178 PRINCIPLES OF ELECTRICAL DESIGN 



a better understanding of the main principles underlying commu- 

 tation phenomena these and similar modifying factors of 

 secondary importance tend to assume a less formidable aspect. 

 The designer, who must of necesssity be an engineer, desires 

 to see clearly what he is doing. If he uses formulas of which 

 he does not know the derivation or physical significance, he is 

 working in the dark. In general, he asks for more physics and 

 less mathematics. If he can picture the short-circuited coil 

 cutting through the various components of the flux in the 

 commutating zone, and understand how these flux components 

 may be calculated within limits of accuracy that are generally 

 satisfactory in practice, he will have a working knowledge of 

 the phenomena of commutation which should be especially 

 valuable in cases where test data cannot be relied upon. 



63. Mechanical Details Affecting Commutation. The quality 

 of the carbon used for the brushes, together with the pressure 

 between brush and commutator surface, will determine the heat- 

 ing due to friction, and therefore, to some extent, the dimensions 

 and proportions of the commutator. The pressure between 

 brush and commutator is usually adjusted by springs so that it 

 shall be from 1 to 2 Ib. per square inch of contact surface. In 

 order to avoid excessive temperature rise, the current density 

 is rarely allowed to exceed 30 or 40 amp. per square inch of 

 brush-contact surface. A sufficient cooling surface is thus 

 provided from which the heat developed through friction and 

 PR loss may be radiated. The brush should cover from 1J4 to 

 3J^ commutator bars, the usual width of brush (brush arc) being 

 something between the limits % in. and \y in.; and as a further 

 check on the desirable dimensions, the width should not exceed 

 }/{ of the pole pitch referred to the commutator surface. Hav- 

 ing determined the width of the brush, and decided upon a suit- 

 able current density, the total axial length per brush set may be 

 calculated, and the length of the commutator decided upon. 



The individual brush rarely exceeds 2 in., measured parallel to 

 the axis of rotation, and when a greater length of contact surface 

 is required, several brush holders are provided on the one spindle 

 or brush arm. Even in small machines, the number of brushes 

 per set should not be less than two, so as to allow of examination 

 and adjustment while running. The final check in the matter 

 of commutator design is the probable temperature rise, which 

 will be considered in the following article. 



