COMMUTATION 



to sparking troubles, and it is not uncommon to groove the 

 commutator between bars, cutting down the mica about J^ in. 

 below the surface, leaving an air space as the insulation between 

 the bars. This undercutting process may have to be repeated 

 as the commutator wears down in use. 



As the effects of any irregularities on the commutator surface 

 are accentuated by high speeds, it is usual to limit the surface 

 velocity of the commutator to about 3,000 ft. per minute when 

 possible. The diameter of the commutator in large machines 

 is generally about 60 per cent, of the armature diameter, while, 

 in small machines, this ratio may be as high as 0.78. The sur- 

 face width of commutator bar may be from 0.2 in. to 0.8 in. : a 

 not uncommon width in modern interpole generators being 0.5 

 in. on machines for 300 to 600 kw. output. 



The design of brushes and holders is a matter of great im- 

 portance; as a general rule, it may be said that the lighter the 

 moving parts of brush and holder, the better the conditions in 

 regard to sparking when the surface of the commutator is not 

 absolutely true. 



64. Heating of Commutator Temperature Rise. In some 

 cases it is necessary to provide special means of ventilation to 

 keep the temperature of the commutator within reasonable 

 limits; but as a rule a sufficiently large cooling surface may be 

 obtained without unduly increasing the size and cost of the 

 commutator. 



The losses to be dissipated consist of: 



1. The PR loss at brush-contact surface. 



2. The loss due to friction of the brushes on commutator 

 surface. 



The PR loss in the commutator segments is relatively small 

 and can usually be neglected. 



The watts lost under item (1) are approximately 27, where / 

 is the total current taken from the machine. This assumes an 

 average value of 2 volts for the total potential drop between, 

 commutator and carbon brushes. For a more exact determina- 

 tion of this electrical loss, the curves of Fig. 69 can be used. As 

 the current passing into all the positive* brushes includes the 

 shunt exciting current, an allowance should be made for this. 

 Moreover, the assumed condition of uniform current density 

 over brush-contact surface will not be fulfilled in practice. The 

 uneven distribution of current density will increase the losses, 



