94 PRINCIPLES OF ELECTRICAL DESIGN 



A large number of bars improves commutation, but increases the 

 cost of the machine; a large diameter of commutator is necessary 

 in order that the individual sector shall not be too thin. The 

 copper bars are insulated from each other by mica, usually 

 about J^2 m - thick, increasing to Mo m - for machines of 1,000 

 volts and upward. It follows that a commutator with a very 

 large number of segments is less easily assembled and less satis- 

 factory from the mechanical standpoint than one with fewer 

 segments. 



The best way to determine the proper number of commutator 

 bars for a particular design of dynamo is to consider the voltage 

 between neighboring bars. This voltage is variable, and depends 

 upon the distribution of the magnetic flux over the armature 

 surface, and upon the position of the armature coil under con- 

 sideration. The maximum potential difference between adjacent 

 commutator bars rarely exceeds 40 volts, and the average voltage 

 should be considerably lower than this. The average voltage 

 between bars may be defined as the potential difference between 

 -f and brush sets divided by the number of commutator 

 segments counted between the brushes of opposite sign. As a 

 rough guide, it may be stated that the value of 15 volts (average) 

 between segments should not be exceeded in machines without 

 interpoles. About double this value is permissible as an upper 

 limit on machines with commutating interpoles, especially if 

 they are provided with compensating pole-face windings which 

 prevent the distortion of flux distribution under load. In 

 practice the allowable average voltage between commutator 

 bars is based upon the machine voltage and, to some extent, 

 upon the kilowatt output, although few designers appear to 

 pay much attention to the influence of the current in determining 

 the number of commutator segments. As an aid to design, the 

 following values may be used for the purpose of deciding upon 

 a suitable number of coils and commutator bars. 



Machine voltage Volts between commutator segments 



110 1 to 6 



220 2.5 to 10 



600 5 to 18 



1,200 9 to 25 



28. Nature and Thickness of Slot Insulation. Since the 

 average voltage between the terminals of an armature coil does 

 not exceed 25 volts, it follows that the potential difference be- 



