PURPOSE: To prevent the wasting consumption of power by suppressing the rise of revolving number of the compressor shaft beyond a given value by a method in which a fluid coupling with an oil path is provided between an electromagnetic clutch and the compressor shaft and an centrifugal weight is provided in the fluid coupling.

CONSTITUTION: A fluid coupling 20 is provided between the drive shaft 23 of the electromagnetic clutch 6 side and a compressor shaft 25 on the compressor body 1 side. An oil path 27 of a labyrinth form is formed between the coupling drive 21 and the coupling idle 22, and a viscous oil 28 is charged into the oil path 27. When the load torque exceeds a give value in such a constitution, rotation force is not transmitted beyond the value. Also, in order to provide a function for controlling the rotation of the compressor to the fluid coupling 20, a centrifugal weight 29 to increase the revolving number and also to expand the width W of the oil path 27 is provided on the coupling idle 22 side of the fluid coupling 20.