Leland Teschler Editor
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.
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.
Getting PM motors to these higher spin speeds necessitates a method called field weakening which the controller must implement. It is essentially a technique that controls current through the stator windings in a manner that effectively weakens the magnetic field of the rotor, reducing its counter EMF, so it can spin faster.
On the other hand, dishwasher motors don’t run at a speed that involves field weakening, so their controllers don’t need to implement the technique. But ground-fault detection may be an issue in high-power air conditioners, so their gate drivers typically include a means of protecting against faults.
Use of a variable-speed approach in residential central-air installations may eliminate the need for dual compressors. It has been typical on central air systems to use one compressor for powering up, a second for managing temperatures once the building has cooled down. A single compressor powered by a variable-speed motor can handle both jobs. “Manufacturers have learned from Japanese makers of air-conditioning systems how to do this,” says IR’s Aengus Murray. “A power controller and dc motor is less expensive than using two compressors. And because the motor is more efficient it can be smaller so there is a savings there as well.”
Control of the compressor speed optimizes the refrigerant flow so the rate of heat transfer matches the load. Other efficiency gains are available by controlling not just the compressor speed but also that of the fan. A variable fan speed ensures the system operates at the right temperatures and pressures through the refrigeration cycle. “Fan makers can buy an integrated motor and controller with the electronics built into the motor. They see no difference in the installation,” says Murray. “That leads them to get involved with a variable-speed drive controlling both the compressor and fan. That is the stage where most U.S. manufacturers are today.”
It isn’t just consumers who are looking for energy savings in motors. Industrial concerns are adopting the same techniques because the incremental gains can be compelling. For example, experts figure that the use of variable-speed motors in the 46 elevators of Shanghai’s new financial center would save enough energy to power 129,000 Chinese households annually.
The problem, though, is that the design of energy-efficient drives is new territory for a lot of engineers. “Many designers have used the same approach for years and this is something new for them,” says Innes. “In many cases they’ve never done the driver design at all and they aren’t comfortable with it.”
That situation will likely change as “high efficiency” gets redefined as “accepted practice.”
Comparing energy-efficient motors
It isn’t just speed control that is making appliances and other motion-based applications more efficient. Motors that excel at variable speeds just operate more efficiently than older models spinning at constant speeds. One indication of the difference comes from an analysis by International Rectifier Inc. which ranked losses of induction motors, synchronous reluctance motors, permanent-magnet synchronous motors, and internal permanent-magnet motors.
One problem with more efficient motors has been that the rare-earth magnets in their construction have been expensive. That situation started improving in the late 1990s and continues today. One analysis by IR compared the relative costs of components found in 750-W induction motors with those of an equivalent internal permanentmagnet motor. Using material costs as of 2006, the IPM came in cheaper by over $1/motor.
There are also subtleties to driving the most-efficient PM motors. Brushless-dc motors are happy with trapezoidal drive signals. On the other hand, PM synchronous motors and IPM motors have a sinusoidal back-EMF, and the drive signals must likewise be sinusoidal. It is only recently that motor-control platforms have had the necessary horse power to synthesize such signals.
The differences in drive signals stem largely from rotor construction. Ordinary PM motors have magnets on the surface of their rotor. In contrast, IPM motors put magnets inside slots on the rotor. This brings efficiencies in managing rotor flux at high speeds when field weakening comes into play.