166 THE MAGNETIC CIRCUIT [ART. 53 



tors come after the commutation. In a motor the armature cur- 

 ivnt flows against the induced e.m.f.; it is therefore the field which 

 cuts the conductors before the commutation that assists the rever- 

 sal of the current. This explains the direction of the shift of the 

 brushes in the two cases. The student should make this clear to 

 himself by considering in detail the directions of the currents and 

 of the induced voltages in a particular case. 



The maximum m.m.f. per pole produced by the distorting 

 belts is equal to (AC) (%rd), but since this m.m.f. acts along the 

 inUTpolar space of high reluctance its effect is not large (except in 

 machines with commutating poles). Of much more importance 

 is the action of the distorting belts under the main poles. At each 

 pole-tip the armature m.m.f. is 



w, ...... (95) 



where w is the width of the pole shoe. This m.m.f. decreases 

 according to the straight line law to the center of each pole and 

 is of opposite signs at the two tips of the same pole. 



Prob. 1. Determine the polarity of the brushes in Fig. 42 for a 

 progressive and a retrogressive winding, in the case of a generator and 

 of a motor. 



Prob. 2. Indicate the D and the T belts in a fractional-pitch winding 

 (a) with the brushes in the geometric neutral, and (6) with the brushes 

 shifted by J. 



Prob. 3. A 500 kw., 230 volt, 10 pole, direct-current machine has 

 a full-pitch multiple winding placed in 165 slots. There are 8 con- 

 ductors per slot, and two turns per commutator segment. What are 

 the demagnetizing ampere-turns per pole when the brushes are shifted 

 by 4 commutator segments? Ans. 3470. 



Prob. 4. What is the amplitude of the broken line ABC in the 

 preceding machine? Ans. 10850 amp-turns. 



Prob. 5. For a given machine, draw the curves of the flux density 

 distribution under a pole, at no-load and at full load, by considering 

 the m.m.fs. acting upon* the individual paths, and the reluctance of the 

 paths. 1 



53. The Calculation of the Field Ampere-turns in a Direct- 

 current Machine under Load. The net ampere-turns per pole are 

 determined from the no-load saturation curve of the machine for 



1 For details and examples of such curves see Pichelmayer, Dynamobau 

 (1908), p. 176; Parshall and Hobart, Electric Machine Design (1906), p. 159; 

 Arnold, Die Gleichstrommachine, Vol. 1 (1906), p. 324. 



