92 PRINCIPLES OF ELECTRICAL DESIGN 



winding because, even in machines of large output, the voltage 

 generated per turn of wire is comparatively small. The diff- 

 erence of potential between the winding as a whole and the 

 armature core may, however, be very great on high-voltage 

 machines, and the slot lining must be designed to withstand 

 this pressure with a reasonable factor of safety. About the 

 same amount of insulation as will be necessary for the slot 

 linings will also have to be provided between the upper and 

 lower coil-sides in each slot, because the potential difference 

 between the two sets of conductors in the slot is the same as that 

 between the terminals of the machine. The space occupied by 

 insulation relatively to the total space available for the winding 

 will depend not only upon the voltage of the dynamo, but also 

 upon such factors as the number of slots and their cross-section 

 and proportions. Even if the total slot area remains constant, 

 the larger number of slots will naturally require the greater 

 amount of insulation, and thus reduce the space available for 

 copper. Again, a wide slot, by reducing the tooth width, may 

 be the cause of unduly high densities in the teeth, while a deep 

 slot is undesirable on account of increased inductance of the 

 windings, and because it may lead to an appreciably reduced 

 iron section at the root of the tooth in armatures of small 

 diameter. 



With the ordinary double-layer winding, the square coil 

 section would give the best winding space factor. Thus if each 

 of the two coil-sides were made of square cross-section, the total 

 depth of slot including space for binding wires or wedge would 

 be from two and one-fourth times to two and one-half times the 

 width. In practice the slot depth is frequently three times the 

 width, but this ratio should not exceed 3H because the design 

 would be uneconomical, and the high inductance of the winding 

 might lead to commutation difficulties. 



Although the calculation of flux densities in the teeth will be 

 dealt with later, it may be stated that it is usual to design the 

 slot with parallel sides and make the slot width from 0.4 to 0.6 

 times the slot pitch. It is very common to make slot and tooth 

 width the same (i.e., one-half the pitch) on the armature surface, 

 especially in small machines. In large machines the ratio 

 tooth width . 



1S fre(uentl about L1 - 



What has been referred to as the slot pitch may be defined as 



