THE SINGLE-PHASE SERIES MOTOR. 311 



simplex type, the necessary thickness of the brushes is determined 

 solely by the necessary area of brush contact to collect the given 

 current. If the armature winding is of the multiplex type, the 

 brush must never touch fewer than n commutator bars, where n 

 is the number of constituent windings in the multiplex winding. 

 In the multiplex winding, however, adjacent commutator bars do 

 not form terminals of an armature section, and therefore the 

 touching of two bars by a brush does not short-circuit the arma- 

 ture section of a multiplex winding ; in fact the brush must touch 

 n 4- I commutator bars of an ;z-plex winding to short-circuit an 

 armature section.* Therefore thick brushes may be used with 

 multiplex armature windings. 



Special devices for preventing sparking. The foregoing dis- 

 cussion refers to those general features of design which tend to 

 reduce short-circuit currents in the armature sections of single- 

 phase series motors. It does not seem to be possible, however, 

 to produce a single-phase series motor which will operate satis- 

 factorily without special devices for preventing excessive short- 

 circuit currents. The simplest device for this purpose is the in- 

 sertion of a moderate amount of resistance in each commutator 

 lead, and a number of special inductive devices have been pro- 

 posed, of which, perhaps, the simplest is the balanced choke-coil 

 arrangement of Mr. S. S. Seyfert 



The resistance leads. The coils bcdef, Fig. 268, represent the 

 sections of an armature winding, and rrrr represent resistances 

 inserted in the commutator leads. The figure shows the arma- 

 ture section d short-circuited by a brush, and the heavy arrow 

 represents the electromotive force induced in the section d by 

 the pulsating field flux. The short-circuit current must flow 



* Short circuits are produced in a multiplex winding by the simultaneous connect- 

 ing of two of the constituent windings by two brushes. Some of these short circuits 

 pass through one constituent winding from a positive to a negative brush and back to 

 the positive brush through another constituent winding ; some of them pass through 

 one constituent winding from one positive (or negative) brush set to another positive 

 (or negative) brush set and back through another constituent winding, and some of 

 them pass through the winding from one positive (or negative) brush set to another 

 positive (or negative) brush set and back through the brush leads. 



