- January 01, 1996, Mykytiuk, Andrew
AC technology is clearly the wave of the future, but its eventual displacement of DC technology is still a long way off. And what about brushless DC? Some leaders in the field offer their opinions.
The DC motor is the converting industry workhorse and has been so for more than 50 years. There are hundreds of thousands of these motors in use by converters throughout the world. The venerable DC motor has stayed around this long because its strengths are two of the critical requirements of converting machinery - accurate speed control and torque control.
These motors can easily achieve and maintain the required speed and torque without sophisticated electronics. Speed is dependent on voltage, and torque is dependent on current; it's a linear proportion. "It's a simple unit, and, in most process controls where precise tension control is required, it can do an excellent job," says Hans Wirth, director of converting drive systems for ABB Industrial Systems, New Berlin, WI. "That's why DC has stayed around so long."
DC certainly is proven technology, but it does have some fundamental limitations: It has internal components, such as brushes, that continually wear out and need replacement; it generates a lot of heat and therefore requires ducting to keep it cool; it requires more electrical energy to operate than an AC motor; it can cost three times as much as AC; and it requires expensive encoders for speed and position feedback.
When AC motors debuted they were used in noncritical applications that didn't require precise speed or torque control, and most of those applications were one-speed pumps or fans. The earliest versions of AC motors had no speed control; it was either off or full on - there was no in between. Research and development on AC motors was driven by the desire to mimic the excellent speed accuracy and torque response of the DC motor while maintaining the AC motor's inherent benefits of virtually no maintenance, ruggedness, small size, and low initial cost. However, for a long time, converters ignored AC motors because of their lack of variable speed capability.
Microprocessors Change the Picture
All that changed with the advent of microprocessors. "To run an AC motor and control it from a speed and torque standpoint, you need powerful electronics that can perform a series of math models to control the motor exactly the way you want to," says Stef Dziekonski, director of marketing for AB Reliance Drive Systems, Mequon, WI. So, it's only within the past 10 to 15 years, as microprocessors have become more powerful, that AC machines could do more than just spin at full blast.
As microprocessor technology advanced, bringing more memory and faster processing capability, software that was specific to the needs of converters could be produced. "We put mathematical models inside the controllers that allow you to precisely control torque and speed," says Dziekonski. "And it's only in the past three or four years that the power of the microprocessor has been large enough to allow you to do this in an effective fashion and, for the first time, actually make the AC motor more accurate than its DC counterpart."
"Three or four years ago you could get speed and torque control that were almost comparable to what DC can do," explains Keith Allen, marketing manager for Allen-Bradley's Converting Industry Drive Systems Group, Mequon, WI, "but within the past one to two years the technology has moved forward to a point where AC will actually give you better torque control than DC."
An AC History Lesson
Earlier AC drives applied in the converting industry featured variable voltage input (VVI) six-step technology. This technology was limited relative to low-speed torque delivery and power utilization and was only suitable for simpler applications that did not require high levels of accuracy or precision, such as fans and pumps.
The next development was the flux vector drive, which controlled flux to simulate torque. It used a pulse encoder to feed rotor position back to the controller and is therefore referred to as a closed-loop system. The motor's electrical characteristics are mathematically modeled, and microprocessors are used to process the information. "The AC motor is controlled by variable frequency; if you change the frequency, the speed changes in proportion to the frequency change," says Wirth. "Torque control is critical to converting applications, because that's how you control web tension. An AC motor has an inherent amount of slip, if you control the flux, and that's how they control the slip - by changing the flux. The result is torque control."
Full torque can be achieved at zero speed, giving this system performance rivaling that of a DC drive. A limiting factor is that a modulator is used, and this slows down the communication between incoming voltage and frequency signals. The motor has to constantly respond to this ever-changing signal.
According to Mark Scharfenort, tension controls product manager for Cleveland Machine Controls (CMC), Cleveland, OH, "AC vector drives are probably a little better than a standard regenerative, three-phase DC drive, better than any nonregenerative DC drives and definitely better than any single-phase regenerative drives."
One of the latest developments in AC motor research has been in the direction of field-oriented control - or making AC motors perform like DC motors. Allen-Bradley's vector control technology deals with the ability to independently control the flux and torque-producing currents in the motor. The result is highly accurate torque and power control. Allen-Bradley has found that a drive using a "self-organized," indirect, field-oriented control approach can provide 300% torque performance for a standard AC motor. This type of drive modifies its control based on a sensed state of the machine instead of forcing the motor to adapt to a model reference that requires parameter identification, which occurs in many flux vector drives. Where an MRAC controller uses a model reference and estimations of parameters to force motor flux and torque to the reference state, a self-organized control scheme for the field-oriented controller recognizes motor state in a generic way. The self-organized, field-organized controller focuses on the result (i.e., precision torque control, flux control) and works with terminal qualities to achieve the desired result.
At the recent Converting Machinery/Materials Show, ABB unveiled its latest development in AC motor drives, a system called Direct Torque Control (DTC). There is no modulator and no need for an encoder to feed back information about motor shaft speed and position. The DTC control incorporates fast digital signal processing hardware, resulting in a torque response that is said to be ten times faster than any AC or DC drive.
AC Costs Come Down
Five years ago AC systems were 50% to 60% more expensive than comparable DC systems, and many customers were not willing to pay that premium. After all, DC technology is proven technology; it does the job. But in the past five years, AC costs have come down dramatically. A fitting analogy would be the personal computer. They're getting more powerful and less expensive every year, and the same thing is happening with power electronics.
"New technological innovations, new manufacturing techniques, and increased capabilities are allowing us to drive the costs down," Dziekonski says. "And AC, which for many years has been a lot more expensive, has come down to the point where today it's on par with DC in terms of cost and superior to DC in terms of performance."
The new systems being designed today are almost exclusively AC. But is DC rapidly becoming a dinosaur as far as new machinery design is concerned?
At a TAPPI engineering conference in Boston in 1970, Wirth heard a seminar speaker predict that in ten years there wouldn't be any DC drives purchased, because AC will have taken over. Obviously that did not happen, but the latest report from the automation research group ARC, based on interviews conducted with people in the manufacturing industry, predicts that DC drive applications will decline at a rate of 3.5% per year. This slow decline is due mainly to the large amount of DC motors in use throughout the industry.
In the meantime, DC remains a strong presence. Says Wirth. "With the thousands and thousands of DC motors out in the field, the customer isn't just going to throw them away, which is why I think, while the changeover to AC will eventually occur, it's off in the future."
The question remains, however, with a clear trend toward AC systems, will companies continue to spend R&D dollars on DC systems? Wirth says the energy conversion aspect of D C technology has not changed in 30 years. "The only developments made in the past 30 years are in the electronics - what you control and how you control it. The portion that converts the energy from the line to the motor is old technology. The only big changes are the ability to program applications into the drive to make the system more reliable and user-friendly, i.e., make it easier to communicate with computerized supervisory control systems. DC motor drives can now send information about themselves back and forth and can also communicate between each other in the line.
When is DC Not DC?
In brushless DC technology an AC stator in a permanent magnet rotor and an encoder device such as a resolver are used for external commutation. With an AC induction motor it's impossible to know the exact position of the poles during rotation - it must be calculated. Brushless motors, because of permanent magnet rotors, allow one to know precisely where the magnets are, so the position of the poles is always known.
"Brushless DC is not DC at all but a completely different technology," says Scott Barlow, national sales manager for Powertec, Rock Hill, SC. The only reason it's called DC, he says, is because of the concept of external commutation. "The commutation cycle always will be smoother, quicker, and more accurate, because we know what AC systems have to calculate. We have been using this technology to replace conventional DC technology since 1987, and, in fact, the growth of our company has been based entirely on the replacement of DC products."
This form of DC motor technology, unlike conventional DC, is virtually maintenance-free; all you have to do is lubricate the oversize bearings. "The brushless DC motor, by design, is the most efficient motor available, and that's documented," Barlow adds. "AC technology has only approached brushless DC as a replacement for DC but has not exceeded it." Just like AC, brushless DC motors offer near unity power factor at all speeds and loads. This technology delivers full torque continuously at any speed, offers precise digital speed regulations, and reportedly provides higher starting torque than AC or brush-type DC drives.
Electronic line shafting eliminates drive shafts and gears and mechanical coupling between all the driven rolls and replaces them with a controller that electronically keeps track of the position of each roll. The computer takes the information from each motor and makes incremental speed adjustments to keep the line in synchronization. Barlow says that brushless DC is the choice for electronic line shafting applications as well.
Scharfenort of CMC says that AC vector drives control motors better than any DC controls but admits that DC servo motors, the type used in motion control applications, are even better, especially in low-speed applications. "In applications such as electronic line shafting, we will only use DC servo motors, both brush and brushless. They are unique in their ability to run smoothly at all rpm ranges, and the trend here is brushless DC."
The breakthroughs that have put AC on a par with DC are relatively recent. The impact of AC motor and drive technology on the converting industry has barely been felt. Hans Wirth says that ABB's DTC system is so new that there are just a few being tested in the "real" world, but expect the number of applications to increase rapidly during the coming years. Converters are business people and will only transition from one technology to the next if it makes sound business sense.
DC Not Obsolete Yet
In this article we have attempted to cover trends in motors and drives. We are not suggesting that the DC equipment you're using is obsolete - it's not. There are, however, big changes on the horizon. An excellent example is the previously mentioned line shafting. While it can be considered a quantum leap offering unprecedented performance and accuracy at any speed, the old mechanical shaft technology has been continually redesigned and perfected since the industrial revolution. Lovejoy Inc., Downers Grove, IL, makes nearly every type of mechanical power transmission coupling imaginable. Its gear couplings offer high horsepower ratings, torque capacity, and axial misalignment capabilities in 27 styles and sizes designed to handle up to 47 million inch-pounds of torque. It's not electronic line shafting, but it's state-of-the-art for a mechanical means of power transfer.
"Even within certain types you can vary the components within a coupling to handle just about any application," says Mark McCullough, Lovejoy's product manager.
One of the reasons AC technology will likely continue to make inroads into DC's dominance is reduced power consumption. Electric utilities impose penalties on converters, depending on how far their operating power factor deviates from the value of 1 (power factor is the relationship between the voltage wave form and the current wave form; the farther below 1, the higher your electric bill).
"On an AC machine/drive system the power factor is constantly at a very high point, almost near unity .97 or .98 regardless of machine speed," says Keith Allen. "With DC systems the power factor varies with speed, so, as you slow down the machine, the power factor drops off substantially, and, as you accelerate, the power factor rises. In applications where the motor must constantly speed up and slow down, the power factor on DC motors fluctuates, while an AC motor's power factor remains constant at any speed, regardless of acceleration or deceleration."
The new technology also allows for increased levels of customer support. With the emphasis on just-in-time production, downtime is unacceptable. With today's systems, if the machine goes down you can, via modem, have a technician at a remote site use system diagnostics software to troubleshoot the entire system from anywhere in the world.
If we could erase history and start out with a clean slate, there is no doubt that AC would be the choice. But the fact is that there are billions of dollars of DC investment running converting operations throughout the world. Some day all motors and drives may be AC, but the conversion will progress at a rate dictated purely by economics.
Because of space constraints, we are unable to discuss all of the companies that manufacture drives and motors for the converting industry. Check the Paper, Film & Foil Converter Directory and Buyers Guide (June 1995) issue for a complete listing, and keep reading PFFC for continuing developments in this very important aspect of our industry.
ABB Industrial Drive Systems Inc., New
Berlin, WI; ph: 414/785-3241; fax: 414/785-8390.
Allen-Bradley, Mequon, WI; ph: 414/242-8631; fax: 414/242-8637.
Cleveland Machine Controls, Cleveland, OH; ph: 216/ 524-8800; fax: 216/642-5155.
Lovejoy Inc., Downers Grove, IL; ph: 708/852-0500; fax: 708/852-2120.
Powertec, Rock Hill, SC; ph: 803/328-1888; fax: 803/328-1870.