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issue: October 2005 APPLIANCE Magazine Part 2: Motors & Air-Moving Devices

Moving Devices
The Efficiency Equation


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by Jill Russell, Associate Editor

As efficiency demands continue to dominate the appliance industry, motor controls are helping motor suppliers maintain a competitive edge.

The ST7MC microcontoller family from STMicroelectronics was designed to work with both three-phase induction and BLDC motors, but can be altered to work with AC PM motors.

With new energy efficiency guidelines from the U.S. Department of Energy on the horizon, appliance makers are looking for new ways to meet them. Historically, the idea of efficiency has essentially been a matter of making an appliance do more with less. In other words, efficiency was making an appliance more powerful and more robust without increasing the size of its motor, and in many ways, this still holds true.
However, appliance producers are now adding new electronic controls, in place of sensors, to their motors to allow them to run more efficiently, with less noise and at a reduced cost. “A solution means that the appliance—the washing machine or air-conditioner—has to be simpler at fewer costs and be more extensive [in function]. That is the goal. Integrated circuits enable this new challenge of energy efficiency,” says Toshio Takahashi, director of the Digital Control IC Design Center for International Rectifier (IR).

Freescale Semiconductor integrates advanced sensor technology and analog/mixed signal components with highly functional embedded controllers to provide total system solutions for new generation refrigerators, washing machines, dryers, cooking products and other household appliances.

Sensorless Solutions

“There is no doubt that the industry is going towards energy efficiency,” Takahashi says. “This is resulting in quite a drastic change in motors.” Takahashi says this change has driven the move from AC induction motors to brushless DC permanent magnet (PM) motors and now to brushless AC PM motors. This, he says, is possible by the use of sensorless motor controls.
“Brushless DC motors have been popular because the motor behaves as a generator and produces voltage. The voltage has a rectangular wave form and is easy to control with a trapezoidal control,” Takahashi explains. “The brushless AC motor has a sinusoidal current control, which requires a sensor to detect the magnet position and is more complicated.”
To make the technology less complicated, IR recently eliminated the need for a motor magnet position sensor and introduced a line of sensorless sinusoidal controls that drive the motor, in addition to detecting the rotor position by analyzing the motor’s voltage wave form. The company’s IRMCK203 is said to offer an integrated smooth start and ramping algorithm, a speed operation of 100,000 rpm with 100 KHz torque control and configurable parameters for different applications.
According to Takahashi, IR was able to eliminate the position sensor by integrating a position estimator algorithm into the silicon chip. “Traditionally, by the time you finish [a motor], you have a control chip in conjunction with sensors, which is attached to the motor with designated wires in addition to the motor power wire going between the motor and controller,” he tells APPLIANCE. “With this device, those wires and sensors can be eliminated.”
Global controller supplier STMicroelectronics agrees, and says it is also seeing a move toward high-efficiency, sensorless solutions. Dennis Nolan, applications engineer for STMicroelectronics, says that the push toward small, three-phase motors in appliances is growing, and with it the implementation of sensorless controls. “In the past, it seemed that motor manufacturers didn’t want to make the small, three-phase motors because the controllers weren’t there, while the controllers weren’t being made because small, three-phase motors weren’t available and were too expensive. We are finally starting to get out of that cycle.”
STMicroelectronics also agrees that the sensorless trend is more cost-effective due to the reduction of both materials and parts associated with eliminating the need for components. “[With sensorless controls], you no longer have to put the traditional set of three hall sensors in the motors, which saves costs,” Nolan says. “It also makes the motor considerably more robust because there is now no electronics in the motor. It has system reliability advantages because when you have five wires along with the normal three power wires, those five wires at a signal level are eliminated when you do sensorless.” This, Nolan says, helps to increase the motor’s reliability, as there are fewer components to create potential faults, in addition to saving costs.

Eliminating Decibels

In addition to cost, sensorless controls also play into the efficiency equation, as sensorless designs help reduce overall noise. Also anticipating a trend in the sensorless market, STMicroelectronics recently introduced its version of a sensorless control with its ST7MC.
The microcontroller (MCU) is augmented with specific hardware macros dedicated to running three-phase motors, which ensures the CPU has enough horsepower left to run switches, lights and other components on the unit, Nolan says. In addition, the MCU, which was designed to work with both three-phase induction and BLDC motors, can be altered to work with AC PM motors, which the supplier says is a naturally quieter motor control.
“Permanent magnet AC motors are driven with a true sine wave drive, rather than a traditional six-step trapezoidal commutation, and that drive technique is an acoustically quieter technique,” Nolan explains.
On the market for a little over 1 year, STMicroelectronics has seen success with its latest MCU. Nolan says the control has been used on motors ranging from 1-1/2 A to 25 A in applications such as a computer server cooler system and commercial refrigeration units. “So far, [the ST7MC] has been able to meet most of the motor control challenges we’ve thrown at it.”
Freescale Semiconductor is also working to provide sinusoidal drive technologies and says the technology greatly reduces noise. According to Dave Wilson, a motion control product specialist for Freescale, a smoother torque ripple is achieved with sinusoidal technology because sine waves are supplied to the motor coils, as opposed to simply turning the stator coils on and off for trapezoidal control.

A New Economic Curve

According to Wilson, one of the main motor control challenges—the overall cost of the technology—has recently been overcome and is allowing control suppliers to offer sensorless technology to a large customer base. “For the first time, new algorithms, which allow quieter and more efficient operation, can be implemented at cost targets that even a few years ago were completely unheard of.”
Since the control algorithm determines how the MCU control drives the motor, the equation has a direct affect on the overall efficiency of the motor and the end appliance product. It is known that in order to achieve the maximum toque per amp, which relates to motor efficiency, the proper angle between the stator field and the rotor flux must be maintained, Wilson says. “This means you have to sample the variables on the motor frequently and if you don’t execute the algorithm fast enough the efficiency is going to suffer,” he tells APPLIANCE.
However, with the price of silicon declining, Wilson says new and advanced algorithms are now readily available and include advanced, sensorless designs at competitive costs. “To do field-orientated control (FOC) on an appliance 5 to 10 years ago was economically impossible,” Wilson says.
“Now that we have silicon that can do FCO very efficiently, our appliance customers are saying they can afford it. Now, because of the dropping price in silicon, new algorithms, sophisticated filters and control techniques can become available to appliance producers economically and they can start putting the technology into their products.”

Suppliers mentioned in this article:
STMicroelectronics
Freescale Semiconductor Inc.
International Rectifier
 

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