88 



THE MAGNETIC CIRCUIT 



[ART. 36 



Prob. 12. Inspect working drawings of electrical machines found 

 rious books and magazine articles; indicate the paths of the main 

 and of the leakage fluxes: and make clear to yourself the reasons for the 

 use of dilYerent kinds of steel and iron in the frame, the core, the pole- 

 pieces, and the pole shoes. 



Prob. 13. Make sketches* of the magnetic circuit of a turbo-alternator 

 with a distributed field winding, of a homopolar machine, of an inductor- 

 type alternator, and of a single-phase commutator motor. Indicate the 

 paths of the useful and of the leakage fluxes. 



36. The Air-gap Ampere Turns. The general character of the 

 distribution of the magnetic flux in the air-gap of a synchronous 

 and of a direct-current machine is shown in Fig. 24, the curvature 

 of the armature being disregarded. The principal features of this 

 flux distribution are as follows : 



Armature 



J-X-l 



/Air-duct 



FIG. 24. The cross-section of a direct-current or synchronous machine, 

 showing the flux in the air-gap. 



(a) The flux per tooth pitch X is practically the same under all 

 the teeth in the middle part of the pole, where the air-gap has a 

 constant length, and is smaller for the teeth near the pole-tips 

 where the air-gap is larger. 



(6) On the armature surface the flux is concentrated mainly 

 at the tooth-tips; very few lines of force enter the armature 

 through the sides and the bottom of the slots. 



(c) There is a considerable spreading, or fringing, of the lines 

 of force at the pole-tips. 



((/) In the planes passing through the axis of the shaft of the 

 machine there is also some spreading or fringing of the lines of 

 force at the flank surfaces of the armature and the pole, and in 

 the ventilating ducts. 



