300 



ALTERNATING CURRENTS 



to start the motor except under the lightest loads. The Wagner 

 Single-Phase Induction Motor starts as a repulsion motor and 

 has a large starting torque. A cross-section of the motor and a 

 view of the armature are shown in Fig. 278. The armature is 

 similar to the type used in the ordinary direct-current motor, 

 except that the brushes J press on the end of the commutator L 

 rather than radially on its surface. These brushes are short- 

 circuited on themselves and are set in a position corresponding 

 to those in Fig. 266 or Fig. 267, pages 290 and 291, so that the 

 motor starts as a repulsion motor. It has a large starting 

 torque and comes up to speed rapidly. As it approaches syn- 

 chronism, a centrifugal device V, Fig. 278 (6), is thrown outward 

 and pushes the brushes away from the commutator, while at the 

 same time a metal ring K presses against the commutator bars 

 on the inside and short-circuits them. The motor now operates 

 as a single-phase induction motor. 



120. The Induction Motor as a Phase Converter. If a 

 three-phase induction motor be operated single-phase, as shown 

 in Fig. 279, three-phase voltages exist across its three terminals. 

 The reason for this is as follows: 



The back emf . in each 

 phase of a polyphase 

 induction motor is in- 

 duced by the rotating 

 field cutting the stator 

 conductors. If the stator 

 is wound for two-phase, 

 the induced emfs. at the 

 stator terminals are two- 

 phase; if the stator is 

 wound for three-phase, 

 the induced emfs. at the stator terminals are three-phase. The 

 induced emf. in each phase of a polyphase induction motor is 

 slightly less than the terminal voltage (per phase) by the amount 

 of the stator impedance drop. 



It was shown in Par. 117, page 294, that in a single-phase in- 

 duction motor, a rotating field exists. At small values of slip 

 this field departs but slightly from a true rotating field such as is 

 produced by polyphase currents in polyphase windings. There- 



OF ELECTRICAL LABORATORY/] 



FACULTY OF APPLIED SCIENCE. 1 



20<)V. 



To 3-Phase 

 nduction Motorg 



Stator of 

 Phase Converter 



FIG. 279. Method of obtaining 3-phase power 

 from single-phase supply, by means of squirrel- 

 cage induction motor operating as phase-con- 

 verter. 



