CHAPTER VI. 



The Single-Phase Motor. 



THE simplified and improved transformer diagram which has 

 stood us in good stead in understanding the phenomena in 

 polyphase motors, will serve our purpose equally well in the 

 treatment of the single-phase motor. The method here employed* 

 is based upon the well-known theorem first made prominent by the 

 late Galileo Ferraris and Andre Blondel, that an oscillating magnetic 

 field can, in all its effects, be replaced by two revolving fields, the 

 amplitude of each of which is equal to half the amplitude of the 

 oscillating field ; the two fields revolve in opposite directions at a 

 frequency equal to that of the oscillating alternating field. 



94. A two-pole armature revolving at a speed of ~ 2 in an oscillat- 

 ing magnetic field of the frequency ~i, has relative to the one mag- 

 netic field, which we will call I, a slip of ~i ~ 2 > relative to the 

 other, II, a slip of ~i + ~ 2 . 



95. Let us now consider the field II. At the immense slip of ~ x H ', 

 the secondary ampere-turns act almost exactly in an opposite direc- 

 tion to the primary ampere-turns ; they thus neutralize each other, 

 leaving only a small field the vector of which coincides with the 

 vectors of the primary and secondary ampere-turns just large 

 enough to balance the voltage which drives the magnetizing current 

 through the field-coils. 



96. We have dissolved the amplitude of the impressed alternating 

 current flowing through the primary into two components of half the 

 amplitude revolving in opposite directions. This is equivalent to 

 having two polyphase motors, the field-coils of which are coupled in 



*See the author's article on "Asynchronous Alternating- Current Motors," in 

 the Elektrotechnische Zeitschrift, Berlin, March 25, 1897. 



54 



