Patent ID: 8651839

Claim:
An oil-free multi-stage compressor comprising: a low-pressure compressor body, a high-pressure compressor body, a motor for driving the low-pressure compressor body and the high-pressure compressor body, the motor including an air suction part for controlling an amount of suction air for the low-pressure compressor body, and a suction throttle valve for controlling air intake into the compressor, controlled by pressure differential of working pressures supplied, moved by the air suction part, and operated by a piston assembly, a first path for supplying one of the working pressures from an air flow passage connecting the suction part and the low-pressure compressor body, a second path for supplying one of the working pressures from an air flow passage connecting the low-pressure compressor body to the high-pressure compressor body, a third path for supplying one of the working pressure from an air flow passage connecting the high-pressure compressor body and a user side, a fourth path for providing communication with atmosphere and the air flow passage connecting the low-pressure compressor body to the high-pressure compressor body and draining condensate included in compressed air discharged from the low-pressure compressor body to outside of the multi-stage compressor, and a control element situated in the fourth path for controlling the communication and the condensate draining, wherein, when the multi-stage compressor is driven in a start up, unloaded condition, the suction throttle valve is fully closed by the working pressure supplied via the third path, wherein, when the multi-stage compressor switches a driving operation from the start up, unloaded operation to a loaded operation after the motor is accelerated to a top speed, the working pressure is supplied to the air suction part via the first path instead of via the third path, and the control element permits the communication between the fourth path and the atmosphere in order to open the throttle valve slightly.