PARTS OF A DYNAMO-ELECTRIC MACHINE 85 



pitch is formed by counting the number of inductors between 

 the two connected to the same commutator bar. In Fig. 43 

 for example, the front pitch is three and the back pitch 

 is five. In Fig. 44 the front pitch and back pitch are both 

 equal to five. The average pitch of any winding should 

 be about the same as the number of inductors between 

 centers of consecutive poles. If this is not the case, there 

 will very likely be some inductors in every path which 

 generate an e.m.f. opposite to that generated by the rest 

 of the inductors in the path. While this is a disadvantage 

 there is an advantage gained by having short end connec- 

 tions when t^he pitch is less than the distance between 

 poles and so this fractional pitch, as it is called, is generally 

 used. If the number of inductors between the centers of 

 consecutive poles is 12, a suitable average pitch would 

 be 10, the front pitch being 11 and the back pitch 9. 



Developed windings. Instead of representing an arma- 

 ture winding as we have done in Figs. 42-45 a so-called 

 developed winding may be given. In such a diagram the 

 winding of the armature is represented in a plane surface, 

 just as though the winding had been peeled off from the 

 armature and laid on a flat surface; the poles are repre- 

 sented in their proper positions and the commutator is 

 shown in developed form also. The winding of Fig. 44 is 

 shown in developed form in Fig. 46. 



Current Capacity of a Winding. The current capacity 

 of an armature is fixed by the safe allowable rise in temper- 

 ature, just as the safe current for a field coil was fixed. 

 The armature is much better ventilated than the field 

 coils, however, because the fan action of the armature 

 comes into play as it revolves. It has been found that the 

 safe current density allowable in armature conductors 

 is reached by providing between 600 and 900 circular mils 

 cross-section of conductor per ampere, the lower figure 

 to be taken for the machine having the better ventila- 

 tion. If an armature winding is made up gf four paths and 



