CHAP. XII] INDUCTANCE OF WINDINGS 239 



The commutator diameter is 52 cm.; the number of segments, 252; 

 the mica insulation is 1 mm. thick; the brushes are 15 mm. thick. 

 What is the average reactance voltage when the total armature current 

 is 320 amp.? Ans. 4.52 volts. 



Prob. 23. Show that the answer to the preceding problem would 

 be nine times larger if by mistake the winding were assumed to be of 

 the two-circuit type. 



Prob. 24. The peripheral velocity of a commutator is 18 meters 

 per sec., the width of each segment (without mica) is 4.5 mm.; the 

 thickness of the mica is 0.9 mm. The commutator is to be used in con- 

 n with a duplex winding. What is the smallest permissible 

 thickness of the brushes if the frequency of commutation must not 

 exceed 800 cycles per sec.? Ans. 17.5 mm. 



Prob. 26. For a perfect commutation and for an imperfect one 

 draw the following curves to time as abscissae : (a) the current in the 

 short-circuited coil; (6) the currents in the leads c and d; (c) the 

 current densities under the heel and toe of the brush. Take the width 

 of the brushes to be equal to that of one commutator segment, and 

 assume the mica insulation to be of a negligible thickness. 



Prob. 26. Show that the width of the brushes has comparatively 

 little net effect upon the commutation of a machine. 



Prob. 27. What flux density is needed in the interpolar zone in 

 prob. 22 to secure perfect commutation? Ans. 2.23 kl./sq. in. 



