in the duct application_number US 201514956432 A description embodiments of the present invention will be described with reference to the drawings components identical or similar to components in a conventional induction motor are indicated by identical reference numerals fig1 is a view of a cooling system of an electric motor with an external fan in an example of a prior art the electric motor 1 includes as a cooling system an external fan 2 and a duct 4 for guiding fluid flow 3 generated by rotation of the external fan 2 the duct 4 is composed of a first flow channel 5 connected to the external fan 2 and a second flow channel 6 connected to the first flow channel here the external fan 2 is assumed to be not a type capable of rotation number control but a type which rotates at a constant rotation number when a constant electric voltage is input the fluid flow 3 generated by the rotation of the external fan 2 is guided by the duct and flows around an electric motor 1 the fluid flow 3 removes heat generated by the electric motor 1 and discharge the heat to outside of the duct 4 at this time since the electric motor 1 is placed in front of the external fan resistance for the fluid flow 3 is large in addition to that a flow channel composed by the duct 4 has narrow cross section such that resistance for the fluid flow 3 is large therefore though the external fan 2 rotates in a rotation number but partial adverse.