An Experimental Study of Frequency-Controlled Compressors

This report describes a study regarding the performance and reliability of frequencycontrolled compressors combined with power frequency inverters, which are considered to be one of the strongest means of providing energy saving for air conditioners. Items Related to Performance Compressor capacity is available in proportion to a change in operating frequency. Moreover, since reduction in efficiency based on a change in operating frequency is very low, frequency-controlled compressors provide an ideal means of capacity control. When the rotary compressor is operated at a relatively high frequency range (such as 30 Hz 90 Hz) and the reciprocating compressor is operated at a relatively low frequency range (25Hz-75Hz), the capacities of these compressors in proportion to a change in operating frequency can be obtained with low input and at high efficiency. Regarding frequency-controlled compressors equipped to air conditioners, the improvement of EER at low load and the reuction of work loss due to ON/OFF cycling become possible. Especially, the difference in required capacity between cooling and heating can be eliminated in heat pumps, and a 20% 40% improvement in SEER can be realized in comparison to the general ON/OFF control method. Items Related to Reliability For rotary compressors, the countermeasures to reduce the amount of oil discharged from the compressor in high operating fre·quency areas and to reduce the wear of the subbearing valve seat were performed. For reciprocating compressors, an improvement of oil pump performance in low operating frequency areas and the prevention of resonance in the discharge piping over the full operating range were obtained. 297 INTRODUCTION In air cor,ditioners, various energy-saving countermeasures have progressed based on recent changes in the energy environment. Regarding an evaluation method for efficiency, not only energy efficiency (EER) during rated operation, but also seasonal energy efficiency (SEER) under actual operating conditions have received a considerable amount of attention. On the other side, in the field of smallsized air conditioners for households, offices and residences, an ON/OFF control system, which is a cheap and simple method, is generally utilized as a capacity control system. However, since the rated capacity of air conditioners is selected based oq the maximum capacity throughout the year at the installation location, there is a disadvantage regarding the ON/OFF control system in that a reduction in SEER could not be avoided due to the large loss caused by frequent ON/OFF cycling under actual loaded conditions. Therefore, compressors for which the capacity can be controlled according to the load to the air conditioners have come to be requested in order to improve the reduction to this ON/OFF control. In order to perfectly perform this capacity control, the following items are considered as conditions. Capacity control shall be continuously performed under loaded conditions. When the capacity decreases, motor input decreases at the same time. During such a partial load operation, the efficiency of the compressor shall not be decreased. The efficiency under a fully-loaded condition shall not decrease due to the capacity control device. The reliability of the compressor shall not decrease. There are various types of actuallyutilized capacity control systems, as shown in Table 1, but the system which optimally matches the conditions described above is the frequency-controlled compressor method. Moreover, an inverter which is applicable to air conditioners has become possible regarding practical utilization from the standpoint of cost and size due to recent rapid progress in the field of electronics, and the continuous capacity control system based on a combination with the inverter has become a very strong method of energy saving. The authors performed study and development of the frequency-controlled compressor, and this type of reciprocating type compressor has been mass-produced since 1980, while this type of rotary type compressor has been mass-produced since 1981. Since it appears that the frequency-controlled compressor will be a strong method for improving SEER in the future, the following report is submitted.