36 ELECTRICAL MACHINERY 



consideration of mechanical clearance* the only way to 

 reduce the air gap reluctance is to increase its area and 

 this is what the pole shoe does. 



Commutating Poles. Besides the main poles, most 

 modern c-c. machines have another set of poles which are 

 very narrow compared to the main poles and are placed 

 midway between the main poles; these small poles are to 

 help commutation and are called commutating poles. A 

 machine equipped with commutating poles is shown in 

 Fig. 13; the extra poles are seen to be very narrow in 



FIG. 12. A Pole, to Show how a Pole Shoe may be Formed of the 

 Laminations of Which the Pole Itself is Formed. Westinghouse 

 Elec. and Mfg. Co. 



comparison with the main poles. Another special field 

 construction has been used on the auxiliary-pole synchronous 

 converter; in this type of machine each main pole is split 



* There are other factors affecting the length of the air gap besides 

 that one mentioned here; as will be explained later the armature 

 exerts a reaction on the magnetic field and the effect of this armature 

 reaction is much exaggerated if the air gap is not of proper length. 

 This armature reaction effect is really the factor controlling the length 

 of air gap. On some high-speed alternators, e.g., the ah- gap length 

 may be an inch or more and evidently mechanical considerations 

 would not necessitate an air gap of such length. 



