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 wa