Variable speed evolution01 March 2004

The most significant advances in AC variable speed drive technology are behind us. Over a decade ago, the shift from bipolar transistors to insulated gate bipolar transistors heralded a quantum leap in efficiency.

Today, changes between one generation of drives and the next are incremental - a process of evolution rather than revolution. Earth-shattering developments have come to an end, but recent trends indicate the technology's future direction.

It's clear that drives are getting smaller. A modern unit is one-tenth the volume of its ten-year-old counterpart. And in another ten years, says Ilpo Ruohonen, technology manager for motors and drives at ABB in Finland, AC drives will be 70% smaller than they are today.

Developments outside the drives industry, chiefly in microelectronics, have made smaller drives possible. The power electronics industry tends to adopt the same technologies and production techniques as the microelectronics industry - five or six years later. More efficient power electronics mean smaller heatsinks - and smaller drives.

Losses in today's insulated gate bipolar transistors are 30-40% lower than a few years ago, but losses per unit surface area are going up as components shrink. Cooling, therefore, remains a crucial consideration in drive design, with computers used to model the flow of air around heatsinks and the drive's other components.

A more exotic potential solution to the heat problem is the 'cool chip', a kind of electronic heat pump. In a cool chip, an applied voltage forces electrons to jump across a microscopically thin vacuum layer. They cannot jump back, so heat is transferred in one direction only, from a power semiconductor to a heatsink, for example. Researchers are still struggling to fabricate large cool chips for practical applications.

A small drive that generates little heat can be fitted in enclosures sealed to IP54 or IP66 and installed outside the control cabinet. Drives suitable for washdown are popular in both the food and beverage, and pharmaceutical industries.

But moving the drive out of the cabinet and closer to the motor is part of a wider trend in automation to move components close to - or even on to - factory machinery. Automation companies make entire ranges of control equipment designed for life outside the control cabinet.

Such decentralised equipment has become popular in niche applications, particularly materials handling, and it can be 30% cheaper to install than a conventional cabinet, largely because there is less cabling.

A related development is the appearance, around five years ago, of the first motors with built-in drives. Drives manufacturers thought that these devices, usually rated below 4kW, would be a fillip for their industry, which had been hit hard by the downturn in manufacturing.

Although in many cases the hoped-for sales have failed to materialise, motors with integrated drives have made an impact in applications spread over a large area - car making, airport luggage handling, materials handling, food processing and warehouses.

The advantages of such devices are obvious: they cut installation and commissioning time, the motor and the drive work together without on-site tweaking and, by eliminating long cable runs between the drive in the control cabinet and the motor, engineers can avoid many electromagnetic compatibility problems.

A disadvantage is that an inherently reliable piece of equipment, an AC motor, is coupled to a complex electronic drive. If either element fails, both will be out of action. Also, the drive electronics in an integrated motor and drive are subject to heat and vibration that they would not experience if safely tucked away in a control cabinet.

In the past few years, several manufacturers have added a 'safety-off' function to their drives, which stops unintentional movement of the motor when power is restored after a failure. This is just the beginning of an effort to design drives for safetycritical applications that will act in a defined way in when a fault occurs, and to eliminate a number of external safety devices.

"The safety-off function has gathered some momentum in certain applications and industries," says Mark Daniels, Rockwell Automation's marketing manager for drives in the UK, "but it's a very simplistic level of safety."

Developing a fully fledged safety drive is extremely difficult because each application has to be considered on its own merits. In some circumstances it may be appropriate to stop the motor dead, in others it may be safer, and less disruptive, to run the process slowly instead.

It will be some time before a true safety drive emerges. Development cannot begin in earnest without a series of agreed, rigorous safety functions that engineers can choose to suit their applications.

The operator panels of modern drives may be less intimidating than those of the past, but many panels have to be set manually during commissioning if the drive is to control the motor properly.

Again, developments in microelectronics have taken some of the drudgery out of setting up multiple drives. It may be possible to remove the operator panel to transfer parameters from one drive to another, or to use a smart card. Connecting a PC will allow more in-depth adjustments.

The user interface is becoming more important to drives designers who want to differentiate their products from those of the competition. Some standard drives, for example, include setup 'assistants' or 'wizards' that guide users through the important settings.

But there is scope to go further. The goal is a selfcommissioning drive that needs no manual setting of parameters. It would detect the characteristics of the driven load or process and then use this data to set up its own parameters. "Self-commissioning drives are now feasible," says Ilpo Ruohonen. "The limit is the understanding of the customer's requirements."

Engineers work with AC vector, servo and DC motors, and in the past each has its own kind of specialist drive. For nearly a decade, drives designers have sought to simplify setup, commissioning and use of different kinds of motor by making drives that can handle them all.

Manufacturers have taken different approaches to this. Some use swappable control board or modules, others achieve the changeover with software, but the principle is the same: programming the drive to run an AC vector, servo or DC motor is an identical procedure. There are collateral savings in training and maintenance because there is only one drive to support.

Some high-end drives have built-in programmable logic controllers. These combined devices perform the control functions for many applications - particularly those that demand only simple logic control and an inverter. Examples include conveyors, positioning stages, winders, hoists and multi-stage pump systems.

Many engineers are wary of combining logic with the inverter. "People know PLCs, and we come along and say 'get rid of it and we'll do it all in the drive'," says Steve Barker, business manager at Siemens' Drive Centre in the UK. "There's a question mark over customer acceptance of that... they like traditional control concepts with PLCs."

Even quite modest drives support a range of fieldbus protocols such as CANopen, DeviceNet, Interbus or Profibus. A standalone drive is quite unusual. The technology to watch here, though, is Ethernet, which is already in use on the factory floor. Before long, drives will have their own IP addresses and will serve web pages of status information for viewing on any web-enabled device.

This process is already starting - at a more basic level. Modern drives store a host of parameters that can be used to trace faults, perform preventive maintenance, or even refine plant performance. "If we do get failures," says Daniels at Rockwell, "the product gives you clues as to what made it finally give up the ghost. People are pulling parameters out over the network and using the data."

At present, only a few stout-hearted early adopters are embracing these technologies. Engineers are notoriously conservative. But they should not let that conservatism and the up-front appeal of cheap, basic drives blind them to some of the alternatives that the evolution of drives technology is making possible.

Key trends

- Control equipment is moving out of the cabinet and closer to, or even onto, machinery

- Drives built into motors cut installation and commissioning time, but are more exposed to heat and vibration

- External safety devices can be eliminated by using drives designed for safety-critical applications, but a true standard is some way off

- Multiple drives can be set up more easily using removable panels or smart cards, or by connecting to a PC

- Self-commissioning drives are now feasible.

SOE

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