A. Anydynamic brakingoperation required for the motor can be realized by means ofbraking DC chopperand resistor shunt connected across the rectifier. Alternatively, an anti-parallel thyristor bridge must be provided in the rectifier section to feed energy back into the AC line. Such phase-controlled thyristor-based rectifiers however have higher AC line distortion and lower power factor at low load than diode-based rectifiers.An AC-AC converter with approximately sinusoidal input currents and bidirectional power flow can be realized by coupling apulse-width modulation(PWM) rectifier and a PWM inverter to the DC-link. The DC-link quantity is then impressed by an energy storage element that is common to both stages, which is a capacitor C for the voltage DC-link or an inductor L for the current DC-link. The PWM rectifier is controlled in a way that a sinusoidal AC line current is drawn, which is in phase or anti-phase (for energy feedback) with the corresponding AC line phase voltage.Due to the DC-link storage element, there is the advantage that both converter stages are to a large extent decoupled for control purposes. Furthermore, a constant, AC line independent input quantity exists for the PWM inverter stage, which results in high utilization of the converters power capability. On the other hand, the DC-link energy storage element has a relatively large physical volume, and when electrolytic capacitors are used, in the case of a voltage DC-link, there is potentially a reduced system lifetime.A.

Simplified block diagram of an electronic inverter. The input is a DC voltage or an AC voltage that is converted to DC. The DC-to-AC Converter transforms the DC voltage into a pulse train (AC) with the requisite frequency and duty cycle. The resulting signal then goes to the Driver, which transforms that AC into the appropriate level. The Output Circuit performs signal conditioning and has functions for implementing safety features. The entire frequency and voltage conversion process is monitored and tightly controlled by the Control Circuit, which typically is a microcontroller.

B. The basic operation of an inverter can be described in a simplified manner (see Figure 5). The inverter takes a DC input voltage directly or can operate from an AC source if it includes a function that converts an AC input voltage into a DC voltage via rectification. The DC-to-AC Converter circuit chops the DC voltage up into pieces, generating a pulse train that constitutes an AC voltage. That AC voltage is subsequently adjusted to the proper voltage level by a Driver function that often includes a transformer. The Output Circuit typically contains a filter and safety protection circuits.A Control Circuit in a feedback loop closely monitors and continually adjusts the timing of the chopping process (frequency and duty cycle), as well as other performance and operational aspects of the inverter, including its output voltage level, overload protection status, etc. The Control Circuit most often is based on a digital processing IC such as a microcontroller chip (MCU).C. Inverter technology is a critically important way to save energy when it is applied in appliances driven by power-hungry motors. The air conditioner is the biggest power consumer in homes in warm climates, a typical unit drawing 1.5 to 2.5 kW. That is more than ten times the power consumed by a refrigerator, which uses about 100 to 150 W. For this reason, the deployment of inverters in air conditioning systems has greatly outpaced their use in other devices, particularly in the Japanese market. Indeed, virtually all air conditioners now being sold in Japan are equipped with inverter control.Manufacturers of air conditioners marketed in other countries havent adopted inverter technology to the same extent, though. Outside Japan, only about 10% of air conditioners are equipped with inverters, and globally the shift to inverter control is still in its early stages. Some markets, however, are moving forward faster than others.In South Korea, the changeover began about four or five years ago, to the point where today just under half of new household air conditioners use inverter control. Manufacturers in China hold a large share of the worldwide air conditioning market, and they have begun to rapidly include inverter technology into new designs. Currently about 30% of new Chinese built air conditioners contain inverters and incorporation of this control technology is predicted to exceed 50% by 2015.The greater efficiency, quieter operation and attractive features of inverter-based air conditioners will spur consumer demand. In turn, this will drive IC makers to ramp up inverter production and accelerate advancements in the technology.