The last decade has witnessed a remarkable transformation in variable frequency drive technology. Jonathan Smith looks at some of the landmark developments of recent years
Drive users can now optimise drive performance to meet a wide range of application requirements without the added cost of customisation or complex programming. New technologies also give users the ability to satisfy their growing and varying needs, from standalone drives to complex, integrated systems. Perhaps the biggest breakthrough over the past ten years was the addition of vector control to ac drives.
At the centre of this technology is the use of a field-oriented control scheme that replaces the volts-per-hertz regulator core found in most ac drives with a high bandwidth current regulator that allows independent control of speed and torque while dynamically adapting to motor and load changes. Monitoring speed and torque separately allows more precise motor control and improves overall performance.
Drive manufacturers continue to improve the performance capabilities of vector control ac drives, taking advantage of progress being made in microprocessor technology. Improved field-oriented control algorithms help optimise drive performance to meet the speed or torque requirements of the application. This enables high-powered ac drives to perform more like dc drives, including delivering torque independent of motor speed, and a quicker reaction to shock loads. As a result, field-oriented control of ac drives combines the best advantages of dc drives - constant torque down to zero speed - with the best of the ac drives - simplified installation, tuning and maintenance.
In hoist and crane applications, tight demands on torque response require the drive to deliver high torque levels without losing control of the load as well as being able to stop the load accurately and precisely. Likewise, applications such as mixers, centrifuges and extruders require a drive capable of delivering full torque immediately upon start-up, constant torque throughout the operating speed range, and full torque down to zero speed. This helps ensure that an injection moulding machine, for example, will generate proper material flow and produce high quality parts from the start, while minimising scrap. For years, users relied on dc drives to meet these challenges, but users are now taking advantage of vector control ac drives that deliver a cost-effective, high-performance alternative.
Other advances in drive technology centre on logic control capabilities. These control features allow users to optimise general-purpose drives for specific applications without having to order a special drive or write additional code in a separate process. Because the embedded control is tightly coupled, users achieve higher speed and throughput in various processes. For example, users may be able to position synchronise a printing press to boost throughput, or more quickly and precisely move items within a work cell, such as lifting and moving vehicle parts on an automotive assembly line.
By embedding the logic in the drive, users also can reduce the size of the drive-control package, and improve reliability by minimising the number of required connections. While applications with large I/O requirements may still require a separate controller, drives today typically support more than ten I/O points locally. Therefore, many simple processes can benefit from the improved speed and efficiency of a standalone drive unit.
Further developments of integrated control capabilities can be seen in today's drives' built-in safety functionality which allow it to fully integrate into a safety related control architecture. Drives including embedded 'safe-off' functionality ensure the start-up of potentially hazardous operations will not occur until a safe condition exists. This in-built functionality offers the user a high degree of motor control and safety integrity, resulting in better performance and capital savings in reduced equipment (contactors would not be required, for example); reduced installation and cabling costs; smaller panel space, and reduced system complexity.
Contributing to these expanded performance capabilities of ac drives are new design techniques and innovative packaging strategies that deliver greater power and efficiency in smaller footprints. For example, drive engineers can now use advanced thermal modelling techniques at the design stage, and are able to create tighter component configurations while delivering improved thermal dissipation. As a result, drive manufacturers minimise heating issues before committing investment to drive production. Users can put ac drives in smaller spaces while also reducing the overheating issues they used to face. In addition, these smaller sizes and advanced design techniques allow EMC requirements to be met without external filters.
While technology has already come a long way in terms of space efficiency and reduced heat load, recent advances in liquid-cooled drive technology are taking it even further. Liquid cooling offers significant space-saving advantages, especially in larger horsepower applications. For example, a 500hp liquid cooled drive uses 65% less space than an air-cooled product. With a smaller cabinet size and reduced plant-floor space requirements, the overall cost of a drive installation can be reduced significantly.
Liquid-cooled drives enable the use of completely sealed, non-ventilated enclosures, making them ideal for high-protection environments. Because the drive doesn't need an air exchange to the cabinet for cooling, it can be placed in virtually any setting, including dusty, wet and outdoor locations. This reduces the heat load in conditioned control rooms and allows it to be transferred to other possible media, such as process water, chilled water or remote or outdoor air. All these improvements allow users to make easier investment decisions where ac drives can conserve process energy.
As microprocessors continue to get faster, more powerful and less expensive, drive manufacturers will continue to look for ways to reduce the size, cost and complexity of ac drives while enhancing performance. On the performance side, new techniques allow higher bandwidths of control even when faced with machine resonances. Still, while some end users demand more application versatility and flexibility, others need ease-of-use features, rather than technological advances. One of the biggest areas of technology innovation is occurring in the area of drive programming and configuration, where simplification and ease of use is driving development. The use of programming wizards in drive start-ups is fast becoming a key focus area . These tools operate much like the set-up programs in new PCs, where after a few prompts, the wizard automatically installs the software and required drivers, sets all the parameters and recognises the connected hardware devices. For example, new drive programming wizards will prompt the user for information about the application, size of the motor, and other critical information. It will then automatically set up all the parameters to meet the defined application and hardware requirements.
For more complex installation, like a centrifuge or lifting processes, the wizard might request some additional parameters, such as the type of application, the speed of the process, and the weight of the load. It will then automatically adjust the parameters to optimise the drive for that application. These tools can dramatically reduce drive start-up and commissioning time and improve setup accuracy by eliminating a significant amount of manual configuration.
One consequence of increased connectivity is that individual devices will no longer be viewed and managed as isolated components, but rather as part of an integrated system. That's because users will be able to program, control and troubleshoot drives, controllers, relays, I/O and motion devices from a common interface using a single programming package. This includes the ability to add logic using common software tools with the same look and feel-no matter whether the application involves a small drive in a standalone process, or a large, high-performance drive as part of an integrated, networked system.
In addition to reduced set-up and operational costs, a key benefit of this integrated environment is that users will be able to more easily save all of their drive parameters and control logic in a single database. In the event of a failure, replacement and restoration of the original drive parameters is a relatively simple process.
The proliferation of high-speed Ethernet and wireless networks on the plant floor has increased the ability to monitor and control drives remotely, and to share information. New technologies such as time synchronised services in Ethernet will provide even higher levels of process control.
Improved diagnostics is another key feature of newer drives allowing more pro-active maintenance practices to be developed. For example, with current technology, users typically have a small window of time (a few minutes) until the drive will trip as a result of an overload condition. Enhanced thermal regulators extend this time by optimising the IGBT switching patterns during periods of thermal stress. Even the new diagnostic tools will allow users to perform a trend analysis over longer periods of time to show that a drive may be drawing more current than normal to achieve the same speed.
With access to more detailed information over longer periods of time, users will be able to better predict potential problems and prevent catastrophic failures. Moreover, the improved quality and availability of data will enable maintenance personnel to be better positioned to troubleshoot, helping to reduce costs and improve up-time. Simple descriptions are given to pinpoint problems, not obscure fault codes.
Advances in ac drive technology are helping manufacturers increase productivity and save energy throughout their facilities. Improvements in technology have delivered more precise speed and torque control, enhanced communications capabilities, reduced downtime and increased throughput, as well as giving a more accurate picture of the manufacturing environment.
Also, as drives manufacturers continue to pack more performance in smaller packages, end users will have greater flexibility in selecting the right drive to meet future application needs. With more simplified set-up tools, improved diagnostics and increased networking capabilities, users will have even greater opportunities to maximise the return on their drive technology investments in the future.
Jonathan Smith is with Rockwell Automation