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The Future Stays Bright For Ac Drives

01 November 2005




Stuart Kemp describes how the need to reduce energy consumption and mains
borne harmonic distortion, not to mention the challenges of a broadening
applications base, has influenced both the drives industry and its
technology developments

Just ten short years ago our industry was proudly announcing incredible
advances in electronic ac variable speed drives on the back of the
digital revolution. New high speed CPUs and DSPs suitable for such
products were being manufactured in such high volumes that the cost of
incorporating them into the drive design had become a commercial reality.
The age of large-scale integration and surface mount technology had
arrived and everything from the physical size of the drive to the cost to
the user was shrinking fast.

During this period, the drives industry had been revolutionised by
digital technology. Bulky mains transformers had given way to tiny
switching power supplies; heat sinks had halved in size as faster
switching, lower loss power devices were introduced, and gone for ever
were the PCB-mounted trimmer 'pots' to be replaced by software supported
touch keys and displays. Performance of the induction motor when
controlled by a digital frequency inverter had far surpassed that of the
old analogue drives and, in closed loop mode, approached the 'holy-grail'
- dc motor performance.

Indeed, there was plenty to crow about at the time. Although there was
clearly room for improvement in dynamic performance, as well as zero
speed holding under full torque conditions and so on, industrial users
were beginning to reap the rewards of lower cost alternatives to
traditional dc drive systems, and in open-loop format.

Variable speed drives remain at the forefront of the government's quest
to minimise energy consumption and the emission of greenhouse gasses. As
they now integrate easily into building management systems and have the
ability to save large amounts of energy, they have become the rule,
rather than the exception, in HVAC projects.

Limitation of harmonic distortion of commercial power supplies and the
increasing trend among power distribution companies to enforce it, means
that the end could be near for the connection of large or multiple
non-linear loads - rectifiers, for example - unless there is a truly
cost-effective filtering solution.

Development of virtually sinusoidal matrix converters, which will
completely eliminate the need for rectifiers and dc link capacitors, and
therefore significantly improve power quality, has been slower than
predicted. The semiconductor technology is now available in a
commercially viable format and early prototype matrix drives have been
displayed, so it will not be too long before we see this type of ac drive
begin to enter the market. Being fully regenerative, it will have big
advantages in applications such as lifts and cranes, and as the cost of
the early models begins to drop, we will see matrix converters used
extensively in these applications.

One technology that has not proved as successful as those early
predictions indicated is the integrated inverter-motor. Higher failure
rates and lack of flexibility have meant that it has not been generally
adopted as an industrial standard as expected and sales have been
mediocre at best.

The design of control circuits and software for modern ac drives though,
has introduced many helpful and cost-cutting features. Secure I/O can now
replace the need for safety relays in some circuit designs, and
integrated PLC functionality gives the user a greater degree of
flexibility while often negating the need for additional devices and
wiring. While the trend towards easy commissioning and self-tuning
increases and advanced graphics and on-board diagnostics have helped
installers and users alike to benefit from today's much more
user-friendly drives.

It is not only in the field of industrial automati


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