SINGLB-PHASE MOTORS 301 



fore, whon a single-phase voltage is applied to one phase of a 

 two-phase or of a three-phase motor, the rotating field is almost 

 identical with that which exists when polyphase voltages are 

 applied to the terminals. Consequently, if a single-phase 

 voltage be applied to one phase of a two-phase stator, a quadra- 

 ture emf. exists across the terminals of the other phase. If a 

 single-phase voltage be applied across one phase of a three-phase 



or, the voltages across the three terminals will very nearly 

 equal one another and will be approximately 120 apart. As 

 the induced emfs. are less than the applied terminal voltage by 

 the amount of the stator impedance drop and as the rotating 

 field is somewhat elliptical, the terminal voltages will not be 

 exactly balanced. For example, in Fig. 279, 220- volts, single- 

 phase, is applied across one phase of a three-phase motor, and 

 voltages of approximately 210 volts and 200 volts are found to 

 exist across the other two phases. 



Polyphase induction motors are often used in this manner to 

 produce polyphase voltages from single-phase supply. That 

 is, single-phase voltage is supplied to one phase of the poly- 

 phase stator and polyphase voltages are obtained from the 

 stator terminals. When so used the motor is called a phase 

 converter. 



The phase converter is used to some extent in railway electri- 

 fication. Although the three-phase induction motor is adapted to 

 railway work, there is considerable disadvantage in using the two 

 trolleys which are required if three-phase power is to be supplied 

 to the locomotive. By using a phase converter, the advantages 



lie three-phase motor for driving may be secured and at the 



,' time all the advantages of a single trolley are retained. 

 The phase converter receives single-phase power, which is pulsat- 

 ing, and delivers three-phase power, which is substantially 



dy. This is made possible by the kinetic energy stored 

 in the rotating armature of the phase converter, tin'- en 

 supplying the power durini: those times when the nn^le-phase 

 IH.U --alive or is less than the average value of the poly- 



phase power. The armature accelerates and BO .-lore- km< 



energy during the periodi when the ring] eds 



the average power. The armature slows down and so gives 



up some of its kinetic energy during the period.- \\ln-n the single- 



