Bill Drury sets the scene on our retrospective and forward looking Anniversary Supplement, celebrating the origins and suggesting future directions for industrial electrical variable speed drives
Ten years ago we were celebrating the 100th anniversary of the birth of the efficient speed control of electric motors pioneered by Harry Ward Leonard. Much had changed in those 100 years with the introduction of solid state switches replacing multiple motors and the microprocessor and ASIC replacing carbon resistor stacks. As well as the significant component changes, the functionality of the drive had developed. It was a speed controller. By 1995 it was, in some applications, a process controller with many drives offering functions such as PID control.
Whilst the functionality had broadened, the core motor shaft control performance had improved and induction motors, were being harnessed in demanding applications ranging from printing and elevators to machine tool spindles. This was achieved by the use of vector control based on rotating reference frame algorithms. Some drive manufacturers created confusion in the market by offering 'voltage vector' technology. Thankfully that 'marketing' has now gone.
Many were starting to view the drive as a commodity product. Availability of drives up to 4kW which would fit on a standard DIN rail alongside contactors and relays plus the common basic functionality of competitor products, fostered this belief. Differentiation was certainly still there in the additional features available to users who looked beyond the speed control functionality.
Those users who were looking to use drives as components in a larger control system, were looking for ever greater connectivity. The fieldbus 'wars' were raging with passionate claims for many systems (I counted over 200 in a 12 month period), most of which have since disappeared. The fieldbus wars have not abated, but have now morphed into the Ethernet wars, with advocates of the different protocols all predicting dominance.
Much has changed, driven by component technology, design techniques and the vision of the industry. Power devices, notably Trench IGBTs have driven improved efficiency, whilst microprocessor performance improvements have yielded not only improved motor shaft performance, but facilitated further significant functionality. Ease-of-use has been, and remains, a key focus. Whilst there are specific motor types and controls best suited to certain applications, users can buy a single product that can meet these different needs. The 'universal' drive meets the diverse requirements of a general purpose open loop vector drive, a closed loop flux vector drive, a servo drive, and a sinusoidal supply converter, with the choice purely led by parameter selection - and without compromise. Ease-of-use and optimisation of-set up is achieved by automatic tuning routines that match the drive to both motor and mechanical load. IEC 61131 compliant PLC functionality is also available for complex system programming, and separate drives can be synchronised together, achieving a control loop jitter of less than 2us without the need for a master controller.
Motor technology, too, is changing. The brushless permanent magnet motor, once only used in high performance applications, is now being considered where efficiency or size is critical. Motors with drives integrated into the housing (mostly in a larger terminal box) are now widely available and have made a significant impact, mainly in replacing mechanical variable speed solutions. Linear motors have made an appearance, but mainly at trade shows. For the most demanding applications, the quality of speed or position measurement is critical. The emergence of sine/cosine encoders have facilitated very high resolution position feedback, while all digital solutions, such as EnDat, point the way forward.
Much has indeed changed in the last ten years. Much will change in the next ten years and beyond. Component developments, particularly in the semiconductor industries, will continue to play a significant role in defining direction. Cost remains a driver of product development, and motor shaft performance improvements continue apace even if they do not always appear centre stage in the marketing brochures.
With the capabilities within a modern drive, users need to consider how to balance elegance and cost over what could be considered over-dependence on a single supplier. The improved interfacing technologies including the emergence of Ethernet as an industrial backbone at the machine level, will certainly act to mitigate these concerns. Customer needs will of course be the key driver to future developments. Significant advances occur when users and drive designers get together and consider system solutions. Some drives are sold to customers who have no intention to connect a motor to it! It has been purchased purely to use the comprehensive auxiliary functions.
The future for drives is as exciting as at any time in its long history. The market demand continues to grow around the globe, driven by energy saving, quality and process demands. Where will we be in another ten years? The answer to that is ....................
Bill Drury is executive vice president, technology at Control Techniques