ARM A TURE WINDINGS AND SLOT INSULA TION 93 



the quotient, armature surface periphery divided by the total 

 number of slots. 



26. Number of Teeth on Armature. It is obvious that a small 

 number of teeth would lead to a reduction of space taken up by 

 insulation and, generally speaking, would lead also to a saving 

 in the cost of manufacture. Other considerations, however, 

 show that there are many points in favor of a large number of 

 teeth. Unless the air gap is large relatively to the slot pitch, 

 there will be appreciable eddy-current loss in the pole pieces on 

 account of the tufting of the flux lines at the tooth top. Again, 

 pulsations of flux in the magnetic circuit are more liable to be of 

 appreciable magnitude with few than with many teeth, and when 

 the tooth pitch is wide in relation to the space between pole tips, 

 commutation becomes difficult because of the variation of air-gap 

 reluctance in the zone of the commutating field. A good practi- 

 cal rule is that the number of slots per pole shall not be less than 

 10, and that there shall be at least three and one-half slots in the 

 space between pole tips. In high-speed machines with large pole 

 pitch, from 14 to 18 slots per pole would usually be provided. 

 With the exception of small generators (machines with armatures 

 of small diameter), the cross-section of the slot is about constant, 

 and approximately equal to 1 sq. in. This corresponds to about 

 1,000 amp. conductors per slot for machines up to 600 volts, 

 on the basis of the current densities to be discussed later. 



27. Number of Commutator Segments Potential Difference 

 between Segments. Machines may be built with a number of 

 commutator bars equal to the number of slots in the armature 

 core. In this case there will be one coil per slot, i.e., two coil- 

 sides in each slot. There is, however, no reason why the number 

 of coils 1 should not be greater than the number m of slots. The 

 usual number of commutator segments per slot is from one to 

 three in low-voltage machines, with a maximum of four or five 

 in low-speed dynamos for high voltages. The number of com- 

 mutator bars may therefore be a multiple of the number of slots. 



i The word coil as here used denotes the number of turns included between 

 the tappings taken to commutator bars. In practice one-half the number < 

 conductors in a slot might be taped up together and handled as a single 

 but if tappings are taken from the ends so as to divide the comple 

 two or more sections electrically, we may speak of four, six, or mor 

 sides in a single slot, notwithstanding the fact that these may be bunched 

 together and treated as a unit when placing the finished coils in position. 



