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.

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