Feature 40 years of drives & motors
Industrial electrical variable speed drives: 40 years of innovation
In 1971, industrial drives were very different to those we use today. Professor Bill Drury of Control Techniques takes a look back at the changing technology, the developments, and the marketplace now and in the future
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ndustrial drives in 1971 were a very different animal to those we are familiar with today. Many of the leading manufacturers of the time no longer dominate the market, and the applications for use have expanded tremendously. It was in 1971 that W.T Grant wrote a definitive survey ‘Electrical Variable Speed Drives’ in Engineers’ Digest, and the bulk of the paper centred on DC drives with novel thyristor based con- verter topologies explained in some detail. While AC motor control did feature, most of this was focused on rotating variable frequency sources, multi-motor configurations and the AC commutator motor, rather that the ubiquitous inverter that we all take for granted today.
These were the days of DC drive dominance, though this survey had great insight, stating: “I suspect that it will not be too many years before the semiconductor-controlled AC motor becomes economically viable for single drives and it will eventually become a competitor for DC equip- ment”. These were also the days of elegant but specific engineering solu- tions, but it is interesting to consider the top two principal factors a user was advised to consider when selecting a drive – cost and efficiency. I am not sure that user behaviour in that respect has changed much in forty years! In 1971, drives were big – and even small drives were big! They were assembled in floor standing cubicles, often with customised analogue con- trol electronics. The advent of isolated thyristor packages heralded a new approach to drives design. Utilising 19-inch rack principles – a cubicle- mounting standard well used in the process industry – compact, high specification ranges of DC drives in modular form became available off the shelf (see image above). Companies such as AEG, Thorn Automation, Mawdsley’s and Control Techniques pioneered this work, and a new era
Design Solutions 1971-2011
processing requirements were such that in its early days commercial exploitation was restricted to large drives such as mill motor drives and boiler feed pump drives.
The 1980s
Utilising 19-inch rack principles, compact, high specification DC drives in modular form became available
of drive design had begun.
The functionality of these modules remained basic. Speed or current/ torque control loops were present with peak limits. The user was advised to connect a 10kH potentiometer to the terminal strip to provide a set point. Good designs provided the 10V supply for the potentiometer!
Industrial drives were DC, with AC only used in specific applications such as textiles where many motors (usually reluctance) were fed from a single inverter. Those inverters employed ‘fast thyristors’ and com- mutation circuits of such complexity that they were instruments of torture for undergraduate students and service engineers, who often had to work on inverter equipment, some of which was oil cooled.
Drives radically changed with plastic mouldings
The 1970s was a time when AC motor drives made great advances but still lacked the dynamic performance to really challenge the DC drive in demanding process applications. Considerable interest was being generated in Field Oriented Control of AC machines. This technique, pioneered by Blaschke and further developed by Leonhard, opened up the opportunity for AC drives not only to match the performance of a DC drive but to improve upon it. The
- 40TH ANNIVERSARY SUPPLEMENT - S17
In the early 1980s drives radically changed when plastic mouldings made their first significant impact. Bipolar transistor technology had also arrived in the mid 70s onwards, which eliminated the bulky auxiliary commutation circuits previously required for AC inverter drives. Control strategies for AC drives remained simple, constant Volts to Hertz with low frequency boost; and field oriented control was known but used only on large drives. It was not until 1989, with the launch of the Control Techniques Vector drive (see image, below) that an AC drive offering superior performance to a DC drive was available to the mass market. In the mid 1980s advances in microprocessors facilitated cost effec- tive digital drives for both DC and AC designs. Drives were introduced containing custom integrated circuits and new plastic materials were intro- duced, giving structural strength,
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