CHAPTER IV 

 INDUCED E.M.F. IN ELECTRICAL MACHINERY 



23. Methods of Inducing E.M.F. The following are the prin- 

 cipal cases of induced e.m.f. in electrical machinery and apparatus: 



(a) In a transformer, an alternating magnetizing current in the 

 primary winding produces an alternating flux which links with 

 both windings and induces in them alternating e.m.f s. A similar 

 case is that of a variable current in a transmission line which 

 induces a voltage in a telephone line which runs parallel to it . 



(6) In a direct-current machine, in a rotary converter, and in 

 a homopolar machine electromotive forces are induced in the 

 armature conductors by moving them across a stationary magnetic 

 field. 



(c) In an alternator and in a synchronous motor, with a sta- 

 tionary armature and a revolving field, electromotive forces are 

 induced by making the magnetic flux travel past the armature 

 conductors. 



((/) In a polyphase induction motor the currents in the stator 

 and in the rotor produce together a resultant magnetomotive force 

 which moves along the air-gap and excites a gliding (revolving) 

 flux. This flux induces voltages in both the primary and the sec- 

 ondary windings. 



(e) In a single-phase motor, with or without a commutator, the 

 r.m.fs. induced in the armature are partly due to the " transformer 

 action/' as under (a), and partly to the "generator action/' as 

 under (6). 



(/) In an inductor-type alternator both the exciting and the 

 armature windings are stationary; the pole pieces alone revolve. 

 The flux linked with the armature coils varies periodically, due to 

 the varying reluctance of the magnetic circuit, because of the 

 motion of the pole pieces. This van-ing flux induces an alternating 

 e.m.i. in the armature winding. Or else, one may say that the 



M 



