## 1 Introduction  
As temperatures are rising owing to global warming and living standards are improving, the use of air conditioners is increasing rapidly. Accordingly, research on ways to improve indoor-space comfort and air conditioner efficiency in actual conditions should be followed by research on energy efficiency. An inverter air conditioner can control the amount of circulating refrigerant by varying compressor driving frequency, thereby modulating cooling ability according to heat load and ultimately maintaining room temperature while reducing deviation. Consequently, unnecessary energy loss can be reduced, and seasonal energy efficiency ratio, which is an energy efficiency index considering environmental temperature changes, is higher in inverter air conditioners than in conventional constant-speed-type air conditioners [1, 2]. [PERSON] et al. [3] demonstrated how control errors can lead to reduced reliability and abnormal cycle problems along with decreased system capability and efficiency. [PERSON] et al. [4] examined the characteristics of compressor efficiency and power consumption and compared the effect of cooling load changes on the energy efficiency ratio. [PERSON] et al. [5] summarized the energy regulations of major countries and emphasized the importance of system efficiency. Control algorithms were developed continuously along with inverter hardware control technology. [PERSON] et al. [6] modeled a system by examining the transient response characteristics of system overheating owing to disturbances using variable area expansion and electronic expansion valves. Moreover, they modeled the transient response caused by changes in the opening degree of an electronic expansion valve mathematically. [PERSON] et al. [7] used compressor speed, compressor outlet superheat and pressure, condenser temperature, evaporator temperature, compressor inlet temperature and pressure, indoor temperature, outdoor temperature, and fan speed to configure an inverter cycle controller. The authors also considered the algorithm technology involved in optimal cycle control. [PERSON] et al. [8] compared the proportional integral derivative (PID) and power factor correction (PFC) control methods with evaporator superheat and confirmed the energy savings effect of 2% through PFC control. The main idea behind inverter air conditioners is variable capacity control based on the cooling load by varying the operating frequency of the compressor. Simultaneously, to match the optimum cycle, the superheat control logic through the electronic expansion valve is operated, and the refrigerant quality of each component is controlled appropriately. As an inverter control method, the fixed-pressure control method controls the refrigerant flow rate to the indoor unit to maintain the refrigerant evaporation temperature at a constant level regardless of the indoor cooling load condition. However, this control method increases the capacity of the indoor unit relative to the cooling indoor load because refrigerant at a constant temperature is supplied to the indoor unit when the indoor cooling load is small (2010, Daikin Industrial Co., Ltd., Osaka, Japan). As shown in Figure 1a, the fixed-pressure control turns off the operation of compressor when the set temperature is reached, and when the room temperature is 3 to 5\({}^{\circ}\)C away from the set temperature, the compressor turns on again and repeats the operation with fixed-pressure control. Thus, as the system repeats on-off, the power consumption of the system increases. As shown in Figure 1b, the cooling-load-estimation control adjusts the operating frequency of the compressor by calculating the cooling load after the room temperature reaches the initial set temperature. Because the compressor operates at low frequencies, the temperature change in the room is minimized, and because the system operates continuously without the compressor on-off at low cooling loads, the power consumption of the system is minimized.