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issue: February 2006 APPLIANCE Magazine

Motor Controls
Controlled Drive


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by David Simpson, Contributing Editor

As appliance companies add sophisticated motors to their products, integrated circuit (IC) suppliers have worked to ease the design process and reduce motor controller costs.

As consumer needs and regulations become more demanding and complex, white goods manufacturers are looking to provide their products with more intelligence, functionality and features. To achieve the challenging requirements, manufacturers increasingly turn to digital signal controllers, like those from Texas Instruments (Houston, Texas, U.S.), to build quieter and more energy-efficient appliances.

Appliance producers around the world are facing stiff regulatory and competitive pressure to produce more energy efficient washers, air-conditioners, refrigerators, and other products. For that reason, it is hardly surprising that makers of motor-driven appliances are attracted to choices such as brushless DC (BLDC) and permanent magnet (PM) motors. In conjunction with their electronic controllers, these motors offer a way to meet the need for higher appliance energy efficiency.
Besides energy efficiency benefits, sophisticated motors and controllers make possible lower appliance noise and vibration levels, higher speeds, smaller motor size, the elimination of gearing, and the incorporation of diagnostics. Significantly, the high level of motor control can also improve function. For instance, for Italian producer Indesit (formerly Merloni), motor and controller changes to its Ariston Super Silent washer line allow faster, better stain removal.

Motor controller functions can also be integrated with user-controlled options in an appliance. Electrolux, for example, has Frigidaire dishwasher models that let the user choose wash cycles that complete in 50 percent less time than in a normal cycle. This is made possible by using a high-voltage PMDC motor to enable a versatile water pump.
Price has always been an issue with the new motors and controllers. However, as is typical with electronics, prices have declined while capabilities have been added. And, as variable speed electronic motor controllers became more prevalent, chip suppliers enhanced their products and developed strategies to make it easier for appliance companies to work with their components.

The dsPIC30F2010 from Microchip (Chandler, Arizona, U.S.) has 12K bytes of enhanced flash memory in a 28-pin SOIC and SPDIP package. It is said to be ideal for controlling a motor using advanced algorithms. This model and the dsPIC30F6010 both have a motor-control pulse width modulation (PWM) module and a 500 KSPS 10-bit analog-to-digital converter.

Efficiency Responses

Energy efficiency standards continue to be tightened as governments strive to reduce energy consumption and power usage.
In the U.S., residential unitary air-conditioners and heat pumps need to meet a 13 SEER (Seasonal Energy Efficiency Ratio) standard as of January of this year. This is a 30-percent increase in efficiency over the previous standard. In response, Toshio Takahashi, director of the Digital Control IC Design Center of International Rectifier (El Segundo, California, U.S.), says U.S. manufacturers are changing outdoor condenser unit fan motors and in-house duct blower motors from induction motors to PM motors. He adds that other motors, from dishwasher pumps to washers to refrigerator compressors, are now rapidly moving to PM motors.
Besides factors such as energy efficiency, he sees the cost of raw materials for motors playing a role in the change. "Since the induction motor is 40 percent oversized in its weight when compared to the same power range of PM motor, it uses more steel. Due to recent price increases of steel, which has gone up
more than 200 percent in the last 2 years, the final cost of the induction motor became more expensive than a PM motor counterpart."
In white goods and home heating appliances, there has been a continuous push to improve motor efficiency, points out Tom Hopkins, director of the industrial applications lab, STMicroelectronics of Schaumburg, Illinois, U.S. "Appliance
companies are looking to bring the efficiency of electronically commutated brushless or three-phase induction motors to the appliance market. Doing this cost-effectively is critical."
Hopkins feels that the digital signal processors (DSPs) sometimes used may be more expensive than necessary for appliances. "We've developed a low-cost microcontroller with a macrocell. This microcontroller is optimized for use with
BLDC motors, and will work with three-phase induction motors. The macrocell handles a lot of the computation timing and sensing. This frees the microprocessor for other tasks, and permits the use of a lower-power, lower-cost microprocessor
core. This approach has been used in refrigerator compressors, for instance, and in home HVAC fans. It has a lower overall system cost than a more complex DSP approach."

iMotion digital controllers, analog interface and power modules from International Rectifier (El Segundo, California, U.S.) provide application-specific integrated design platforms for variable speed, sensorless motor control in permanent magnet synchronous motors (PMSM) for appliances.

More for Less

"The trend in controllers is toward more features and lower cost, as with everything in electronics," observes Dr. Arefeen Mohammed, systems applications, Texas Instruments (Houston, Texas, U.S.). Prices have been going down pretty aggressively. Until, today, a controller, including everything-PC board, digital signal processor, terminals, and connectors-may cost in the $10 to $20 range."
One reason for declining costs is that controllers and systems are using fewer or less costly components. Instead, more functions are appearing on the chips.
For instance, EMI filters can be reduced in size through pulse width modulation techniques. "These require advanced algorithms, and a good math engine like our DSPs," observes Arefeen.
"When we first began talking to appliance companies more than 10 years ago, some of them thought that DSPs were overkill," Arefeen continues. "Why, they wondered, did they need algorithms that precisely control torque accuracy?
After all, a mechanical system such as a washer drum isn't going to change speeds as fast as the current. But without the algorithms, you are going to have higher current spikes. This means that you need larger, more costly MOSFETs in your
system. MOSFETs, not your controller chips, can be your most expensive components."
"Many chip makers are offering multi-chip modules," says Stephen Caldwell, director, Home Appliance Solutions Group, Microchip Technology Inc., Chandler, Arizona, U.S. "These modules integrate the power portion of the design in
a single chip, which eases the system design. Chip makers are also offering specialized peripherals on the microcontroller, which reduce overall component count, easy system design and reduce electro-magnetic interference (EMI)."
"OEMs want platform designs that they can reuse across markets," says Fraser McHenry, MCU division, Freescale Semiconductor (East Kilbridge, Scotland,United Kingdom). "Often there are necessary regional variations in design.
Using a platform helps reduce the time to market in different countries. One way semiconductor companies can help is with non-volatile memory, which enables an appliance to be customized for each market. Our chips use flash or flash
plus EPROM memory. We also include security protection options to keep people from hacking the customer's intellectual property (IP)."
International Rectifier's Takahashi tells APPLIANCE that his company developed a motor drive solution that enables full scale conversion from electromechanical control to variable speed electronic motor drive control for appliance applications
based upon application-specific integrated design platforms. His company's iMotion platform integrates digital controllers, analog interface and power modules to provide application-specific integrated design platforms for variable speed sensorless
motor control in permanent magnet synchronous motors (PMSMs).
"This enables a designer to quickly create a very low-cost design platform that is tailored to specific types of motor control applications with application-specific features and design tools," Takahashi said. "One significant trend is a PM motor with integrated inverter control. This is particularly a new trend
for washer and dishwasher applications. If motor manufacturers can supply this to the end appliance producers, this will create a win-win situation for both motor and appliance producers. Motor manufacturers can increase their value content by integrating intelligence and power control to their motor. Therefore, this can be a more value-added product for them. At the same time, for appliance producers it is a simplification of logistics. It simplifies component procurement, its maintenance and quality control. This also helps eliminate wiring."

Addressing IC Issues

High performance data converters from suppliers like Analog Devices solve design challenges associated with current and voltage monitoring, optical encoder feedback and resolver-to-digital conversion. The processes are vital to industrial applications, such as assembly-line robots, which require precise control of motor speed and mechanical movements.

Takahashi sees three key issues that semiconductor manufacturers need to deal with: quiet operation, torque per amp and the elimination of motor position sensors.
"Sinusoidal current control is the only way to achieve quiet PM motor control," he says. "Without sinusoidal excitation, the acoustic noise level becomes unsatisfactory, especially at high-speed operation. Consider the spin mode of a washer: if the control uses traditional trapezoidal current waveform control,
the acoustic noise combined with excess amounts of vibration make it impossible for high speed spin operation of the washer. The motion controller has to be able to produce sinusoidal current control."
The issue for power electronics the challenge of increasing utilization of torque-torque per amp, or how much torque can be attained by a given per-unit ampere flow to the motor. "If torque per amp can be improved, then appliance producers
can use smaller-size power devices," Takahashi says. "This also results in a smaller heat sink structure. The question is how to increase torque per amp for the PM motor. The PM motor, specifically the interior permanent magnet motor (IPM) has a so-called reluctance torque component. Reluctance torque is
additional torque due to inductance variation as a function of rotor position."
The third issue that Takahashi sees is how to eliminate the position sensor from the PM motor. "The PM motor essentially requires an accurate rotor position in order for control to properly align the voltage excitation to the motor stator winding depending on the rotor position," Takahashi explains. "Particularly for sinusoidal current control of a PM motor, accurate position sensing has been an essential part of the motor drive system. Traditionally the Hall Effect sensors
have been widely used to sense the rotor position. However, if control can achieve elimination of position sensing while providing smooth sinusoidal current control, then it becomes an effective solution for this latest appliance application.
However, achieving this sensorless control requires intensive R&D effort and, even if achieved, it may not be practical. It is, therefore, desirable for the control IC to integrate these functions."
Microchip's Caldwell
explains that, in a basic design, the motor control is done in the main microcontroller, but as electronics become more pervasive
in appliances the motor control can be offloaded to another microcontroller. "Control algorithm sophistication can also lead the motor-control designer to a separate microcontroller," Caldwell says. "A simple V/F (voltage frequency) algorithm on an ACIM requires 2 to 3 MIPS. Slip limited control requires approximately 5 MIPS. Vector control requires 10 to 15 MIPS. Depending on the maximum operation of the microcontroller, the designer may need to offload the motor algorithm to another microcontroller.
"Another option is to use a microcontroller with built-in DSP capability," he adds. "We offer the PIC18F1330, an ideal chip for ACIM control, which can operate to 10 MIPS. The dsPIC30F2010 has similar, albeit more advanced features, and can operate to 40 MIPS with built-in DSP capabilities. Using the latter chip
may allow the designer to keep a one-chip solution, even for designs requiring complicated algorithms."

Industry Participation

"There is tremendous motor-control knowledge being developed in the appliance supply chain," says Caldwell. "Keeping the development in-house enables the appliance producer to protect and enhance its IP (intellectual property). However, contracting the development makes sense when a partner already has the knowledge, thus reducing development
cost and risk."
As the appliance industry adds more high technology motor
approaches, IC suppliers offer a variety of assists. Many provide starter kits.
There are reference designs that have complete hardware listings and software files. Web tutorials may be available. Depending on the application, the IC supplier may do feasibility studies or work directly with the OEM to develop a solution.
"The chip makers are also offering integrated design environment systems with advanced graphical user interfaces (GUIs)," says Caldwell. "These GUIs help the system designers finalize the motor's operation parameters, which not only reduces development lead time, but also enables better appliance performance. These GUIs can also provide the microcode necessary for the microcontroller firmware."
"We are finding that, during the last 10 years, appliance producers have been taking over more and more of their IP ownership and development is being brought in house," says Freescale's McHenry. "A decade ago, there was a whole raft of suppliers who provided modules for the appliance companies. Now, increasingly, OEMs have their own electronic design centers.
"In general, we have seen that OEMs really began looking at three-phase motor control. They are moving from slot algorithms to vector control and space vector control, which provides more accurate estimation from the sensors. The
changes require better CPU performance. Initially, OEMs looked at DSPs, but they were not optimized for the applications. Now, however, we've got optimized DSPs for washers, dishwashers and compressors, and we are seeing an increasing rate of three-phase vector control solutions which are being driven by efficiency, noise and lower overall cost."
One looming issue that will affect programmable motors is the IEC 60730 legislation, which deals with household appliance safety. "It will require that white goods with motor controls include diagnostics," Freescale Semiconductor's Dugald Campbell tells APPLIANCE. "Manufacturers will use memory space in large appliances to check and verify the system integrity. And, if a component fails, it must fail safely. Already some companies are working toward meeting the standards, even though the implementation date is not yet set. In Europe,
we are working with standards bodies such as VDE, IMQ and BSI to discuss how the standards can be met."

Lower Power


While much of the attention has been on motor controllers for white goods and comfort conditioning appliances where efficiency is an issue, low-cost ICs can be beneficial in smaller appliances like mixers, blenders and food processors.
"Some manufacturers want to go beyond the traditional switch and triac control and speed regulation," says STMicroelectronics' Hopkins. "With a typical setup, when a user throws some food into the appliance the motor pulls down. We are now developing low-end microprocessors for speed regulation. Since manufacturers generally don't want to go to the expense of having a tachometer or sensing device on the shaft, these microprocessors will provide sensorless control. This approach uses a very low-cost microprocessor and triac on a brushed
universal motor. When a user adds food to the appliance, the motor will pull down much less."
Even as higher performing motor controller ICs are being
developed, there remains a wide range of less powerful silicon options, often at significantly lower price than a few years ago. Given the scope of options and motor controllers' advantages, it seems likely that ever-evolving motor control will gain an increasing role in tomorrow's appliances.

 

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