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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 automation and building services that the modern ac drive is king. During the next few years, the technology will also become widespread in domestic markets. Low-cost OEM drives with specific functionality will be mass produced without keypads, heat sinks and covers for white-goods markets. Washing machines, dryers, air conditioners, ovens and many other appliances will use low-power ac drives as standard. Domestic central heating pumps and gardening equipment can all benefit from such technology.
In extra low-voltage format we may also begin to see the introduction of fieldbus controlled ac drives for traction applications in electric powered vehicles. IMO has made significant progress worldwide using standard ac drives for marine applications, replacing hydraulic motors with induction motors for propulsion, steering, thrusters, winching and many more applications.
Whether low voltage or medium voltage, high power or low power, land or sea,
the future is indeed bright for ac drives!

Stuart Kemp is business manager, drives, IMO Precision Controls

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