CONSTRUCTION OF SQUIRREL CAGE ARMA'I 



TURKS 1 



05 



TABLE X. 

 AIR-GAP DEPTH FOR INDUCTION MOTORS. 



Diameter at Air-gap 

 ill cms. 



20 



30 



40 



50 



60 



80 

 100 

 200 



Radial Depth of Air-gap 

 in mm. 



0-8 

 I'O 

 1-1 

 1-2 

 1-4 

 1-6 

 2-0 

 3-0 



For rotors of smaller diameter than 20 cm., which are used in 

 very small ratings, air-gaps 

 even smaller than 0'8 mm. 

 have been used, and in some 

 cases as small as 0'5 mm. To 

 obtain accurate machining, the 

 surface of the rotor iron is fre- 

 quently ground up by an emery 

 wheel rotating and mounted 

 on the slide rest of the lathe 

 in which the rotor itself is 

 being machined, as in Fig. 117. 



It is important for the air- 

 gap length to be constant at 

 all points around the peri- 

 phery, and inequalities in the 

 air-gap will be more marked 

 and serious when the air-gap 



is small, as is the case with induction motors. When the bearings 

 of the motor begin to wear there is a danger of the rotor fouling 

 the stator core at the bottom, and it is not now uncommon with 

 induction motors to make the end brackets which carry the bear- 

 ings adjustable, so that the rotor may be maintained central with 

 regard to the stator, to compensate for any wear of the bearings. 



These small gaps are employed for induction motors for reasons 

 connected with the electrical design and qualities of the motor, 

 prominent among which are the power-factor, and -the maximum 

 permissible value for the current at no-load. For the same 

 reasons semi-closed slots (or, sometimes, totally closed slots) are 



FIG. 116. Squirrel Cage Rotor, showing 

 Construction of Rotor Spider and 

 Screwed End Rings. 



