Appliance motors turn green
Through legislative and social pressures, public attention is focusing more on the energy used to perform basic domestic functions such as food preparation, cleaning, and climate control. Government-developed energy guidelines now influence consumer preferences.
PMSMs contain less steel and copper than induction motors of the same power rating. That makes the relative price of permanent-magnet motors become competitive because the price of copper has more than tripled since 2003 and steel prices are again on the rise. Volatile commodity markets have less impact on permanent-magnet electric motors. In fact, growing use of permanent magnets in applications ranging from automotive drives to cellular-phone-vibrator motors has actually reduced magnet prices thanks to greater production.
However, the need to measure shaft angles with such position sensors as Hall-effect devices or resolvers tends to restrict PMSMs to high-end industrial servos. More recently, sensorless control algorithms have emerged that open the door to low-cost PMSM controls suitable for home appliances.
Early sensorless controllers used six-step commutation sequences for motor windings. They estimated rotor position by monitoring the back-EMF of the open winding. Speed control using this method is generally robust. But the back-EMF makes current in the outgoing winding drop faster while opposing current rise in the incoming phase during commutation. The result is uneven torque with high harmonic content. Those harmonics become audible noise as they resonate through the appliance's mechanical system. And the problem worsens at higher speeds.
DSP or Risc-based microcontrollers permit more sophisticated sensorless control. Examples are the sensorless controller that measures motor currents to estimate rotor position. The sinusoidal voltage and current waveforms produced by this controller improve torque quality while reducing audible noise. However, since control calculations are time critical, the processor may lack power to support additional features like reluctance-torque control or field-oriented control (FOC) to manage motor current. The additional software-development tasks also add extra cost and risk to the controller-design project.
Implementing a sensorless control algorithm in dedicated hardware eliminates these restrictions while providing more sophisticated control modes. For example, International Rectifier offers a hardware-based controller that combines rotor-angle estimation, phase-current reconstruction, and FOC as well as a phase-advance feature to enable reluctance-torque control when driving an IPM motor. Moreover, the hardware implementation executes the control algorithm up to 2.5 times faster than a DSP or Risc-based system. The FOC algorithm uses vector rotations to decouple the ac-motor-winding currents into two dc components controlling torque (IQ) and flux (ID). This simplified design lets the controller adjust the current loop independently of motor speed and helps control motor acceleration.
IR consolidates these hardware-based control algorithms along with a library of motor-control and general-purpose functions into a configurable block called the Motion Control Engine or MCE. Designers can configure the library functions to meet specific system needs. A graphical configuration tool selects functions and can include PI compensators, limit functions, and vector rotations as well as analog inputs and space-vector PWM control. The included compiler translates the control design into sequencer instructions that connect the hardware macro blocks in the proper sequence.
Efficient use of electricity and water is now a key selling point for domestic washing machines. Motor-speed control is a critical facet in meeting the demand to minimize consumption of resources.
For example, front-loading washers have a critical drum speed above which the centripetal force balances the weight of the clothes — the clothes stick to the side of the drum. Slowing drum rotation below this critical speed lets the clothes fall to the bottom of the drum. However, a drum turning too slowly does not properly open the clothes to the wash water, trapping dirt inside the folds. Maintaining proper drum speed as the heavy clothes shift position requires fast torque response from the controller. The drum must turn rapidly enough to provide vigorous washing of soiled garments yet provide gentle-wash cycles for delicate items. Finally, a high-spin-mode speed wrings excess water from the fabrics, boosting drying efficiency and energy savings.
In the past, appliance makers used a transmission to get different drum speeds in washers from fixed-speed motors. There were inherent inefficiencies from electrical losses in the motor to mechanical losses in the transmission gears. It is more efficient to drive the drum directly by a PMSM with electronic speed control. The more flexible and versatile motor-speed modes let designers better manage the washing action and develop wash programs that use less water.
A washing-machine motor-control system, such as the type formed from IR's MCE and its associated motor-drive chipset, gives fast torque response over a wide speed range. The controller efficiently drives a washer's permanent-magnet motor with single-shunt architecture and no sensors.
International Rectifier, (310) 726-8000, irf.com
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