Trends: Drive technology for ac motors

Zurich, Switzerland—Traditionally, the primary use of drives has been in such applications as powering pumps, fans and conveyors.

By Control Engineering Staff June 28, 2007

Zurich, Switzerland —Traditionally, the primary use of drives has been in such applications as powering pumps, fans and conveyors. And while they will continue to be used in these applications, today’s end-users have a very different approach to that of a decade ago. This shift in attitude brings with it the assumption that the drives are simple to buy, install, start-up, commission, and own and run.
At the same time, drives are finding new applications: in exercise machines, pizza ovens, honey centrifuges and car washes. In these applications, the drive is considered a commodity, and the original equipment manufacturers (OEMs), who may not traditionally have used drives, are again demanding simplicity. In fact, a recent survey showed that simple controls and set-ups (70%) and convenient operator interfaces (53%) were rated as “very important” by ac drives users.
Smaller is better
Use of fewer components, greater packing density, improvements in semiconductor technology, and improved cooling techniques have combined to reduce drive size. In fact, there has been a 10-fold decrease in the size of drives over the past 10 years. The fact that drives are now being used in domestic washing machines is a testimony to their extreme compactness. Drives have become smaller, more -capable, easier to use and cheaper, by orders of magnitude.
Smaller drives are easier to install. Panel builders are able to fit more drives into a standard cubicle, so the whole panel can be smaller. This allows the use of smaller and less costly control rooms. It also becomes easier for OEMs to fit drives into their equipment. A classic example of this is in cranes, an application that has always had very limited space for the drive.
An additional benefit of reducing the component count in a drive is that it cuts costs. ABB predicts that, over the next few years, the parts count of its drives will be reduced by approximately 20% through use of integrated electronics to eliminate separate components such as external flash and RAM memories and analog/digital converters. Mechanical parts are also being integrated, for example, by combining frames and enclosures, allowing them to perform multiple functions. Reducing part count also enhances reliability: fewer parts mean fewer interfaces and fewer mechanical fixings, which are often a source of failures.
Advances in the development of power semiconductors have also helped to improve drives. A reduction in the power losses per-unit-area-of-silicon used means that the same silicon area can handle more power. This has enabled smaller semiconductors and reduced the need for cooling within the drive. This, in turn, allows use of smaller heat sinks and reduced air volumes inside the drive—further reducing drive size. The only limitations are the terminals, which must accommodate cables large enough to carry sufficient current to the drive.
But it is not just the development of power semiconductors that has enabled miniaturization of drives. Of prime importance is the technology used for cooling. Considerable R&D effort is being put into developing new cooling techniques, as well as into reducing the need for cooling. While air cooling is likely to remain the dominant technique, liquid cooling is finding increasing use in areas, such as wind power, transportation and marine applications, as reflected by the recently launched, liquid-cooled ABB industrial drive.
Ever-shrinking drives feature ever-expanding functionality thanks to software development. Today’s software monitors, diagnoses, configures, and archives information and parameters concerning drives in industrial plants. Set-ups are performed using software functions then downloaded to the drives. Set-up information is archived for future retrieval.
To obtain the full benefit of this technology, however, operators must still refer to the user manual. Intelligent control panels will significantly decrease the need for paper-based manuals. The secret, though, is to find easy access. Enter the keypad.
The ideal keypad
Even the actual buttons on the keypad are carefully selected to ensure that just the right level of built-in resistance gives the user the feeling of stability and accurate key-press detection.
ABB compiled 11 guiding usability principles that consider all aspects of visibility and readability; text and terminology; icons used; and common look and feel that allow users to switch between different drives without a time-consuming learning process for each.
Advantages are not all aesthetic. Most equipment investment decisions carry a proviso for fast installation to ensure that production will start rapidly and smoothly. Paramount is speed with which a machine can be up and running after the installation of new equipment, or after maintenance shut down. If a machine breaks down, it can cost the company $20,000 per hour, so easy set-up and commissioning are a priority. Such urgency increases the risk of errors in installation and commissioning. These can be overcome by eliminating manual intervention wherever possible, and keypad design is central to this aim.
These guiding principles suggest the need for intuitive assistance through “wizards” aimed at guiding the user through various procedures, such as maintenance, diagnostics, and start-up.
The start-up assistant, for example, guides the user through both start-up and commissioning by asking questions in plain text language. There are no complex parameter numbers or codes. Software intelligence helps the user through the commissioning process. For an OEM, who might buy 4,000 ac drives per year, the time saved by using an easy wizard-based start-up system can cut an estimated 15 minutes from the commissioning time of each drive, equating to a time saving of 1,000 hours per year. For an engineer working 2,000 hours per year, this is half a person-year.
Specialist HMI
Another tool that makes life easier for the OEM is a hand-held human-machine interface (HMI) that allows drive parameters to be installed rapidly. ABB’s FlashDrop, for example, can be used to select and set parameters and to copy configurations between drives without powering up the drive, allowing users to download a set of parameters in two seconds. No specialized knowledge is required to use FlashDrop, and the user interface will be familiar to ABB drive users.
Manufacturers of ac drives can reduce costs for users by creating application-specific drive solutions. These drives incorporate incremental functionality that supports specific applications, such as fan and pump control, mixers, or crane controls. They can reduce the total cost of ownership through shorter start-up times, lower integration costs, and improved machine productivity. Time savings during commissioning can range from one to several hours. The process does not require expert programmers and, therefore, saves the considerable expense of sending commissioning engineers around the globe to fine-tune individual drives.
Take, for example, ABB’s new pump control software module, Intelligent Pump Control (IPC), which eliminates the need for an external PLC (programmable logic controller) and can help to save energy, reduce downtime, and prevent pump jamming and pipeline blocking.
The IPC is a software add-on containing all the common functions needed by water and waste utilities, industrial plants and other pump users through six pump control functions. The software also incorporates an adaptive programming utility, enabling users to customize drives for specific applications. This utility consists of a set of simple-to-use function blocks that can be combined to perform any operation. All common mathematical and logical functions as well as switches, comparators, filters and timers are available.
Programming can be carried out using the standard control panel. No special hardware or software programming tools are needed. As a result, the programming takes only a few minutes and can be carried out on-site, during commissioning.
Eleven years since ABB launched direct torque control (DTC) algorithms have been tweaked to move standard ac drive technology into the servo arena. The result is the launch of the ABB machinery drive. It uses standard ac drive technology, but, by tuning DTC to include a motor model designed for servo motors and using very fast torque control loops within DTC, the drive can reach servo-drive performance levels. It can control synchronous or asynchronous motors, either open- or closed-loop. With the new drive, depending on the application, machinery builders need specify only one drive for a variety of motor types along with an appropriate feedback device.
Modularity is the use of plug-in modules to house the heart and brains of the drive. On the hardware side, there is a power module and a control interface module, while the software module provides easy programming of the relay logic or PLC routines that an OEM may require for its own applications. In this way, modules can reduce commissioning times, eliminate the need for external maintenance engineers and minimize unpredicted production line stoppages.
By providing separate units, the power module and control interface can be shipped to site and installed ahead of the software memory unit. When the installation is complete, the memory unit, which can be pre-programmed with the OEM’s specific application code, can be delivered and simply plugged into the control interface on site. Without on-site programming or connecting communication cables to PCs, time savings during commissioning can range from one to several hours. The process does not require expert programmers, so saves the expense of sending commissioning engineers around the globe to fine tune individual drives.
Future ac drives are set to be smaller, more intelligent, easier to install and control, have better communications, and be suitable for many more applications, particularly at the low power end of the range, all at a constantly reducing price.
Ilpo Ruohonen, Mika Paakkonen, Mikko S. Koskinen, ABB
Edited by C.G. Masi , senior editor,
Control Engineering
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