402 THE INDUCTION MOTOR. 



It is interesting to note that the rotating field will 

 now be capable of acting on the idle windings of the 

 stator which we assumed to be cut out of circuit. Thus, 

 if one wire of a three-phase motor be interrupted, while the 

 rotor is revolving, so as to convert the motor into a single- 

 phase motor, and voltmeters are connected across the 

 two idle phases, they will be found to have voltages 

 induced in them of practically the same value as that 

 of the supply. Further, the phase of the voltages 

 induced in these windings by the rotating field will be 

 exactly 120 respectively in advance and behind the 

 phase of the voltage in the winding which is still supplied 

 with current. 



Starting Devices of a Single-phase Motor It has been 

 explained that a single-phase motor with a short- 

 circuited rotor is not self-starting, since no rotating 

 field is set up in the stationary rotor. The various 

 methods employed for starting such motors consist in 

 the addition of an extra winding to the stator between 

 the main windings, i.e., so situated as to produce a field 

 in a direction perpendicular to the field of the main wind- 

 ing. This extra winding must then be supplied by some 

 means with an alternating current out of phase with the 

 current of the main winding, since this is the condition 

 for the production of a rotating field. The chief difference 

 in the starting devices employed by different makers 

 lies in the method of producing the phase-difference 

 in the " starting phase." The usual arrangements con- 

 sist of either an inductive resistance or condenser in the 

 starting circuit, or a non-inductive resistance in the 

 main circuit. By any of these means a difference in the 

 relative inductance of the main and auxiliary circuits is 

 produced, and a difference of phase in the currents in 

 them is brought about. 



The starting torque of a single-phase motor is usually 

 low and the starting current is relatively high ; it is 

 consequently desirable whenever possible to start up 

 under light loads only. The difference in phase pro- 

 duced between the currents in the main and auxiliary 

 phases is only small, and consequently the flux 

 perpendicular to the main flux is weak. It is not 

 of much use to make the auxiliary winding itself highly 



