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A new category of AC drives is set to compete head-on with servodrives

As standard AC drives approach their 40th birthday, and move into their fifth generation, the technology they use has already evolved to challenge the dominance of other methods of variable speed such as hydraulic, pneumatic and DC drives. Now, it would appear, it is the turn of the servodrive to battle with AC drive technology.

But what, exactly, is meant by a servodrive? The applications for servo technology are vast. And as manufacturers jockey to position their products, the result is a plethora of names trying to describe the different technologies available. In the high performance, high functionality range of applications, names such as ‘intelligent’, ‘smart’ and ‘servo’ drives are congregating to take a slice of a lucrative market with an estimated worldwide value of US$1.78B*.

This can lead to a confusing picture for the user. So what do these different terms mean and where does AC drive technology fit in? As a rule of thumb, the following helps define the different terms:

• ‘Intelligent’ drive has built-in motion control capability

High end drive offers complex functions such as synchronization between multiple axes

Low end drives is for basic positioning

• ‘Smart’ drive has built-in PLC: This is now incorporated into most modern drives

• ‘Servodrive’ is accurate and dynamic but may not necessarily be ‘smart’ or ‘intelligent’, depending on its functionality within a machine system

So the new category of drive is one that attempts to combine all of the above into a true universal solution. The term ‘machinery drive’ has been coined for this new category. And the recent launch of a new range of AC drives by ABB, the so called ‘high performance machinery drives’, is indicative of a shift in the market.

According to ABB, the main features of such a drive include:

• Dynamic: extremely rapid changes in speed and direction

• Accurate: follows precisely the demanded changes in speed and direction

• Flexible: has supporting functions such as:

Basic position control

Advanced motion control including multiaxis synchronization

PLC (Programmable Logic Control)

Serial communications to overriding process control

Drives any type of AC motor

Uses a variety of feedback speed/position devices

“Replacing servodrives with conventional AC drives is a dream of several manufacturers,” says professor José Mario Pacas, from the Siegen University in Germany. “To some extent it is truly possible but you need to be quite clear which applications you are talking about. You also need to be clear about the differences and limitations of each technology.”

Until recently many machine builders relied on servodrive technologies to meet their demands for positional variable speed control. Now they are being encouraged to look at the emerging machinery drives. Because machinery drives use any type of AC motor and because these drives use components that are produced in high volumes, they promise the machine builder improved reliability at a lower cost.

“The transition has been underway for the last five years,” argues Graham Barlow, engineered drives manager at ABB. “The main change happened when very high open loop performance became available in ordinary drives. This really meant that servos and, generally speaking, closed loop systems were replaced by low cost inverters in certain applications.”

Today, AC drives have a relatively low market share in applications currently served by servodrives. However, some experts predict that within 10 years, AC drives will replace some 20 per cent of the applications where servodrives currently dominate.

Where an AC drive really benefits is above 5kW. “The biggest cost benefits come with the larger applications,” explains Graham Barlow. “Here, AC drives will replace moderately dynamic, multi kW type applications.”

Applications for machine builders where machinery drives can compete with servos include converting, packaging, and printing – including registration control.

The ABB machinery drive’s Direct Torque Control is now said to offer servo performance from any type or make of AC motor, either specialist permanent magnet brushless servomotors, low inertia square frame AC induction motors or standard AC motors, utilizing a variety of feedback devices. The choice of motor depends on the application, but needs only one type of drive – the ABB model.

“We are just at the beginning of a new trend,” concludes Graham Barlow. “Machinery drives are applied by customers who appreciate the small size, high performance, user friendliness and quality of these products. With DTC, we can achieve servo performance, whereas the standard AC drive technology offers many benefits over servodrives.”

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