Mikko Koskinen argues that a new range of ac drives – the so-called ‘machinery drives’ – is indicative of a shift in the drives market.
Mikko Koskinen argues that a new range of ac drives – the so-called ‘machinery drives’ – is indicative of a shift in the drives market.
Anew category of ac drives is emerging that is set to compete head-on with servo drives. As standard ac drives approach their 40th birthdayand move into their fifth generationthe technology they use has already evolved to challenge the dominance of other methods of variable speed such as hydraulicpneumatic and dc drives. Nowit would appearit 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.”
The term ‘machinery drive’ has been coined for this new category. And the recent launch of a range of ac drives by ABBthe so-called ‘general machinery drives’is indicative of a shift in the drives market.
“Today our ACS150ACS350 and ACS800 with motion control software are all used in machinery applications. Now if we can group these togetherthey fall into this new category called machinery drives” says Roelof Timmermarketing director for ABB. “They are not technically servo drives because we are coming from a different starting pointthat isthe 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 volumesthey promise the machine builder improved reliability at a lower cost.
“The transition has been underway for the last five years” argues Juha Saloriuttaproduct 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.”
Today ac drives have a relatively low market share in applications currently served by servo drives. Some expertshoweverpredict that in the next five to 10 years ac drives will replace some 20percent 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 sincethen it’s time to consider your alternatives. Designers need to break their habit. The world has moved on in five years. 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 Webbmanaging director of motion control specialistsQuin Systems. “Hereac drives will replace moderately dynamicmulti-kW type applications.”
Applications where ac drives can compete with servos are steel rolling millslarge carpet machinesguillotines and flying shears. It is at the bigger powers where the cost difference begins to favour ac drive technology.
HoweverProfessor 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? Todayconventional ac drives have some important functions for machine builders and provided they are affordablethey will sell.”
Functions of interest include basic motion control tasks; communications – is it affordable and easy to use; what diagnostic features are included; and what is the drives reliable? Howevernot to be out doneservo manufacturers are hitting back and the prices for servos have dropped in recent years. Servo drive manufacturers will respond and produce more competitive and cheaper products which will be good news for customers.
The traditional servo drive may disappear in time due to advances in ac drive technology. Pragmatists and servo drive manufacturers who make both ac and servo drives may delay the processbut pressures on companies to cut costs and downsize will ultimately lead to change.
There are many low demand applications which use only servos becauseuntil nowthere has been nothing else. Provided the application is an easy positioning systemsuch as conveyorsthat does not have high dynamics or demanding positional requirements then ac driveswith position controlwill 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 application is registration controla solution for which has been developed by Quin Systems. Registration involves keeping one product or material in register with another reference pointelectronically rather than mechanically. Ensuring a rotary knife cuts at the edge of a labelfor exampleand not in the middle or registering a sucker head to a moving target on a conveyor.
“We never considered ac motors previously for this application because their low speed characteristics are traditionally very poor” says Webb. “But nowsuch is the technologythat when used in a closed loopac 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. DTC makes ac drives a better candidate because you can achieve the low speed torque characteristics that you are looking for” says Webb.
Most pumps and fans run at nominal speed whereas servo applications require regular start/stop routines and this is where the characteristics of DTC make it a better solution for low speedlow torque applications.
Another advantage is that field weakening can be used with the ac motor which can extend the speed range if high torque is not needed at full speed. If you know the profile characteristics of the ac drive it can be better and give a faster machine.
“We are just at the beginning of a new trend” believes Timmer. “Machinery drives are applied by customers who appreciate the small sizehigh performanceuser-friendliness and quality of these products. With DTC we can achieve servo-like performancewhereas the standard ac drive technology offers many benefits over servo drives.”
The future of ac drives within the servo arena looks goodaccording to Webb: “We believe in it as long as it is applied in the right way. Howeverdon’t try and use it in high dynamic applications or with small inertias.”
According to Mikko Koskinenmarketing manager for ABB’s machinery drivesthe trendtodaywithin the high performance segment is to have a multifunctional concept. “This means that the same product is able to control all kinds of motorsincluding high performance servo motors.”
Mikko Koskinen is with ABBHelsinkiFinland.
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