Permanent-magnet variable-speed motors point the way to high-efficiency appliance motors.
El Segundo, Calif.
Washing machines, refrigerators, dishwashers, air conditioners, and other appliances sell more on their "green" credentials, like a high-energy-efficiency rating, than other factors.
The use of speed control alone in appliances such as refrigerators and washing machines boosts electrical efficiency more than 30% compared to traditional fixed-speed units. Variable-speed systems also let designers specify smaller and less-expensive motors and thereby continue to offer attractive prices to consumers.
Low-cost induction motors dominated the fixed-speed appliance era, but are not necessarily the best choice for variable-speed schemes. For example, controllers, for those motors able to manage the high dynamic torque washing machines require, are expensive to create. An alternative motor type, such as a permanent-magnet synchronous motor (PMSM), makes it easier and cheaper to get the necessary performance. Sensorless control algorithms and new semiconductor technologies now streamline the development of variable-speed PMSM controls for inexpensive consumer applications.
Induction motors have a stator that induces field currents in the rotor through transformer action. As a result, induction motors always run at some speed less than stator-synchronous speed. The difference is known as the slip frequency. Higher loads make rotor speed drop and slip frequency rise, producing larger rotor currents that generate greater torque.
For this reason, induction motors usually operate at a fixed speed derived from the line frequency. Motor control is limited to simple on-and-off cycling as when keeping temperatures inside a refrigerator within target range. This wastes energy at the beginning of each operating cycle because it takes time for the refrigerant to hit optimum operating temperature and pressure.
By running motors at lower speeds, energy savings accrue from the reduced power requirements. Conversion percentages are higher as motors operate at lower temperatures and give off less waste heat. And the motor runs quieter, lessening the need for noise abatement materials.
Techniques for variable-speed operation in inverter-driven induction motors include open-loop volts-per-Hertz speed control and closed-loop control with speed sensors. Both types control motor torque by varying the motor-slip frequency. However, it's tough to dynamically control induction motors because rotor currents aren't available for measurement and because the rotor circuit has a large time constant.
By contrast, dynamic-torque control for permanent-magnet synchronous motors is straightforward given the rotor's angular position. PMSMs also benefit from greater torque-per-amp with smaller losses, as well as higher continuous torque compared to similar-sized induction motors.
Construction of permanent-magnet motors also influences the maximum torque the machine can produce. For example, an Interior Permanent-Magnet (IPM) motor produces more torque than a surface-type (SPM) motor, because it exhibits an additional reluctance-torque component. By using reluctance-torque control with an IPM motor, machine designers can get even higher torque for a given operating current.
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.