In accordance with the recent trend of up-scale of buildings, consumer's requirements for multi-air conditioners with several indoor units commonly connected to one outdoor unit have increased. In a multi-air conditioner, the required cooling capacities of the indoor units are different from each other and the indoor units are independently operated, and so the total cooling capacity of the air conditioner, which is calculated by the sum cooling capacities of the indoor units, is also variable. It is thus necessary to control the capacity of a compressor and control the opening ratio of the electric expansion valve, installed at the upstream of an evaporator used as a heat exchanger of each indoor unit, in accordance with a variation in the total cooling capacity of the air conditioner.
In order to accomplish the variable cooling capacity of such multi-air conditioners, variable capacity compressors with variable refrigerant compressing capacity have been used. Such variable capacity compressors are typically classified into two types: variable rpm compressors and pulse width modulation compressors. In a variable rpm compressor, the motor's rpms are controlled in accordance with a variation in the sum of the required cooling capacities of indoor units by changing, through an inverter control, the frequency of the current applied to the compressor. The capacity of the variable rpm compressor is thus controlled and accomplishes the variable cooling capacity in a multi-air conditioner. In a pulse width modulation compressor, the capacity of the compressor is controlled in response to a duty control signal which determines the loading time when the compressor discharges compressed refrigerant and the unloading time when the compressor ceases to discharge compressed refrigerant. The pulse width modulation compressor thus accomplishes the variable cooling capacity of a multi-air conditioner.
However, the variable capacity of such compressors typically used in multi-air conditioners is limited. During the operation of such a multi-air conditioner, the sum of the required cooling capacities of the indoor units, which are commonly connected to one outdoor unit, may exceed the capacity of one variable capacity compressor installed in the air conditioner. Therefore, it is necessary to install an additional variable capacity compressor in the air conditioner.
However, such variable capacity compressors are expensive and difficult to install in multi-air conditioners in comparison with fixed capacity compressors which have fixed refrigerant compressing capacity. Therefore, in the prior art, both a variable capacity compressor and a fixed capacity compressor are installed in a multi-air conditioner and are parallely connected to each other in order to meet the demands of the variable cooling capacities of the indoor units.
In the operation of a multi-air conditioner with two such types of compressors, the cooling operation of the indoor units is performed by using only the variable capacity compressor when the sum of the required cooling capacities of the indoor units varies within the allowable capacity range of the variable capacity compressor. When the sum of the required cooling capacities of the indoor units exceeds the capacity of the variable capacity compressor, the fixed capacity compressor is operated at the same time to meet the required cooling capacities of the indoor units.
However, the conventional multi-air conditioner with both compressors is designed such that the fixed capacity compressor is started regardless of the operational condition of the variable capacity compressor, and so an excessive starting current may be induced in the fixed capacity compressor. Such a starting current prevents the smooth start of the fixed capacity compressor, and reduces the starting performance of said compressor. Furthermore, the excessive starting current sometimes damages the fixed capacity compressor, and causes a reduction in the operational reliability of the air conditioner.