Efficient AC

Energy efficiency in appliances is becoming a hot topic. Several countries have now enacted legislation aimed at boosting the efficiency of products such as clothes washers, water heaters, and in particular air conditioners. Air conditioners are a special target because they are becoming more popular in Japan, Europe, and emerging countries such as China and India.

Modern air conditioners incorporate power factor correction (PFC) to meet ever-more stringent efficiency standards. New circuits combine PFC with rectification to help meet these requirements.

In the U.S., for example, the Dept. of Energy recently issued energy-efficiency standards for numerous appliance products. Similar initiatives have been in force in Europe and Japan. In this regard, the residential air-conditioning market (about 35 million units manufactured worldwide) is by its nature a "high-impact product" for energy-efficiency programs. An air conditioner must have a SEER of at least 10 to be sold in the U.S. Higher efficiency models have a SEER of 11, 12, 13 or 14. (SEER is Seasonal Energy Efficiency Rating. It is the most commonly used measure of the efficiency of consumer central-airconditioning systems. EER, or Energy Efficiency Rating, is the most commonly used measure of efficiency for commercial air-conditioning systems.)

U.S. legislation on air conditioners and heat pumps establishes more stringent minimum efficiency standards that become effective January 23, 2006. These standards are tough to hit without the use of advanced technology and are quite challenging for manufacturers. Without the use of more advanced electronic technology these standards may be potentially impractical. Such technology includes variable-speed drives powering either a standard ac induction or brushless dc compressor.

Most variable-speed drives have used electronic inverters for controlling the motor. These inverters (as well as linear and switch-mode power supplies) contain a bridge rectifier/capacitor frontend. But the front end presents a highly nonlinear load to the main line: The input bulk capacitor charges only toward peaks of the voltage sine wave and thus induces a peak of current in the load.

This nonsinusoidal pulse current contains harmonic multiples of the fundamental line frequency. And each of these harmonics has a significant energy content.

These combined effects of only drawing line current during voltage peaks and high harmonic content contribute to realizing a circuit that has an extremely poor power factor.

Now consider what happens when a multitude of similar systems are on the same ac lines and all draw current at the same time. This phenomenon reduces ac network capacity, in essence aggravating any energy problems on the power grid. So to head off the problem, electronic motion controls of this type must use an input section that incorporates a power-factor correction circuit.

POWER FACTOR LIMITS
The IEC standard EN/IEC61000-3-2 spells out the maximum allowable harmonicson the ac line for various classes of equipment. The standard applies to all products up to 16 A/phase and classifies all motor-driven equipment as Class A. Thus air conditioners fall into Class A of the standard as well and must exhibit harmonics that fall below the levels spelled out there.