322 



ARMATURK CONSTRUCTION 



slot, it is not serious. These latter, however, are usually carried 

 out as bar windings, and, with such, semi-closed or tunnel slots 

 can be used without disadvantage. 



A better alternative is Fig. 397 ; but here the double thickness 

 of insulation at the mouth of the slot results in a considerable 

 loss of space, and low space factor in the slot. 



With hand windings the slot lining may consist of a seamless 

 tube of insulation moulded and pressed into one piece, which 

 renders it equally reliable at all points. 



A further point in connection with Figs. 395, 396, and 397 is 

 that the insulating tubes have to be opened out to admit the coils, 

 which subjects the insulation to mechanical strain at the corners 

 and weakens it. 



FIG. 394. 



FIG. 395. FIG. 396. 



Methods of Slot Insulating. 



FIG. 397. 



One advantage of form - wound coils is that they may be 

 wound, packed, and transported separately, and not assembled on 

 the armature until the machine is erected on site, which is more 

 safe than transporting a large wound armature. 



Also, on the question of repairs on site, it is generally easier 

 to replace a form-wound coil. To wind a coil on site by hand is, 

 however, not a long nor difficult process. The hand-wound machine 

 with closed or semi-closed slots gives a more even wave shape than 

 the open slots, but the inductance of the winding is rather greater. 



B. Lap-coil Windings. When the conductor is wire or small 

 strip, the coils will be very similar to certain coils of continuous- 

 current armatures. Examples of these types of windings may be 

 seen in Chap. XL The style of coil and winding form will be very 

 similar to those described in that chapter. 



When there is only one conductor per slot (or at most two) the 

 winding will be carried out as a bar winding, and the process will 

 be precisely the same as for bar-wound wave windings except that 

 the coils are lap instead of wave connected. 



