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.
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.
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