Patent ID: 8753076

Claim:
A centrifugal fan comprising: an electric motor including a rotary shaft; a casing including a suction port opening in an axial direction of the rotary shaft; and an impeller fixed to the rotary shaft of the electric motor to rotate therewith and including: an impeller body which rotates about the rotary shaft; a plurality of stems arranged at intervals in a direction of rotation of the rotary shaft with one end of each stem fixed to a portion of the impeller body in the vicinity of the suction port; an annular blade mounting member arranged concentrically with the impeller body with the other end of each stem fixed thereto; a plurality of blades including: a plurality of main blades arranged at intervals in the direction of rotation of the rotary shaft with one end of each main blade fixed to the blade mounting member, the main blades extending along an axial line of the rotary shaft in a direction away from the suction port, and shaped to suck air into the casing through the suction port when the impeller rotates in a direction of normal rotation; and a plurality of sub blades arranged at intervals in the direction of rotation of the rotary shaft with one end of each sub blade fixed to the blade mounting member, the sub blades extending along the axial line toward the suction port, and shaped to suck air into the casing through the suction port when the impeller rotates in a direction of reverse rotation opposite to the direction of normal rotation; the number of the sub blades is equal to or less than half the number of the main blades, and is equal to or more than one fourth of the number of the main blades; and defining a maximum length of each main blade in the axial direction as L 1 and a maximum length of each sub blade in the axial direction as L 2 , L 1 and L 2 are determined so that a relationship of ⅕<L 2 /L 1 <½ holds.