|
|
|
|
|
|
01/11/2010
 Email to a friend
 Comment on this article
|
A new category of AC drives is emerging that is set to compete head-on with servo drives. As standard AC drives approach their 40th birthday, and move into their 5th 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 servo drive to do battle with AC drive technology.
 "Replacing servo drives with conventional AC drives is a dream of several manufacturers," says Professor 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."
A new category of drive
The term "machinery drive" has been coined for this new drives category. And the recent launch of a new range of AC drives by ABB, the so-called "general machinery drives", is indicative of a shift in the drives market.
"Today our most of our drives with motion control software are used in machinery applications. Grouping these together, they fall into this new category called machinery drives," says Roelof Timmer, marketing director for ABB. "They are not technically servo drives because we are coming from a different starting point, that is, the technology is based on standard AC drives."
Until recently many machine builders relied on servo drive 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 a standard squirrel cage 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 Juha Saloriutta, product manager for ABB's machinery drives. "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.
"It's all down to shaft performance: low speed torque performance and dynamic response - these are the key features."
Market share
Today, AC drives have a relatively low market share in applications currently served by servo drives. Some experts, however, predict that within 10 years, AC drives will replace some 20% of the applications where servo drives currently dominate.
"It is a mindset change," says Timmer. "If you designed a machine five years ago and have not upgraded since, then it's time to consider your alternatives. When the applications do not have high dynamics or demanding positional requirements then users should consider AC drives as an alternative. Think more broadly."
Where an AC drive really benefits is above 5kW. "The biggest cost benefits come with the larger applications," explains Mike Webb, managing director of motion control specialists, Quin Systems. "Here, AC drives will replace moderately dynamic, multi-kW type applications."
Applications where AC drives can compete with servos are steel rolling mills, large carpet machines, guillotines and flying shears. It is at the bigger powers where the cost difference begins to favour AC drive technology.
What cost
However, Professor Pacas believes the key parameter remains cost and not performance. Machine builders always ask the question: "Am I getting what I need for the minimum cost?"
"Today, conventional AC drives have some important functions for machine builders and provided they are affordable, they will sell."
Functions of interest include basic motion control tasks; communications – is it affordable and easy to use and what diagnostic features are included.
However, not to be out done, servo manufacturers are hitting back and the prices for servos have dropped dramatically in recent years.
Applications
There are many low demand applications which use only servos because, until now, there has been nothing else. Provided the application is an easy positioning system, such as conveyors, that does not have high dynamics or demanding positional requirements then AC drives, with position control, will do the job well.
This is a role for the recently launched ABB general machinery drive. "We have the possibility to compete against servos because sometimes the servos are offered to applications that do not really need the high performance," says Leo Annala. "I once visited a customer producing meat products. They had a servo drive that was turning sausages in a grill at an extremely slow pace. I could not believe my eyes."
Another suitable application is registration control, a solution for which has been developed by motion control specialists, Quin Systems. Registration involves keeping one product or material in register with another reference point, electronically rather than mechanically. Ensuring a rotary knife cuts at the edge of a label, for example, and not in the middle.
"We never considered AC motors previously for this application because their low speed characteristics are traditionally very poor," says Webb. "But now, such is the technology, that when used in a closed loop, AC motors give good torque characteristics at a low speed. This is an area where Direct Torque Control (DTC) and flux sensorless have widened the scope for AC drives. DTC gives velocity and current loop but without an encoder we could not do registration."
Quin has developed a card that slots into an ABB drive and provides closed loop control. "It is now very easy to turn a drive into a registration controller or flying shear controller simply by plugging in a card."
The future
"We are just at the beginning of a new trend," believes Timmer. "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-like performance, whereas the standard AC drive technology offers many benefits over servo drives."
The future of AC drives within the servo arena looks good, according to Webb: "We believe in it as long as it is applied in the right way. However, don't try and use it in high dynamic applications or with small inertias."
|
|
| |
Author
ABB Ltd
|
| |
| |
|
| |
This material is protected by Findlay Media copyright 2012.
See Terms and Conditions.
One-off usage is permitted but bulk copying is not.
For multiple copies contact the sales team.
|
| |
|
|
| |
To comment on news stories or blogs you need to complete our 60 second registration
process. Once completed this then allows you to download any and all white papers,
register for e-zines and access our detailed supplier directory for FREE.
If you are all ready a registered user then enter your e-mail address and login.
You will need to have logged in prior to entering your comments in the boxes provided.
|
|
|
|