184 PRINCIPLES OF ELECTRICAL DESIGN 



where W = the total watts to be dissipated. 



A = the cooling area computed as above (square inches). 

 v c = the peripheral velocity of the cylindrical surface of 



the commutator in feet per minute. 

 T = the temperature rise in degrees Centigrade. 



The allowable temperature rise, i.e., the limiting value of T 

 in formula (90) is 50 to 60C. 



The mechanical construction of the commutator is a matter 

 of great importance, and when the peripheral velocity exceeds 

 4,000 ft. per minute, special means may have to be adopted 

 to ensure satisfactory operation. For instance, if the axial 

 length is great, it may be necessary to provide one or more 

 steel rings which can be slipped over the surface of the bars and 

 shrunk on, to prevent displacement of the bars or loosening of 

 the mica insulation owing to vibration or centrifugal action. 

 These mechanical details must, however, be studied elsewhere 

 as their discussion is not included in the scope of this book. A 

 sufficient radial depth of commutator bar must be provided in 

 order that the strength and stiffness may be sufficient to resist 

 the effects of centrifugal force. This dimension of the copper 

 segments should include an allowance of % to % in. for wear, 

 as the commutator must be large enough in diameter to allow 

 of its being turned down occasionally without reducing the 

 cross-section of the segments to a dangerous extent. The 

 mechanical considerations are the controlling factors in this 

 connection, as the cross-section is usually ample to carry the 

 required current. The pitch of the bars at the commutator 

 surface should not be less than 0.2 in., because, with the mica 

 of the usual thickness (0.03 to 0.035 in.) the bar would be 

 mechanically unsatisfactory if the thickness were reduced below 

 this limit. 



