292 



ALTERNATING CURRENTS 



Repulsion motors have characteristics similar to those of 

 series motors and have large starting torque. The sparking is 

 very small at synchronous speed (3,600 r.p.m. for a two-pole, 

 60-cycle motor) but at speeds differing greatly from this, the 

 sparking may be excessive. It will be noted that the motor of 

 Fig. 268 is similar to the inductively compensated series motor 

 of Fig. 259, with the connections of the compensating winding 

 and of the armature interchanged. There are several types of 

 repulsion motor on the market which, while differing in detail 

 from the motor just described, involve identical principles. 



116. Single-phase Induction Motor. Figure 269 shows a two- 

 pole motor whose magnetic field is produced by single-phase 

 current flowing in a simple field winding. The current in this 



A.C. 



Line 



FIG. 269. Single-phase, alternating field. FIG. 270. Time-variation of a 



single-phase alternating field. 



field is assumed to vary sinusoidally with time and if the iron 

 be assumed to operate at moderate flux densities, the flux through 

 the armature will vary practically sinusoidally with time. The 

 variation of this field with time may be represented by the pro- 

 jection of a rotating vector <f> max upon a vertical axis XX, shown 

 in Fig. 270. The vector <j> max is equal to the maximum value of 

 the flux and its speed of rotation in revolutions per second is 

 equal to the line frequency in cycles per second. 



It may also be assumed that this single-phase field is made up of 

 two equal and oppositely rotating fields represented by two equal 

 and oppositely rotating vectors, Fig. 271 (a), the maximum value 

 of each of these fields or vectors being equal to one-half <t> max - 

 The resultant of two such vectors always lies along the vertical 

 axis and is equal in magnitude at any instant to the field actually 



