development simply driven by the drives
manufacturers' marketing departments? Click here to read his article