Trends: The ever-shrinking ac drive

01 September 2007

The attitude of end users towards ac drives has shifted over the past ten years. Today they want drives that are simple to buy, install, start-up, commission, and to own and run.

Taditionally, the primary use of ac drives, or inverters, has been for applications such as pumps, fans and conveyors. They will continue to be used in these applications, but, 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.

Why 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 ten-fold decrease in the size of drives over the past ten years. The fact that drives are now being used in domestic
washing machines is a testimony to their extreme compactness.

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
analogue/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 miniaturisation 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 everexpanding 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 and then downloaded to the drives. Set-up information is archived for future retrieval.

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 (photo at right), 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 specialised knowledge is required to use
FlashDrop. The user interface is familiar to ABB drive users.

Application-specific drives
Manufacturers of ac drives can reduce costs for users by creating applicationspecific 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 startup 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 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 customise 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.

The machinery drive
Eleven years since ABB launched direct torque control (DTC), algorithms have been improved 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 minimise unpredicted production line stoppages.

By providing separate units, the power module and control interface can be shipped to the site and installed
ahead of the software memory unit. When the installation is complete, the memory unit, which can be preprogrammed with the OEM’s specific application code, can be delivered and simply plugged into the control interface. 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 it 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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