PARTS OF A DYNAMO-ELECTRIC MACHINE 47 



Advantage of Semi-closed Slots. It is easily seen that 

 a semi-closed slot armature is more difficult to wind than 

 an open slot armature and the question arises, what advan- 

 tage does the semi-closed slot have over the open slot? 

 Evidently the semi-closed slot affords better protection and 

 support for the coils than does the open slot; it may also be 

 seen by refererce to Fig. 23 that the effective area of the 

 air gap is much larger when a semi-closed slot is used. 



Fi<;. 2. Winding a Core with Partially Formed Coils, on a Core with 

 Semi-closed Slots. Westinghouse Elec. and Mfg. Co. 



This decreases the magnetic reluctance of the machine, so 

 that smaller field coils may be used. When open slots are 

 used, the effective area of the air gap (i.e., the area of that 

 part of the air gap through which the magnetic field goes) 

 is not much more than the area of the teeth situated under 

 a pole face, as seen by Fig. 23 (a). When the semi-closed 

 slot is used, the flux utilizes practically all of the area of 

 the air gap; the reluctance of the gap in (a) Fig. 23 is 

 much greater than that of (6). 



