On hot sweltering days, it is easy to appreciate why air conditioners for 1.3 billion Chinese are a driving force behind more efficient motor control.

“Almost every household in China wants a room air conditioner. They will buy an air conditioner before they get a refrigerator,” says Fairchild Semiconductor Marketing Director Claudia Innes.

And those air conditioners must be energy misers. “The Chinese like efficient appliances because they generally have only a 4-kW capacity in their homes,” explains International Rectifier Inc. iMotion Product Management Director Aengus Murray. (Residential capacity in the U.S. is about 10 kW.)

Nevertheless, Asia isn’t the only place where energy-efficient appliance motors are gaining traction. Appliances billed with green credentials are starting to sell in North America as well. “Two or three years ago, people looked for the cheapest clothes washer on the showroom floor,” says IR’s Murray. “Now they more often look for washers with an Energy Star rating.”

Whether the motor is in an air conditioner, clothes washer, dishwasher, or refrigerator, the general approach is the same: replace an old-style constant- speed motor with one capable of operating at variable speeds. Then optimize its operation for the conditions at hand.

The quest for efficiency has appliance makers moving away from induction motors and toward switched reluctance and brushless-dc motors. Benefits include not only energy efficiency but also longer motor life. “In an old refrigerator, for example, there is motor wear and tear because of the switching at rated speed,” says Innes. “A variable-speed motor spends less time at top speed so it lasts longer, and the system operates at a more consistent temperature.”

Not all of the energy savings is in the motor and drive. “The motor only accounts for about 15% of the energy used in a washing cycle. Most of the rest of it is associated with water consumption,” says IR’s Murray. “With an intelligent agitator, you can get more washing action with less water.

The typical way of driving energyefficient appliance motors is with an inverter circuit — IGBT switches for high-power applications such as washing machines and air conditioners, MOSFETs for those such as dishwashers with lower voltage and current demands. Then add modules that handle specific conditions characterizing appliances of a given type.

Permanent-magnet synchronous motors can be smaller than the split-phase induction motors they replace, making them less expensive. An example of a rotor from an internal PM motor comes from Oak Ridge National Laboratory which fabricated the device to study efficient motor design at high speeds.

For example, electronics for washing- machine motors typically have ratings of 600 V and 30 A; those for air conditioners, ratings of 600 V and 75 A. That means a different set of thermal demands for each set of modules. Drivers for dishwasher motors are typically 3-A, 250-V devices. Thus their thermal management is less of an issue.

“The topology for all those drivers are quite similar,” says Fairchild’s Innes. “We modify the components we make for the application, then match them for efficiency.”

The modular approach addresses the fact that there are differences between applications that concern more than just the size of the motor. Different appliances have different modes of operation that need special handling. For example, superefficient washers spin clothes at much higher speeds than older models. The faster the spin, the less moisture left in clothes. “We can halve the amount of heat you have to blow through clothes to get rid of the moisture,” says Murray.