Motors Move Toward More Efficiency

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.

Front-loading washers use less water than their top-loading counterparts, but some of their energy efficiency also comes from the use of variable-speed motors that can agitate clothes efficiently and spin the drum superfast to squeeze out moisture before drying. This Japanese washer uses special motion-control chips from International Rectifier Inc. to handle a synchronous reluctance motor.

“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.