Save your energy

DN Staff

November 18, 2002

9 Min Read
Save your energy

There are a lot of ways to reduce energy consumption in appliances, and appliance designers don't miss many of them. Whirlpool's front-loading Duet clothes washer, for example, uses 67% less electricity than conventional top-loading washers, primarily because it uses less water that has to be heated. In refrigerators, energy consumption has declined dramatically due to numerous simple design improvements like using a microprocessor, rather than a timer, to control the defrost cycle.

But the one thing common to most appliances is a motor, and motors are significant energy users themselves. As a result, there's a growing trend to equip appliances with motors that are more efficient, such as the brushless dc (BLDC) and the brushless ac (BLAC) motor. And because these motors require electronic control, they're being accompanied in appliances by motor-control microcontrollers and digital signal processors (DSPs). The software-driven appliance motor is becoming, if not yet the norm, then the reality of the future.

The switch to new motor types with electronic control is practically inevitable, say industry experts. You can redesign traditional ac induction motors and universal motors for greater efficiency, but motor prices rise as a result, and the marginal gain in efficiency simply isn't enough to meet increasingly tough energy regulations and guidelines. In addition, the single-phase ac induction motors long used in most American air conditioners, refrigerators, washers, and dryers-the largest household energy consumers-run at a single speed. Single-speed operation makes these motors not only wasteful of energy, but handicapped in providing features that many consumers want, such as quiet operation. Electronically controlled variable-speed motors don't have these problems.

The switch to new motors in not without disadvantages, however. Brushless motors cost more, and that's a problem in the price-sensitive appliance market. Compared to ac induction motors, says Motor Consultant Thomas Kaporch of Drives Research Corp. (San Juan Capistrano, CA), brushless dc motors have about a 50% price premium. On top of that is the cost of electronic control, roughly equivalent to the cost of the motor itself. In a washer, says Kaporch, motor and drive together can run $70 to $85, a big chunk of the total bill of materials.

A brushless dc motor needs electronic control because it can't, by itself, generate the sequenced curents in stator windings that are needed to create a rotating electromagnetic field in the stator.

Advantages. Fortunately, electronic control offers advantages that offset many disadvantages. For example, because an electronically controlled BLDC motor has variable speed, a washing machine design can eliminate the belts and gears normally needed to reduce motor speed (typically 1800 rpm) to tub speed (perhaps 300 rpm). Eliminating belts and gears doesn't completely negate the higher cost of a brushless motor and electronics, says Kaporch, but it does significantly offset it.

Electronically controlled variable-speed motors also enable noise reduction. In a washer, the elimination of belts and gears reduces noise. In an air conditioner or a refrigerator, a compressor can run constantly with a speed that varies with cooling load, rather than intermittently off and on at a higher and noisier speed. European appliances, most of which use variable-speed motors, typically are significantly quieter than their American counterparts.

Energy conservation, of course, is the biggest advantage of the new motors. Brushless motors are inherently more efficient than ac induction motors because their rotors contain permanent magnets, not electromagnets. Also, variable-speed operation results in system efficiency improvements. In a refrigerator or an air conditioner, for example, a constantly running, variable-speed compressor that supplies just the right amount of cooling typically uses 30% less energy than a compressor that runs intermittently at a higher speed. In a washing machine, a variable-speed motor can provide a high spin-cycle that extracts extra water from clothing and thus reduces the amount of energy needed for subsequent drying. In Maytag's Neptune washer, for example, a switched-reluctance (SR) motor runs at 800 rpm to extract 30% more water than conventional washers. Subsequent energy for drying drops accordingly, by 25 to 30%.

Without electronic control, however, none of these benefits are possible. Brushless motors, in fact, won't even run without electronic control. Their simplified constructions, relative to ac induction motors, are incapable of generating a rotating electromagnetic field that, in turn, provides mechanical rotation. This field comes-indirectly, at least-from electronics.

Consider, for example, how electronics drive a brushless dc motor. (This motor is actually a permanent-magnet ac synchronous motor. It runs on ac, but is often called a dc brushless motor, because its principle of operation is similar to that of a dc motor). A brushless dc motor has permanent magnets in the rotor (the rotating part) and electrical windings in the stator (the fixed part). As an electromagnetic field rotates around the stator and attracts the permanent-magnetic rotor, the rotor follows this field and thus rotates. A BLDC motor can't generate this rotating stator field by itself, however. Additional electronic components must produce field rotation by appropriately sequencing the flow of current into the different phase windings around the stator. In most cases, the key component for controlling this sequence is a microcontroller or a DSP. Because microcontrollers and DSPs are programmable, they offer design flexibility that hardwired control components can't.

Choices. The choice between a microcontroller or a DSP for motor control isn't always simple, however, because the two devices have many similarities. Microcontrollers, which are essentially micro-processors with on-chip functions such as memory, timers, and analog-to-digital (A/D) converters, have always been geared to the control of various types of devices, including motors. In contrast, the original purpose for DSPs was signal processing-as in radar or communications-which requires multiple and parallel high-speed computations. However, DSPs that specifically targeted motor control began appearing in the late 1990s.

These new DSPs had on-chip peripherals similar to those on microcontrollers. All three of the big DSP companies-Texas Instruments (Dallas, TX), Motorola (Austin, TX), and Analog Devices (Norwood, MA)-now provide these special DSPs. Motor-control microcontrollers are available from a number of companies, including Motorola, STMicroelectronics (Carrollton, TX), Microchip Technology (Chandler, AZ), Hitachi Semiconductor America (San Jose, CA), and Fujitsu Microelectronics America (San Jose, CA).

Essentially, DSPs designed for motor control are like microcontrollers with additional on-chip hardware for number crunching. This edge in processing power, say motor-control experts, makes them well-suited for precise closed-loop control of high-speed motors. According to Texas Instruments, DSPs' superior number crunching reduces control-loop delays to allow high-resolution control, thus enabling the reduction of torque ripple and motor vibration. Microcontrollers, although less capable, are well suited for open-loop motor control and simple closed-loop systems.

Microcontrollers are gaining on DSPs' high-end superiority, however. For example, Hitachi's 32-bit SuperH microcontroller is a reduced-instruction-set-computer (RISC) device that runs at 50 MHz and delivers 65 million instructions per second (MIPS) processing performance. In the past, says Drives Research's Kaporch, motor-control designers avoided RISC architectures because of their lack of determinism-the ability to reliably accomplish certain tasks in a known time interval. Now, though, controllers such as the Hitachi SuperH are fast enough and powerful enough to make determinism essentially irrelevant.

When it comes to motor control, specifications such as MIPS are not necessarily a sufficient yardstick for measuring microcontroller or DSP performance. "You have to look at how many of those MIPS are used up doing the necessary mathematical operations," says Victor Berrios, a marketing account manager in Motorola's DSP division. DSPs have a single-cycle multiply-and-accumulate (MAC) instruction that is crucial for fast computing, he says, whereas a microcontroller might have no MAC instruction or a slower one. "You can put a MAC instruction on a microcontroller," Berrios says, "but not all MACs are created equal."

Lower prices. DSPs are also getting closer to microcontrollers in price. Several years ago, DSPs couldn't be had for less than about $20. Now, although microcontrollers still have a price edge, motor-control DSPs and microcontrollers both sell for less than $5. With their higher processing power, some DSPs also gain a cost advantage by handling general-purpose control tasks other than motor control, such as user interfaces and overall appliance control. By combining these tasks, DSPs can sometimes eliminate one, or possibly two, additional microcontrollers from an appliance's bill of materials.

Not every neat feat is in the DSP's bag of tricks, though. Microcontrollers might actually play a role in keeping alive the venerable and ubiquitous single-phase ac induction motor in appliances. Anacon Systems (Mountain View, CA) has developed technology to provide variable-speed control to these motors, and, according to Kaporch, they've implemented it on an 8-bit RISC microcontroller. "This is a relatively simple control, and it could be a potential breakthrough," Kaporch says.

On the other hand, motors used in appliances could conceivably take a completely different turn. Using technology developed by Sunpower, Inc. (Athens, OH), LG Electronics has developed a refrigerator, now being sold in Korea, that uses a highly efficient linear compressor. Unlike a conventional compressor that converts a motor's rotary motion to reciprocating motion, a linear compressor gets its mechanical energy from a linear motor or actuator. "It has about a 2-to-1 advantage in efficiency versus a standard reciprocating compressor," Kaporch says.

Inevitable electronics. Regardless of which motor types gain prominence in appliances, electronic control seems destined to be included. Its variable-speed capability alone can reduce energy consumption and acoustic noise, two things that are at the top of many appliance buyers' shopping lists. With programmable microcontrollers and DSPs, you can also make software changes to tailor appliances for different markets and regulatory requirements.

Is such a dramatic switch to new motors and electronic control really necessary, though? Is that the best way, or even a good way, to save energy? These issues and many others related to energy use are subject to debate. According to Motorola, however, refrigeration alone accounts for 10% of the world's energy consumption. With better control, the company claims, 30 to 50% of that energy can be saved. Better control in the whole range of appliances can only add to those savings.

Comments about this article? Contact the author at [email protected].

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