The external motor fan is mounted at the top of the air duct, driven by a V-belt, and the center distance can be adjusted appropriately. The drive assembly is fixed in a sealed cylinder at the center of the air duct, isolated from the medium, thus allowing the conveyance of moist gases and gases harmful to the motor. Suitable for conveying gases harmful to electric motors at temperatures below 60°C in chemical, papermaking, and other factories.