THE SINGLE-PHASE SERIES MOTOR. 303 



to the field winding as the short-circuited secondary coil of a 

 transformer of which the field winding is the primary coil. There- 

 fore an excessive short-circuit current is produced through the 

 armature coil and through the tip of the brush ; and the result 

 is an increased heating of the armature coils, commutator, and 

 brushes, and an excessive sparking at the brush tips as the com- 

 mutator bars pass from under the brushes. 



Low power factor. A less serious difficulty in the operation 

 of a series motor by alternating current is that the inductance of 

 the field and armature windings cause the current to lag behind 

 the supply voltage in phase, so that the power factor of the series 

 motor when driven by alternating current is always considerably 

 less than unity. 



The single-phase series motor. The ordinary series motor when 

 designed so as to operate satisfactorily with alternating current 

 is called the single-phase commutator motor, or the single-phase 

 series motor. The peculiar features of design and the special de- 

 vices for reducing sparking and for increasing the power factor of 

 such a motor are described in a subsequent article. Every part 

 of the magnetic circuit of a single-phase series motor must be 

 made of laminated iron. 



141, The clock diagram of the single-phase series motor. Let 

 the line Of, Fig. 260, represent the current flowing through the 



field winding and armature of a 

 series motor, let X f be the re- 

 actance of the field winding, X a 

 the reactance of the armature 

 winding, R f the resistance of the 

 field winding, and R a the resist- 

 ance of the armature winding. 

 The components of the electro- 

 Fig 260 motive forced which is required 



to force the current / through 



the field winding are XJ and RJ, as shown in Fig. 260, and 

 the components of the electromotive force A which is required 



