118 ELECTRICAL MACHINERY 



line DE and the total current in the short-circuit coil is the 

 sum of the currents CA and DE, shown by CFB. This is 

 evidently the current required for sparkless commutation. 



If the load on the machine changes so that the current 

 per path is 20 amperes instead of 10 amperes, the e.m.f. 

 required for sparkless commutation is much greater than 

 before. If the short-circuited coil still lies in the same 

 field strength as before, the current in B during short 

 circuit follows the curve C'G. The resistance variation, as 

 explained above, gives the current C'A' and the addition 

 of the short-circuit current, DE (the same as before because 

 the e.m.f. producing it is the same) gives the current C'G. 

 At the end of the short-circuit period the current in coil B 

 should be at B r (negative), and it really is at G (positive). 



Necessity of Shifting the Brushes with Load Variation. 

 If the brush is advanced more, so that coil B is in a denser 

 field than before, a greater e.m.f. is induced in it while 

 it is short circuited and the short-circuit current becomes 

 DE'. Now DE' added to C'A' gives the current C'F'B', 

 which is just right for sparkless commutation. Hence by 

 means of brush shifting, sparkless commutation may be 

 obtained but the amount of shift required varies with the 

 load so that if brush shifting is used the operator has to 

 change the position of the brushes (by moving the brush- 

 holder yoke) as the load changes. The amount of brush 

 shift necessary depends upon the design of the machine. 

 A modern well-designed machine operates fairly well with 

 no brush shift at all. The brushes are placed somewhere 

 between the proper no load position and proper full load 

 position and are left there no matter what the load may be. 

 Use of Commutating Poles. When commutating poles 

 are used, brush shifting is unnecessary. Small poles, 

 wound with a series winding, are placed midway between 

 the main poles and the flux produced by them acts in the 

 same way as does the pole fringe just discussed. The 

 brushes short circuit the coils lying directly under the 



