Drive up energy efficiency

The constant-speed approach to AC motors made a lot of sense when energy was cheap and drives technology was in its infancy. With their latest advances, adjustable speed drives can provide one of the best energy efficiency options for a plant.

By Mike Spear, editor at large

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All industries use energy, but large energy consumers, such as chemicals manufacturers, refiners and others at the heavier end of the processing spectrum, have a bigger incentive than most to look at ways of using energy as efficiently as possible. Chemical engineers might extol the virtues of nonthermal separation processes, or push process intensification techniques as ways of reducing the demand for energy. These can certainly reap dividends; but electrical engineers might counter that one of the best energy efficiency measures is to fit a plant’s electric motors with adjustable speed drives (ASDs).

ASDs are hardly new technology, although the terminology does seem to change with the seasons. Some vendors and users, particularly in Europe, call them variable speed drives or frequency inverters, but essentially they all perform the same function of allowing you to control the speed and torque of AC motors that would otherwise constantly be running at full speed. Typical examples are pump and fan applications, where flow control is generally achieved not by varying the motor speed, but by throttling the flow with control valves or dampers while the motor runs flat out.

The constant-speed approach to AC motors made a lot of sense when energy was cheap and drives technology was in its infancy. Many a plant engineer might have argued that process control was better without introducing yet another variable – but technology and economics have moved on significantly since those distant days.

Sales of AC drives technology are now growing at a rate of more than 5% per year, according to Steve Ruddell, U.K.-based general manager of drives and motors for ABB, New Berlin, Wis., yet market penetration is still surprisingly low. Although ABB believes the global drives market to be worth around $6 billion — and its own market share to be nearly twice as big as its nearest competitor — it says only about 5% of all motors are controlled. That figure increases to nearly 20% for higher power applications. The U.S. Department of Energy estimates that the 40 million motors used by U.S. industry account for 70% of its electricity consumption — and that up to 18% of that energy could be saved by applying proven efficiency technologies such as ASDs.

Payback paradox
With potential energy savings of that order, it’s not surprising that short payback time on capital investment has long been a selling point put forward by drives suppliers. “Price remains a factor, but overall, prices are falling as drives become more of an off-the-shelf commodity,” says Geraint Evans of Control Techniques, Newtown, Wales, the U.K. manufacturing division of Emerson Control Techniques, Eden Prairie, Minn. “The paradox here, of course, is that purchasing decisions are therefore more likely to be made on capex [capital expenditure or cost] rather than opex [operating costs] grounds, so short payback periods stemming from reduced running costs become less of a consideration.” In such a price-sensitive market, functionality becomes more of a key factor with several technology trends emerging as the drives manufacturers strive to make their products as user-friendly as possible.

Last year’s introduction by Emerson Control Techniques of its Commander SK AC drive series is a good example of this. “The SK is designed to meet the needs of the mainstream distribution and OEM markets,” says the company’s Phil Sewell, executive vice president of sales and marketing. “It has the easiest possible installation and setup that take in some 80% of applications. Yet, scratch the surface and there’s a very smart, versatile product that is also suitable for more complex system applications.” The first phase of the worldwide launch covered sizes from 1/3 hp to 5 hp, with this year’s second phase introducing larger sizes of up to 135 hp (110 kW).

With its “simple and compact” label, the Commander SK range takes aim at the commodity, general-purpose drives market, but it also includes many added-value features. Control functionality is substantial, with five digital inputs, one digital output, 4-20 mA or 0-10 V analog inputs that allow PID control, a 24-V backup and ModBus RTU as standard. With optional SM plug-in modules (which are interchangeable with the company’s Unidrive SP series), a range of fieldbus configurations can be catered for Profibus-DP, DeviceNet, CANbus, Modbus TCP/IP and Ethernet IP and HSE, for example.
Among customer-requested features, the SK also has the option of a NEMA-1 gland plate enclosure, principally to meet U.S. market requirements, but also reflecting its growing prevalence as a requirement for global machine designers.

More than general use
   Figure 1
Figure 1. The Unidrive SP series is the basis for the Commander SK line, which is aimed at the commodity market, but offers features suitable for process applications.
Source: Emerson Control Technologies

Enthusiasm for Ethernet
According to a customer survey commissioned by Control Techniques, 75% of automaton users say they are already using or investigating Ethernet in industrial applications — a trend that is reflected in other recent drive introductions. For instance, Baldor, Fort Smith, Ark., launched at this year’s National Manufacturing Week in Chicago, and at the Interkama exhibition in Hanover, Germany, its new H2 series of inverter drives that communicate via Ethernet TCP/IP, Ethernet/IP (DeviceNet on Ethernet) or Ethernet Powerlink.

According to Mark Crocker, marketing director for Baldor Europe, the Powerlink option provides the possibility of true deterministic control, whereas the Ethernet service connection expansion board makes remote monitoring and adjusting of the H2 drives’ parameters easier than ever before.
The drives are an “all new” successor to Baldor’s established H range, and, says Crocker, “have been engineered for reliability and ease of use.” The operator keypads are removable and interchangeable, interfacing with all H2 power bases, control and expansion boards. The keypad enclosure is rated NEMA 4X (equivalent to IP66) when mounted on a panel, although it is also designed for remote mounting up to 100 ft. away.

With a choice of open- or closed-loop control, including inverter, encoderless vector, and closed-loop vector operational modes, the H2 series is said by Baldor to offer a solution to all drive application requirements. The company also maintains that the drives are the most reliable industrial units on the market — a claim that is easy to make so early in a product’s lifespan, but one backed up by Baldor’s design strategy that includes failure mode and effect analysis and a manufacturing process that function tests every sub-assembly, as well as stress and full-load testing of the final products. The initial units in the H2 series cover ratings from 1 hp to 60 hp (0.75 kW to 45 kW), with the full range expected to extend to more than 1250 hp (1000 kW) to handle the vast majority of AC-drive applications.

Late last year, Hitachi, Tarrytown, N.Y., announced an optional Ethernet board for its SJ300 and L300P families of AC inverters. The SJ-EN board can be installed into one of the drive’s internal expansion slots, connecting to the user’s network through a standard RJ45 connector. It implements the Modbus/TCP protocol, providing a high-performance, easy-to-use Ethernet interface with no need for special software or hardware, says Charlie Takeuchi, general sales manager of industrial components and equipment.

Priming the pump
As mentioned earlier, one of the most common process applications for AC drives is for pump control. Also at Interkama in April, ABB launched an optional software package for its low-voltage AC industrial drives. Dubbed “Intelligent Pump Control,” the package includes six functions together with ABB’s own Adaptive Programming utility. The company’s Nanette Baur says this enables users to customize drives for specific applications.

Its “Multipump Control” function, initially aimed at water and wastewater utilities, suits any application where several pumps are operated together and the required flow rate is variable. Each pump is controlled by its own drive, with the speed of one being adjusted while the rest run at constant speed. Baur says this results in smooth control with no pressure peaks.

Another of the functions, dubbed “Level Control,” is typically used to control the filling or emptying of wastewater storage tanks. A special feature of the software prevents sediment buildup on the tank walls by randomly varying the surface level within a user-preset range. Fast-ramp starting flushes pipelines clear, and the pump is operated at the optimum point on its efficiency curve to minimize energy consumption. The level-control function can be applied to a single pump, or two to three pumps and drives in parallel.

ABB’s flagship industrial drives form the ACS800 series, which boasts the extraordinarily wide power range of 2/3 hp to 7,000 hp (0.55 kW to 5.6 MW). All feature the company’s Direct Torque Control (DTC) technology, unveiled 10 years ago, but still regarded as something of a benchmark in terms of speed. DTC regulates motor torque and speed directly without the need for separate control of voltage and frequency.

Designed for use in tough environments, the modular range of drives features enclosure protection up to IP54 (which approximates NEMA 4) against dust ingress and splashing water. This generally means they can be used outdoors without the need for additional cabinets. The IP54-protected drive range has recently been extended to 110 kW (135 hp) from the previous maximum of 45 kW (55 hp).
Again in April, Rockwell Automation, Bloomington, Minn., introduced a new line of Allen-Bradley ArmorStart distributed motor controllers that now feature a built-in Powerflex sensorless vector drive. Housed in a robust IP67 (NEMA Type 4) enclosure, the compact drives can be mounted close to the motor (“On-Machine” to use Rockwell’s branding) and come pretested with quick-connect wiring; these features can provide savings on installation time of up to 40%, according to Marketing Manager, Claude Joye. “Manufacturers at all levels are searching for high-performance control devices that will give them greater application flexibility, faster installation and lower engineering costs,” he says.
The ArmorStart drive can be used with motors ranging from 0.5 hp to 5 hp. Factory-installed options include an HOA (hand/off/auto) keypad with jog function, dynamic brake contactor, source brake contactor, EMI filter and shielded motor cable.

Most of today’s drives look quite different from previous generations. Thanks to advances in power electronics, such as insulated-gate bipolar transistors, they can offer the same power as before, but in much smaller packages. However, it is at the user-interface level where the changes are most notable. Whether keypads are detachable or integral with the enclosure, many have the familiar look and feel of today’s cell phone technology — offering up graphical displays and intuitive menus for users to navigate the increasing variety of operational settings.

Supportive software
For those users whose intuition might fail them at a critical time, manufacturers like Yasakawa Electric, New Berlin, Wis., provide software wizards. Downloadable at no charge at www.yasakawa.com, version 6.0 of the company’s DriveWizard now features a greater capacity for remotely viewing, manipulating, troubleshooting and exchanging data with its drives. Parameters can be downloaded to any Yasakawa drive irrespective of its size or rating, making life easier for users working with specific applications on different drives. The DriveWizard can now also store multiple IP addresses for when drives are being networked using Ethernet communication.

Another piece of downloadable software shown at Interkama by Siemens Automation and Drives, Alpharetta, Ga., could be just the thing to put in front of those purchasing managers who think drives are just a commodity item and who simply buy the cheapest option. Sinasave (downloadable at www.siemens.de/energiesparprogramm) can calculate the potential energy savings when using frequency converters for variable-speed systems. For fixed-speed applications, it can show the savings to be accrued from switching to energy-efficient motors. The program takes into account the entire drive train — not just the individual drive.

For constant-speed drives, Sinasave calculates the energy savings and amortization involved in changing to energy-saving motors of a higher class. For variable-speed systems, you input the characteristic plant data and parameters, such as delivery volume and delivery heads in the case of pumps, or mass flow and total pressure difference for fans and blowers, along with data on working shifts, duration of operation and so on. The program then calculates the energy saving effect of using the correct drive system and will provide the payoff period for the investment.
So, now you can prove the benefits of the drive to energy efficiency.

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