Air-conditioning system for vehicle and its control method

An air-conditioning system for a vehicle including a refrigerant circuit and an engine (EG), has a compressor (CP) in the refrigerant circuit, a battery, an electric motor (MG) and a mechanism (PT) operative to controllably connect and disconnect a power transmitting path between the engine and the motor. The motor (MG) is connected to drive the compressor (CP) when energized by the battery. A first controller is operative to control the electric power supplied to the motor from the battery. A capacity control mechanism is responsive to the controller to maintain the discharge capacity of the compressor below a predetermined value upon starting the engine by the motor.