Abstract: An invertor room air-conditioner for transmitting/receiving data between indoor and outdoor microprocessors, in which one microprocessor is synchronized with the remaining microprocessor using its internal clock pulse, while the microprocessors perform the data transmission/reception using dummy data previously stored therein, respectively, thereby removing the data transmission/reception errors and the influence of the exterior noises.

Abstract: Line voltage of any two phases of a three phase AC power supply is detected through a high pass filter. The line voltage is amplified by an amplifier. A microprocessor receives the amplified line voltage via an input terminal. The microprocessor determines whether the line voltage is below a first set value, between the first set value and a second set value or above the second set value. When the line voltage is below the first set value, the compressor operates in the previous operation state. When the line voltage is between the first set value and the second set value, the compressor operates in the operation state 1 step lower than the previous speed. When the line voltage is above the second set value, the compressor operates in the lowest operation state. Whereby the noise and vibrations generated in the compressor are suppressed.