A Diverse Drives Market Depends On PE Engineers
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Motion Control System block diagram. (Courtesy of Infineon Technologies) By replacing the diode- or SCR-based ac-dc rectifier with an active front end, drive manufacturers can provide high power factor and reduced harmonics on the line. Active front ends also enable regeneration—the ability to take the energy produced in braking the motor and put it back on the ac line rather than wasting it as heat.
The market for industrial drives encompasses a very wide and varied range of motor and motion control applications. From fractional horsepower to 1000+ hp motors with widely varying voltage levels and wide-ranging control requirements, industrial drives must serve a broad spectrum of needs in powering all types of industrial machinery.
On top of this diversity, there are some common market pressures that add to the challenge of designing and manufacturing industrial drives. Since motors are big energy consumers, it’s imperative that the drives (and the machines they’re part of) be as efficient as possible. And by virtue of their role in production, drives must be highly reliable with long life expectancy. Meanwhile, as with just about all power supplies, customers want drives to be made as small as possible to help shrink the size of their equipment. All of these goals must be met while balancing other demands such as electromagnetic compliance (EMC) and cost.
When you couple the diversity of application needs with the market pressures for improved performance, you end up with potential opportunities for power electronics (PE) engineers in the industrial automation (IA) field. A recent survey of some of the IA company websites reveals that a number of these organizations are seeking to hire PE engineers. You can see some example job openings in the online version of this article.
In pursuit of the many drive-design goals, PE engineers who develop industrial drives are working to evaluate and deploy the latest generation of silicon-based power semiconductors, while also keeping close tabs on the emerging silicon carbide devices, which promise performance leaps in the near future. These engineers are also working to apply new topology options such as active front ends, which offer multiple application benefits.
A Maturing Drives Market with Diverse Needs
At a high level, the design of power electronics for industrial drives is driven by the same types of technical requirements common to nearly all power electronics applications, explains Bill Drury, a consultant with Emerson’s Control Techniques division. Power converters, says Drury are “defined by the tasks they have to perform (and the required performance), the environment they have to operate in, the reliability and lifetime, and finally the cost and price.”
In IA, the nature of the tasks being performed can be quite varied, resulting in very different requirements for motor drives.
“Consider, for example, a machine tool axis,” says Drury. “In this application, very high levels of acceleration and deceleration are required, often at high repetition rates. The power converter (and motor) have to be able to cope with very high overloads, and indeed are likely to be dimensioned on the basis of those overloads rather than the average loading. By contrast, a fan or pump type of load, where dynamic performance is rarely a requirement, would be dimensioned on the maximum continuous loading.”
Meanwhile, the actual power levels and voltages of the motors, as well as their control requirements can also vary widely as described by Tom Lenk, director of Drives Development at Rockwell Automation.
“We are working across the spectrum that goes from fractional horse powers all the way up to a megawatt…and at different voltages, anywhere from 230 V or 480 V all the way up to 15 kV. We’re also looking at different control performances,” says Lenk.
In addition to the wide range of power levels required, you have a maturing drives market that is demanding more segmentation by power level, according to Lenk. “So the power electronics continues to spread in spectrum as to the number of products that need to be created to fit the more intelligent buyer who is trying to find the specific, right power device for his application.”
Drury observes that the drives market is also migrating to higher voltages. “As power ratings for industrial automation products increase, there is a move to products operating at higher supply voltages. This brings with it a range of novel power electronic circuit topologies and power semiconductor devices,” says Drury.
Efficiency In Pursuit of Other Goals
High efficiency, high reliability, and small size are high on the list of design goals for industrial drives and pursuing the first tends to support the pursuit of the other two objectives. Achieving these goals depends heavily on the PE engineer’s ability to leverage the best available (yet cost effective) power semiconductors including the designer’s ability to accurately model these devices.
“Efficiency and reliability are paramount. They have always been paramount, but the numbers that we’re looking at for PPMs and uptime have dramatically improved with the improvement of the power semiconductor device and our knowledge of that semiconductor device,” says Lenk.
Rather than being an end in itself, achieving higher efficiency is a means to other ends including smaller size and new capabilities. “This is not simply a low carbon issue, but also reflects into product size (less heat means smaller size) and cost of cooling, etc,” says Drury. “A lot of work is being undertaken to reduce losses in power electronic equipment, and this will continue.”
Lenk observes how the size issue becomes more important for higher-power drives.
“There is a constant market pressure to increase power density. And as you go larger in power, the more dramatic that pressure becomes. On a small 1-hp or 1-kW drive, the control boards and the user interface are quite a bit larger than the power section. But as we go up in power, the power cooling system starts to become a major factor, the capacitors that are supporting the dc bus are a major factor and just the heat removal from the semiconductors is a factor.”
The PE engineer focuses on these thermal management issues.
“The power engineer today is continually playing a thermal tradeoff game with making the devices switch faster to reduce losses, but as we switch faster, we create more noise, so then you need a larger filter. So we’re continually working that spectrum of ‘where’s the sweet spot between losses within the device due to switching versus the cooling system.’ And we’re working very closely—the top tier companies are all working—with the power semiconductor device makers to get the optimal power device to suite their equation for smallest footprint for the given power level.”
Drury ties the size issue to the customer’s space requirements in the industrial cubicles or cabinets where drives are housed.
“Machine builders are looking to reduce the size of their machines and control cabinets which contain drives are often the focus of their attention. In the extreme, they are eliminated and a distributed solution is adopted or the available size is reduced,” says Drury. “There is also a trend to more and more drives being used on machines and so even if the cabinet is not being shrunk, then more has to be fit into the existing space. Packages exist with more than one drive in a module, and in some situations this can be helpful, but there is continual pressure on size.”
Hearing this emphasis on improving efficiency, you might be misled to think industrial drives are relatively inefficient. However, that’s not the case. Lenk points out that existing drives already achieve efficiencies of 97% to 98%.
However, there is a challenge even in maintaining this level of efficiency as drive manufacturers work to add new new capabilities to their products. For example, replacing a conventional diode- or SCR-based front end with an active front end offers big benefits to customers (see the figure.)
With active front ends, which are IGBT-based power converters, the energy produced in slowing down a motor can be put back on the ac line instead of wasting that energy as heat. This process, which is similar to the technique being implemented in hybrid and electric vehicles, is known as regeneration. This capability supports the pursuit of higher (energy) efficiency on a “macro” level for the customer. Meanwhile, active front ends also clean up the line, reducing harmonics drawn by the drive, and raising its power factory to unity, which will also help to reduce the customer’s utility bill.
One more windfall from using active front ends—the drive can now accommodate a wider input voltage range. As an example, Lenk points to HVAC applications.
“We have OEMs in the HVAC space who sell a product in the United States. It’s a 480-V product and they sell a product in Europe which is a 400-V product. Today, when they don’t use drives they have to use two different motors to make up for the 50 Hz and 400 V when they’re in Europe. If we put in an active front end, we can actually use that active front end to take the 400 V and boost it up to a dc voltage which is equivalent to the 480 V. Hence, they can use the same motor and same drive for the European skew numbers as they do for the U.S. skew numbers.”
Lenk notes that active front ends have tended to be used in the past on higher power drives where energy savings are greatest. But as the associated silicon costs have come down, says Lenk, active front end technology has been applied at lower and lower power levels.
This discussion continues in the online version of this article where you can read about the quest for improved power semiconductor devices and better device models in industrial drive development. This section also looks at other trends in industrial automation and their impact on the drives’ power electronics. The article concludes with a listing of recent job openings for power electronics engineers in the industrial automation industry.
About the Author
When he’s not writing this career development column, David G. Morrison is busy building an exotic power electronics portal called How2Power.com. Do not visit this website if you’re looking for the same old, same old. Do come here if you enjoy discovering free technical resources that may help you develop power systems, components, or tools. Also, do not visit How2Power.com if you fancy annoying pop-up ads or having to register to view all the good material. How2Power.com was designed with the engineer’s convenience in mind, so it does not offer such features. For a quick musical tour of the website and its monthly newsletter, watch the videos at www.how2power.com and www.how2power.com/newsletters/.
Better Devices and Better Modeling
Whether working to improve efficiency or reliability, the PE engineer continually seeks better power semiconductor devices and better models for those devices.
“We are very interested in the status of the silicon and the bond wires inside the device,” says Lenk. “And we go to great lengths in our business within Rockwell to have real-time models of that device to ensure its wellness and to ensure its reliability.
Drury notes the importance of device modeling in achieving higher performance. “This places greater demand not only on control strategies but also the power electronic circuits,” says Drury. “Higher overloads are being sought for some applications, which means much better thermal modeling of the power semiconductor gate regions to ensure protection under extreme operation.”
For now the focus is on getting the best silicon components (mainly IGBTs), but drives manufacturers are looking ahead to a time when silicon carbide (IGBTs or MOSFETs) are available and cost effective. Although SiC power components are starting to become available and feasible at lower power levels for other applications such as high-efficiency power supplies, they’re not ready for most industrial drives. “They’re way too expensive for the standard market today, but that trend is upon us and it will be entering the drives market over the next decade” says Lenk.
Drury observes that SiC and gallium nitride (GaN) devices both have potential to reduce losses, but there are device and application issues that need to be addressed.
“GaN and SiC both show benefits in terms of reduced losses compared with silicon devices. Some types of devices based on these technologies are now available and are finding uses in some niche applications and aspects of design,” says Drury. “However, problems exist—their gates appear to be “delicate” and a new approach to driving and protection is likely to be required. Little work has been published on these practical areas—data sheets for those devices which are available appear to show the manufacturers are struggling to be able to recommend how best to use their devices.”
When other drive requirements are factored in, silicon devices come out ahead, at least for now.
“Power electronic design is a compromise; a balance often between minimizing losses, the EMC generated and cost of the power semiconductor,” says Drury. “In terms of [drives] products for the main market areas in coming years, getting the best out of silicon is the name of the game.”
Nevertheless, when the power semiconductors based on new substrate materials become practical, they will enable significant improvements in performance. Speaking of the anticipated SiC devices Lenk explains, “The value proposition here is that when the losses go down on a semiconductor we can switch it [at a higher switching frequency] and increase the losses a little. But increasing the switching frequency allows us to make all those filters smaller, and makes some heatsinks smaller. So, things start collapsing again and the size and footprint go way down.”
Though not necessarily restricted to SiC and GaN devices, which do allow higher operating temperatures than silicon, there is a general trend on the part of device manufacturers to permit operation of power semiconductors at higher temperatures. According to Drury, this trend could further advance efforts to integrate motors and motor drives. “It may not be too far-fetched to imagine power semiconductor devices located beside or within the end windings of the motor,” says Drury.
Other Trends Impacting Drives Design
In IA, there’s a migration from centralized to distributed control. While the control function falls outside of power electronics design, the adoption of distributed control has implications for PE engineers as Drury describes.
“This can place equipment in more onerous environments, perhaps with higher temperatures and potential contamination with anything from textile fibres to explosive gasses. Equipment may need to be regularly washed down. And the electrical supply may be less well regulated at on-site locations,” says Drury.
Distributed control also means more complexity in terms of achieving electromagnetic compliance (EMC). Under centralized control, multiple motor drives are housed in a single cubicle (cabinet) and in such cases “a single ‘bulk’ EMC filter can be used to address conducted emissions issues,” says Drury. However, under a distributed control scheme, the motor drives are moved out of the cabinet and closer to the motors, so individual filtering will be needed on each drive.
In general, distributed control will dictate greater complexity. “Control and communications will grow ever more important and synchronization of control loops between industrial automation equipment will be critical in high-performance motion systems,” says Drury.
Though the focus in this article has been on power electronics, mainly the power stage devices, certainly there’s more to the drive design than power. Industrial drives have evolved over time to integrate more and more functionality to suit the needs of IA. “Communications, PLC (programmable logic control) functions, safety features, motion control, HMI (human machine interface), condition monitoring, modem & text messaging are all present in many drives,” says Drury.
Lenk also emphasizes the importance of incorporating intelligence within drive products. As an example, he cites a feature called DeviceLogix that Rockwell offers in its drives. This feature enables users to customize the functionality of the drive at the drive level, while also simplifying the task of connecting the drive to a network so that the drive is essentially plug and play.
“We see that localization and we think OEMs like it because they can sell a machine with the drives in it and possibly not the PLC. So the OEMs can put their IP into their product, programming it locally at the drive level and sell it to an end user who will have a network with Logix PLCs that connects right into the drives. That integrated architecture connection between the PLC and the drive is where Rockwell excels.”
Meanwhile, the continued development of the PLC has potential implications for the motor drive. “The PLC functionality allows users the ability to create their own machine-specific control strategies. It is likely that PLC programming will continue to develop in terms of ease of programming and scope of control, including powerful distributed control techniques,” says Drury.
“It is possible that the drive will eventually become so flexible and so functional that someone will hit on the idea of developing a very simple power converter block with bolt on functions –industrial Lego!” Although Drury concedes that such a vision for industrial drives may be impractical, and not necessarily lead to lower cost or greater flexibility, it is a dream of some customers.
