THE CONTINUOUS CURRENT GENERATOR 111 



low. For a given brush width and commutator speed the 

 rate of current change during commutation increases directly 

 with the current the armature is carrying. This rate is very 

 large on big machines and therefore the coefficient of self- 

 induction must be kept correspondingly low. It will be 

 noticed that on large armatures the coils have very few 

 turns; sometimes only one turn per coil is used. 



Methods for Getting Sparkless Commutation. As the 

 coil moves under the brush the variation of the contact 

 resistance between the brush and the two segments to which 

 the coil is attached tends to make the current in the coil 

 reverse. In low voltage, slow-speed machines this effect is 

 great enough to produce sparkless commutation and it is 

 unnecessary to introduce into the short circuited coil an 

 e.m.f. to overcome that of self-induction. Commutation 

 which depends only upon this resistance effect to eliminate 

 sparking is sometimes called resistance commutation. If 

 some means is employed to generate in the short circuited 

 coil an e.m.f. equal and opposite to that of self-induction, 

 the machine is said to have e.m.f. commutation. Of course, 

 even if a machine employs e.m.f. commutation, the resist- 

 ance effect is also present, helping the e.m.f. effect. 



Resistance Commutation. The idea of resistance com- 

 mutation may be understood by studying the variation 

 of the contact resistance as the commutator bars move 

 under a brush. Consider only a few coils of the armature 

 shown in Fig. 66; the coil undergoing commutation and a 

 few on each side of it are shown in Fig. 67, where B 

 is the coil about to be commutated. Actually the coil 

 B moves and the brush is stationary; in the diagram we 

 have shown the brush as moving backward on the com- 

 mutator and the coil as stationary because it is easier to 

 represent the relative motion of the two in this way. The 

 brush marked (1) gives the first position of the brush, that 

 marked (2) the second position we wish to consider, etc. 



Position (1) shows coil B before it begins to be com- 



