Motor and Control Advancements Save Energy

Over the last few years, many equipmentdesigners have moved away from low-efficiency motors, such as single-phaseinduction motors or brushed dc motors. But often this transition results in ahigher cost for motion systems that comprise the motor, an inverter and acontroller, and require development of new control algorithms. But enterprisesmust balance the higher initial cost of efficiency with the long-term overallcost of ownership. Over its lifetime, a more efficient motor can save theconsidA-erable cost of otherwise-wasted electricity.

"You can buy a one-horsepower 1,800-rpmac-induction motor for $40 to $50, but a comparable industrial-grade permaA-nent-magnetbrushless motor could cost as much as $400," says Scott Evans, an appliA-cationengineer with Kollmorgen. "Large machine manufacturers have started to includeenergy-efficiency criteria such as an upper current limit, power use and a highpower factor in their design specifications. And, of course, companies wanttheir customers to see them as a euro ~green.'"

"So rather than using less-expensive,less-efficient motors to save on the initial purchase price of, say, amillion-dollar packaging line, they'll pay a fair price for highly efficientmotors that help save electrical costs long term," he continues.

"Just because you specify apermanent-magnet brushless motor doesn't ensure a more-efficient system," Evansstresses. "A permanent-magnet motor could use a ferrite magnet typical of thosein older brushed-dc motors. Those motors are a bit more efficient than acomparable ac-induction motor but you don't get much for the increase in cost.On the other hand, samarium-cobalt or neodymium magnets produce a much strongerfield, which manifests itself in higher thrust or torque. Specify one of theserare-earth-type magnets in a brushless motor to improve efficiency."

"When you use BLDC motors or PMSMs(permanent-magnet synchroA-nous motors), you must control them efficiently togenerate the highest torque," says Jorge Zambada, senior applications engineerin the High Performance MicroA-controller Div. at Microchip Technology. "So youuse field-oriented control rather than the basic six-step control that justspins a motor. In field-oriented control, or FOC, the controller measurescurrent in the stator coils and uses that information to precisely a euro ~position'the fields so you obA-tain the highest torque possible with the same current youwould use for a six-step controller." For more information about field-orientedcontrol. (Ref. 1)

"Engineers refer to the current meaA-surementtechnique as a euro ~sensorless,' but you can use sensor-based techniques instead,"says Zambada. "In theory, a sensor, such as an encoder, could give you betterposition information, but unless you have excellent motor coil-to-sensoralignment, you won't get good feedback information. So in many cases, engineersuse sensorless techniques,

although they increase thecomplexity of control algorithms."

"In a sensor-based design, the engiA-neers could use motors withbuilt-in Hall Effect sensors that detect magnetic fields," says RajuKaringattil, MCU business development manager for moA-tor-control applicationsat Texas InstruA-ments. "But this approach adds wiring complexity and costbecause the motor requires more wires for the sensors and if a sensor burnsout, you must replace the motor. Sensorless designs simplify wiring andmechanical challenges, but they inA-crease the challenge of knowing the exactposition of the rotor."

"You can apply FOC to 3-phase inducA-tion motors, too," continuesZambada. "Those motors traditionally have used what we call volts-per-hertzcontrol that employs a basic configuration of MOSA-FETs or IGBTs in an invertercircuit to control the stator-coil currents. Engineers can use a basic 8-bitmicrocontroller (MCU) to create a dedicated volts-per-hertz controller thatdoesn't use feedback information. If they want to change to FOC, they mustmeasure field positions and choose a more-capable MCU that can run the FOCalgorithms. But they can use the same MOSFET- or IGBT-based inverter circuit todrive the stator fields." In steady-state operation, for a motor under volts-per-hertzcontrol, the air-gap flux is approximately related to the ratio Vs/fs, where Vsrepresents the amplitude of motor phase voltage and fs represents thesynchronous electrical frequency applied to the motor. Thus the volts-per-hertzdesignation. (Ref. 2.)

"Engineers might say, a euro ~ourbrushless application is 70-percent efficient now, what efficiency can weexpect from FOC?'" says Zambada. "It depends on their algorithm, the type ofmotor, the type of load and other factors. Engineers can use a dynamometer toanalyze efficiency from electrical-energy input to mechanical-energy output."

"You can always look atthe pulse-width modulator (PWM) signals and the type of inverter circuit usedto control a motor for ways to save energy," says Zambada. "If you operate theinverter at, say, 20 kilohertz you might get comparable results by dropping to16 kilohertz. That represents a small change, but it eliminates some switchingloss at the MOSFETs or IGBTs. As a rule of thumb, use as low a PWM frequency asyou can to get the performance you need. The lower frequencies can increaseaudible noise, but if you build the motor into a noisy compressor, for example,a bit of added noise won't make much difA-ference but you do save power."

"Put special emphasis oninverter deA-sign," stresses Zambada. "The slower the MOSFETs turn on, thehigher the switchA-ing losses due to the IR drop through the MOSFET. Aim for alow gate resistance on the inverter MOSFETs so they turn on quickly. But if yougo too low, you can create switching noise that can adversely affect otherelectronic circuits."

In a product such as awashing machine, engineers can design a direct-drive or a belt-and-gear-drivetub. The latter often puts restraints on spin speeds. "In that sitA-uation,engineers can use a field-weakening algorithm to drive a motor athigher-than-normal speeds, but at a loss of some efA-ficiency," says Zambada."They can reduce the size of the motor and eliminate the gears and belts,however." Motor speeds can exceed 1.5 times the rated speed. Up to that ratedspeed, a motor operates in constant-torque mode. But above its rated speed, itoperates in constant-power mode. That means the increased speed comes at theprice of reduced torque. But in a washA-ing machine, for example, the spinningtub does not need a lot of torque to keep it going. (Ref. 3.)

"We have seen a four- orfive-fold increase above a motor's rated speed," says Zambada. "If a customerhas used an 8-bit MCU to control the motor they would need to change to ahigher-perA-formance MCU, perhaps a digital signal controller. The algorithmrequires a lot of feedback information and the MCU must continuously adjust thefields to enA-sure the right commutation of the motor to raise it to thosehigher speeds."

Semiconductor companiesrealize these design techniques sound complicated, so they offer engineersdevelopment kits, software libraries and reference designs to give them a headstart. "Motor control involves knowledge of the motor - an electromechanicaldevice - analog components, the MCUs, knowledge of

the software and powerelectronics. So engineers have a lot to think about and a development kit orreference design lets them start with from 50 to 75 percent of the electronicsand software already tested and ready to go," says Karingattil.

"Sometimes engineers need some help understanding the differentapproaches they can take to control motors efficiently," he says. "They mightnot understand how to compare field-oriented control with 6-step BLDC motorcontrol. For example, you can control a BLDC motor with either trapezoidal orsinusoidal signals. But by usA-ing a sinusoidal drive you get better control andless vibration. Vibration means less efficient control of a motor."

"Engineers mustunderstand, though, that the motor that comes with a reference design isusually not the same one they will use in their equipment," cautionsKaringattil. "TI provides the parameters for the motors in its kits, so whendesigners change to a different moA-tor, they must characterize it to obtain itsassociated parameters and characteristics. Motor suppliers will give you a datasheet, but you need the characteristics of the motor for your load conditionsand your operating environment so you can determine the control parameters foryour equipment. Those parameters lead to the values you use in the control softA-wareyou create. So you must know what happens when the motor starts, stops, stallsand so on. Don't take the data sheet as the a euro ~last word' on a motor."

"When you develop acontrol algorithm, the motor exists as a mathematical model," continuesKaringattil. "If you plug in inA-correct values, the motor will vibrate; have ashort life and waste power. So, characterA-ize a motor as accurately aspossible."

With that mathematicalmodel in mind, engineers could look to modelA-ing and simulation tools to lend abig hand. "Our simulation tools let these engineers model more and more of themotor down to nonlinearities," says Tony Lennon, industry marketing manager atThe MathWorks. "Modeling supports the capability to create algorithms thatprovide better motor control and reduce the amount of power a motor uses."

Engineers may find,though, that a vendor's prototype motor might have somewhat differentcharacteristics than the motors assembled later on a manuA-facturing line. Also,the characteristics of a production motor can change due to variations in thewire used to wind the coils and magnetic materials used in rotors. So, thosesmall changes can cause significant variations in the overall systemperformance.

"Experienced engineershave learned about these variations through a lot of lab testing and theyunderstand the

need to include thosevariations in their simulation models," says Lennon. "Simulation tools canincorporate these kinds of tolerances in the model and you can run parametersweeps to see how the system performance will change with the variations in themotors that you receive. Being able to assess the motor variation in thesimulation lets you develop more robust motor control algorithms that can betested against operational profiles. It helps you take a systematic and repeatA-ableapproach to making more informed design and cost trade-offs."

"If you put a motor on adynamomA-eter and you excite it, you can measure inputs and outputs for manyoperating conditions," says Lennon. "Then you have data that represents thedynamics of motor operation. Optimization routines can use this data to tuneparameters, such as rotor inertia and torque constant, in the motor model,resulting in a more accurate model. "A more accurate model lets you make betterestimates of how the motor will perform with a variety of real operating loads,which gives you better insight into the kind of controller needed to achievethe desired efficiency of the motor-drive combination," says Lennon.

"Then in a simulation you drive the model with a euro ~real' currentso you can deA-termine how many kilowatts the motor will use. And you can putdifferent loads on the simulated motor to examine how it behaves."

In addition to a thorough analysisof electronics and control algorithms, engineers must realize improvements inefficiency don't stop at the end of a motor shaft. "People will spend a lot ofmoney on their motors and controller," says Kollmorgen's Evans. "Then theypurchase an inexpensive 50:1 worm-gear box, for example, instead of a moreexpensive and high-efficiency helical-bevel, cycloidal or planetary gear box.Instead of getting 0.95 horsepower from a 1 horsepower motor, they only get abit more than 0.5 horsepower after the worm gearbox. The wasted energy goes intoheat and audible noise, resulting in premature motor failure."

For more information:

a euro cent "Induction Motors," Baldor ElectricCorp. http://designnews.hotims.com/27744-503

a euro cent "Motor Operation aboveBase Speed: Field Weakening," Technical Note 128. Emerson Industrial Automation http://designnews.hotims.com/27744-504

a euro cent "AC Induction MotorSlip: What It Is and How to Minimize It," http://www.plantservices.com/articles/2002/48.html

a euro cent "Developing a Permanent Magnet Synchronous Motor Controllerusing Model-Based Design," The MathWorks http://www.mathworks.com/wbnr39764