Abstract: A selectively driven compressor for stopping the power generating operation of a motor when an internal combustion engine is operating to make it unnecessary to provide the motor that can withstand high voltage. The system reduces the load on the internal combustion engine. The system includes a pulley driven by a main drive source, an electric motor, which includes an armature and a field system, driven by a power source. The compressor is driven selectively by the pulley or the electric motor. The armature and the field system of the electric motor are independently supported. The pulley is mechanically connected to either of the armature and the field system, and the compressor is mechanically connected to the other.