COMMUTATION 



183 



Some designers consider only the outside cylindrical surface 

 of the commutator when calculating temperature rise; but this 

 leads to unsatisfactory results in the case of short commutators. 

 In the formula here proposed, it is assumed that the risers add 

 to the effective cooling surface up to a limiting radial distance 

 of 2 in.; that is to say, if the risers are longer than 2 in., the area 

 beyond this distance will be considered ineffective in the matter 

 of dissipating heat losses occurring at the commutator surface. 

 The external surface of the carbon-brush holders is helpful in 

 keeping down the temperature and it will be taken into account 

 by assuming that the cooling surface of the commutator is in- 

 creased by an amount equal to 2l c b sq. in.; where l c is the total 

 axial length of one set of brushes, and b is the total number of 

 brush sets. 



FIG. 70. Cooling surface of commutator. 



The cooling area, as indicated in Fig. 70, will therefore con- 

 sist of the cylindrical surface irDcL, c ] the surface of the risers 



^ (D r 2 - .Dc 2 ) ; the surface of the exposed ends (if any) of the 

 copper bars, of value ;, (D c 2 - D 2 ) ; and the allowance of 



<x 



2l c b for the brush holders. 



The empirical formula here proposed for calculating the tem- 

 perature rise of the commutator is 



(90) 



W = TA (0.025 + 



