104 



ARMATURE CONSTRUCTION 



thus leaving the internal surface of the core almost completely 

 exposed. The bolts connecting the end flanges just clear the 

 interior of the core, as in Fig. 104. Each end flange carries 

 three projecting vanes for driving the air through. In this rotor 



FIG. 114. Typical methods of fastening end Rings to rectangular Rotor Bars. 



there are no ventilating ducts in the core ; and the effectiveness 

 of the end vanes, and of the open type of spider, would be much 

 increased if one or two ventilating ducts were present, as the air 

 driven in by the blades would circulate through the core ducts and 

 effectively cool the body of the motor. 



FIG. 115. Typical methods of fastening end Rings to rectangular Rotor Bars. 



A matter of great importance in connection with induction 

 motor squirrel cage armatures (and, indeed, any induction motor 

 rotor) is the accurate machining of the external surface of the core 

 necessitated by the very small air-gaps employed. The values 

 employed for the radial depth of air-gap, between the surface of 

 the stator and rotor laminations, are shown for various diameters 

 in the following table, which represents the writers' average 

 practice. 



